Publications

Preprints [4]:

D. Jaksch, P. Givi, A.J. Daley and T. Rung,
Variational Quantum Algorithms for Computational Fluid Dynamics,
preprint arXiv:2209.04915.
Variational Quantum Algorithms for Computational Fluid Dynamics
Quantum computing uses the physical principles of very small systems to develop computing platforms which can solve problems that are intractable on conventional supercomputers. There are challenges not only in building the required hardware, but also in identifying the most promising application areas and developing the corresponding quantum algorithms. The availability of intermediate-scale noisy quantum computers is now propelling the developments of novel algorithms, with applications across a variety of domains, including in aeroscience. Variational quantum algorithms are particularly promising since they are comparatively noise tolerant and aim to achieve a quantum advantage with only a few hundred qubits. Furthermore, they are applicable to a wide range of optimization problems arising throughout the natural sciences and industry. To demonstrate the possibilities for the aeroscience community, we give a perspective on how variational quantum algorithms can be utilized in computational fluid dynamics. We discuss how classical problems are translated into quantum algorithms and their logarithmic scaling with problem size. As an explicit example we apply this method to Burgers Equation in one spatial dimension. We argue that a quantum advantage over classical computing methods could be achieved by the end of this decade if quantum hardware progresses as currently envisaged and emphasize the importance of joining up development of quantum algorithms with application-specific expertise to achieve real-world impact.

created: 13-09-2022
Z. Zhang, D. Dreon, T. Esslinger, D. Jaksch, B. Buca and T. Donner,
Tunable Non-equilibrium Phase Transitions between Spatial and Temporal Order through Dissipation,
preprint arxiv:2205.01461.
Tunable Non-equilibrium Phase Transitions between Spatial and Temporal Order through Dissipation
We propose an experiment with a driven quantum gas coupled to a dissipative optical cavity that realizes a novel kind of far-from-equilibrium phase transition between spatial and temporal order. The control parameter of the transition is the detuning between the drive frequency and the cavity resonance. For negative detunings, the system features a spatially ordered phase, while positive detunings lead to a phase with both spatial order and persistent oscillations, which we call dissipative spatio-temporal lattice. We give numerical and analytical evidence for this superradiant phase transition and show that the spatio-temporal lattice originates from cavity dissipation. In both regimes the atoms are subject to an accelerated transport, either via a uniform acceleration or via abrupt transitions to higher momentum states. Our work provides perspectives for temporal phases of matter that are not possible at equilibrium.

created: 04-05-2022
G. Guarnieri, M.T. Mitchinson, A. Purkayastha, D. Jaksch, B. Buca and J. Goold,
Time periodicity from randomness in quantum systems,
preprint arXiv:2104.13402.
Time periodicity from randomness in quantum systems
Many complex systems can spontaneously oscillate under non-periodic forcing. Such self-oscillators are commonplace in biological and technological assemblies where temporal periodicity is needed, such as the beating of a human heart or the vibration of a cello string. While self-oscillation is well understood in classical non-linear systems and their quantized counterparts, the spontaneous emergence of periodicity in quantum systems without a semi-classical limit is more elusive. Here, we show that this behaviour can emerge within the repeated-interaction description of open quantum systems. Specifically, we consider a many-body quantum system that undergoes dissipation due to sequential coupling with auxiliary systems at random times. We develop dynamical symmetry conditions that guarantee an oscillatory long-time state in this setting. Our rigorous results are illustrated with specific spin models, which could be implemented in trapped-ion quantum simulators.

created: 04-05-2021
B. Buca, A. Purkayastha, G. Guarnieri, M.T. Mitchinson, D. Jaksch and J. Goold,
Quantum many-body attractor with strictly local dynamical symmetries,
preprint arXiv:2008.11166.
Quantum many-body attractor with strictly local dynamical symmetries
Dynamical symmetries are algebraic constraints on quantum dynamical systems, which are often responsible for persistent temporal periodicity of observables. In this work, we discuss how an extensive set of strictly local dynamical symmetries can exist in an interacting many-body quantum system. These strictly local dynamical symmetries lead to spontaneous breaking of continuous time-translation symmetry, i.e. the formation of extremely robust and persistent oscillations when an infinitesimal time-dependent perturbation is applied to an arbitrary initial (stationary) state. Observables which do not overlap with the local (dynamical) symmetry operators can relax, losing memory of their initial conditions. The remaining observables enter highly robust non-equilibrium limit cycles, signalling the emergence of a non-trivial quantum many-body attractor. We provide an explicit recipe for constructing Hamiltonians featuring local dynamical symmetries. As an example, we introduce the XYZ spin-lace model, which is a model of a quasi-1D quantum magnet.

created: 26-08-2020

Refereed publications in international journals [193]:

M. Hughes, A.U.J. Lode, D. Jaksch and P. Molignini,
Accuracy of quantum simulators with ultracold dipolar molecules: a quantitative comparison between continuum and lattice descriptions,
Phys. Rev. A 107, 033323 (2023).
Accuracy of quantum simulators with ultracold dipolar molecules: a quantitative comparison between continuum and lattice descriptions
With rapid progress in control and manipulation of ultracold magnetic atoms and dipolar molecules, the quantum simulation of lattice models with strongly interacting dipole-dipole interactions (DDI) and high densities is now within experimental reach. This rapid development raises the issue about the validity of quantum simulation in such regimes. In this study, we address this question by performing a full quantitative comparison between the continuum description of a one-dimensional gas of dipolar bosons in an optical lattice, and the single-band Bose-Hubbard lattice model that it quantum simulates. By comparing energies and density distributions, and by calculating direct overlaps between the continuum and lattice many-body wavefunctions, we demonstrate that in regimes of strong DDI and high densities the continuum system fails to recreate the desired lattice model. Two-band Hubbard models become necessary to reduce the discrepancy observed between continuum and lattice descriptions, but appreciable deviations in the density profile still remain. Our study elucidates the role of strong DDI in generating physics beyond lowest-band descriptions and should offer a guideline for the calibration of near-term dipolar quantum simulators.

created: 22-11-2022, last modified: 23-03-2023
J. Tindall, D. Jaksch and C. Sanchez Munoz,
On the generality of symmetry breaking and dissipative freezing in quantum trajectories,
SciPost Phys. Core 6, 004 (2023).
On the generality of symmetry breaking and dissipative freezing in quantum trajectories
Recently, several studies involving open quantum systems which possess a strong symmetry have observed that every individual trajectory in the Monte Carlo unravelling of the master equation will dynamically select a specific symmetry sector to freeze into in the long-time limit. This phenomenon has been termed dissipative freezing, and in this paper we demonstrate how it is a general consequence of the presence of a strong symmetry in an open system and independent of any microscopic details. Specifically, through several simple, plausible, mathematical arguments, we detail how this phenomenon will happen in most setups with only a few exceptions. Using a number of example systems we illustrate these arguments, highlighting the relationship between the spectral properties of the Liouvillian in off-diagonal symmetry sectors and the time it takes for freezing to occur. In the limiting case that eigenmodes with purely imaginary eigenvalues are manifest in these sectors, freezing fails to occur. Such modes indicate the preservation of information and coherences between symmetry sectors of the system and are associated with phenomena such as non-stationarity and synchronisation. The absence of freezing at the level of a single quantum trajectory provides a simple, computationally efficient way of identifying these traceless modes.

created: 18-04-2022, last modified: 30-01-2023
M. Hughes and D. Jaksch,
Dipolar Bose-Hubbard model in finite-size real-space cylindrical lattices,
Phys. Rev. A 105, 053301 (2022).
Dipolar Bose-Hubbard model in finite-size real-space cylindrical lattices
Recent experimental progress in magnetic atoms and polar molecules has created the prospect of simulating dipolar Hubbard models with off-site interactions. When applied to real-space cylindrical optical lattices, these anisotropic dipole-dipole interactions acquire a tunable spatially dependent component while they remain translationally invariant in the axial direction, creating a sublattice structure in the azimuthal direction. We numerically study how the coexistence of these classes of interactions affects the ground state of hard-core dipolar bosons at half filling in a finite-size cylindrical optical lattice with octagonal rings. When these two interaction classes cooperate, we find a solid state where the density order is determined by the azimuthal sublattice structure and builds smoothly as the interaction strength increases. For dipole polarizations where the axial interactions are sufficiently repulsive, the repulsion competes with the sublattice structure, significantly increasing entanglement and creating two distinct ordered density patterns. The spatially varying interactions cause the emergence of these ordered states in small lattices as a function of interaction strength to be staggered according to the azimuthal sublattices.

created: 01-06-2022
J. Tindall, A. Searle, A. Alhajri and D. Jaksch,
Quantum Physics in Connected Worlds,
Nature Communications 13, 7445 (2022).
Quantum Physics in Connected Worlds
Theoretical research into many-body quantum systems has focused almost exclusively on regular structures which have a small, simple unit cell and where only a vanishingly small number of pairs of the constituents directly interact. Motivated by rapid advances in control over the pairwise interactions and geometries in many-body simulators, we determine the fate of many-body spin systems on more general, arbitrary graphs. When placing the minimum possible constraints on the underlying graph, we prove and observe how, with certainty in the thermodynamic limit, such systems behave like a single collective spin and exhibit no many-body physics. We thus understand the emergence of complex many-body physics as dependent on exceptional, geometrically constrained structures such as the low-dimensional, regular ones found in nature. Within the space of highly connected graphs we identify hitherto unknown exceptions via their inhomogeneity and use state-of-the-art Matrix Product State algorithms to observe how complexity is heralded in these systems by a large degree of entanglement and highly non-uniform correlation functions. By bringing a graph-theoretic approach to the realm of many-body physics, our work paves the way for the discovery and exploitation of a whole class of geometries which can host uniquely complex phases of matter.

created: 18-05-2022, last modified: 10-01-2023
B. Jaderberg, A. Eisfeld, D. Jaksch and S. Mostame,
Recompilation-enhanced simulation of electron-phonon dynamics on IBM Quantum computers,
New J. Phys. 24, 093017 (2022).
Recompilation-enhanced simulation of electron-phonon dynamics on IBM Quantum computers
Simulating quantum systems is believed to be one of the first applications for which quantum computers may demonstrate a useful advantage. For many problems in physics, we are interested in studying the evolution of the electronphonon Hamiltonian, for which efficient digital quantum computing schemes exist. Yet to date, no accurate simulation of this system has been produced on real quantum hardware. In this work, we consider the absolute resource cost for gate-based quantum simulation of small electron-phonon systems as dictated by the number of Trotter steps and bosonic energy levels necessary for the convergence of dynamics. We then apply these findings to perform experiments on IBM quantum hardware for both weak and strong electron-phonon coupling. Despite significant device noise, through the use of approximate circuit recompilation we obtain electron-phonon dynamics on current quantum computers comparable to exact diagonalisation. Our results represent a significant step in utilising near term quantum computers for simulation of quantum dynamics and highlight the novelty of approximate circuit recompilation as an error mitigation tool.

created: 22-02-2022, last modified: 22-11-2022
N. Gourianov, M. Lubasch, S. Dolgov, Q.Y. van den Berg, H. Babaee, P. Givi, M. Kiffner and D. Jaksch,
A quantum-inspired approach to exploit turbulence structures,
Nature Computational Science 2, 30 (2022).
A quantum-inspired approach to exploit turbulence structures
Understanding turbulence is the key to our comprehension of many natural and technological flow processes. At the heart of this phenomenon lies its intricate multi-scale nature, describing the coupling between different-sized eddies in space and time. Here we introduce a new paradigm for analyzing the structure of turbulent flows by quantifying correlations between different length scales using methods inspired from quantum many-body physics. We present results for interscale correlations of two paradigmatic flow examples, and use these insights along with tensor network theory to design a structure-resolving algorithm for simulating turbulent flows. With this algorithm, we find that the incompressible Navier-Stokes equations can be accurately solved within a computational space reduced by over an order of magnitude compared to direct numerical simulation. Our quantum-inspired approach provides a pathway towards conducting computational fluid dynamics on quantum computers.

created: 29-10-2021, last modified: 28-03-2022
L.W. Anderson, M. Kiffner, P. Kl. Barkoutsos, I. Tavernelli, J. Crain and D. Jaksch,
Coarse grained intermolecular interactions on quantum processors,
Phys. Rev. A 105, 062409 (2022).
Coarse grained intermolecular interactions on quantum processors
Variational quantum algorithms (VQAs) are increasingly being applied in simulations of strongly-bound (covalently bonded) systems using full molecular orbital basis representations. The application of quantum computers to the weakly-bound intermolecular and non-covalently bonded regime however has remained largely unexplored. In this work, we develop a coarse-grained representation of the electronic response that is ideally suited for determining the ground state of weakly interacting molecules using a VQA. We require qubit numbers that grow linearly with the number of molecules and derive scaling behaviour for the number of circuits and measurements required, which compare favourably to traditional variational quantum eigensolver methods. We demonstrate our method on IBM superconducting quantum processors and show its capability to resolve the dispersion energy as a function of separation for a pair of non-polar molecules - thereby establishing a means by which quantum computers can model Van der Waals interactions directly from zero-point quantum fluctuations. Within this coarse-grained approximation, we conclude that current-generation quantum hardware is capable of probing energies in this weakly bound but nevertheless chemically ubiquitous and biologically important regime. Finally, we perform simulations on an extended version of the model which includes multipole interactions and anharmonicity - the consequences of the latter are unexamined in large systems using classical computational methods but can be incorporated here with low computational overhead.

created: 12-10-2021, last modified: 06-06-2022
P. Molignini, C. Leveque, H. Kessler, D. Jaksch, R. Chitra and A.U.J. Lode,
Crystallization via cavity-assisted infinite-range interactions,
Phys. Rev. A 106, L011701 (2022).
Crystallization via cavity-assisted infinite-range interactions
We study a one-dimensional array of bosons with infinite-range interactions mediated by a laser-driven dissipative optical cavity. The cavity-mediated infinite-range interactions open up a new pathway to fermionization, hitherto only known for dipolar bosons due to their long-range interactions. In parameter ranges attainable in state-of-the-art experiments, we systematically compare observables for bosons and fermions with infinite-range interactions. At large enough laser pump power, many observables, including density distributions in real and momentum space, correlation functions, eigenvalues of the one-body density matrix, and superradiance order parameter, become identical for bosons and fermions. We map out the emergence of this cavity-induced fermionization as a function of pump power and contact interactions. We discover that cavity-mediated interactions can compensate a reduction by several orders of magnitude in the strength of the contact interactions needed to trigger fermionization.

created: 20-07-2021, last modified: 13-07-2022
B. Jaderberg, L.W. Anderson, W. Xi, S. Albanie, M. Kiffner and D. Jaksch,
Quantum Self-Supervised Learning,
Quantum Sci. Technol. 7, 035005 (2022).
Quantum Self-Supervised Learning
The popularisation of neural networks has seen incredible advances in pattern recognition, driven by the supervised learning of human annotations. However, this approach is unsustainable in relation to the dramatically increasing size of real-world datasets. This has led to a resurgence in self-supervised learning, a paradigm whereby the model generates its own supervisory signal from the data. Here we propose a hybrid quantum-classical neural network architecture for contrastive self-supervised learning and test its effectiveness in proof-of-principle experiments. Interestingly, we observe a numerical advantage for the learning of visual representations using small-scale quantum neural networks over equivalently structured classical networks, even when the quantum circuits are sampled with only 100 shots. Furthermore, we apply our best quantum model to classify unseen images on the ibmq_paris quantum computer and find that current noisy devices can already achieve equal accuracy to the equivalent classical model on downstream tasks.

created: 30-03-2021, last modified: 09-05-2022
B. Buca, C. Booker and D. Jaksch,
Theory of Quantum Synchronization and Limit Cycles under Dissipation,
SciPost Physics 12, 097 (2022).
Theory of Quantum Synchronization and Limit Cycles under Dissipation
Synchronization is a dynamical phenomenon found in complex systems ranging from biological systems to human society and is characterized by the constituents parts of a system locking their motion so that they have the same phase and frequency. Recent intense efforts have focused on understanding synchronization in quantum systems without clear semi-classical limits but no comprehensive theory providing a systematic basis for the underlying physical mechanisms has yet been found. Through a complete characterization of the non-decaying persistently oscillating eigenmodes of smooth quantum evolutions corresponding to quantum limit cycles, we provide a general algebraic theory of quantum synchronization which provides detailed criteria for various types of synchronization to occur. These results constitute the previously absent framework within which to pursue a structured study of quantum synchronization. This framework enables us to study both stable synchronization which lasts for infinitely long times limit, and metastable synchronization which lasts for long, but finite, times. Moreover, we give compact algebraic criteria that may be checked in a given system to prove the absence of synchronization. We use our theory to demonstrate the anti-synchronization of two spin-1/2 and discuss synchronization in the Fermi-Hubbard model and its generalisations which are relevant for various fermionic cold atom experiments, including multi-species, multi-orbital and higher spin atoms.

created: 03-03-2021, last modified: 28-03-2022
H. Gao, F. Schlawin and D. Jaksch,
Higgs mode stabilization by photo-induced long-range interactions in a superconductor,
Phys. Rev. B 104, L140503 (2021).
Higgs mode stabilization by photo-induced long-range interactions in a superconductor
We show that low-lying excitations of a 2D BCS superconductor are significantly altered when coupled to an externally driven cavity, which induces controllable long-range attractive interactions between the electrons. We find that they combine non-linearly with intrinsic local interactions to increase the Bogoliubov quasiparticle excitation energies, thus enlarging the superconducting gap. The long-range nature of the driven-cavity-induced attraction qualitatively changes the collective excitations of the superconductor. Specifically, they lead to the appearance of additional collective excitations of the excitonic modes. Furthermore, the Higgs mode is pushed into the gap and now lies below the Bogoliubov quasiparticle continuum such that it cannot decay into quasiparticles. This way, the Higgs mode lifetime is greatly enhanced.

created: 11-06-2021, last modified: 12-10-2021
J. Tindall, F. Schlawin, M. Sentef and D. Jaksch,
Lieb Theorem and Maximum Entropy Condensates,
Quantum 5, 610 (2021).
Lieb Theorem and Maximum Entropy Condensates
The maximum entropy steady states which form upon Floquet heating of the ground state of the Hubbard model on unbalanced bi-partite lattices are shown to possess uniform finite off-diagonal long-range order in the thermodynamic limit. For repulsive interactions the induced order corresponds to the formation of a spin-wave condensate, whilst for attractive interactions it instead corresponds to the formation of a superconducting, eta-paired condensate. This creation of a hot condensate can occur on any periodically driven unbalanced lattice where the relevant SU(2) symmetry is preserved. Our results provide an understanding of how strong driving can expose order which has been suppressed by the lattice geometry - independent of any microscopic parameters. We discuss the implications of this for recent experiments observing emergent superconductivity in solid-state photoexcited compounds.

created: 09-03-2021, last modified: 24-12-2021
J. Tindall, F. Schlawin, M. Sentef and D. Jaksch,
Analytical Solution for the Steady States of the Driven Hubbard model,
Phys. Rev. B 103, 035146 (2021).
Analytical Solution for the Steady States of the Driven Hubbard model
Under the action of coherent periodic driving a generic quantum system will undergo Floquet heating and continuously absorb energy until it reaches a featureless thermal state. The phase-space constraints induced by certain symmetries can, however, prevent this and allow the system to dynamically form robust steady states with off-diagonal long-range order. In this work, we take the Hubbard model on an arbitrary lattice with arbitrary filling and, by simultaneously diagonalising the two possible SU(2) symmetries of the system, we analytically construct the correlated steady states for different symmetry classes of driving. This construction allows us to make verifiable, quantitative predictions about the long-range particle-hole and spin-exchange correlations that these states can possess. In the case when both SU(2) symmetries are preserved in the thermodynamic limit we show how the driving can be used to form a unique condensate which simultaneously hosts particle-hole and spin-wave order.

created: 10-11-2020, last modified: 01-02-2021
A.U.J. Lode, R. Lin, M. Buettner, L. Papariello, C. Leveque, R. Chitra, M.C. Tsatsos, D. Jaksch and P. Molignini,
Optimized Observable Readout from Single-shot Images of Ultracold Atoms via Machine Learning,
Phys. Rev. A 104, L041301 (2021).
Optimized Observable Readout from Single-shot Images of Ultracold Atoms via Machine Learning
Single-shot images are the standard readout of experiments with ultracold atoms - the tarnished looking glass into their many-body physics. The efficient extraction of observables from single-shot images is thus crucial. Here, we demonstrate how artificial neural networks can optimize this extraction. In contrast to standard averaging approaches, machine learning allows both one- and two-particle densities to be accurately obtained from a drastically reduced number of single-shot images. Quantum fluctuations and correlations are directly harnessed to obtain physical observables for bosons in a tilted double-well potential at an unprecedented accuracy. Strikingly, machine learning also enables a reliable extraction of momentum-space observables from real-space single-shot images and vice versa. This obviates the need for a reconfiguration of the experimental setup between in-situ and time-of-flight imaging, thus potentially granting an outstanding reduction in resources.

created: 29-10-2020, last modified: 25-11-2021
C. Sanchez Munoz and D. Jaksch,
Squeezed Lasing,
Phys. Rev. Lett. 127, 183603 (2021).
Squeezed Lasing
We introduce the idea of a squeezed laser, in which a squeezed cavity mode develops a macroscopic photonic occupation powered by stimulated emission. Above the lasing threshold, the emitted light retains both the spectral purity inherent of a laser and the photon correlations characteristic of a photonic mode with squeezed quadratures. Our proposal, which can be implemented in optical setups, relies on the parametric driving of the cavity and dissipative stabilization by a broadband squeezed vacuum. We show that the squeezed laser can find applications going beyond those of standard lasers thanks to the squeezed character, such as enhanced operation in multi-photon microscopy or Heisenberg scaling of the Fisher information in quantum parameter estimation.

created: 10-08-2020, last modified: 29-10-2021
M. Budden, T. Gebert, M. Buzzi, G. Jotzu, E. Wang, T. Matsuyama, G. Meier, Y. Laplace, D. Pontiroli, M. Ricco, F. Schlawin, D. Jaksch and A. Cavalleri,
Evidence for metastable photo-induced superconductivity in K3C60,
Nature Physics 17, 611 (2021).
Evidence for metastable photo-induced superconductivity in K3C60
Far and mid infrared optical pulses have been shown to induce non-equilibrium unconventional orders in complex materials, including photo-induced ferroelectricity in quantum paraelectrics, magnetic polarization in antiferromagnets and transient superconducting correlations in the normal state of cuprates and organic conductors. In the case of non-equilibrium superconductivity, femtosecond drives have generally resulted in electronic properties that disappear immediately after excitation, evidencing a state that lacks intrinsic rigidity. Here, we make use of a new optical device to drive metallic K3C60 with mid-infrared pulses of tunable duration, ranging between one picosecond and one nanosecond. The same superconducting-like optical properties observed over short time windows for femtosecond excitation are shown here to become metastable under sustained optical driving, with lifetimes in excess of ten nanoseconds. Direct electrical probing becomes possible at these timescales, yielding a vanishingly small resistance. Such a colossal positive photo-conductivity is highly unusual for a metal and, when taken together with the transient optical conductivities, it is rather suggestive of metastable light-induced superconductivity.

created: 14-04-2020, last modified: 18-05-2022
H. Gao, J.R. Coulthard, D. Jaksch and J. Mur-Petit,
Anomalous spin-charge separation in a driven Hubbard system,
Phys. Rev. Lett. 125, 195301 (2020).
Anomalous spin-charge separation in a driven Hubbard system
Spin-charge separation (SCS) is a striking manifestation of strong correlations in low-dimensional quantum systems, whereby a fermion splits into separate spin and charge excitations that travel at different speeds. Here, we demonstrate that periodic driving enables control over SCS in a Hubbard system near half-filling. In one dimension, we predict analytically an exotic regime where charge travels slower than spin and can even become frozen, in agreement with numerical calculations. In two dimensions, the driving slows both charge and spin, and leads to complex interferences between single-particle and pair-hopping processes.

created: 23-05-2020, last modified: 10-11-2020
J. Tindall, F. Schlawin, M. Buzzi, D. Nicoletti, J.R. Coulthard, H. Gao, A. Cavalleri, M. Sentef and D. Jaksch,
Dynamical Order and Superconductivity in a Frustrated Many-Body System,
Phys. Rev. Lett. 125, 137001 (2020).
Dynamical Order and Superconductivity in a Frustrated Many-Body System
In triangular lattice structures, spatial anisotropy and frustration can lead to rich equilibrium phase diagrams with regions containing complex, highly entangled states of matter. In this work, we study the driven two-rung triangular Hubbard model and evolve these states out of equilibrium, observing how the interplay between the driving and the initial state unexpectedly shuts down the particle-hole excitation pathway. This restriction, which symmetry arguments fail to predict, dictates the transient dynamics of the system, causing the available particle-hole degrees of freedom to manifest uniform long-range order. We discuss possible implications of our results for a recent experiment on photo-induced superconductivity in k-(BEDT-TTF)2Cu[N(CN)2]Br molecules.

created: 20-05-2020, last modified: 22-09-2020
C. Booker, B. Buca and D. Jaksch,
Non-stationarity and Dissipative Time Crystals: Spectral Properties and Finite-Size Effects,
New J. Phys. 22, 085007 (2020).
Non-stationarity and Dissipative Time Crystals: Spectral Properties and Finite-Size Effects
We discuss the emergence of non-stationarity in open quantum many-body systems. This leads us to the definition of dissipative time crystals which display experimentally observable, persistent, time-periodic oscillations induced by noisy contact with an environment. We use the Loschmidt echo and local observables to indicate the presence of a dissipative time crystal. Starting from the closed Hubbard model we then provide examples of dissipation mechanisms that yield experimentally observable quantum limit cycles and allow analysis of the emergence of dissipative time crystals. For a disordered Hubbard model including two-particle loss and gain we find a dark Hamiltonian driving oscillations between GHZ states in the long-time limit. Finally, we discuss how the presented examples could be experimentally realized.

created: 12-05-2020, last modified: 09-10-2020
B. Buca, C. Booker, M. Medenjak and D. Jaksch,
Bethe ansatz approach for dissipation: exact solutions of quantum many-body dynamics under loss,
New J. Phys. 22, 123040 (2020).
Bethe ansatz approach for dissipation: exact solutions of quantum many-body dynamics under loss
We develop a Bethe ansatz based approach to study dissipative systems experiencing loss. The method allows us to exactly calculate the spectra of interacting, many-body Liouvillians. We discuss how the dissipative Bethe ansatz opens the possibility of analytically calculating the dynamics of a wide range of experimentally relevant models including cold atoms subjected to one and two body losses, coupled cavity arrays with bosons escaping the cavity, and cavity quantum electrodynamics. As an example of our approach we study the relaxation properties in a boundary driven XXZ spin chain. We exactly calculate the Liouvillian gap and find different relaxation rates with a novel type of dynamical dissipative phase transition. This physically translates into the formation of a stable domain wall in the easy-axis regime despite the presence of loss. Such analytic results have previously been inaccessible for systems of this type.

created: 14-04-2020, last modified: 09-03-2021
Y. Ashida, A. Imamoglu, J. Faist, D. Jaksch, A. Cavalleri and E. Demler,
Quantum Electrodynamic Control of Matter: Cavity-Enhanced Ferroelectric Phase Transition,
Phys. Rev. X 10, 041027 (2020).
Quantum Electrodynamic Control of Matter: Cavity-Enhanced Ferroelectric Phase Transition
The light-matter interaction can be utilized to qualitatively alter physical properties of materials. Recent theoretical and experimental studies have explored this possibility of controlling matter by light based on driving many-body systems via strong classical electromagnetic radiation, leading to a time-dependent Hamiltonian for electronic or lattice degrees of freedom. To avoid inevitable heating, pump-probe setups with ultrashort laser pulses have so far been used to study transient light-induced modifications in materials. Here, we pursue yet another direction of controlling quantum matter by modifying quantum fluctuations of its electromagnetic environment. In contrast to earlier proposals on light-enhanced electron-electron interactions, we consider a dipolar quantum many-body system embedded in a cavity composed of metal mirrors and formulate a theoretical framework to manipulate its equilibrium properties on the basis of quantum light-matter interaction. We analyze hybridization of different types of the fundamental excitations, including dipolar phonons, cavity photons, and plasmons in metal mirrors, arising from the cavity confinement in the regime of strong light-matter interaction. This hybridization qualitatively alters the nature of the collective excitations and can be used to selectively control energy-level structures in a wide range of platforms. Most notably, in quantum paraelectrics, we show that the cavity-induced softening of infrared optical phonons enhances the ferroelectric phase in comparison with the bulk materials. Our findings suggest an intriguing possibility of inducing a superradiant-type transition via the light-matter coupling without external pumping. We also discuss possible applications of the cavity-induced modifications in collective excitations to molecular materials and excitonic devices.

created: 01-04-2020, last modified: 10-11-2020
H. Gao, F. Schlawin, M. Buzzi, A. Cavalleri and D. Jaksch,
Photo-induced electron pairing in a driven cavity,
Phys. Rev. Lett. 125, 053602 (2020).
Photo-induced electron pairing in a driven cavity
We demonstrate how virtual scattering of laser photons inside a cavity via two-photon processes can induce controllable long-range electron interactions in two-dimensional materials. We show that laser light that is red(blue)-detuned from the cavity yields attractive(repulsive) interactions, whose strength is proportional to the laser intensity. Furthermore, we find that the interactions are not screened effectively except at very low frequencies. For realistic cavity parameters, laser-induced heating of the electrons by inelastic photon scattering is suppressed and coherent electron interactions dominate. When the interactions are attractive, they cause an instability in the Cooper channel at a temperature proportional to the square root of the driving intensity. Our results provide a novel route for engineering electron interactions in a wide range of two-dimensional materials including AB-stacked bilayer graphene and the conducting interface between LaAlO3 and SrTiO3.

created: 12-03-2020, last modified: 28-07-2020
B. Jaderberg, A. Agarwal, K. Leonhardt, M. Kiffner and D. Jaksch,
Minimum Hardware Requirements for Hybrid Quantum-Classical DMFT,
Quantum Sci. Technol. 5, 034015 (2020).
Minimum Hardware Requirements for Hybrid Quantum-Classical DMFT
We numerically emulate noisy intermediate-scale quantum (NISQ) devices and determine the minimal hardware requirements for two-site hybrid quantum-classical dynamical mean-field theory (DMFT). We develop a circuit recompilation algorithm which significantly reduces the number of quantum gates of the DMFT algorithm and find that the quantum-classical algorithm converges if the two-qubit gate fidelities are larger than 99 percent. The converged results agree with the exact solution within 10 percent, and perfect agreement within noise-induced error margins can be obtained for two-qubit gate fidelities exceeding 99.9 percent. By comparison, the quantum-classical algorithm without circuit recompilation requires a two-qubit gate fidelity of at least 99.999 percent to achieve perfect agreement with the exact solution. We thus find quantum-classical DMFT calculations can be run on the next generation of NISQ devices if combined with the recompilation techniques developed in this work.

created: 13-02-2020, last modified: 18-06-2020
M. Hughes, M.D. Frye, R. Sawant, G. Bhole, J.A. Jones, S.L. Cornish, M.R. Tarbut, J.M. Hutson, D. Jaksch and J. Mur-Petit,
Robust entangling gate for polar molecules using magnetic and microwave fields,
Phys. Rev. A 101, 062308 (2020).
Robust entangling gate for polar molecules using magnetic and microwave fields
Polar molecules are an emerging platform for quantum technologies based on their long-range electric dipole-dipole interactions, which open new possibilities for quantum information processing and the quantum simulation of strongly correlated systems. Here, we use magnetic and microwave fields to design a fast entangling gate with >0.999 fidelity and which is robust with respect to fluctuations in the trapping and control fields and to small thermal excitations. These results establish the feasibility to build a scalable quantum processor with a broad range of molecular species in optical-lattice and optical-tweezers setups.

created: 07-02-2020, last modified: 03-06-2020
H. Gao, J.R. Coulthard, D. Jaksch and J. Mur-Petit,
Controlling magnetic correlations in a driven Hubbard system far from half-filling,
Phys. Rev. A 101, 053634 (2020).
Controlling magnetic correlations in a driven Hubbard system far from half-filling
We propose using ultracold fermionic atoms trapped in a periodically shaken optical lattice as a quantum simulator of the t-J Hamiltonian, which describes the dynamics in doped antiferromagnets and is thought to be relevant to the problem of high-temperature superconductivity in the cuprates. We show analytically that the eï¬€ective Hamiltonian describing this system for oï¬€-resonant driving is the t-J model with additional pair hopping terms, whose parameters can all be controlled by the drive. We then demonstrate numerically that a slow modiï¬cation of the driving strength allows near-adiabatic transfer of the system from the ground state of the underlying Hubbard model to the ground state of the eï¬€ective t-J Hamiltonian. Finally, we report time-dependent density matrix renormalization group and exact diagionalization calculations illustrating the control achievable on the dynamics of spin-singlet pairs utilising this technique with current cold-atom quantumsimulation technology. These results open new routes to explore the interplay between density and spin in strongly-correlated fermionic systems through their out-of-equilibrium dynamics.

created: 07-02-2020, last modified: 01-06-2020
M. Buzzi, D. Nicoletti, M. Fechner, N. Tancogne-Dejean, M. Sentef, A Georges, M. Dressel, A. Henderson, T. Siegrist, J.A. Schlueter, K. Miyagawa, K. Kanoda, M.-S. Nam, A. Ardavan, J.R. Coulthard, J. Tindall, F. Schlawin, D. Jaksch and A. Cavalleri,
Photomolecular High-Temperature Superconductivity,
Phys. Rev. X 10, 031028 (2020).
Photomolecular High-Temperature Superconductivity
Superconductivity in organic conductors is often tuned by the application of chemical or external pressure. With this type of tuning, orbital overlaps and electronic bandwidths are manipulated, whilst the properties of the molecular building blocks remain virtually unperturbed.Here, we show that the excitation of local molecular vibrations in the charge-transfer salt K−(BEDT−TTF)2 Cu[N(CN)2]Br induces a colossal increase in carrier mobility and the opening of a superconducting-like optical gap. Both features track the density of quasi-particles of the equilibrium metal, and can be achieved up to a characteristic coherence temperature T* ~ 50K, far higher than the equilibrium transition temperature TC=12.5K. Notably, the large optical gap achieved by photo-excitation is not observed in the equilibrium superconductor, pointing to a light induced state that is different from that obtained by cooling. First-principle calculations and model Hamiltonian dynamics predict a transient state with long-range pairing correlations, providing a possible physical scenario for photo-molecular superconductivity.

created: 27-01-2020, last modified: 10-08-2020
Y Zhang, J. Tindall, J. Mur-Petit, D. Jaksch and B. Buca,
Stationary State Degeneracy of Open Quantum Systems with Non-Abelian Symmetries,
J. Phys. A: Math. Gen. 53, 215304 (2020).
Stationary State Degeneracy of Open Quantum Systems with Non-Abelian Symmetries
We study the null space degeneracy of open quantum systems with multiple non-Abelian, strong symmetries. By decomposing the Hilbert space representation of these symmetries into an irreducible representation involving the direct sum of multiple, commuting, invariant subspaces we derive a tight lower bound for the stationary state degeneracy. We apply these results within the context of open quantum many-body systems, presenting three illustrative examples: a fully-connected quantum network, the XXX Heisenberg model and the Hubbard model. We find that the derived bound, which scales at least cubically in the system size the SU(2) symmetric cases, is often saturated. Moreover, our work provides a theory for the systematic block-decomposition of a Liouvillian with non-Abelian symmetries, reducing the computational difficulty involved in diagonalising these objects and exposing a natural, physical structure to the steady states - which we observe in our examples.

created: 27-01-2020, last modified: 23-05-2020
P. Secular, N. Gourianov, M. Lubasch, S. Dolgov, S.R. Clark and D. Jaksch,
Parallel time-dependent variational principle algorithm for matrix product states,
Phys. Rev. B 101, 235123 (2020).
Parallel time-dependent variational principle algorithm for matrix product states
Combining the time-dependent variational principle (TDVP) algorithm with the parallelization scheme introduced by Stoudenmire and White for the density matrix renormalization group (DMRG), we present the first parallel matrix product state (MPS) algorithm capable of time evolving one-dimensional (1D) quantum lattice systems with long-range interactions. We benchmark the accuracy and performance of the algorithm by simulating quenches in the long-range Ising and XY models. We show that our code scales well up to 32 processes, with parallel efficiencies as high as 86 percent. Finally, we calculate the dynamical correlation function of a 201-site Heisenberg XXX spin chain with quadratically decaying interactions, which is challenging to compute sequentially. These results pave the way for the application of tensor networks to increasingly complex many-body systems.

created: 27-01-2020, last modified: 06-06-2020
J. Mur-Petit, A. Relano, R.A. Molina and D. Jaksch,
Fluctuations of work in realistic equilibrium states of quantum systems with conserved quantities,
SciPost Phys. Proc. 3, 024 (2020).
Fluctuations of work in realistic equilibrium states of quantum systems with conserved quantities
The out-of-equilibrium dynamics of quantum systems is one of the most fascinating problems in physics, with outstanding open questions on issues such as relaxation to equilibrium. An area of particular interest concerns few-body systems, where quantum and thermal fluctuations are expected to be especially relevant. In this contribution, we present numerical results demonstrating the impact of conserved quantities (or charges) in the outcomes of out-of-equilibrium measurements starting from realistic equilibrium states on a few-body system implementing the Dicke model.

created: 26-01-2020, last modified: 26-02-2020
M. Mizoguchi, Y. Zhang, M. Kunimi, A. Tanaka, S. Takeda, N. Takei, V. Bharti, K. Koyasu, T. Kishimoto, D. Jaksch, A. Glaetzle, M. Kiffner, G. Masella, G. Pupillo, M. Weidemueller and K. Ohmori,
Ultrafast creation of overlapping Rydberg electrons in an atomic BEC and Mott-insulator lattice,
Phys. Rev. Lett. 124, 253201 (2020).
Ultrafast creation of overlapping Rydberg electrons in an atomic BEC and Mott-insulator lattice
An array of ultracold atoms in an optical lattice (Mott insulator) excited to a state where single electron wave-functions spatially overlap would represent a new and ideal platform to simulate exotic electronic many-body phenomena in the condensed phase. However, this highly excited non-equilibrium system is expected to be so short-lived that it has eluded observation so far. Here, we demonstrate the first step toward its realization by exciting high-lying electronic (Rydberg) states of the atomic Mott insulator with a coherent ultrashort laser pulse. Beyond a threshold principal quantum number where Rydberg orbitals of neighboring lattice sites overlap with each other, the atoms efficiently undergo spontaneous Penning ionization resulting in a drastic change of ion-counting statistics, sharp increase of avalanche ionization and the formation of an ultracold plasma. These observations signal the actual creation of exotic electronic states with overlapping wave functions, which is further confirmed by a significant difference in ionization dynamics between a Bose-Einstein condensate and a Mott insulator.

created: 15-10-2019, last modified: 23-06-2020
P. Rosson, M. Kiffner, J. Mur-Petit and D. Jaksch,
Characterizing the phase diagram of finite-size dipolar Bose-Hubbard systems,
Phys. Rev. A 101, 013616 (2020).
Characterizing the phase diagram of finite-size dipolar Bose-Hubbard systems
We use state-of-the-art density matrix renormalization group calculations in the canonical ensemble to determine the phase diagram of the dipolar Bose-Hubbard model on a finite cylinder. We consider several observables that are accessible in typical optical lattice setups and assess how well these quantities perform as order parameters. We find that, especially for small systems, the occupation imbalance is less susceptible to boundary effects than the structure factor in uncovering the presence of a periodic density modulation. By analysing the non-local correlations, we find that the appearance of supersolid order is very sensitive to boundary effects, which may render it difficult to observe in quantum gas lattice experiments with a few tens of particles. Finally, we show how density measurements readily obtainable on a quantum gas microscope allow distinguishing between superfluid and solid phases using unsupervised machine-learning techniques.

created: 21-09-2019, last modified: 14-01-2020
R. Sawant, J.A. Blackmore, P.D. Gregory, J. Mur-Petit, D. Jaksch, J. Aldegunde, J.M. Hutson, M.R. Tarbut and S.L. Cornish,
Ultracold molecules as qudits,
New J. Phys. 22, 013027 (2020).
Ultracold molecules as qudits
We discuss how the internal structure of ultracold molecules, trapped in the motional ground state of optical tweezers, can be used to implement qudits. We explore the rotational, fine and hyperfine structure of 40Ca19F and 87Rb133Cs, which are examples of molecules with 2Σ and 1Σ electronic ground states, respectively. In each case we identify a subset of levels within a single rotational manifold suitable to implement a 4-level qudit. Quantum gates can be implemented using two-photon microwave transitions via levels in a neighboring rotational manifold. We discuss limitations to the usefulness of molecular qudits, arising from off-resonant excitation and decoherence. As an example, we present a protocol for using a molecular qudit of dimension d = 4 to perform the Deutsch algorithm.

created: 18-09-2019, last modified: 26-01-2020
J. Tindall, C. Sanchez Munoz, B. Buca and D. Jaksch,
Quantum Synchronisation Enabled by Dynamical Symmetries and Dissipation,
New J. Phys. 22, 013026 (2020).
Quantum Synchronisation Enabled by Dynamical Symmetries and Dissipation
In nature, instances of synchronisation abound across a diverse range of environments. In the quantum regime, however, synchronisation is typically observed by identifying an appropriate parameter regime in a specific system. In this work we show that this need not be the case, identifying symmetry-based conditions which, when satisfied, guarantee completely synchronous, entangled limit cycles between the individual constituents of a generic open quantum system - no restrictions are placed on its microscopic details. We describe these systems as posssessing a strong dynamical symmetry and we prove that, to first order, they are completely robust to symmetry-breaking perturbations. Using these ideas we identify two central examples where synchronisation arises via this qualitatively new mechanism: a chain of quadratically dephased spin-1s and the many-body charge-dephased Hubbard model. In both cases, due to their dynamical symmetries, perfect phase-locking occurs throughout the system, regardless of the specific microscopic parameters or initial states. Furthermore, when these systems are perturbed, their non-linear responses elicit long-lived signatures of both phase and frequency-locking.

created: 31-07-2019, last modified: 07-02-2020
M. Lubasch, J. Joo, P. Moinier, M. Kiffner and D. Jaksch,
Variational Quantum Algorithms for Nonlinear Problems,
Phys. Rev. A 101, 010301(R) (2020).
Variational Quantum Algorithms for Nonlinear Problems
We show that nonlinear problems including nonlinear partial differential equations can be efficiently solved by variational quantum computing. We achieve this by utilizing multiple copies of variational quantum states to treat nonlinearities efficiently and by introducing tensor networks as a programming paradigm. The key concepts of the algorithm are demonstrated for the nonlinear Schrödinger equation as a canonical example. We numerically show that the variational quantum ansatz can be exponentially more efficient than matrix product states and present experimental proof-of-principle results obtained on an IBM Q device.

created: 25-07-2019, last modified: 14-01-2020
M. Medenjak, B. Buca and D. Jaksch,
Isolated Heisenberg magnet as a quantum time crystal,
Phys. Rev. B 102, 041117(R) (2020).
Isolated Heisenberg magnet as a quantum time crystal
We demonstrate analytically and numerically that the paradigmatic model of quantum magnetism, the Heisenberg XXZ spin chain, does not equilibrate. It constitutes an example of persistent nonstationarity in a quantum many-body system that does not rely on external driving or coupling to an environment. We trace this phenomenon to the existence of extensive dynamical symmetries. We discuss how the ensuing persistent oscillations that seemingly violate one of the most fundamental laws of physics could be observed experimentally.

created: 23-05-2019, last modified: 25-07-2020
F. Schlawin and D. Jaksch,
Cavity-mediated unconventional pairing in ultracold fermionic atoms,
Phys. Rev. Lett. 123, 133601 (2019).
Cavity-mediated unconventional pairing in ultracold fermionic atoms
We investigate long-range pairing interactions between ultracold fermionic atoms confined in an optical lattice which are mediated by the coupling to a cavity. In the absence of other perturbations, we find three degenerate pairing symmetries for a two-dimensional square lattice. By tuning a weak local atomic interaction via a Feshbach resonance or by tuning a weak magnetic field, the superfluid system can be driven from a topologically trivial s-wave to topologically ordered, chiral superfluids containing Majorana edge states. Our work points out a novel path towards the creation of exotic superfluid states by exploiting the competition between long-range and short-range interactions.

created: 25-06-2019, last modified: 24-09-2019
B. Buca and D. Jaksch,
Dissipation Induced Nonstationarity in a Quantum Gas,
Phys. Rev. Lett. 123, 260401 (2019).
Dissipation Induced Nonstationarity in a Quantum Gas
Nonstationary longtime dynamics was recently observed in a driven two-component Bose-Einstein condensate coupled to an optical cavity [N. Dogra, M. Landini, K. Kroeger, L. Hruby, T. Donner, and T. Esslinger, arXiv:1901.05974] and analyzed in mean-field theory. We solve the underlying model in the thermodynamic limit and show that this system is always dynamically unstable—even when mean-field theory predicts stability. Instabilities always occur in higher-order correlation functions leading to squeezing and entanglement induced by cavity dissipation. The dynamics may be understood as the formation of a dissipative time crystal. We use perturbation theory for finite system sizes to confirm the nonstationary behavior.

created: 31-05-2019, last modified: 06-01-2020
F. Schlawin, A.S.D. Dietrich and D. Jaksch,
Optical control of the current-voltage relation in stacked superconductors,
Phys. Rev. B 100, 134510 (2019).
Optical control of the current-voltage relation in stacked superconductors
We simulate the current-voltage relation of short layered superconductors, which we model as stacks of capacitively coupled Josephson junctions. The system is driven by external laser fields, in order to optically control the voltage drop across the junction. We identify parameter regimes in which supercurrents can be stabilized against thermally induced phase slips, thus reducing the effective voltage across the superconductor. Furthermore, single driven Josephson junctions are known to exhibit phase-locked states, where the superconducting phase is locked to the driving field. We numerically observe their persistence in the presence of thermal fluctuations and capacitive coupling between adjacent Josephson junctions. Our results indicate how macroscopic material properties can be manipulated by exploiting the large optical nonlinearities of Josephson plasmons.

created: 23-05-2019, last modified: 02-11-2019
M. Kiffner, J.R. Coulthard, F. Schlawin, A. Ardavan and D. Jaksch,
Mott polaritons in cavity-coupled quantum materials,
New J. Phys. 21, 073066 (2019).
Mott polaritons in cavity-coupled quantum materials
We show that strong electron-electron interactions in cavity-coupled quantum materials can enable collectively enhanced light-matter interactions with ultrastrong effective coupling strengths. As a paradigmatic example we consider a Fermi-Hubbard model coupled to a single-mode cavity and find that resonant electron-cavity interactions result in the formation of a quasi-continuum of polariton branches. The vacuum Rabi splitting of the two outermost branches is collectively enhanced and scales with the square root of L which is the number of electronic sites. The maximal achievable value for the coupling is determined by the volume of the unit cell of the crystal. We find that for existing quantum materials it can by far exceed the width of the first excited Hubbard band. This effect can be experimentally observed via measurements of the optical conductivity and does not require ultra-strong coupling on the single-electron level. Quantum correlations in the electronic ground state as well as the microscopic nature of the light-matter interaction enhance the collective light-matter interaction compared to an ensemble of independent two-level atoms interacting with a cavity mode.

created: 23-05-2019, last modified: 01-08-2019
M. Kiffner, J.R. Coulthard, F. Schlawin, A. Ardavan and D. Jaksch,
Erratum: Manipulating quantum materials with quantum light [Phys. Rev. B 99, 085116 (2019)],
Phys. Rev. B 99, 099907 (2019).
Erratum: Manipulating quantum materials with quantum light [Phys. Rev. B 99, 085116 (2019)]
Second order terms neglected in the original paper are included into the calculation of the magnetic interaction.

created: 26-03-2019
C. Sanchez Munoz, B. Buca, J. Tindall, A. Gonzalez-Tudela, D. Jaksch and D. Porras,
Symmetries and conservation laws in quantum trajectories: Dissipative freezing,
Phys. Rev. A 100, 042113 (2019).
Symmetries and conservation laws in quantum trajectories: Dissipative freezing
In driven-dissipative systems, the presence of a strong symmetry guarantees the existence of several steady states belonging to different symmetry sectors. Here we show that when a system with a strong symmetry is initialized in a quantum superposition involving several of these sectors, each individual stochastic trajectory will randomly select a single one of them and remain there for the rest of the evolution. Since a strong symmetry implies a conservation law for the corresponding symmetry operator on the ensemble level, this selection of a single sector from an initial superposition entails a breakdown of this conservation law at the level of individual realizations. Given that such a superposition is impossible in a classical stochastic trajectory, this is a purely quantum effect with no classical analog. Our results show that a system with a closed Liouvillian gap may exhibit, when monitored over a single run of an experiment, a behavior completely opposite to the usual notion of dynamical phase coexistence and intermittency, which are typically considered hallmarks of a dissipative phase transition. We discuss our results on a coherently driven spin ensemble with a squeezed superradiant decay, a simple model that presents a wealth of nonergodic dynamics.

created: 14-03-2019, last modified: 05-12-2019
J. Tindall, B. Buca, J.R. Coulthard and D. Jaksch,
Heating-Induced Long-Range eta-Pairing in the Hubbard Model,
Phys. Rev. Lett. 123, 030603 (2019).
Heating-Induced Long-Range eta-Pairing in the Hubbard Model
We show how heating the spin degrees of freedom of the Hubbard model to infinite temperature can be used to melt the order within this sector and reach steady states, in any dimension, which have completely uniform long-range correlations between eta pairs. We induce this heating with either dissipation or periodic driving and evolve the system towards a non-equilibrium steady state. This steady state is identical in both cases and displays distance-invariant off-diagonal eta correlations. These correlations were first recognised in the superconducting eigenstates described in a seminal paper by C. N. Yang [Physical Review Letters, 63, 2144 (1989)], which are a subset of our steady states. Finally, we show that our results are a consequence of the symmetry properties of the model and independent of the microscopic details of the heating mechanism.

created: 14-02-2019, last modified: 18-07-2019
P. Rosson, M. Lubasch, M. Kiffner and D. Jaksch,
Bosonic Fractional Quantum Hall States on a Finite Cylinder,
Phys. Rev. A 99, 033603 (2019).
Bosonic Fractional Quantum Hall States on a Finite Cylinder
We investigate the ground state properties of a bosonic Harper-Hofstadter model with local interactions on a finite cylindrical lattice with filling fraction of 1/2. We find that our system supports topologically ordered states by calculating the topological entanglement entropy, and its value is in good agreement with the theoretical value for the corresponding Laughlin state. By exploring the behaviour of the density profiles, edge currents and single-particle correlation functions, we find that the ground state on the cylinder shows all signatures of a fractional quantum Hall state even for large values of the magnetic flux density. Furthermore, we determine the dependence of the correlation functions and edge currents on the interaction strength. We find that depending on the magnetic flux density, the transition towards Laughlin-like behaviour can be either smooth or happens abruptly for some critical interaction strength.

created: 14-01-2019, last modified: 11-03-2019
W.C. Yu, J. Tangpanitanon, A. Glaetzle, D. Jaksch and D.G. Angelakis,
Discrete time crystal in globally driven interacting quantum systems without disorder,
Phys. Rev. A 99, 033618 (2019).
Discrete time crystal in globally driven interacting quantum systems without disorder
Time crystals in periodically driven systems have initially been studied assuming either the ability to quench the Hamiltonian between different many-body regimes, the presence of disorder or long-range interactions. Here we propose the simplest scheme to observe discrete time crystal dynamics in a one-dimensional driven quantum system of the Ising type with short-range interactions and no disorder. The system is subject only to a periodic kick by a global magnetic field, and no extra Hamiltonian quenching is performed. We analyze the emerging time crystal stabilizing mechanisms via extensive numerics as well as using an analytic approach based on an off-resonant transition model. Due to the simplicity of the driven Ising model, our proposal can be implemented with current experimental platforms including trapped ions, Rydberg atoms, and superconducting circuits.

created: 23-07-2018, last modified: 26-03-2019
J. Tangpanitanon, S.R. Clark, V.M. Bastidas, R. Fazio, D. Jaksch and D.G. Angelakis,
Hidden Order in Quantum Many-body Dynamics of Driven-Dissipative Nonlinear Photonic Lattices,
Phys. Rev. A 99, 043808 (2019).
Hidden Order in Quantum Many-body Dynamics of Driven-Dissipative Nonlinear Photonic Lattices
We study the dynamics of nonlinear photonic lattices driven by two-photon parametric processes. By means of matrix-product state based calculations, we show that a quantum many-body state with long-range hidden order can be generated from the vacuum. This order resembles that characterizing the Haldane insulator. A possible explanation highlighting the role of the symmetry of the drive, and the effect of photon loss are discussed. An implementation based on superconducting circuits is proposed and analysed.

created: 29-06-2018, last modified: 23-05-2019
M. Kiffner, J.R. Coulthard, F. Schlawin, A. Ardavan and D. Jaksch,
Manipulating quantum materials with quantum light,
Phys. Rev. B 99, 085116 (2019).
Manipulating quantum materials with quantum light
We show that the macroscopic magnetic and electronic properties of strongly correlated electron systems can be manipulated by coupling them to a cavity mode. As a paradigmatic example we consider the Fermi-Hubbard model and find that the electron-cavity coupling enhances the magnetic interaction between the electron spins in the ground-state manifold. At half filling this effect can be observed by a change in the magnetic susceptibility. At less than half filling, the cavity introduces a next-nearest-neighbor hopping and mediates a long-range electron-electron interaction between distant sites. We study the ground-state properties with tensor network methods and find that the cavity coupling can induce a phase characterized by a momentum-space pairing effect for electrons.

created: 19-06-2018, last modified: 13-02-2019
F. Schlawin, A. Cavalleri and D. Jaksch,
Cavity-mediated electron-photon superconductivity,
Phys. Rev. Lett. 122, 133602 (2019).
Cavity-mediated electron-photon superconductivity
Pairing between fermionic quasi-particles in solids through the exchange of virtual bosonic excitations is one of the most studied phenomena in the solid state because it can lead to remarkable emergent phases of matter like superconductivity. This phenomenon has been extensively investigated for the case of attractive interactions induced by phonons and magnetic excitations. Vacuum fluctuations of the electromagnetic field can also provide attractive interactions, albeit with far lower efficiency. Here, we show that photon-mediated superconductivity, which is not observed in free space, is possible in unpumped optical cavities in which spontaneous vacuum fluctuations may lead to pairing at equilibrium. We study the case of a generic material with the parameters of GaAs, embedded in a nanoplasmonic terahertz cavity. We show the possibility of electron-photon superconductivity with critical temperatures on the order of 0.5 Kelvin. Because the electron-photon pairing interaction is effectively long-range, both singlet and triplet pairing are possible. Consequently, cavity-mediated electron-photon superconductors with nontrivial topology may be induced in 2D materials.

created: 20-04-2018, last modified: 23-05-2019
B. Buca, J. Tindall and D. Jaksch,
Complex coherent quantum many-body dynamics through dissipation,
Nature Communications 10, 1730 (2019).
Complex coherent quantum many-body dynamics through dissipation
The assumption that physical systems relax to a stationary state in the long-time limit underpins statistical physics and much of our intuitive understanding of scientific phenomena. For isolated systems this follows from the eigenstate thermalization hypothesis. When an environment is present the expectation is that all of phase space is explored, eventually leading to stationarity. Notable exceptions are decoherence-free subspaces that have important implications for quantum technologies. These have been studied for systems with a few degrees of freedom only. Here we identify simple and generic conditions for dissipation to prevent a quantum many-body system from ever reaching a stationary state. We go beyond dissipative quantum state engineering approaches towards controllable long-time non-stationary dynamics typically associated with macroscopic complex systems. This coherent and oscillatory evolution constitutes a dissipative version of a quantum time-crystal. We discuss the possibility of engineering such complex dynamics with fermionic ultracold atoms in optical lattices.

created: 19-04-2018, last modified: 23-05-2019
J.A. Blackmore, L. Caldwell, P.D. Gregory, E.M. Bridge, R. Sawant, J. Aldegunde, J. Mur-Petit, D. Jaksch, J.M. Hutson, B.E. Sauer, M.R. Tarbut and S.L. Cornish,
Ultracold molecules for quantum simulation: rotational coherences in CaF and RbCs,
Quantum Sci. Technol. 4, 014010 (2019).
Ultracold molecules for quantum simulation: rotational coherences in CaF and RbCs
We explore the uses of ultracold molecules as a platform for future experiments in the field of quantum simulation, focusing on two molecular species, 40Ca 19F and 87Rb 133Cs. We report the development of coherent quantum state control using microwave fields in both molecular species; this is a crucial ingredient for many quantum simulation applications. We demonstrate proof-of-principle Ramsey interferometry measurements with fringe spacings of approx. 1 kHz and investigate the dephasing time of a superposition of N=0 and N=1 rotational states when the molecules are confined. For both molecules, we show that a judicious choice of molecular hyperfine states minimises the impact of spatially varying transition-frequency shifts across the trap. For magnetically trapped 40Ca1 9F we use a magnetically insensitive transition and observe a coherence time of 0.61(3)ms. For optically trapped 87Rb 133Cs we exploit an avoided crossing in the AC Stark shift and observe a maximum coherence time of 0.75(6)ms.

created: 09-04-2018, last modified: 13-01-2019
J.R. Coulthard, S.R. Clark and D. Jaksch,
Ground state phase diagram of the one-dimensional t-J model with pair hopping terms,
Phys. Rev. B 98, 035116 (2018).
Ground state phase diagram of the one-dimensional t-J model with pair hopping terms
The t-J model is a standard model of strongly correlated electrons, often studied in the context of high-Tc superconductivity. However, most studies of this model neglect three-site terms, which appear at the same order as the superexchange J. As these terms correspond to pair-hopping, they are expected to play an important role in the physics of superconductivity when doped sufficiently far from half-filling. In this paper we present a density matrix renormalisation group study of the one-dimensional t-J model with the pair hopping terms included. We demonstrate that that these additional terms radically change the one-dimensional ground state phase diagram, extending the superconducting region at low fillings, while at larger fillings, superconductivity is completely suppressed. We explain this effect by introducing a simplified effective model of repulsive hardcore bosons.

created: 26-04-2018, last modified: 13-07-2018
M. Kiffner, D. Jaksch and D. Ceresoli,
A polynomial Ansatz for Norm-conserving Pseudopotentials,
J. Phys.: Condens. Matter 30, 275501 (2018).
A polynomial Ansatz for Norm-conserving Pseudopotentials
We show that efficient norm-conserving pseudopotentials for electronic structure calculations can be obtained from a polynomial Ansatz for the potential. Our pseudopotential is a polynomial of degree ten in the radial variable and fulfills the same smoothness conditions imposed by the Troullier-Martins method [Phys. Rev. B 43, 1993 (1991)] where pseudopotentials are represented by a polynomial of degree twenty-two. We compare our method to the Troullier-Martins approach in electronic structure calculations for diamond and iron in the bcc structure and find that the two methods perform equally well in calculations of the total energy. However, first and second derivatives of the total energy with respect to atomic coordinates converge significantly faster with the plane wave cutoff if the standard Troullier-Martins potentials are replaced by the pseudopotentials introduced here.

created: 22-03-2018, last modified: 14-06-2018
A.S.D. Dietrich, M. Kiffner and D. Jaksch,
Probing microscopic models for system-bath interactions via parametric driving,
Phys. Rev. A 98, 012122 (2018).
Probing microscopic models for system-bath interactions via parametric driving
We show that strong parametric driving of a quantum harmonic oscillator coupled to a thermal bath allows one to distinguish between different microscopic models for the oscillator-bath coupling. We consider a bath with an Ohmic spectral density and a model where the system-bath interaction can be tuned continuously between position and momentum coupling via the coupling angle. We derive a master equation for the reduced density operator of the oscillator in Born-Markov approximation and investigate its quasisteady state as a function of the driving parameters, the temperature of the bath and the coupling angle. We find that the driving introduces a strong dependence of the time-averaged variance of position and momentum on these parameters. In particular, we identify parameter regimes that maximize the angle dependence and provide an intuitive explanation of our results.

created: 22-03-2018, last modified: 19-07-2018
M. Lubasch, P. Moinier and D. Jaksch,
Multigrid Renormalization,
J. Comp. Phys. 372, 587 (2018).
Multigrid Renormalization
We combine the multigrid method with state-of-the-art concepts from the variational formulation of the numerical renormalization group. The resulting renormalization method is a natural generalization of the multigrid method for solving partial differential equations. When the solution on a grid of N points is sought, our method has a computational cost scaling as O(log(N)), as opposed to O(N) for the best standard MG method. Therefore it can exponentially speed up standard computations. To illustrate our method, we develop a novel algorithm for the ground state computation of the nonlinear Schroedinger equation. Our algorithm acts variationally on tensor products and updates the tensors one after another by solving a local nonlinear optimization problem. We compare several different methods for the nonlinear tensor update and find that the Newton method is the most efficient as well as precise. The combination of our method with the nonlinear ground state algorithm produces accurate results for the nonlinear Schroedinger equation on N=10^18 grid points in three spatial dimensions.

created: 08-03-2018, last modified: 30-06-2018
M. Lubasch, A.A. Valido, J.J. Renema, W.S. Kolthammer, D. Jaksch, M. S. Kim, I.A. Walmsley and R. Garcia-Patron,
Tensor network states in time-bin quantum optics,
Phys. Rev. A 97, 062304 (2018).
Tensor network states in time-bin quantum optics
The current shift in the quantum optics community towards experiments with many modes and photons necessitates new classical simulation techniques that efficiently encode many-body quantum correlations and go beyond the usual phase-space formulation. To address this pressing demand we formulate linear quantum optics in the language of tensor network states. We extensively analyze the quantum and classical correlations of time-bin interference in a single fiber loop. We then generalize our results to more complex time-bin quantum setups and identify different classes of architectures for high-complexity and low-overhead boson sampling experiments.

created: 30-01-2018, last modified: 11-06-2018
J. Mur-Petit, A. Relano, R.A. Molina and D. Jaksch,
Revealing missing charges with generalised quantum fluctuation relations,
Nature Communications 9, 2006 (2018).
Revealing missing charges with generalised quantum fluctuation relations
The non-equilibrium dynamics of quantum many-body systems is one of the most fascinating problems in physics. Open fundamental and practical questions range from how they relax to equilibrium, to how to extract useful work from them. Here, we derive a set of exact results that relate out-of-equilibrium fluctuations in the energy and other observables of a quantum system to its equilibrium properties for a very general family of initial conditions. These quantum fluctuation relations generalise the Jarzynski and Crooks relations to quantum systems with conserved quantities, and can be applied to protocols driving the system between integrable and chaotic regimes, or coupling it to different reservoirs. We illustrate our results with simulations of an integrable model subject to quenches realisable with current technology. Our findings will help guiding research on the interplay of quantum and thermal fluctuations in quantum simulation, and their exploitation in the design of new quantum devices.

created: 06-11-2017, last modified: 21-09-2019
F. Cosco, M. Borrelli, J. J. Mendoza-Arenas, F. Plastina, D. Jaksch and S. Maniscalco,
Bose-Hubbard lattice as a controllable environment for open quantum systems,
Phys. Rev. A 97, 040101(R) (2018).
Bose-Hubbard lattice as a controllable environment for open quantum systems
We investigate the open dynamics of an atomic impurity embedded in a one-dimensional Bose-Hubbard lattice. We derive the reduced evolution equation for the impurity and show that the Bose-Hubbard lattice behaves as a tunable engineered environment allowing one to simulate both Markovian and non-Markovian dynamics in a controlled and experimentally realizable way. We demonstrate that the presence or absence of memory effects is a signature of the nature of the excitations induced by the impurity, being delocalized or localized in the two limiting cases of a superfluid and Mott insulator, respectively. Furthermore, our findings show how the excitations supported in the two phases can be characterized as information carriers.

created: 29-06-2017, last modified: 10-04-2018
J. Han, T. Vogt, C. Gross, D. Jaksch, M. Kiffner and W. Li,
Free-space microwave-to-optical conversion via six-wave mixing in Rydberg atoms,
Phys. Rev. Lett. 120, 093201 (2018).
Free-space microwave-to-optical conversion via six-wave mixing in Rydberg atoms
We present an experimental demonstration of converting a microwave field to an optical field via frequency mixing in a cloud of cold 87Rb atoms, where the microwave field strongly couples to an electric dipole transition between Rydberg states. We show that the conversion allows the phase information of the microwave field to be coherently transferred to the optical field. With the current energy level scheme and experimental geometry, we achieve a photon-conversion efficiency of ~0.3 percent at low microwave intensities and a broad conversion bandwidth of more than 4MHz. Theoretical simulations agree well with the experimental data, and they indicate that near-unit efficiency is possible in future experiments.

created: 30-01-2017, last modified: 05-03-2018
F. Schlawin, A.S.D. Dietrich, M. Kiffner, A. Cavalleri and D. Jaksch,
Terahertz field control of interlayer transport modes in cuprate superconductors,
Phys. Rev. B 96, 064526 (2017).
Terahertz field control of interlayer transport modes in cuprate superconductors
We theoretically show that terahertz pulses with controlled amplitude and frequency can be used to switch between stable transport modes in layered superconductors, modeled as stacks of Josephson junctions. We find pulse shapes that deterministically switch the transport mode between superconducting, resistive, and solitonic states. We develop a simple model that explains the switching mechanism as a destabilization of the center-of-mass excitation of the Josephson phase, made possible by the highly nonlinear nature of the light-matter coupling.

created: 02-08-2017, last modified: 20-09-2017
J. J. Mendoza-Arenas, F.J. Gomez-Ruiz, M. Eckstein, D. Jaksch and S.R. Clark,
Ultra-fast control of magnetic relaxation in a periodically driven Hubbard model,
Annals of Physics1700024 (2017).
Ultra-fast control of magnetic relaxation in a periodically driven Hubbard model
Motivated by cold atom and ultra-fast pump-probe experiments we study the melting of long-range antiferromagnetic order of a perfect Neel state in a periodically driven repulsive Hubbard model. The dynamics is calculated for a Bethe lattice in infinite dimensions with non-equilibrium dynamical mean-field theory. In the absence of driving melting proceeds differently depending on the quench of the interactions to hopping ratio U/J0 from the atomic limit. For U much larger than J0 decay occurs due to mobile charge-excitations transferring energy to the spin sector, while for J0 approximately equal to U it is governed by the dynamics of residual quasi-particles. Here we explore the rich effects strong periodic driving has on this relaxation process spanning three frequency omega regimes:(i) high-frequency omega much larger than J0, (ii) resonant l times omega equal U greater J0 with integer l, and (iii) in-gap U greater omega greater J0 away from resonance. In case (i) we can quickly switch the decay from quasi-particle to charge-excitation mechanism through the suppression of J0. For (ii) the interaction can be engineered, even allowing an effective U=0 regime to be reached, giving the reverse switch from a charge-excitation to quasi-particle decay mechanism. For (iii) the exchange interaction can be controlled with little effect on the decay. By combining these regimes we show how periodic driving could be a potential pathway for controlling magnetism in antiferromagnetic materials. Finally, our numerical results demonstrate the accuracy and applicability of matrix product state techniques to the Hamiltonian DMFT impurity problem subjected to strong periodic driving.

created: 17-01-2017, last modified: 19-07-2017
S. Al-Assam, S.R. Clark and D. Jaksch,
The tensor network theory library,
J. Stat. Mech. 2017, 093102 (2017).
The tensor network theory library
In this technical paper we introduce the tensor network theory (TNT) library - an open-source software project aimed at providing a platform for rapidly developing robust, easy to use and highly optimised code for TNT calculations. The objectives of this paper are (i) to give an overview of the structure of TNT library, and (ii) to help scientists decide whether to use the TNT library in their research. We show how to employ the TNT routines by giving examples of ground-state and dynamical calculations of one-dimensional bosonic lattice system. We also discuss different options for gaining access to the software available at www.tensornetworktheory.org.

created: 10-10-2016, last modified: 13-07-2018
M. Kiffner, E. OBrien and D. Jaksch,
Topological spin models in Rydberg lattices,
Appl. Phys. B 123, 46 (2017).
Topological spin models in Rydberg lattices
We show that resonant dipole-dipole interactions between Rydberg atoms in a triangular lattice can give rise to artificial magnetic fields for spin excitations. We consider the coherent dipole-dipole coupling between np and ns Rydberg states and derive an effective spin-1/2 Hamiltonian for the np excitations. By breaking time-reversal symmetry via external fields we engineer complex hopping amplitudes for transitions between two rectangular sub-lattices. The phase of these hopping amplitudes depends on the direction of the hop. This gives rise to a staggered, artificial magnetic field which induces non-trivial topological effects. We calculate the single-particle band structure and investigate its Chern numbers as a function of the lattice parameters and the detuning between the two sub-lattices. We identify extended parameter regimes where the Chern number of the lowest band is C=1 or C=2

created: 17-09-2016, last modified: 10-05-2017
J.R. Coulthard, S.R. Clark, S. Al-Assam, A. Cavalleri and D. Jaksch,
Enhancement of super-exchange pairing in the periodically-driven Hubbard model,
Phys. Rev. B 96, 085104 (2017).
Enhancement of super-exchange pairing in the periodically-driven Hubbard model
Recent experiments performed on cuprates and alkali-doped fullerides have demonstrated that key signatures of superconductivity can be induced above the equilibrium critical temperature by optical modulation. These observations in disparate physical systems may indicate a general underlying mechanism. Multiple theories have been proposed, but these either consider specific features, such as competing instabilities, or focus on conventional BCS-type superconductivity. Here we show that periodic driving can enhance electron pairing in strongly correlated systems. Focusing on the strongly repulsive limit of the doped Hubbard model, we investigate in-gap, spatially inhomogeneous, on-site modulations. We demonstrate that such modulations substantially reduce electronic hopping, while simultaneously sustaining superexchange interactions and pair hopping via driving-induced virtual charge excitations. We calculate real-time dynamics for the one-dimensional case, starting from zero- and finite-temperature initial states, and we show that enhanced singlet-pair correlations emerge quickly and robustly in the out-of-equilibrium many-body state. Our results reveal a fundamental pairing mechanism that might underpin optically induced superconductivity in some strongly correlated quantum materials.

created: 18-08-2016, last modified: 01-08-2017
M. Streif, A. Buchleitner, D. Jaksch and J. Mur-Petit,
Measuring correlations of cold-atom systems using multiple quantum probes,
Phys. Rev. A 94, 053634 (2016).
Measuring correlations of cold-atom systems using multiple quantum probes
We present a nondestructive method to probe a complex quantum system using multiple-impurity atoms as quantum probes. Our protocol provides access to different equilibrium properties of the system by changing its coupling to the probes. In particular, we show that measurements with two probes reveal the system nonlocal two-point density correlations, for probe-system contact interactions. We illustrate our findings with analytic and numerical calculations for the Bose-Hubbard model in the weakly and strongly interacting regimes, under conditions relevant to ongoing experiments in cold-atom systems.

created: 11-10-2016, last modified: 28-11-2016
M.T. Mitchinson, T.H. Johnson and D. Jaksch,
Probing the dynamic structure factor of a neutral Fermi superfluid along the BCS-BEC crossover using atomic impurity qubits,
Phys. Rev. A 94, 063618 (2016).
Probing the dynamic structure factor of a neutral Fermi superfluid along the BCS-BEC crossover using atomic impurity qubits
We study an impurity atom trapped by an anharmonic potential, immersed within a cold atomic Fermi gas with attractive interactions that realizes the crossover from a Bardeen-Cooper-Schrieffer superfluid to a Bose- Einstein condensate. Considering the qubit comprising the lowest two vibrational energy eigenstates of the impurity, we demonstrate that its dynamics probes the equilibrium density fluctuations encoded in the dynamic structure factor of the superfluid. Observing the impurity evolution is thus shown to facilitate nondestructive measurements of the superfluid order parameter and the contact between collective and single-particle excitation spectra. Our setup constitutes a model of an open quantum system interacting with a thermal reservoir, the latter supporting both bosonic and fermionic excitations that are also coupled to each other.

created: 27-09-2016, last modified: 16-12-2016
J. Tangpanitanon, V.M. Bastidas, S. Al-Assam, P. Roushan, D. Jaksch and D.G. Angelakis,
Topological pumping of photons in nonlinear resonator arrays,
Phys. Rev. Lett. 117, 213603 (2016).
Topological pumping of photons in nonlinear resonator arrays
We show how to implement topological or Thouless pumping of interacting photons in one-dimensional nonlinear resonator arrays by simply modulating the frequency of the resonators periodically in space and time. The interplay between the interactions and the adiabatic modulations enables robust transport of Fock states with few photons per site. We analyze the transport mechanism via an effective analytic model and study its topological properties and its protection to noise. We conclude by a detailed study of an implementation with existing circuit-QED architectures.

created: 15-07-2016, last modified: 19-11-2016
J.M. Kreula, L. Garcia Alvarez, L. Lamata, S.R. Clark, E. Solano and D. Jaksch,
Few-qubit quantum-classical simulation of strongly correlated lattice fermions,
EPJ Quantum Technology 3, 11 (2016).
Few-qubit quantum-classical simulation of strongly correlated lattice fermions
We study a proof-of-principle example of the recently proposed hybrid quantum-classical simulation of strongly correlated fermion models in the thermodynamic limit. In a two-site dynamical mean-field theory (DMFT) approach we reduce the Hubbard model to an effective impurity model subject to self-consistency conditions. The resulting minimal two-site representation of the non-linear hybrid setup involves four qubits implementing the impurity problem, plus an ancilla qubit on which all measurements are performed. We outline a possible implementation with superconducting circuits feasible with near-future technology.

created: 16-06-2016, last modified: 13-08-2016
P.-L. Giscard, Z. Choo, S.J. Thwaite and D. Jaksch,
Exact Inference on Gaussian Graphical Models of Arbitrary Topology using Path-Sums,
Journal of Machine Learning Research 17, 1 (2016).
Exact Inference on Gaussian Graphical Models of Arbitrary Topology using Path-Sums
We present the path-sum formulation for exact statistical inference of marginals on Gaussian graphical models of arbitrary topology. The path-sum formulation gives the covariance between each pair of variables as a branched continued fraction of finite depth and breadth. Our method originates from the closed- form resummation of infinite families of terms of the walk-sum representation of the covariance matrix. We prove that the path- sum formulation always exists for models whose covariance matrix is positive definite: i.e. it is valid for both walk-summable and non-walk-summable graphical models of arbitrary topology. We show that for graphical models on trees the path-sum formulation is equivalent to Gaussian belief propagation. We also recover, as a corollary, an existing result that uses determinants to calculate the covariance matrix. We show that the path-sum formulation formulation is valid for arbitrary partitions of the inverse covariance matrix. We give detailed examples demonstrating our results.

created: 13-06-2016
S. Rajasekaran, E. Casandruc, Y. Laplace, D. Nicoletti, G.D. Gu, S.R. Clark, D. Jaksch and A. Cavalleri,
Parametric Amplification of a Terahertz Quantum Plasma Wave,
Nature Physics 12, 1012 (2016).
Parametric Amplification of a Terahertz Quantum Plasma Wave
Many applications in photonics require all-optical manipulation of plasma waves, which can concentrate electromagnetic energy on sub-wavelength length scales. This is difficult in metallic plasmas because of their small optical nonlinearities. Some layered superconductors support weakly damped plasma waves, involving oscillatory tunnelling of the superfluid between capacitively coupled planes. Such Josephson plasma waves (JPWs) are also highly nonlinear, and exhibit striking phenomena like cooperative emission of coherent terahertz radiation, superconductor-metal oscillations and soliton formation. We show here that terahertz JPWs in cuprate superconductors can be parametrically amplified through the cubic tunnelling nonlinearity. Parametric amplification is sensitive to the relative phase between pump and seed waves and may be optimized to achieve squeezing of the order parameter phase fluctuations or single terahertz-photon devices.

created: 11-06-2016, last modified: 03-11-2016
M. Mitrano, A. Cantaluppi, D. Nicoletti, S. Kaiser, A. Perucchi, S. Lupi, P. Di Pietro, D. Pontiroli, M. Ricco, S.R. Clark, D. Jaksch and A. Cavalleri,
Possible light-induced superconductivity in K3C60 at high temperature,
Nature 530, 461-464 (2016).
Possible light-induced superconductivity in K3C60 at high temperature
The non-equilibrium control of emergent phenomena in solids is an important research frontier, encompassing effects such as the optical enhancement of superconductivity. Nonlinear excitation of certain phonons in bilayer copper oxides was recently shown to induce superconducting-like optical properties at temperatures far greater than the superconducting transition temperature, Tc. This effect was accompanied by the disruption of competing charge-density-wave correlations which explained some but not all of the experimental results. Here we report a similar phenomenon in a very different compound, K3C60. By exciting metallic K3C60 with mid-infrared optical pulses, we induce a large increase in carrier mobility, accompanied by the opening of a gap in the optical conductivity. These same signatures are observed at equilibrium when cooling metallic K3C60 below Tc (20 kelvin). Although optical techniques alone cannot unequivocally identify non-equilibrium high-temperature superconductivity, we propose this as a possible explanation of our results.

created: 26-02-2016
M. Kiffner, A. Feizpour, K.T. Kaczmarek, D. Jaksch and J. Nunn,
Two-way interconversion of millimeter-wave and optical fields in Rydberg gases,
New J. Phys. 18, 093030 (2016).
Two-way interconversion of millimeter-wave and optical fields in Rydberg gases
We show that cold Rydberg gases enable an efficient six-wave mixing process where terahertz or microwave fields are coherently converted into optical fields and vice versa. This process is made possible by the long lifetime of Rydberg states, the strong coupling of millimeter waves to Rydberg transitions and by a quantum interference effect related to electromagnetically induced transparency. Our frequency conversion scheme applies to a broad spectrum of millimeter waves due to the abundance of transitions within the Rydberg manifold, and we discuss two possible implementations based on focussed terahertz beams and millimeter wave fields confined by a waveguide, respectively. We analyse a realistic example for the interconversion of terahertz and optical fields in rubidium atoms and find that the conversion efficiency can in principle exceed 90%.

created: 27-01-2016, last modified: 15-09-2016
T.H. Johnson, Y. Yuan, W. Bao, S.R. Clark, C.J. Foot and D. Jaksch,
Hubbard Model for Atomic Impurities Bound by the Vortex Lattice of a Rotating Bose-Einstein Condensate,
Phys. Rev. Lett. 116, 240402 (2016).
Hubbard Model for Atomic Impurities Bound by the Vortex Lattice of a Rotating Bose-Einstein Condensate
We investigate cold bosonic impurity atoms trapped in a vortex lattice formed by condensed bosons of another species. We describe the dynamics of the impurities by a bosonic Hubbard model containing occupation-dependent parameters to capture the effects of strong impurity-impurity interactions. These include both a repulsive direct interaction and an attractive effective interaction mediated by the Bose-Einstein condensate. The occupation dependence of these two competing interactions drastically affects the Hubbard model phase diagram, including causing the disappearance of some Mott lobes.

created: 04-01-2016, last modified: 19-06-2016
T.H. Johnson, F. Cosco, M.T. Mitchinson, D. Jaksch and S.R. Clark,
Thermometry of ultracold atoms via nonequilibrium work distributions,
Phys. Rev. A 93, 053619 (2016).
Thermometry of ultracold atoms via nonequilibrium work distributions
Estimating the temperature of a cold quantum system is difficult. Usually one measures a well-understood thermal state and uses that prior knowledge to infer its temperature. In contrast, we introduce a method of thermometry that assumes minimal knowledge of the state of a system and is potentially nondestructive. Our method uses a universal temperature dependence of the quench dynamics of an initially thermal system coupled to a qubit probe that follows from the Tasaki-Crooks theorem for nonequilibrium work distributions. We provide examples for a cold-atom system, in which our thermometry protocol may retain accuracy and precision at subnano-Kelvin temperatures.

created: 29-10-2015, last modified: 25-05-2016
J.M. Kreula, S.R. Clark and D. Jaksch,
Non-linear quantum-classical scheme to simulate non-equilibrium strongly correlated fermionic many-body dynamics,
Scientific Reports 6, 32940 (2016).
Non-linear quantum-classical scheme to simulate non-equilibrium strongly correlated fermionic many-body dynamics
We propose a non-linear, hybrid quantum-classical scheme for simulating non-equilibrium dynamics of strongly correlated fermions described by the Hubbard model in a Bethe lattice in the thermodynamic limit. Our scheme implements non-equilibrium dynamical mean field theory (DMFT) and uses a digital quantum simulator to solve a quantum impurity problem whose parameters are iterated to self-consistency via a classically computed feedback loop where quantum gate errors can be partly accounted for. We analyse the performance of the scheme in an example case.

created: 29-10-2015, last modified: 15-09-2016
J. J. Mendoza-Arenas, S.R. Clark, S. Felicetti, G. Romero, E. Solano, D.G. Angelakis and D. Jaksch,
Beyond mean-field bistability in driven-dissipative lattices: bunching-antibunching transition and quantum simulation,
Phys. Rev. A 93, 023821 (2016).
Beyond mean-field bistability in driven-dissipative lattices: bunching-antibunching transition and quantum simulation
In the present work we investigate the existence of multiple nonequilibrium steady states in a coherently driven XY lattice of dissipative two-level systems. A commonly used mean-field ansatz, in which spatial correlations are neglected, predicts a bistable behavior with a sharp shift between low- and high-density states. In contrast one-dimensional matrix product methods reveal these effects to be artifacts of the mean-field approach, with both disappearing once correlations are taken fully into account. Instead, a bunching-antibunching transition emerges. This indicates that alternative approaches should be considered for higher spatial dimensions, where classical simulations are currently infeasible. Thus we propose a circuit QED quantum simulator implementable with current technology to enable an experimental investigation of the model considered.

created: 29-10-2015, last modified: 26-02-2016
M. Kiffner, D. Ceresoli, W. Li and D. Jaksch,
Quantum mechanical calculation of Rydberg - Rydberg autoionization rates,
J. Phys. B: At. Mol. Opt. Phys. 49, 204004 (2016).
Quantum mechanical calculation of Rydberg - Rydberg autoionization rates
We present quantum mechanical calculations of autoionization rates for two rubidium Rydberg atoms with weakly overlapping electron clouds. We neglect exchange effects and consider tensor products of independent atom states forming an approximate basis of the two-electron state space. We consider large sets of two-atom states with randomly chosen quantum numbers and find that the charge overlap between the two Rydberg electrons allows one to characterise the magnitude of the autoionization rates. If the electron clouds overlap by more than one percent, the autoionization rates increase approximately exponentially with the charge overlap. This finding is independent of the energy of the initial state. Cover of the print version of Special Issue on Addressing Quantum Many-body Problems with Cold Atoms and Molecule

created: 14-07-2015, last modified: 19-10-2016
M Foerst, A.D. Caviglia, R. Scherwitzl, R. Mankowsky, P. Zubko, V. Khanna, H. Bromberger, S.B. Wilkins, Y.-D. Chuang, W.S. Lee, W.F. Schlotter, J.J. Turner, G.L. Dakovski, M.P. Minitti, J. Robinson, S.R. Clark, D. Jaksch, J.-M. Triscone, J.P. Hill, S.S. Dhesi and A. Cavalleri,
Spatially resolved ultrafast magnetic dynamics initiated at a complex oxide heterointerface,
Nature Materials 14, 883 (2015).
Spatially resolved ultrafast magnetic dynamics initiated at a complex oxide heterointerface
Static strain in complex oxide heterostructures has been extensively used to engineer electronic and magnetic properties at equilibrium. In the same spirit, deformations of the crystal lattice with light may be used to achieve functional control across heterointerfaces dynamically. Here, by exciting large-amplitude infrared-active vibrations in a LaAlO3 substrate we induce magnetic order melting in a NdNiO3 film across a heterointerface. Femtosecond resonant soft X-ray diffraction is used to determine the spatiotemporal evolution of the magnetic disordering. We observe a magnetic melt front that propagates from the substrate interface into the film, at a speed that suggests electronically driven motion. Light control and ultrafast phase front propagation at heterointerfaces may lead to new opportunities in optomagnetism, for example by driving domain wall motion to transport information across suitably designed devices.

created: 06-07-2015, last modified: 26-08-2015
P.-L. Giscard, K. Lui, S.J. Thwaite and D. Jaksch,
An Exact Formulation of the Time-Ordered Exponential using Path-Sums,
Journal of Mathematical Physics 56, 053503 (2015).
An Exact Formulation of the Time-Ordered Exponential using Path-Sums
We present the path-sum formulation for the time-ordered exponential of a time-dependent matrix. The path-sum formulation gives the time-ordered exponential as a branched continued fraction of finite depth and breadth. The terms of the path-sum have an elementary interpretation as self-avoiding walks and self-avoiding polygons on a graph. Our result is based on a representation of the time-ordered exponential as the inverse of an operator, the mapping of this inverse to sums of walks on a graphs, and the algebraic structure of sets of walks. We give examples demonstrating our approach. We establish a super-exponential decay bound for the magnitude of the entries of the time-ordered exponential of sparse matrices. We give explicit results for matrices with commonly encountered sparse structures.

created: 24-04-2015, last modified: 11-05-2015
S.J. Denny, S.R. Clark, Y. Laplace, A. Cavalleri and D. Jaksch,
Proposed Parametric Cooling of Bilayer Cuprate Superconductors by Terahertz Excitation,
Phys. Rev. Lett. 114, 137001 (2015).
Proposed Parametric Cooling of Bilayer Cuprate Superconductors by Terahertz Excitation
We propose and analyse a scheme for parametrically cooling bilayer cuprates based on the selective driving of a c-axis vibrational mode. The scheme exploits the vibration as a transducer making the Josephson plasma frequencies time-dependent. We show how modulation at the difference frequency between the intra- and interbilayer plasmon substantially suppresses interbilayer phase fluctuations, responsible for switching c-axis transport from a superconducting to resistive state. Our calculations indicate that this may provide a viable mechanism for stabilizing non-equilibrium superconductivity even above Tc, provided a finite pair density survives between the bilayers out of equilibrium. Editors` suggestion and Physics Synopsis: Making Superconductors Sturdier

created: 18-11-2014, last modified: 31-03-2015
D. Hangleiter, M.T. Mitchinson, T.H. Johnson, M. Bruderer, M.B. Plenio and D. Jaksch,
Nondestructive selective probing of phononic excitations in a cold Bose gas using impurities,
Phys. Rev. A 91, 013611 (2015).
Nondestructive selective probing of phononic excitations in a cold Bose gas using impurities
We introduce a detector that selectively probes the phononic excitations of a cold Bose gas. The detector is composed of a single impurity atom confined by a double-well potential, where the two lowest eigenstates of the impurity form an effective probe qubit that is coupled to the phonons via density-density interactions with the bosons. The system is analogous to a two-level atom coupled to photons of the radiation field. We demonstrate that tracking the evolution of the qubit populations allows probing both thermal and coherent excitations in targeted phonon modes. The targeted modes are selected in both energy and momentum by adjusting the impurity`s potential. We show how to use the detector to observe coherent density waves and to measure temperatures of the Bose gas down to the nanokelvin regime. We analyze how our scheme could be realized experimentally, including the possibility of using an array of multiple impurities to achieve greater precision from a single experimental run.

created: 18-11-2014, last modified: 13-01-2015
J. J. Mendoza-Arenas, S.R. Clark and D. Jaksch,
Coexistence of energy diffusion and local thermalization in nonequilibrium XXZ spin chains with integrability breaking,
Phys. Rev. E 91, 042129 (2015).
Coexistence of energy diffusion and local thermalization in nonequilibrium XXZ spin chains with integrability breaking
In this work we analyze the simultaneous emergence of diffusive energy transport and local thermalization in a nonequilibrium one-dimensional quantum system, as a result of integrability breaking. Specifically, we discuss the local properties of the steady state induced by thermal boundary driving in a XXZ spin chain with staggered magnetic field. By means of efficient large-scale matrix product simulations of the equation of motion of the system, we calculate its steady state in the long-time limit. We start by discussing the energy transport supported by the system, finding it to be ballistic in the integrable limit and diffusive when the staggered field is finite. Subsequently we examine the reduced density operators of neighboring sites and find that for large systems they are well approximated by local thermal states of the underlying Hamiltonian in the nonintegrable regime, even for weak staggered fields. In the integrable limit, on the other hand, this behavior is lost, and the identification of local temperatures is no longer possible. Our results agree with the intuitive connection between energy diffusion and thermalization.

created: 18-11-2014, last modified: 24-04-2015
T.H. Johnson, T. Elliot, S.R. Clark and D. Jaksch,
Capturing Exponential Variance Using Polynomial Resources: Applying Tensor Networks to Nonequilibrium Stochastic Processes,
Phys. Rev. Lett. 114, 090602 (2015).
Capturing Exponential Variance Using Polynomial Resources: Applying Tensor Networks to Nonequilibrium Stochastic Processes
Estimating the expected value of an observable appearing in a non-equilibrium stochastic process usually involves sampling. If the observable`s variance is high, many samples are required. In contrast, we show that performing the same task without sampling, using tensor network compression, efficiently captures high variances in systems of various geometries and dimensions. We provide examples for which matching the accuracy of our efficient method would require a sample size scaling exponentially with system size. In particular, the high variance observable exp(-beta W, with W the work done quenching from equilibrium at inverse temperature beta, is exactly and efficiently captured by tensor networks.

created: 17-11-2014, last modified: 09-03-2015
R. Singla, G. Cotugno, S. Kaiser, M Foerst, M. Mitrano, H.Y. Liu, A. Cartella, C. Manzoni, H. Okamoto, T. Hasegawa, S.R. Clark, D. Jaksch and A. Cavalleri,
THz-Frequency Modulation of the Hubbard U in an Organic Mott Insulator,
Phys. Rev. Lett. 115, 187401 (2015).
THz-Frequency Modulation of the Hubbard U in an Organic Mott Insulator
We use midinfrared pulses with stable carrier-envelope phase offset to drive molecular vibrations in the charge transfer salt ET&#8722;F2TCNQ, a prototypical one-dimensional Mott insulator. We find that the Mott gap, which is probed resonantly with 10 fs laser pulses, oscillates with the pump field. This observation reveals that molecular excitations can coherently perturb the electronic on-site interactions (Hubbard U ) by changing the local orbital wave function. The gap oscillates at twice the frequency of the vibrational mode, indicating that the molecular distortions couple quadratically to the local charge density.

created: 04-09-2014, last modified: 29-10-2015
T.H. Johnson, S.R. Clark and D. Jaksch,
What is a quantum simulator?,
EPJ Quantum Technology 1, 10 (2014).
What is a quantum simulator?
Quantum simulators are devices that actively use quantum effects to answer questions about model systems and, through them, real systems. Here we expand on this definition by answering several fundamental questions about the nature and use of quantum simulators. Our answers address two important areas. First, the difference between an operation termed simulation and another termed computation. This distinction is related to the purpose of an operation, as well as our confidence in and expectation of its accuracy. Second, the threshold between quantum and classical simulations. Throughout, we provide a perspective on the achievements and directions of the field of quantum simulation.

created: 14-05-2014, last modified: 06-09-2014
J. J. Mendoza-Arenas, M.T. Mitchinson, S.R. Clark, J. Prior, D. Jaksch and M.B. Plenio,
Transport enhancement from incoherent coupling between one-dimensional quantum conductors,
New J. Phys. 16, 053016 (2014).
Transport enhancement from incoherent coupling between one-dimensional quantum conductors
We study the non-equilibrium transport properties of a highly anisotropic two dimensional lattice of spin&#8722;1/2 particles governed by a Heisenberg XXZ Hamiltonian. The anisotropy of the lattice allows us to approximate the system at finite temperature as an array of incoherently coupled one-dimensional chains. We show that in the regime of strong intrachain interactions, the weak interchain coupling considerably boosts spin transport in the driven system. Interestingly, we show that this enhancement increases with the length of the chains, which is related to superdiffusive spin transport. We describe the mechanism behind this effect, compare it to a similar phenomenon in single chains induced by dephasing, and explain why the former is much stronger.

created: 08-05-2014, last modified: 09-05-2014
M. Kiffner, M. Huo, W. Li and D. Jaksch,
Few-body bound states of dipole-dipole-interacting Rydberg atoms,
Phys. Rev. A 89, 052717 (2014).
Few-body bound states of dipole-dipole-interacting Rydberg atoms
We show that the resonant dipole-dipole interaction can give rise to bound states between two and three Rydberg atoms with nonoverlapping electron clouds. The dimer and trimer states arise from avoided level crossings between states converging to different fine-structure manifolds in the limit of separated atoms. We analyze the angular dependence of the potential wells, characterize the quantum dynamics in these potentials, and discuss methods for their production and detection. Typical distances between the atoms are of the order of several micrometers which can be resolved in state-of-the-art experiments. The potential depths and typical oscillation frequencies are about one order of magnitude larger as compared to the dimer and trimer states investigated by M. Kiffner et al. [Phys. Rev. A 86, 031401(R) (2012)] and M. Kiffner, W. Li, and D. Jaksch [Phys. Rev. Lett. 111, 233003 (2013)], respectively. We find that the dimer and trimer molecules can be aligned with respect to the axis of a weak electric field.

created: 23-04-2014, last modified: 01-06-2014
M. Mitrano, G. Cotugno, S.R. Clark, R. Singla, S. Kaiser, J. Staehler, R. Beyer, M. Dressel, L. Baldassarre, D. Nicoletti, A. Perucchi, T. Hasegawa, H. Okamoto, D. Jaksch and A. Cavalleri,
Pressure-Dependent Relaxation in the Photoexcited Mott Insulator ET-F2TCNQ: Influence of Hopping and Correlations on Quasiparticle Recombination Rates,
Phys. Rev. Lett. 112, 117801 (2014).
Pressure-Dependent Relaxation in the Photoexcited Mott Insulator ET-F2TCNQ: Influence of Hopping and Correlations on Quasiparticle Recombination Rates
We measure the ultrafast recombination of photoexcited quasiparticles (holon-doublon pairs) in the one dimensional Mott insulator ET-F2TCNQ as a function of external pressure, which is used to tune the electronic structure. At each pressure value, we first fit the static optical properties and extract the electronic bandwidth t and the intersite correlation energy V. We then measure the recombination times as a function of pressure, and we correlate them with the corresponding microscopic parameters. We find that the recombination times scale differently than for metals and semiconductors. A fit to our data based on the time-dependent extended Hubbard Hamiltonian suggests that the competition between local recombination and delocalization of the Mott-Hubbard exciton dictates the efficiency of the recombination.

created: 19-03-2014
S. Kaiser, S.R. Clark, D. Nicoletti, G. Cotugno, R.I. Tobey, N. Dean, S. Lupi, H. Okamoto, T. Hasegawa, D. Jaksch and A. Cavalleri,
Optical Properties of a Vibrationally Modulated Solid State Mott Insulator,
Scientific Reports 4, 3823 (2014).
Optical Properties of a Vibrationally Modulated Solid State Mott Insulator
Optical pulses at THz and mid-infrared frequencies tuned to specific vibrational resonances modulate the lattice along chosen normal mode coordinates. In this way, solids can be switched between competing electronic phases and new states are created. Here, we use vibrational modulation to make electronic interactions (Hubbard-U) in Mott-insulator time dependent. Mid-infrared optical pulses excite localized molecular vibrations in ET-F2TCNQ, a prototypical one-dimensional Mott-insulator. A broadband ultrafast probe interrogates the resulting optical spectrum between THz and visible frequencies. A red-shifted charge-transfer resonance is observed, consistent with a time-averaged reduction of the electronic correlation strength U. Secondly, a sideband manifold inside of the Mott-gap appears, resulting from a periodically modulated U. The response is compared to computations based on a quantum-modulated dynamic Hubbard model. Heuristic fitting suggests asymmetric holon-doublon coupling to the molecules and that electron double-occupancies strongly squeeze the vibrational mode.

created: 31-07-2012, last modified: 22-01-2014
I.A. Walmsley, K.C. Lee, M.R. Sprague, B.J. Sussman, J. Nunn, N.K. Langford, X.-M. Jin, T. Champion, P. Michelberger, K.F. Reim, D. England and D. Jaksch,
Entang-bling: Observing quantum correlations in room-temperature solids,
J. Phys.: Conf. Ser. 442, 012004 (2013).
Entang-bling: Observing quantum correlations in room-temperature solids
Quantum entanglement in the motion of macroscopic solid bodies has implications both for quantum technologies and foundational studies of the boundary between the quantum and classical worlds. Entanglement is usually fragile in room-temperature solids, owing to strong interactions both internally and with the noisy environment. We generated motional entanglement between vibrational states of two spatially separated, millimeter-sized diamonds at room temperature. By measuring strong nonclassical correlations between Raman-scattered photons, we showed that the quantum state of the diamonds has positive concurrence with 98% probability. Our results show that entanglement can persist in the classical context of moving macroscopic solids in ambient conditions.

created: 28-08-2013, last modified: 01-06-2014
M. Kiffner, W. Li and D. Jaksch,
Three-Body Bound States in Dipole-Dipole Interacting Rydberg Atoms,
Phys. Rev. Lett. 111, 233003 (2013).
Three-Body Bound States in Dipole-Dipole Interacting Rydberg Atoms
We show that the dipole-dipole interaction between three identical Rydberg atoms can give rise to bound trimer states. The microscopic origin of these states is fundamentally different from Efimov physics. Two stable trimer configurations exist where the atoms form the vertices of an equilateral triangle in a plane perpendicular to a static electric field. The triangle edge length typically exceeds R approximately 2 microns, and each configuration is twofold degenerate due to Kramers degeneracy. The depth of the potential wells and the triangle edge length can be controlled by external parameters. We establish the Borromean nature of the trimer states, analyze the quantum dynamics in the potential wells, and describe methods for their production and detection.

created: 27-08-2013, last modified: 04-12-2013
M. Kiffner, W. Li and D. Jaksch,
Abelian and non-Abelian gauge fields in dipole-dipole interacting Rydberg atoms,
J. Phys. B: At. Mol. Opt. Phys. 46, 134008 (2013).
Abelian and non-Abelian gauge fields in dipole-dipole interacting Rydberg atoms
We show that the dipole-dipole interaction between two Rydberg atoms can lead to substantial Abelian and non-Abelian gauge fields acting on the relative motion of the two atoms. We demonstrate how the gauge fields can be evaluated by numerical techniques. In the case of adiabatic motion in a single internal state, we show that the gauge fields give rise to a magnetic field that results in a Zeeman splitting of the rotational states. In particular, the ground state of a molecular potential well is given by the first excited rotational state. We find that our system realizes a synthetic spin-orbit coupling where the relative atomic motion couples to two internal two-atom states. The associated gauge fields are non-Abelian. Article included in IOPselect

created: 24-06-2013
N. Schetakis, T. Grujic, S.R. Clark, D. Jaksch and D.G. Angelakis,
Frozen photons in Jaynes Cummings arrays,
J. Phys. B: At. Mol. Opt. Phys. 46, 224025 (2013).
Frozen photons in Jaynes Cummings arrays
We study the origin of frozen states in coupled Jaynes-Cummings-Hubbard arrays in the presence of losses. For the case of half the array initially populated with photons while the other half is left empty we show the emergence of self-localized photon or frozen states for specific values of the local atom-photon coupling. We analyze the dynamics in the quantum regime and discover important additional features appear not captured by a semiclassical treatment, which we analyze for different array sizes and filling fractions. We trace the origin of this interaction-induced photon freezing to the suppression of excitation of propagating modes in the system at large interaction strengths. We discuss in detail the possibility to experimentally probe the relevant transition by analyzing the emitted photon correlations. We find a strong signature of the effect in the emitted photons statistics.

created: 07-06-2013, last modified: 08-11-2013
M. Kiffner, W. Li and D. Jaksch,
Magnetic Monopoles and Synthetic Spin-Orbit Coupling in Rydberg Macrodimers,
Phys. Rev. Lett. 110, 170402 (2013).
Magnetic Monopoles and Synthetic Spin-Orbit Coupling in Rydberg Macrodimers
We show that sizable Abelian and non-Abelian gauge fields arise in the relative motion of two dipole-dipole interacting Rydberg atoms. Our system exhibits two magnetic monopoles for adiabatic motion in one internal two-atom state. These monopoles occur at a characteristic distance between the atoms that is of the order of one micron. The deflection of the relative motion due to the Lorentz force gives rise to a clear signature of the effective magnetic field. In addition, we consider nonadiabatic transitions between two near-degenerate internal states and show that the associated gauge fields are non-Abelian. We present quantum mechanical calculations of this synthetic spin-orbit coupling and show that it realizes a velocity-dependent beam splitter.

created: 24-04-2013, last modified: 09-05-2013
R. Walters, G. Cotugno, T.H. Johnson, S.R. Clark and D. Jaksch,
Ab initio derivation of Hubbard models for cold atoms in optical lattices,
Phys. Rev. A 87, 043613 (2013).
Ab initio derivation of Hubbard models for cold atoms in optical lattices
We derive ab initio local Hubbard models for several optical-lattice potentials of current interest, including the honeycomb and kagome lattices, verifying their accuracy on each occasion by comparing the interpolated band structures against the originals. To achieve this, we calculate the maximally localized generalized Wannier basis by implementing the steepest-descent algorithm of Marzari and Vanderbilt [ Phys. Rev. B 56 12847 (1997)] directly in one and two dimensions. To avoid local minima we develop an initialization procedure that is both robust and requires no prior knowledge of the optimal Wannier basis.

created: 16-04-2013
J. J. Mendoza-Arenas, S. Al-Assam, S.R. Clark and D. Jaksch,
Heat transport in the XXZ spin chain: from ballistic to diffusive regimes and dephasing enhancement,
J. Stat. Mech. P07007 (2013).
Heat transport in the XXZ spin chain: from ballistic to diffusive regimes and dephasing enhancement
In this work we study the heat transport in an XXZ spin-1/2 Heisenberg chain with homogeneous magnetic field, incoherently driven out of equilibrium by reservoirs at the boundaries. We focus on the effect of bulk dephasing (energy-dissipative) processes in different parameter regimes of the system. The non-equilibrium steady state of the chain is obtained by simulating its evolution under the corresponding Lindblad master equation, using the time evolving block decimation method. In the absence of dephasing, the heat transport is ballistic for weak interactions, while being diffusive in the strongly-interacting regime, as evidenced by the heat-current scaling with the system size. When bulk dephasing takes place in the system, diffusive transport is induced in the weakly-interacting regime, with the heat current monotonically decreasing with the dephasing rate. In contrast, in the strongly-interacting regime, the heat current can be significantly enhanced by dephasing for systems of small size.

created: 27-03-2013, last modified: 16-07-2013
J. J. Mendoza-Arenas, T. Grujic, D. Jaksch and S.R. Clark,
Dephasing enhanced transport in non-equilibrium strongly-correlated quantum systems,
Phys. Rev. B 87, 235130 (2013).
Dephasing enhanced transport in non-equilibrium strongly-correlated quantum systems
A key insight from recent studies is that noise, such as dephasing, can improve the efficiency of quantum transport by suppressing coherent single-particle interference effects. However, it is not yet clear whether dephasing can enhance transport in an interacting many-body system. Here we address this question by analysing the transport properties of a boundary driven spinless fermion chain with nearest-neighbour interactions subject to bulk dephasing. The many-body non-equilibrium stationary state is determined using large scale matrix product simulations of the corresponding quantum master equation. We find dephasing enhanced transport only in the strongly interacting regime, where it is shown to induce incoherent transitions bridging the gap between bound dark-states and bands of mobile eigenstates. The generic nature of the effect is illustrated and shown not to depend on the integrability of the model considered. As a result dephasing enhanced transport is expected to persist in more realistic driven systems of strongly-correlated particles.

created: 22-03-2013, last modified: 24-06-2013
T. Grujic, S.R. Clark, D. Jaksch and D.G. Angelakis,
Repulsively induced photon super-bunching in driven resonator arrays,
Phys. Rev. A 87, 053846 (2013).
Repulsively induced photon super-bunching in driven resonator arrays
We analyze the nonequilibrium behavior of driven nonlinear photonic resonator arrays under the selective excitation of specific photonic many-body modes. Targeting the unit-filled ground state, we find a counterintuitive superbunching in the emitted photon statistics in spite of relatively strong on-site repulsive interaction. We consider resonator arrays with Kerr nonlinearities described by the Bose-Hubbard model, but also show that an analogous effect is observable in near-future experiments coupling resonators to two-level systems as described by the Jaynes-Cummings-Hubbard Hamiltonian. For the experimentally accessible case of a pair of coupled resonators forming a photonic molecule, we provide an analytical explanation for the nature of the effect.

created: 04-01-2013, last modified: 07-06-2013
T.H. Johnson, J.D. Biamonte, S.R. Clark and D. Jaksch,
Solving search problems by strongly simulating quantum circuits,
Scientific Reports 3, 1235 (2013).
Solving search problems by strongly simulating quantum circuits
Simulating quantum circuits using classical computers lets us analyse the inner workings of quantum algorithms. The most complete type of simulation, strong simulation, is believed to be generally inefficient. Nevertheless, several efficient strong simulation techniques are known for restricted families of quantum circuits and we develop an additional technique in this article. Further, we show that strong simulation algorithms perform another fundamental task: solving search problems. Efficient strong simulation techniques allow solutions to a class of search problems to be counted and found efficiently. This enhances the utility of strong simulation methods, known or yet to be discovered, and extends the class of search problems known to be efficiently simulable. Relating strong simulation to search problems also bounds the computational power of efficiently strongly simulable circuits; if they could solve all problems in $mathrm{P}$ this would imply the collapse of the complexity hierarchy $mathrm{P} subseteq mathrm{NP} subseteq # mathrm{P}$.

created: 10-08-2012, last modified: 07-06-2013
P.-L. Giscard, S.J. Thwaite and D. Jaksch,
Evaluating Matrix Functions by Resummations on Graphs: the Method of Path-Sums,
SIAM. J. Matrix Anal. & Appl 34, 445 - 469 (2013).
Evaluating Matrix Functions by Resummations on Graphs: the Method of Path-Sums
We introduce the method of path-sums, which is a tool for analytically evaluating a primary function of a finite square discrete matrix based on the closed-form resummation of infinite families of terms in the corresponding Taylor series. Provided the required inverse transforms are available, our approach yields the exact result in a finite number of steps. We achieve this by combining a mapping between matrix powers and walks on a weighted directed graph with a universal graph-theoretic result on the structure of such walks. We present path-sum expressions for a matrix raised to a complex power, the matrix exponential, the matrix inverse, and the matrix logarithm. We present examples of the application of the path-sum method.

created: 08-12-2011, last modified: 07-06-2013
T. Grujic, S.R. Clark, D. Jaksch and D.G. Angelakis,
Non-equilibrium many-body effects in driven nonlinear resonator arrays,
New J. Phys. 14, 103025 (2012).
Non-equilibrium many-body effects in driven nonlinear resonator arrays
We study the non-equilibrium behavior of optically driven dissipative coupled resonator arrays. Assuming each resonator is coupled with a two-level system via a Jaynes-Cummings interaction, we calculate the many-body steady state behavior of the system under coherent pumping and dissipation. We propose and analyze the many-body phases using experimentally accessible quantities such as the total excitation number, the emitted photon spectra and photon coherence functions for different parameter regimes. In parallel, we also compare and contrast the expected behavior of this system assuming the local nonlinearity in the cavities is generated by a generic Kerr effect rather than a Jaynes-Cummings interaction. We nd that the behavior of the experimentally accessible observables produced by the two models differs for realistic regimes of interactions even when the corresponding nonlinearities are of similar strength. We analyze in detail the extra features available in the Jaynes-Cummings-Hubbard (JCH) model originating from the mixed nature of the excitations and investigate the regimes where the Kerr approximation would faithfully match the JCH physics. We nd that the latter is true for values of the light-matter coupling and losses beyond the reach of current technology. Throughout the study we operate in the weak pumping, fully quantum mechanical regime where approaches such as mean field theory fail, and instead use a combination of quantum trajectories and the time evolving block decimation algorithm to compute the relevant steady state observables. In our study we have assumed small to medium size arrays (from 3 up to 16 sites) and values of the ratio of coupling to dissipation rate g = 20 which makes our results implementable with current designs in Circuit QED and with near future photonic crystal set ups.

created: 01-06-2012, last modified: 17-10-2012
M. Pino, J. Prior, A.M. Somoza, D. Jaksch and S.R. Clark,
Re-entrance and entanglement in the one-dimensional Bose-Hubbard model,
Phys. Rev. A 86, 023631 (2012).
Re-entrance and entanglement in the one-dimensional Bose-Hubbard model
Re-entrance is a novel feature where the phase boundaries of a system exhibit a succession of transitions between two phases A and B, like A-B-A-B, when just one parameter is varied monotonically. This type of re-entrance is displayed by the 1D Bose Hubbard model between its Mott insulator (MI) and superfluid phase as the hopping amplitude is increased from zero. Here we analyse this counter-intuitive phenomenon directly in the thermodynamic limit by utilizing the infinite time-evolving block decimation algorithm to variationally minimize an infinite matrix product state (MPS) parameterized by a matrix size chi. Exploiting the direct restriction on the half-chain entanglement imposed by fixing chi, we determined that re-entrance in the MI lobes only emerges in this approximate when chi >= 8. This entanglement threshold is found to be coincident with the ability an infinite MPS to be simultaneously particle-number symmetric and capture the kinetic energy carried by particle-hole excitations above the MI. Focussing on the tip of the MI lobe we then applied, for the first time, a general finite-entanglement scaling analysis of the infinite order Kosterlitz-Thouless critical point located there. By analysing chi's up to a very moderate chi = 70 we obtained an estimate of the KT transition as t_KT = 0.30 +/- 0.01, demonstrating the how a finite-entanglement approach can provide not only qualitative insight but also quantitatively accurate predictions.

created: 01-06-2012, last modified: 25-08-2012
M. Kiffner, U. Dorner and D. Jaksch,
Dissipative quantum light field engineering,
Phys. Rev. A 85, 023812 (2012).
Dissipative quantum light field engineering
We put forward a dissipative preparation scheme for strongly correlated photon states. Our approach is based on a two-photon loss mechanism that is realized via a single four-level atom inside a bimodal optical cavity. Each elementary two-photon emission event removes one photon out of each of the two modes. The dark states of this loss mechanism are given by NOON states and arbitrary superpositions thereof. We find that the steady state of the two cavity modes exhibits entanglement and, for certain parameters, a mixture of two coherent entangled states is produced. We discuss how the quantum correlations in the cavity modes and the output fields can be measured.

created: 19-12-2011, last modified: 12-02-2012
S. Broadfoot, U. Dorner and D. Jaksch,
The Optical Excitation of Zigzag Carbon Nanotubes with Photons Guided in Nanofibres,
Phys. Rev. B 85, 195455 (2012).
The Optical Excitation of Zigzag Carbon Nanotubes with Photons Guided in Nanofibres
We consider the excitation of electrons in semiconducting carbon nanotubes by photons from the evanescent field created by a subwavelength-diameter optical fiber. The strongly changing evanescent field of such nanofibers requires dropping the dipole approximation. We show that this leads to novel effects, especially a high dependence of the photon absorption on the relative orientation and geometry of the nanotube-nanofiber setup in the optical and near infrared domain. In particular, we calculate photon absorption probabilities for a straight nanotube and nanofiber depending on their relative angle. Nanotubes orthogonal to the fiber are found to perform much better than parallel nanotubes when they are short. As the nanotube gets longer the absorption of parallel nanotubes is found to exceed the orthogonal nanotubes and approach 100% for extremely long nanotubes. In addition, we show that if the nanotube is wrapped around the fiber in an appropriate way the absorption is enhanced. We find that optical and near infrared photons could be converted to excitations with efficiencies that may exceed 90%. This may provide opportunities for future photodetectors and we discuss possible setups.

created: 09-12-2011, last modified: 28-08-2012
T.H. Johnson, M. Bruderer, Y. Cai, S.R. Clark, W. Bao and D. Jaksch,
Breathing oscillations of a trapped impurity in a Bose gas,
Europhys. Lett. 98, 26001 (2012).
Breathing oscillations of a trapped impurity in a Bose gas
Motivated by a recent experiment [J. Catani et al., arXiv:1106.0828v1 preprint, 2011], we study breathing oscillations in the width of a harmonically trapped impurity interacting with a separately trapped Bose gas. We provide an intuitive physical picture of such dynamics at zero temperature, using a time-dependent variational approach. In the Gross-Pitaevskii regime we obtain breathing oscillations whose amplitudes are suppressed by self trapping, due to interactions with the Bose gas. Introducing phonons in the Bose gas leads to the damping of breathing oscillations and non-Markovian dynamics of the width of the impurity, the degree of which can be engineered through controllable parameters. Our results reproduce the main features of the impurity dynamics observed by Catani et al. despite experimental thermal effects, and are supported by simulations of the system in the Gross-Pitaevskii regime. Moreover, we predict novel effects at lower temperatures due to self-trapping and the inhomogeneity of the trapped Bose gas.

created: 29-11-2011, last modified: 03-01-2013
S.J. Denny, J.D. Biamonte, D. Jaksch and S.R. Clark,
Algebraically contractible topological tensor network states,
J. Phys. A: Math. Theor. 45 015309 (2012).
Algebraically contractible topological tensor network states
We adapt the bialgebra and Hopf relations to expose internal structure in the ground state of a Hamiltonian with $Z_2$ topological order. Its tensor network description allows for exact contraction through simple diagrammatic rewrite rules. The contraction property does not depend on specifics such as geometry, but rather originates from the non-trivial algebraic properties of the constituent tensors. We then generalise the resulting tensor network from a spin-half lattice to a class of exactly contractible states on spin-S degrees of freedom, yielding the most efficient tensor network description of finite Abelian lattice gauge theories. We gain a new perspective on these states as examples of two-dimensional quantum states with algebraically contractible tensor network representations. The introduction of local perturbations to the network is shown to reduce the von Neumann entropy of string-like regions, creating an unentangled sub-system within the bulk in a certain limit. We also show how perturbations locally extinguish topological order. This class of tensor networks is readily translated onto any lattice, and we differentiate between the physical consequences of bipartite and non-bipartite lattices on the properties of the corresponding quantum states. We explicitly show this on the hexagonal, square, kagome and triangular lattices.

created: 04-08-2011, last modified: 05-05-2013
K.C. Lee, B.J. Sussman, M.R. Sprague, P. Michelberger, K.F. Reim, J. Nunn, N.K. Langford, P Bustard, D. Jaksch and I.A. Walmsley,
Macroscopic nonclassical states and terahertz quantum processing in room-temperature diamond,
Nature Photonics 6, 41 (2012).
Macroscopic nonclassical states and terahertz quantum processing in room-temperature diamond
The nature of the transition between the familiar classical, macroscopic world and the quantum, microscopic one continues to be poorly understood. Expanding the regime of observable quantum behaviour to large-scale objects is therefore an exciting open problem1. In macroscopic systems of interacting particles, rapid thermalization usually destroys any quantum coherence before it can be measured or used at room temperature2. Here, we demonstrate quantum processing in the vibrational modes of a macroscopic diamond sample under ambient conditions. Using ultrafast Raman scattering, we create an extended, highly non-classical state in the optical phonon modes of bulk diamond. Direct measurement of phonon coherence and correlations establishes the non-classical nature of the crystal dynamics. These results show that optical phonons in diamond provide a unique opportunity for the study of large-scale quantum behaviour, and highlight the potential for diamond as a micro-photonic quantum processor capable of operating at terahertz rates.

created: 16-06-2011, last modified: 03-01-2012
K.C. Lee, M.R. Sprague, B.J. Sussman, J. Nunn, N.K. Langford, X.-M. Jin, T. Champion, P. Michelberger, K.F. Reim, D. England, D. Jaksch and I.A. Walmsley,
Entangling Macroscopic Diamonds at Room Temperature,
Science 334, 1253 (2011).
Entangling Macroscopic Diamonds at Room Temperature
Quantum entanglement in the motion of macroscopic solid bodies has implications both for quantum technologies and foundational studies of the boundary between the quantum and classical worlds. Entanglement is usually fragile in room-temperature solids, owing to strong interactions both internally and with the noisy environment. We generated motional entanglement between vibrational states of two spatially separated, millimeter-sized diamonds at room temperature. By measuring strong nonclassical correlations between Raman-scattered photons, we showed that the quantum state of the diamonds has positive concurrence with 98% probability. Our results show that entanglement can persist in the classical context of moving macroscopic solids in ambient conditions.

created: 02-12-2011
S. Al-Assam, S. R. Clark, C. J. Foot, D. Jaksch,
Capturing long range correlations in two-dimensional quantum lattice systems using correlator product states,
Phys. Rev. B 84, 205108 (2011).
Capturing long range correlations in two-dimensional quantum lattice systems using correlator product states
We study the suitability of correlator product states for describing ground-state properties of two-dimensional spin models. Our ansatz for the many-body wave function takes the form of either plaquette or bond correlator product states and the energy is optimized by varying the correlators using Monte Carlo minimization. For the Ising model we find that plaquette correlators are best for estimating the energy while bond correlators capture the expected long-range correlations and critical behavior of the system more faithfully. For the antiferromagnetic Heisenberg model, however, plaquettes outperform bond correlators at describing both local and long-range correlations because of the substantially larger number of local parameters they contain. These observations have quantitative implications for the application of correlator product states to other more complex systems, and give important heuristic insights: in particular the necessity of carefully tailoring the choice of correlators to the system considered, its interactions and symmetries.

created: 06-07-2011, last modified: 10-11-2011
T.H. Johnson, S.R. Clark, M. Bruderer and D. Jaksch,
Impurity transport through a strongly interacting bosonic quantum gas,
Phys. Rev. A 84, 023617 (2011).
Impurity transport through a strongly interacting bosonic quantum gas
Using near-exact numerical simulations we study the propagation of an impurity through a one-dimensional Bose lattice gas for varying bosonic interaction strengths and filling factors at zero temperature. The impurity is coupled to the Bose gas and confined to a separate tilted lattice. The precise nature of the transport of the impurity is specific to the excitation spectrum of the Bose gas which allows one to measure properties of the Bose gas non-destructively, in principle, by observing the impurity; here we focus on the spatial and momentum distributions of the impurity as well as its reduced density matrix. For instance we show it is possible to determine whether the Bose gas is commensurately filled as well as the bandwidth and gap in its excitation spectrum. Moreover, we show that the impurity acts as a witness to the cross-over of its environment from the weakly to the strongly interacting regime, i.e., from a superfluid to a Mott insulator or Tonks-Girardeau lattice gas and the effects on the impurity in both of these strongly-interacting regimes are clearly distinguishable. Finally, we find that the spatial coherence of the impurity is related to its propagation through the Bose gas, giving an experimentally controllable example of noise-enhanced quantum transport.

created: 21-06-2011, last modified: 14-10-2011
J.D. Biamonte, S.R. Clark and D. Jaksch,
Categorical Tensor Network States,
AIP Advances 1, 042172 (2011).
Categorical Tensor Network States
We examine the use of the mathematics of category theory in the description of quantum states by tensor networks. This approach enables the development of a categorical framework allowing a solution to the quantum decomposition problem. Specifically, given an n-body quantum state |psi>, we present a general method to factor |psi> into a tensor network. Moreover, this decomposition of |psi> uses building blocks defined mathematically in terms of purely diagrammatic laws. We use the solution to expose a previously unknown and large class of quantum states which we prove can be sampled efficiently and exactly. This general framework of categorical tensor network states, where a combination of generic and algebraically defined tensors appear, enhances the theory of tensor network states.

created: 03-12-2010, last modified: 12-12-2011
S. Wall, D. Brida, S.R. Clark, H.P. Ehrke, D. Jaksch, A. Ardavan, S. Bonora, H. Uemura, Y. Takahashi, T. Hasegawa, H. Okamoto, G. Cerullo and A. Cavalleri,
Quantum interference between charge excitation paths in a solid-state Mott insulator,
Nature Physics 7, 114 (2011).
Quantum interference between charge excitation paths in a solid-state Mott insulator
By using nearly-single-cycle optical pulses in the near infrared, we measure the time-dependent optical properties of the Mott insulator ET-F2TCNQ after prompt photo-excitation. We observe a rapid drop in spectral weight at the Mott gap, and oscillations in the optical conductivity, revealing coherent electronic-structural rearrangements on the timescale of electron correlations. A quantum dynamic model based on the Fermi Hubbard Hamiltonian predicts recurrence oscillations between bound and unbound holon-doublon pairs, closely matching the experimentally observed frequency. To date, coherent many body processes of this kind have only been experimentally accessible in ultracold gases, where the correlation energies are re-normalized to lower values and the dynamics is correspondingly slower. We bridge the gap between optics on extreme timescales and the control of strongly correlated quantum gases, and identify the first steps in the coherent response of a Mott insulator.

created: 21-10-2009, last modified: 01-02-2011
S. Broadfoot, U. Dorner and D. Jaksch,
Long distance entanglement generation in two-dimensional networks,
Phys. Rev. A 82, 042326 (2010).
Long distance entanglement generation in two-dimensional networks
We consider two-dimensional networks composed of nodes initially linked by two-qubit mixed states. In these networks we develop a global error correction scheme that can generate distance-independent entanglement from arbitrary network geometries using rank-2 states. By using this method and combining it with the concept of percolation, we also show that the generation of long-distance entanglement is possible with rank-3 states. Entanglement percolation and global error correction have different advantages depending on the given situation. To reveal the trade-off between them we consider their application to networks containing pure states. In doing so we find a range of pure-state schemes, each of which has applications in particular circumstances: For instance, we can identify a protocol for creating perfect entanglement between two distant nodes. However, this protocol cannot generate a singlet between any two nodes. In contrast, we can also construct schemes for creating entanglement between any nodes, but the corresponding entanglement fidelity is lower.

created: 24-08-2010, last modified: 26-07-2011
M. Bruderer, T.H. Johnson, S.R. Clark, D. Jaksch, A. Posazhennikova and W. Belzig,
Phonon resonances in atomic currents through lattice Bose-Fermi mixtures,
Phys. Rev. A 82, 043617 (2010).
Phonon resonances in atomic currents through lattice Bose-Fermi mixtures
We present an analysis of Bose-Fermi mixtures in optical lattices for the case where the lattice potential of the fermions is tilted and the bosons (in the superfluid phase) are described by Bogoliubov phonons. It is shown that the Bogoliubov phonons enable hopping transitions between fermionic Wannier-Stark states; these transitions are accompanied by energy dissipation into the superfluid and result in a net atomic current along the lattice. We derive a general expression for the drift velocity of the fermions and find that the dependence of the atomic current on the lattice tilt exhibits negative differential conductance (NDC) and phonon resonances. Numerical simulations of the full dynamics of the system based on the time-evolving block decimation (TEBD) algorithm reveal that the phonon resonances should be observable under the conditions of a realistic measuring procedure.

created: 23-07-2010, last modified: 14-10-2011
L. Heaney, S.-W. Lee and D. Jaksch,
Bell inequality for pairs of particle-number-superselection-rule restricted states,
Phys. Rev. A 82, 042116 (2010).
Bell inequality for pairs of particle-number-superselection-rule restricted states
Proposals for Bell-inequality tests on systems restricted by the particle-number-superselection rule often require operations that are difficult to implement in practice. In this article, we derive a Bell inequality, where measurements on pairs of states are used as a method to bypass this superselection rule. In particular, we focus on mode entanglement of an arbitrary number of massive particles and show that our Bell inequality detects the entanglement in an identical pair of states when other inequalities fail. However, as the number of particles in the system increases, the violation of our Bell inequality decreases due to the restriction in the measurement space caused by the superselection rule. This Bell test can be implemented using techniques that are routinely used in current experiments.

created: 21-06-2010, last modified: 26-07-2011
T.H. Johnson, S.R. Clark and D. Jaksch,
Dynamical simulations of classical stochastic systems using matrix product states,
Phys. Rev. E 82, 036702 (2010).
Dynamical simulations of classical stochastic systems using matrix product states
We adapt the time-evolving block decimation (TEBD) algorithm, originally devised to simulate the dynamics of 1D quantum systems, to simulate the time-evolution of non-equilibrium stochastic systems. We describe this method in detail; a system's probability distribution is represented by a matrix product state (MPS) of finite dimension and then its time-evolution is efficiently simulated by repeatedly updating and approximately re-factorizing this representation. We examine the use of MPS as an approximation method, looking at parallels between the interpretations of applying it to quantum state vectors and probability distributions. In the context of stochastic systems we consider two types of factorization for use in the TEBD algorithm: non-negative matrix factorization (NMF), which ensures that the approximate probability distribution is manifestly non-negative, and the singular value decomposition (SVD). Comparing these factorizations we find the accuracy of the SVD to be substantially greater than current NMF algorithms. We then apply TEBD to simulate the totally asymmetric simple exclusion process (TASEP) for systems of up to hundreds of lattice sites in size. Using exact analytic results for the TASEP steady state, we find that TEBD reproduces this state such that the error in calculating expectation values can be made negligible, even when severely compressing the description of the system by restricting the dimension of the MPS to be very small. Out of the steady state we show for specific observables that expectation values converge as the dimension of the MPS is increased to a moderate size.

created: 15-06-2010, last modified: 17-09-2010
S.R. Clark, A.J. Connor, D. Jaksch and S. Popescu,
Entanglement consumption of instantaneous nonlocal quantum measurements,
New J. Phys. 12, 083034 (2010).
Entanglement consumption of instantaneous nonlocal quantum measurements
Relativistic causality has dramatic consequences on the measurability of nonlocal variables and poses the fundamental question of whether it is physically meaningful to speak about the value of nonlocal variables at a particular time. Recent work has shown that by weakening the role of the measurement in preparing eigenstates of the variable it is in fact possible to measure all nonlocal observables instantaneously by exploiting entanglement. However, for these measurement schemes to succeed with certainty an infinite amount of entanglement must be distributed initially and all this entanglement is necessarily consumed. In this work we sharpen the characterisation of instantaneous nonlocal measurements by explicitly devising schemes in which only a finite amount of the initially distributed entanglement is ever utilised. This enables us to determine an upper bound to the average consumption for the most general cases of nonlocal measurements. This includes the tasks of state verification, where the measurement verifies if the system is in a given state, and verification measurements of a general set of eigenstates of an observable. Despite its finiteness the growth of entanglement consumption is found to display an extremely unfavourable exponential of an exponential scaling with either the number of qubits needed to contain the Schmidt rank of the target state or total number of qubits in the system for an operator measurement. This scaling is seen to be a consequence of the combination of the generic exponential scaling of unitary decompositions combined with the highly recursive structure of our scheme required to overcome the no-signalling constraints of relativistic causality.

created: 06-04-2010, last modified: 17-08-2010
M. Rosenkranz, A. Klein and D. Jaksch,
Simulating and detecting artificial magnetic fields in trapped atoms,
Phys. Rev. A 81, 013607 (2010).
Simulating and detecting artificial magnetic fields in trapped atoms
A Bose-Einstein condensate exhibiting a nontrivial phase induces an artificial magnetic field in immersed impurity atoms trapped in a stationary, ring-shaped optical lattice. We present an effective Hamiltonian for the impurities for two condensate setups: the condensate in a rotating ring and in an excited rotational state in a stationary ring. We use Bogoliubov theory to derive analytical formulas for the induced artificial magnetic field and the hopping amplitude in the limit of low condensate temperature where the impurity dynamics is coherent. As methods for observing the artificial magnetic field we discuss time-of-flight imaging and mass current measurements. Moreover, we compare the analytical results of the effective model to numerical results of a corresponding two-species Bose-Hubbard model. We also study numerically the clustering properties of the impurities and the quantum chaotic behavior of the two-species Bose-Hubbard model.

created: 17-02-2010
K.F. Reim, J. Nunn, V.O. Lorenz, B.J. Sussman, K.C. Lee, D. Jaksch and I.A. Walmsley,
Towards high-speed optical quantum memories,
Nature Photonics 4, 218 - 221 (2010).
Towards high-speed optical quantum memories
Quantum memories, capable of controllably storing and releasing a photon, are a crucial component for quantum computers and quantum communications. To date, quantum memories have operated with bandwidths that limit data rates to megahertz. Here we report the coherent storage and retrieval of sub-nanosecond low-intensity light pulses with spectral bandwidths exceeding 1|[nbsp]|GHz in caesium vapour. The novel memory interaction takes place through a far off-resonant two-photon transition in which the memory bandwidth is dynamically generated by a strong control field. This should allow data rates more than 100 times greater than those of existing quantum memories. The memory works with a total efficiency of 15|[percnt]|, and its coherence is demonstrated through direct interference of the stored and retrieved pulses. Coherence times in hot atomic vapours are on the order of microseconds, the expected storage time limit for this memory.

created: 29-01-2010, last modified: 07-04-2010
S. Broadfoot, U. Dorner and D. Jaksch,
Singlet Generation in Mixed State Quantum Networks,
Phys. Rev. A 81, 042316 (2010).
Singlet Generation in Mixed State Quantum Networks
We study the generation of singlets in quantum networks with nodes initially sharing a finite number of partially entangled bipartite mixed states. We prove that singlets between arbitrary nodes in such networks can be created if and only if the initial states connecting the nodes have a particular form. We then generalize the method of entanglement percolation, previously developed for pure states, to mixed states of this form. As part of this, we find and compare different distillation protocols necessary to convert groups of mixed states shared between neighboring nodes of the network into singlets. In addition, we discuss protocols that only rely on local rules for the efficient connection of two remote nodes in the network via entanglement swapping. Further improvements of the success probability of singlet generation are developed by using particular forms of “quantum preprocessing” on the network. This includes generalized forms of entanglement swapping and we show how such strategies can be embedded in regular and hierarchical quantum networks.

created: 27-01-2010, last modified: 09-05-2010
S.R. Clark, J. Prior, M. J. Hartmann, D. Jaksch and M.B. Plenio,
Exact matrix product solutions in the Heisenberg picture of an open quantum spin chain,
New J. Phys. 12, 025005 (2010).
Exact matrix product solutions in the Heisenberg picture of an open quantum spin chain
In recent work Hartmann et al [Phys. Rev. Lett. 102, 057202 (2009)] demonstrated that the classical simulation of the dynamics of open 1D quantum systems with matrix product algorithms can often be dramatically improved by performing time evolution in the Heisenberg picture. For a closed system this was exemplified by an exact matrix product operator solution of the time-evolved creation operator of a quadratic fermi chain with a matrix dimension of just two. In this work we show that this exact solution can be significantly generalized to include the case of an open quadratic fermi chain subjected to master equation evolution with Lindblad operators that are linear in the fermionic operators. Remarkably even in this open system the time-evolution of operators continues to be described by matrix product operators with the same fixed dimension as that required by the solution of a coherent quadratic fermi chain for all times. Through the use of matrix product algorithms the dynamical behaviour of operators in this non-equilibrium open quantum system can be computed with a cost that is linear in the system size. We present some simple numerical examples which highlight how useful this might be for the more detailed study of open system dynamics. Given that Heisenberg picture simulations have been demonstrated to offer significant accuracy improvements for other open systems that are not exactly solvable our work also provides further insight into how and why this advantage arises.

created: 03-08-2009, last modified: 12-03-2010
R. Walters, S.R. Clark and D. Jaksch,
Decoherence of a quantum memory coupled to a collective spin bath,
Int. J. Quant. Info. 8, 271 (2010).
Decoherence of a quantum memory coupled to a collective spin bath
We study the quantum dynamics of a single qubit coupled to a bath of interacting spins as a model for decoherence in solid state quantum memories. The spin bath is described by the Lipkin-Meshkov-Glick model and the bath spins are subjected to a transverse magnetic field. We investigate the qubit interacting via either an Ising- or an XY-type coupling term to subsets of bath spins of differing size. The large degree of symmetry of the bath allows us to find parameter regimes where the initial qubit state is revived at well defined times after the qubit preparation. These times may become independent of the bath size for large baths and thus enable faithful qubit storage even in the presence of strong coupling to a bath. We analyze a large range of parameters and identify those which are best suited for quantum memories. In general we find that a small number of links between qubit and bath spins leads to less decoherence and that systems with Ising coupling between qubit and bath spins are preferable.

created: 09-07-2009, last modified: 27-05-2010
K.C. Lee, B.J. Sussman, J. Nunn, V.O. Lorenz, K.F. Reim, D. Jaksch, I.A. Walmsley, P. Spizziri and S. Prawer,
Comparing Vibrational Dephasing Lifetimes in Diamond using Transient Coherent Ultrafast Phonon Spectroscopy,
Diamond and Related Materials 19, 1289 (2010).
Comparing Vibrational Dephasing Lifetimes in Diamond using Transient Coherent Ultrafast Phonon Spectroscopy
Transient Coherent Ultrafast Phonon Spectroscopy (TCUPS) is utilized to study phonon dephasing lifetimes in various diamond types. Samples of natural, chemical vapour deposited, and high pressure high temperature diamond are compared showing significant differences. Dephasing mechanisms are discussed.

created: 16-06-2009, last modified: 31-08-2010
S.-W. Lee, H. Jeong and D. Jaksch,
Witnessing entanglement in phase space using inefficient detectors,
Phys. Rev. A 81, 012302 (2010).
Witnessing entanglement in phase space using inefficient detectors
We propose a scheme for witnessing entanglement in phase space by significantly inefficient detectors. The implementation of this scheme does not require any additional process for correcting errors, in contrast to previous proposals. Moreover, it allows us to detect entanglement without full a priori knowledge of the detection efficiency. It is shown that entanglement in single-photon entangled and two-mode squeezed states can be witnessed with detection efficiency as low as 40 percent. Our approach enhances the possibility of witnessing entanglement in various physical systems using current detection technologies.

created: 08-04-2009, last modified: 07-01-2010
J. Nunn, U. Dorner, P. Michelberger, K.F. Reim, K.C. Lee, N.K. Langford, I.A. Walmsley and D. Jaksch,
Quantum memory in an optical lattice,
Phys. Rev. A 82, 022327 (2010).
Quantum memory in an optical lattice
Arrays of atoms trapped in optical lattices are appealing as storage media for photons, since motional dephasing of the atoms is eliminated. The regular lattice is also associated with band structure in the dispersion experienced by incident photons. Here we study the in uence of this band structure on the efficiency of quantum memories based on electromagnetically induced transparency (EIT) and on Raman absorption. We observe a number of interesting eects, such as both reduced and superluminal group velocities, enhanced atom-photon coupling and anomalous transmission. These effects are ultimately deleterious to the memory efficiency, but they are easily avoided by tuning the optical fields away from the band edges.

created: 27-11-2007, last modified: 27-08-2010
S. Broadfoot, U. Dorner and D. Jaksch,
Entanglement Percolation with Bipartite Mixed States,
EuroPhys. Lett. 88, 50002 (2009).
Entanglement Percolation with Bipartite Mixed States
We develop a concept of entanglement percolation for long-distance singlet generation in quantum networks with neighboring nodes connected by partially entangled bipartite mixed states. We give a necessary and sufficient condition on the class of mixed network states for the generation of singlets. States beyond this class are insufficient for entanglement percolation. We find that neighboring nodes are required to be connected by multiple partially entangled states and devise a rich variety of distillation protocols for the conversion of these states into singlets. These distillation protocols are suitable for a variety of network geometries and have a sufficiently high success probability even for significantly impure states. In addition to this, we discuss possible further improvements achievable by using quantum strategies including generalized forms of entanglement swapping.

created: 11-12-2009, last modified: 29-01-2010
S.-W. Lee, H. Jeong and D. Jaksch,
Testing quantum nonlocality by generalized quasiprobability functions,
Phys. Rev. A 80, 022104 (2009).
Testing quantum nonlocality by generalized quasiprobability functions
We derive a Bell inequality based on a generalized quasiprobability function which is parameterized by one non-positive real value. Two types of known Bell inequalities formulated in terms of the Wigner- and Q-functions are included as limiting cases. We investigate violations of our Bell inequalities for single photon entangled states and two-mode squeezed vacuum states when varying the detector efficiency. We show that the Bell inequality for the Q-function allows the lowest detection efficiency for violations of local realism.

created: 04-08-2009
M. Rosenkranz and D. Jaksch,
Parameter estimation with cluster states,
Phys. Rev. A 79, 022103 (2009).
Parameter estimation with cluster states
We propose a scheme for parameter estimation with cluster states. We find that phase estimation with cluster states under a many-body Hamiltonian and separable measurements leads to a precision at the Heisenberg limit. As noise models we study the dephasing, depolarizing, and pure damping channels. Decoherence reduces the sensitivity but our scheme remains superior over several reference schemes with states such as maximally entangled states and product states. For small cluster states and fixed evolution times it remains at the Heisenberg limit for approximately 2 times as many qubits than alternative schemes.

created: 09-12-2008, last modified: 04-02-2009
A. Klein and D. Jaksch,
Phonon-induced artificial magnetic fields,
EuroPhys. Lett. 85, 13001 (2009).
Phonon-induced artificial magnetic fields
We investigate the effect of a rotating Bose-Einstein condensate on a system of immersed impurity atoms trapped by an optical lattice. We analytically show that for a one-dimensional, ring-shaped setup the coupling of the impurities to the Bogoliubov phonons of the condensate leads to a non-trivial phase in the impurity hopping. The presence of this phase can be tested by observing a drift in the transport properties of the impurities. These results are quantitatively confirmed by a numerically exact simulation of a two-mode Bose-Hubbard model. We also give analytical expressions for the occurring phase terms for a two-dimensional setup. The phase realises an artificial magnetic field and can for instance be used for the simulation of the quantum Hall effect using atoms in an optical lattice.

created: 15-08-2008, last modified: 19-01-2009
S.-W. Lee and D. Jaksch,
Maximal violation of tight Bell inequalities for maximal high-dimensional entanglement,
Phys. Rev. A 80, 010103(R) (2009).
Maximal violation of tight Bell inequalities for maximal high-dimensional entanglement
We propose a Bell inequality for high-dimensional bipartite systems obtained by binning local measurement outcomes and show that it is tight. We find a binning method for even d-dimensional measurement outcomes for which this Bell inequality is maximally violated by maximally entangled states. Furthermore, we demonstrate that the Bell inequality is applicable to continuous variable systems and yields strong violations for two-mode squeezed states.

created: 20-03-2008, last modified: 30-07-2009
J. Nunn, K.F. Reim, K.C. Lee, V.O. Lorenz, B.J. Sussman, I.A. Walmsley and D. Jaksch,
Multimode Memories in Atomic Ensembles,
Phys. Rev. Lett. 101, 260502 (2008).
Multimode Memories in Atomic Ensembles
The ability to store multiple optical modes in a quantum memory allows for increased effciency of quantum communication and computation. Here we compute the multimode capacity of a variety of quantum memory protocols based on light storage in ensembles of atoms. We find that adding a ontrolled inhomogeneous broadening improves this capacity significantly.

created: 26-11-2008, last modified: 19-01-2009
B Vaucher, S.J. Thwaite and D. Jaksch,
Ultra-large Rydberg dimers in optical lattices,
Phys. Rev. A 78, 043415 (2008).
Ultra-large Rydberg dimers in optical lattices
We investigate the dynamics of Rydberg electrons excited from the ground state of ultracold atoms trapped in an optical lattice. We first consider a lattice comprising an array of double-well potentials, where each double well is occupied by two ultracold atoms. We demonstrate the existence of molecular states with equilibrium distances of the order of experimentally attainable inter-well spacings and binding energies of the order of 103 GHz. We also consider the situation whereby ground-state atoms trapped in an optical lattice are collectively excited to Rydberg levels, such that the charge-density distributions of neighbouring atoms overlap. We compute the hopping rate and interaction matrix elements between highly-excited electrons separated by distances comparable to typical lattice spacings. Such systems have tunable interaction parameters and a temperature ~10^{-4} times smaller than the Fermi temperature, making them potentially attractive for the study and simulation of strongly correlated electronic systems.

created: 16-08-2008, last modified: 13-07-2009
F.C. Waldermann, B.J. Sussman, J. Nunn, V.O. Lorenz, K.C. Lee, K. Surmacz, K.H. Lee, D. Jaksch, I.A. Walmsley, P. Spizziri, P. Olivero and S. Prawer,
Measuring Phonon Dephasing with Ultrafast Pulses using Raman spectral interference,
Phys. Rev. B 78, 155201 (2008).
Measuring Phonon Dephasing with Ultrafast Pulses using Raman spectral interference
A technique to measure the decoherence time of optical phonons in a solid is presented. Phonons are excited with a pair of time delayed 80 fs, near infrared pulses via spontaneous, transient Raman scattering. The fringe visibility of the resulting Stokes pulse pair, as a function of time delay, is used to measure the phonon dephasing time. The method avoids the need to use either narrow band or few femtosecond pulses and is useful for low phonon excitations. The dephasing time of phonons created in bulk diamond is measured to be tau=6.8ps (Delta nu=1.56 cm-1).

created: 17-07-2008, last modified: 09-12-2008
R.N. Palmer, A. Klein and D. Jaksch,
Optical lattice quantum Hall effect,
Phys. Rev. A 78, 013609 (2008).
Optical lattice quantum Hall effect
We explore the behavior of interacting bosonic atoms in an optical lattice subject to a large artificial magnetic field. We extend earlier investigations of this system where the number of magnetic flux quanta per unit cell alpha is close to a simple rational number [Phys. Rev. Lett. 96, 180407 (2006)]. Interesting topological states such as the Laughlin and Read-Rezayi states can occur even if the atoms experience a weak trapping potential in one direction. An explicit numerical calculation near alpha = 1/2 shows that the system exhibits a striped vortex lattice phase of one species, which is analogous to the behavior of a two-species system for small alpha. We also investigate methods to probe the encountered states. These include spatial correlation functions and the measurement of noise correlations in time of flight expanded atomic clouds. Characteristic differences arise which allow for an identification of the respective quantum Hall states. We furthermore discuss that a counterintuitive flow of the Hall current occurs for certain values of alpha.

created: 08-07-2008, last modified: 15-07-2008
M. Rosenkranz, D. Jaksch, F.Y. Lim and W. Bao,
Self-trapping of Bose-Einstein condensates expanding into shallow optical lattices,
Phys. Rev. A 77, 063607 (2008).
Self-trapping of Bose-Einstein condensates expanding into shallow optical lattices
We observe a sudden breakdown of the transport of a strongly repulsive Bose-Einstein condensate through a shallow optical lattice of finite width. We are able to attribute this behavior to the development of a self-trapped state by using accurate numerical methods and an analytical description in terms of nonlinear Bloch waves. The dependence of the breakdown on the lattice depth and the interaction strength is investigated. We show that it is possible to prohibit the self-trapping by applying a constant offset potential to the lattice region. Furthermore, we observe the disappearance of the self-trapped state after a finite time as a result of the revived expansion of the condensate through the lattice. This revived expansion is due to the finite width of the lattice.

created: 08-04-2008, last modified: 15-07-2008
M. Bruderer, W. Bao and D. Jaksch,
Self-trapping of impurities in Bose-Einstein condensates: Strong attractive and repulsive coupling,
EuroPhys. Lett. 82, 30004 (2008).
Self-trapping of impurities in Bose-Einstein condensates: Strong attractive and repulsive coupling
We study the interaction-induced localization -- the so-called self-trapping -- of a neutral impurity atom immersed in a homogeneous Bose-Einstein condensate (BEC). Based on a Hartree description of the BEC we show that -- unlike repulsive impurities -- attractive impurities have a singular ground state in 3d and shrink to a point-like state in 2d as the coupling approaches a critical value. Moreover, we find that the density of the BEC increases markedly in the vicinity of attractive impurities in 1d and 2d, which strongly enhances inelastic collisions between atoms in the BEC. These collisions result in a loss of BEC atoms and possibly of the localized impurity itself.

created: 28-01-2008, last modified: 15-07-2008
M. Rodriguez, S.R. Clark and D. Jaksch,
Adiabatic melting of two-component Mott-insulator states,
Phys. Rev. A 77, 043613 (2008).
Adiabatic melting of two-component Mott-insulator states
We analyze the outcome of a Mott insulator to superfluid transition for a two-component Bose gas with two atoms per site in an optical lattice in the limit of slow ramping down the lattice potential. This manipulation of the initial Mott insulating state transforms local correlations between hyperfine states of atom pairs into multiparticle correlations extending over the whole system. We show how to create macroscopic twin Fock states in this way an that, in general, the obtained superfluid states are highly depleted even for initial ground Mott insulator states.

created: 22-01-2008, last modified: 11-04-2008
M. Rodriguez, S.R. Clark and D. Jaksch,
Adiabatic evolution of on-site superposition states in a completely-connected optical lattice,
J. Phys.: Conf. Ser. 99, 012017 (2008).
Adiabatic evolution of on-site superposition states in a completely-connected optical lattice
We analyze the dynamical melting of two-component atomic Mott-Insulator states in a completely-connected optical lattice within the adiabatic approximation. We examine in detail the effect of the dynamical phase acquired by the state during the adiabatic melting of the lattice potential. We show how for certain limits an on-site superposition state with two particles per site melts into a macroscopic superposition state, while an on-site superposition state with only one particle per site melts into a coherent state.

created: 06-12-2007, last modified: 06-03-2008
B Vaucher, A Nunnenkamp and D. Jaksch,
Creation of robust entangled states and new resources for measurement-based quantum computation using optical superlattices,
New J. Phys. 10, 023005 (2008).
Creation of robust entangled states and new resources for measurement-based quantum computation using optical superlattices
We investigate how to create robust entangled states of ultracold atoms trapped in optical lattices by dynamically manipulating the shape of the lattice potential. We consider an additional potential (the superlattice) that allows both the splitting of each site into a double well potential, and the control of the height of potential barrier between sites. We use superlattice manipulations to perform entangling operations between neighbouring qubits encoded on the Zeeman levels of the atoms without having to perform transfers between the different vibrational states of the atoms. We show how to use superlattices to engineer robust many-body entangled states. Also, we present a method to realize a 2D resource for measurement-based quantum computing via Bell-pair measurements that is resilient to collective dephasing noise. We analyze measurement networks that allow the execution of quantum algorithms while maintaining the resilience properties of the system throughout the computation.

created: 26-10-2007, last modified: 07-02-2008
M. Bruderer, A. Klein, S.R. Clark and D. Jaksch,
Transport of strong-coupling polarons in optical lattices,
New J. Phys. 10, 033015 (2008).
Transport of strong-coupling polarons in optical lattices
We study the transport of ultracold impurity atoms immersed in a Bose-Einstein condensate (BEC) and trapped in a tight optical lattice. Within the strong-coupling regime, we derive an extended Hubbard model describing the dynamics of the impurities in terms of polarons, i.e. impurities dressed by a coherent state of Bogoliubov phonons. Using a generalized master equation based on this microscopic model we show that inelastic and dissipative phonon scattering results in (i) a crossover from coherent to incoherent transport of impurities with increasing BEC temperature and (ii) the emergence of a net atomic current across a tilted optical lattice. The dependence of the atomic current on the lattice tilt changes from ohmic conductance to negative differential conductance within an experimentally accessible parameter regime. This transition is accurately described by an Esaki-Tsu-type relation with the effective relaxation time of the impurities as a temperature-dependent parameter.

created: 25-10-2007, last modified: 11-03-2008
U. Dorner, A. Klein and D. Jaksch,
A quantum repeater based on decoherence free subspaces,
Quant. Inf. Comp. 8, 0468 (2008).
A quantum repeater based on decoherence free subspaces
We study a quantum repeater which is based on decoherence free quantum gates recently proposed by Klein et al. [Phys. Rev. A, 73, 012332 (2006)]. A number of operations on the decoherence free subspace in this scheme makes use of an ancilla qubit which undergoes dephasing and thus introduces decoherence to the system. We examine how this decoherence affects entanglement swapping and purification as well as the performance of a quantum repeater. We compare the decoherence free quantum repeater with a quantum repeater based on qubits that are subject to decoherence and show that it outperforms the latter when decoherence due to long waiting times of conventional qubits becomes significant. Thus, a quantum repeater based on decoherence free subspaces is a possibility to greatly improve quantum communication over long or even intercontinental distances.

created: 01-06-2007, last modified: 17-04-2008
K. Surmacz, J. Nunn, K.F. Reim, K.C. Lee, V.O. Lorenz, B.J. Sussman, I.A. Walmsley and D. Jaksch,
Efficient spatially-resolved multimode quantum memory,
Phys. Rev. A 78, 033806 (2008).
Efficient spatially-resolved multimode quantum memory
Light storage in atomic ensembles has been implemented successfully, but the retrieval efficiency can be low. We propose to improve this efficiency with appropriately phase-matched backward propagating retrieval. This method allows for easy spatial filtering of the retrieved light; in addition, multiple optical modes can be stored in the transverse momentum of the ensemble. We model walk-off effects with a full numerical simulation, and confirm the applicability of the scheme.

created: 02-04-2007, last modified: 10-09-2008
A. Klein, M. Bruderer, S.R. Clark and D. Jaksch,
Dynamics, dephasing and clustering of impurity atoms in Bose-Einstein condensates,
New J. Phys. 9, 411 (2007).
Dynamics, dephasing and clustering of impurity atoms in Bose-Einstein condensates
We investigate the influence of a Bose-Einstein condensate (BEC) on the properties of immersed impurity atoms, which are trapped in an optical lattice. Assuming a weak coupling of the impurity atoms to the BEC, we derive a quantum master equation (QME) for the lattice system. In the special case of fixed impurities with two internal states the atoms represent a quantum register and the QME reproduces the exact evolution of the qubits. We characterize the qubit dephasing which is caused by the interspecies coupling and show that the effect of sub- and super-decoherence is observable for realistic experimental parameters. Furthermore, the BEC phonons mediate an attractive interaction between the impurities, which has an important impact on their spatial distribution. If the lattice atoms are allowed to move, there occurs a sharp transition with the impurities aggregating in a macroscopic cluster at experimentally achievable temperatures. We also investigate the impact of the BEC on the transport properties of the impurity atoms and show that a crossover from coherent to diffusive behaviour occurs with increasing interaction strength.

created: 21-11-2007, last modified: 26-11-2007
A. Klein, D. Jaksch, Y Zhang and W. Bao,
Dynamics of vortices in weakly interacting Bose-Einstein condensates,
Phys. Rev. A 76, 043602 (2007).
Dynamics of vortices in weakly interacting Bose-Einstein condensates
We study the dynamics of vortices in ideal and weakly interacting Bose-Einstein condensates using a Ritz minimization method to solve the two-dimensional Gross-Pitaevskii equation. For different initial vortex configurations we calculate the trajectories of the vortices. We find conditions under which a vortex-antivortex pair annihilates and is created again. For the case of three vortices we show that at certain times two additional vortices may be created, which move through the condensate and annihilate each other again. For a noninteracting condensate this process is periodic, whereas for small interactions the essential features persist, but the periodicity is lost. The results are compared to exact numerical solutions of the Gross-Pitaevskii equation confirming our analytical findings.

created: 03-10-2007
M. Bruderer, A. Klein, S.R. Clark and D. Jaksch,
Polaron Physics in Optical Lattices,
Phys. Rev. A 76, 011605(R) (2007).
Polaron Physics in Optical Lattices
We investigate the effects of a nearly uniform Bose-Einstein condensate (BEC) on the properties of immersed trapped impurity atoms. Using a weak-coupling expansion in the BEC-impurity interaction strength, we derive a model describing polarons, i.e., impurities dressed by a coherent state of Bogoliubov phonons, and apply it to ultracold bosonic atoms in an optical lattice. We show that, with increasing BEC temperature, the transport properties of the impurities change from coherent to diffusive. Furthermore, stable polaron clusters are formed via a phonon-mediated off-site attraction.

created: 06-03-2007, last modified: 24-07-2007
B Vaucher, S.R. Clark, U. Dorner and D. Jaksch,
Fast initialization of a high-fidelity quantum register using optical superlattices,
New J. Phys. 9, 221 (2007).
Fast initialization of a high-fidelity quantum register using optical superlattices
We propose a method for the fast generation of a quantum register of addressable qubits consisting of ultracold atoms stored in an optical lattice. Starting with a half filled lattice we remove every second lattice barrier by adiabatically switching on a superlattice potential which leads to a long wavelength lattice in the Mott insulator state with unit filling. The larger periodicity of the resulting lattice could make individual addressing of the atoms via an external laser feasible. The dynamics of the transition from a half filled to a commensurately filled lattice is analyzed numerically with the help of the Time Evolving Block Decimation algorithm and analytically using the Kibble-Zurek theory. We show that the time scale for the whole process, i.e. creating the half filled lattice and subsequent doubling of the lattice periodicity, is significantly faster than adiabatic direct quantum freezing of a superfluid into a Mott insulator for large lattice periods. Our method therefore provides a high fidelity quantum register of addressable qubits on a fast time scale.

created: 27-02-2007, last modified: 27-11-2007
S.R. Clark, A. Klein, M. Bruderer and D. Jaksch,
Graph state generation with noisy mirror-inverting spin chains,
New J. Phys. 9, 202 (2007).
Graph state generation with noisy mirror-inverting spin chains
We investigate the influence of noise on a graph state generation scheme which exploits a mirror inverting spin chain. Within this scheme the spin chain is used repeatedly as an entanglement bus (EB) to create multi-partite entanglement. The model we consider comprises of each spin of this EB being exposed to independent local noise which degrades the capabilities of the EB. Here we concentrate on quantifying its performance as a single-qubit channel and as a mediator of a two-qubit entangling gate, since these are basic operations necessary for graph state generation using the EB. In particular, for the single-qubit case we numerically calculate the average channel fidelity and whether the channel becomes entanglement breaking, i.e., expunges any entanglement the transferred qubit may have with other external qubits. We find that neither local decay nor dephasing noise cause entanglement breaking. This is in contrast to local thermal and depolarizing noise where we determine a critical length and critical noise coupling, respectively, at which entanglement breaking occurs. The critical noise coupling for local depolarizing noise is found to exhibit a power-law dependence on the chain length. For two qubits we similarly compute the average gate fidelity and whether the ability for this gate to create entanglement is maintained. By considering the concatenation of these noisy gates for the construction of a linear cluster state we demonstrate that there are severe constraints on the level of noise that can be tolerated for graph state generation.

created: 16-02-2007, last modified: 26-11-2007
A. Griessner, A.J. Daley, S.R. Clark, D. Jaksch and P. Zoller,
Dissipative dynamics of atomic Hubbard models coupled to a phonon bath: Dark state cooling of atoms within a Bloch band of an optical lattice,
New J. Phys. 9, 44 (2007).
Dissipative dynamics of atomic Hubbard models coupled to a phonon bath: Dark state cooling of atoms within a Bloch band of an optical lattice
We analyse a laser assisted sympathetic cooling scheme for atoms within the lowest Bloch band of an optical lattice. This scheme borrows ideas from sub-recoil laser cooling, implementing them in a new context in which the atoms in the lattice are coupled to a BEC reservoir. In this scheme, excitation of atoms between Bloch bands replaces the internal structure of atoms in normal laser cooling, and spontaneous emission of photons is replaced by creation of excitations in the BEC reservoir. We analyse the cooling process for many bosons and fermions, and obtain possible temperatures corresponding to a small fraction of the Bloch band width within our model. This system can be seen as a novel realisation of a many-body open quantum system.

created: 12-12-2006, last modified: 24-07-2007
F.C. Waldermann, J. Nunn, K. Surmacz, Z. Wang, D. Jaksch, R. A. Taylor, I.A. Walmsley, P. Olivero, M. Draganski, B. Fairchild, P. Reichart, A. Greentree, D. Jamieson and S. Prawer,
Creating diamond color centers for quantum optical applications,
Diamond and Related Materials 16, 1887 (2007).
Creating diamond color centers for quantum optical applications
Nitrogen vacancy (NV) centers in diamond have distinct promise as solid-state qubits. This is because of their large dipole moment, convenient level structure and very long room-temperature coherence times. In general, a combination of ion irradiation and subsequent annealing is used to create the centers, however for the rigorous demands of quantum computing all processes need to be optimized, and decoherence due to the residual damage caused by the implantation process itself must be mitigated. To that end we have studied photoluminescence (PL) from NV$^-$, NV$^0$ and GR1 centers formed by ion implantation of 2MeV He ions over a wide range of fluences. The sample was annealed at $600^{\circ}$C to minimize residual vacancy diffusion, allowing for the concurrent analysis of PL from NV centers and irradiation induced vacancies (GR1). We find non-monotic PL intensities with increasing ion fluence, monotonic increasing PL in NV$^0$/NV$^-$ and GR1/(NV$^0$ + NV$^1$) ratios, and increasing inhomogeneous broadening of the zero-phonon lines with increasing ion fluence. All these results shed important light on the optimal formation conditions for NV qubits. We apply our findings to an off-resonant photonic quantum memory scheme using vibronic sidebands.

created: 14-11-2006, last modified: 29-11-2007
W. Bao, Y. Ge, D. Jaksch, P. Markowich and R.M. Weishaupl,
Convergence rate of dimension reduction in Bose-Einstein condensates,
Comput. Phys. Comm. 177, 832 (2007).
Convergence rate of dimension reduction in Bose-Einstein condensates
In this paper, we study dimension reduction of the three-dimensional (3D) Gross-Pitaevskii equation (GPE) modelling Bose-Einstein condensation under different limiting interaction and trapping requencies parameter regimes. Convergence rates for the dimension reduction of 3D ground state and dynamics of the GPE in the case of disk-shaped condensation and cigar-shaped condensation are reported based on our asymptotic and numerical results. In addition, the parameter regimes in which the 3D GPE cannot be reduced to lower dimensions are identified.

created: 27-09-2006, last modified: 27-10-2007
M. Rodriguez, S.R. Clark and D. Jaksch,
Generation of twin Fock states via transition from a two-component Mott insulator to a superfluid,
Phys. Rev. A 75, 011601(R) (2007).
Generation of twin Fock states via transition from a two-component Mott insulator to a superfluid
We propose the dynamical creation of twin Fock states, which exhibit Heisenberg-limited interferometric phase sensitivities, in an optical lattice. In our scheme a two-component Mott insulator with two bosonic atoms per lattice site is melted into a superfluid. This process transforms local correlations between hyperfine states of atom pairs into multiparticle correlations extending over the whole system. The melting time does not scale with the system size which makes our scheme experimentally feasible.

created: 15-07-2006, last modified: 24-07-2007
J. Nunn, I.A. Walmsley, M.G. Raymer, K. Surmacz, F.C. Waldermann, Z. Wang and D. Jaksch,
Mapping broadband single-photon wave packets into an atomic memory,
Phys. Rev. A 75, 011401(R) (2007).
Mapping broadband single-photon wave packets into an atomic memory
We analyze a quantum optical memory based on the off-resonant Raman interaction of a single broadband photon, copropagating with a classical control pulse, with an atomic ensemble. The conditions under which the memory can perform optimally are found, by means of a universal mode decomposition. This enables the memory efficiency to be specified in terms of a single parameter, and the control field pulse shape to be determined via a simple nonlinear scaling. We apply the same decomposition to determine the optimal configurations for read-out.

created: 11-04-2006, last modified: 23-01-2007
K. Surmacz, J. Nunn, F.C. Waldermann, Z. Wang, I.A. Walmsley and D. Jaksch,
Entanglement fidelity of quantum memories,
Phys. Rev. A 74, 050302(R) (2006).
Entanglement fidelity of quantum memories
We introduce a figure of merit for a quantum memory which measures the preservation of entanglement between a qubit stored in and retrieved from the memory and an auxiliary qubit. We consider a general quantum memory system consisting of a medium of two level absorbers, with the qubit to be stored encoded in a single photon. We derive an analytic expression for our figure of merit taking into account Gaussian fluctuations in the Hamiltonian parameters, which, for example, model inhomogeneous broadening and storage time dephasing. Finally we specialize to the case of an atomic quantum memory where fluctuations arise predominantly from Doppler broadening and motional dephasing.

created: 12-08-2006, last modified: 14-11-2006
A. Griessner, A.J. Daley, S.R. Clark, D. Jaksch and P. Zoller,
Dark state cooling of atoms by superfluid immersion,
Phys. Rev. Lett. 97, 220403 (2006).
Dark state cooling of atoms by superfluid immersion
We propose and analyse a scheme to cool atoms in an optical lattice to ultra-low temperatures within a Bloch band, and away from commensurate filling. The protocol is inspired by ideas from dark state laser cooling, but replaces electronic states with motional levels, and spontaneous emission of photons by emission of phonons into a Bose-Einstein condensate, in which the lattice is immersed. In our model, achievable temperatures correspond to a small fraction of the Bloch band width, and are much lower than the reservoir temperature.

created: 10-07-2006, last modified: 29-11-2006
S.R. Clark and D. Jaksch,
Signatures of the superfluid to Mott-insulator transition in the excitation spectrum of ultracold atoms,
New J. Phys. 8, 160 (2006).
Signatures of the superfluid to Mott-insulator transition in the excitation spectrum of ultracold atoms
We present a detailed analysis of the dynamical response of ultra-cold bosonic atoms in a one-dimensional optical lattice subjected to a periodic modulation of the lattice depth. Following the experimental realization by Stoferle et al [Phys. Rev. Lett. 92, 130403 (2004)] we study the excitation spectrum of the system as revealed by the response of the total energy as a function of the modulation frequency Omega. By using the Time Evolving Block Decimation algorithm, we are able to simulate one-dimensional systems comparable in size to those in the experiment, with harmonic trapping and across many lattice depths ranging from the Mott-insulator to the superfluid regime. Our results produce many of the features seen in the experiment, namely a broad response in the superfluid regime, and narrow discrete resonances in the Mott-insulator regime. We identify several signatures of the superfluid-Mott insulator transition that are manifested in the spectrum as it evolves from one limit to the other.

created: 27-04-2006, last modified: 30-08-2006
R.N. Palmer and D. Jaksch,
High field fractional quantum Hall effect in optical lattices,
Phys. Rev. Lett. 96, 180407 (2006).
High field fractional quantum Hall effect in optical lattices
We consider interacting bosonic atoms in an optical lattice subject to a large simulated magnetic field. We develop a model similar to a bilayer fractional quantum Hall system valid near simple rational numbers of magnetic flux quanta per lattice cell. Then we calculate its ground state, magnetic lengths, fractional fillings, and find unexpected sign changes in the Hall current. Finally we study methods for detecting these novel features via shot noise and Hall current measurements.

created: 11-04-2006, last modified: 30-05-2006
M. Bruderer and D. Jaksch,
Probing BEC phase fluctuations with atomic quantum dots,
New J. Phys. 8, 87 (2006).
Probing BEC phase fluctuations with atomic quantum dots
We consider the dephasing of two internal states |0> and |1> of a trapped impurity atom, a so-called atomic quantum dot (AQD), where only state |1> couples to a Bose-Einstein condensate (BEC). A direct relation between the dephasing of the internal states of the AQD and the temporal phase fluctuations of the BEC is established. Based on this relation we suggest a scheme to probe BEC phase fluctuations nondestructively via dephasing measurements of the AQD. In particular, the scheme allows to trace the dependence of the phase fluctuations on the trapping geometry of the BEC.

created: 31-01-2006, last modified: 06-06-2006
A. Klein and D. Jaksch,
Simulating high-temperature superconductivity model Hamiltonians with atoms in optical lattices,
Phys. Rev. A 73, 053613 (2006).
Simulating high-temperature superconductivity model Hamiltonians with atoms in optical lattices
We investigate the feasibility of simulating different model Hamiltonians used in high-temperature superconductivity. We briefly discuss the most common models and then focus on the simulation of the so-called t-J-U Hamiltonian using ultra-cold atoms in optical lattices. For this purpose, previous simulation schemes to realize the spin interaction term J are extended. We especially overcome the condition of a filling factor of exactly one, which otherwise would restrict the phase of the simulated system to a Mott-insulator. Using ultra-cold atoms in optical lattices allows simulation of the discussed models for a very wide range of parameters. The time needed to simulate the Hamiltonian is estimated and the accuracy of the simulation process is numerically investigated for small systems.

created: 13-01-2006, last modified: 29-05-2006
S.R. Clark and D. Jaksch,
Efficient dynamical simulation of strongly correlated one-dimensional quantum systems,
Lecture Notes in Computer Science 3743, 555 (2006).
Efficient dynamical simulation of strongly correlated one-dimensional quantum systems
Studying the unitary time evolution of strongly correlated quantum systems is one of the most challenging theoretical and experimental problems in physics. For an important class of one-dimensional (1D) systems dynamical simulations have become possible since the advent of the time-evolving block decimation (TEBD) algorithm. We study the computational properties of TEBD using the Bose-Hubbard model (BHM) as a test-bed. We demonstrate its efficiency and verify its accuracy through comparisons with an exactly solvable small system and via the convergence of one- and two-particle observables in a larger system.

created: 31-12-2005, last modified: 16-02-2006
A. Klein, U. Dorner, C. Moura Alves and D. Jaksch,
Robust implementations of Quantum Repeaters,
Phys. Rev. A 73, 012332 (2006).
Robust implementations of Quantum Repeaters
We show how to efficiently exploit decoherence free subspaces, which are immune to collective noise, for realizing quantum repeaters with long lived quantum memories. Our setup consists of an assembly of simple modules and we show how to implement them in systems of cold, neutral atoms in arrays of dipole traps. We develop methods for realizing robust gate operations on qubits encoded in a DFS using collisional interactions between the atoms. We also give a detailed analysis of the performance and stability of all required gate operations and emphasize that all modules can be realized with current or near future experimental technology.

created: 08-11-2005, last modified: 24-01-2006
S.R. Clark, C. Moura Alves and D. Jaksch,
Efficient generation of graph states for quantum computation,
New J. Phys. 7, 124 (2005).
Efficient generation of graph states for quantum computation
We present an entanglement generation scheme which allows arbitrary graph states to be efficiently created in a linear quantum register via an auxiliary entangling bus (EB). The dynamical evolution of the EB is described by an effective non-interacting fermionic system undergoing mirror-inversion in which qubits, encoded as local fermionic modes, become entangled purely by Fermi statistics. We discuss a possible implementation using two species of neutral atoms stored in an optical lattice and find that the scheme is realistic in its requirements even in the presence of noise.

created: 08-08-2005, last modified: 26-08-2005
D. Jaksch and P. Zoller,
The cold atom Hubbard toolbox, review article,
Annals of Physics 315, 52 (2005).
The cold atom Hubbard toolbox, review article
We review recent theoretical advances in cold atom physics concentrating on strongly correlated cold atoms in optical lattices. We discuss recently developed quantum optical tools for manipulating atoms and show how they can be used to realize a wide range of many body Hamiltonians. Then we describe connections and differences to condensed matter physics and present applications in the fields of quantum computing and quantum simulations. Finally we explain how defects and atomic quantum dots can be introduced in a controlled way in optical lattice systems.

created: 08-08-2005
A. Griessner, A.J. Daley, D. Jaksch and P. Zoller,
Fault-Tolerant Dissipative Preparation of Atomic Quantum Registers with Fermions,
Phys. Rev. A 72, 032332 (2005).
Fault-Tolerant Dissipative Preparation of Atomic Quantum Registers with Fermions
We propose a fault tolerant loading scheme to produce an array of fermions in an optical lattice of the high fidelity required for applications in quantum information processing and the modelling of strongly correlated systems. A cold reservoir of Fermions plays a dual role as a source of atoms to be loaded into the lattice via a Raman process and as a heat bath for sympathetic cooling of lattice atoms. Atoms are initially transferred into an excited motional state in each lattice site, and then decay to the motional ground state, creating particle-hole pairs in the reservoir. Atoms transferred into the ground motional level are no longer coupled back to the reservoir, and doubly occupied sites in the motional ground state are prevented by Pauli blocking. This scheme has strong conceptual connections with optical pumping, and can be extended to load high-fidelity patterns of atoms.

created: 08-08-2005, last modified: 10-10-2005
R.N. Palmer, C. Moura Alves and D. Jaksch,
Detection and characterization of multipartite entanglement in optical lattices,
Phys. Rev. A 72, 042335 (2005).
Detection and characterization of multipartite entanglement in optical lattices
We investigate the detection and characterization of entanglement based on the quantum network introduced in [Phys. Rev. Lett. 93, 110501 (2004)] for different experimental scenarios. We first give a detailed discussion of the ideal scheme where no errors are present and full spatial resolution is available. Then we analyze the implementation of the network in an optical lattice. We find that even without any spatial resolution entanglement can be detected and characterized in various kinds of states including cluster states and macroscopic superposition states. We also study the effects of detection errors and imperfect dynamics on the detection network. For our scheme to be practical these errors have to be on the order of one over the number of investigated lattice sites. Finally, we consider the case of limited spatial resolution and conclude that significant improvement in entanglement detection and characterization compared to having no spatial resolution is only possible if single lattice sites can be resolved.

created: 02-07-2005, last modified: 08-11-2005
A.J. Daley, S.R. Clark, D. Jaksch and P. Zoller,
Numerical Analysis of Coherent Many-Body Currents in a Single Atom Transistor,
Phys. Rev. A 72, 043618 (2005).
Numerical Analysis of Coherent Many-Body Currents in a Single Atom Transistor
We study the dynamics of many atoms in the recently proposed Single Atom Transistor setup [A. Micheli, A. J. Daley, D. Jaksch, and P. Zoller, Phys. Rev. Lett. 93, 140408 (2004)] using recently developed numerical methods. In this setup, a localised spin 1/2 impurity is used to switch the transport of atoms in a 1D optical lattice: in one state the impurity is transparent to probe atoms, but in the other acts as a single atom mirror. We calculate time-dependent currents for bosons passing the impurity atom, and find interesting many body effects. These include substantially different transport properties for bosons in the strongly interacting (Tonks) regime when compared with fermions, and an unexpected decrease in the current when weakly interacting probe atoms are initially accelerated to a non-zero mean momentum. We also provide more insight into the application of our numerical methods to this system, and discuss open questions about the currents approached by the system on long timescales.

created: 02-07-2005, last modified: 11-07-2006
S.R. Clark and D. Jaksch,
Dynamics of the Superfluid to Mott Insulator Transition in One Dimension,
Phys. Rev. A 70, 043612 (2004).
Dynamics of the Superfluid to Mott Insulator Transition in One Dimension
We numerically study the superfluid to Mott insulator transition for bosonic atoms in a one dimensional lattice by exploiting a recently developed simulation method for strongly correlated systems. We demonstrate this methods accuracy and applicability to Bose-Hubbard model calculations by comparison with exact results for small systems. By utilizing the efficient scaling of the numerical algorithm we then concentrate on systems of comparable size to those studied in experiments and in the presence of a magnetic trap. We investigate spatial correlations and fluctuations of the ground state as well as nature and speed at which a superfluid component is built up when dynamically melting a Mott insulating state by ramping down the lattice potential. This is performed for slow ramping, where we find that the superfluid builds up on a timescale consistent with single atom hopping, and for rapid ramping where the build up is much faster than can be explained by this simple mechanism. Our calculations are in remarkable agreement with the experimental results obtained by M. Greiner et al. [Nature 415, 39 (2002)].

created: 08-08-2005
W. Bao, D. Jaksch and P. Markowich,
Three Dimensional Simulation of Jet Formation in Collapsing Condensates,
J. Phys. B: At. Mol. Opt. Phys. 37, 329 (2004).
Three Dimensional Simulation of Jet Formation in Collapsing Condensates
We numerically study the behavior of collapsing and exploding condensates using the parameters of the experiments by E.A. Donley et al. [Nature, 412, 295, (2001)]. Our studies are based on a full three-dimensional numerical solution of the Gross-Pitaevskii equation (GPE) including three body loss. We determine the three body loss rate from the number of remnant condensate atoms and collapse times and obtain only one possible value which fits with the experimental results. We then study the formation of jet atoms by interrupting the collapse and find very good agreement with the experiment. Furthermore we investigate the sensitivity of the jets to the initial conditions. According to our analysis the dynamics of the burst atoms is not described by the GPE with three body loss incorporated.

created: 08-08-2005
A. Griessner, D. Jaksch and P. Zoller,
Cavity Assisted Nondestructive Laser Cooling of Atomic Qubits,
J. Phys. B: At. Mol. Opt. Phys. 37, 1419 (2004).
Cavity Assisted Nondestructive Laser Cooling of Atomic Qubits
We analyze two configurations for laser cooling of neutral atoms whose internal states store qubits. The atoms are trapped in an optical lattice which is placed inside a cavity. We show that the coupling of the atoms to the damped cavity mode can provide a mechanism which leads to cooling of the motion without destroying the quantum information.

created: 08-08-2005
D. Jaksch,
Optical Lattices, Ultracold Atoms and Quantum Information Processing,
Contemporary Physics 45, 367 (2004).
Optical Lattices, Ultracold Atoms and Quantum Information Processing
We review novel methods to investigate, control and manipulate neutral atoms in optical lattices. These setups allow unprecedented quantum control over large numbers of atoms and thus are very promising for applications in quantum information processing. After introducing optical lattices we discuss the superfluid (SF) and Mott insulating (MI) states of neutral atoms trapped in such lattices and investigate the SF-MI transition as recently observed experimentally. In the second part of the paper we give an overview of proposals for quantum information processing and show different ways to entangle the trapped atoms, in particular the usage of cold collisions and Rydberg atoms. Finally, we also briefly discuss the implementation of quantum simulators, entanglement enhanced atom interferometers, and ideas for robust quantum memory in optical lattices.

created: 08-08-2005
C. Moura Alves and D. Jaksch,
Multipartite entanglement detection in bosons,
Phys. Rev. Lett. 93, 110501 (2004).
Multipartite entanglement detection in bosons
We propose a simple quantum network to detect multipartite entangled states of bosons, and show how to implement this network for neutral atoms stored in an optical lattice. We investigate the special properties of cluster states, multipartite entangled states and superpositions of distinct macroscopic quantum states in Bose-Einstein condensates that can be identified by the network.

created: 08-08-2005
C. Simon and D. Jaksch,
Possibility of observing energy decoherence due to quantum gravity,
Phys. Rev. A 70, 052104 (2004).
Possibility of observing energy decoherence due to quantum gravity
It has recently been proposed that quantum gravity might lead to the decoherence of superpositions in energy, corresponding to a discretization of time at the Planck scale. At first sight the proposal seems amenable to experimental verification with methods from quantum optics and atomic physics. However, we argue that the predicted decoherence is unobservable in such experiments if it acts globally on the whole experimental setup. This is related to the unobservability of the global phase in interference. We also show how local energy decoherence, which acts separately on system and phase reference, could be detected with remarkable sensitivity and over a wide range of length scales by long-distance Ramsey interferometry with metastable atomic states. The sensitivity of the experiments can be further enhanced using multi-atom entanglement.

created: 08-08-2005, last modified: 25-10-2007
A. Micheli, A.J. Daley, D. Jaksch and P. Zoller,
A Single Atom Transistor in a 1D Optical Lattice,
Phys. Rev. Lett. 93, 140408 (2004).
A Single Atom Transistor in a 1D Optical Lattice
We propose a scheme utilizing a quantum interference phenomenon to switch the transport of atoms in a 1D optical lattice through a site containing an impurity atom. The impurity represents a qubit which in one spin state is transparent to the probe atoms, but in the other acts as a single atom mirror. This allows a single-shot quantum non-demolition measurement of the qubit spin.

created: 08-08-2005
W. Bao, D. Jaksch and P. Markowich,
Numerical solution of the Gross-Pitaevskii equation for Bose-Einstein condensation,
J. Comp. Phys. 187, 318 (2003).
Numerical solution of the Gross-Pitaevskii equation for Bose-Einstein condensation
In this paper, we study the numerical solution of the time-dependent Gross-Pitaevskii equation (GPE) describing a Bose-Einstein condensate (BEC) at zero or very low temperature. We take the 3d Gross-Pitaevskii equation, and as preparatory steps scale it to obtain a four-parameter model and use an approach well known in the physical literature to reduce it to 2d and 1d GPEs in certain limiting regimes. We provide approximate ground state solutions of the GPE in two extreme regimes: (very) weak interactions and strong repulsive interactions. Then we use a time-splitting spectral method to solve the time-dependent GPE in 1d, 2d and 3d. Extensive numerical examples in 1d, 2d and 3d for weak/strong interactions, defocusing/focusing nonlinearity, and zero/nonzero initial phase data are presented to demonstrate the novel numerical method and to discuss the physics of Bose-Einstein condensation.

created: 08-08-2005, last modified: 11-08-2006
U. Dorner, P. Fedichev, D. Jaksch, M. Lewenstein and P. Zoller,
Entangling strings of neutral atoms in 1D atomic pipeline structures,
Phys. Rev. Lett. 91, 073601 (2003).
Entangling strings of neutral atoms in 1D atomic pipeline structures
We study a string of neutral atoms with nearest neighbor interaction in a 1D beam splitter configuration, where the longitudinal motion is controlled by a moving optical lattice potential. The dynamics of the atoms crossing the beam splitter maps to a 1D spin model with controllable time dependent parameters, which allows the creation of maximally entangled states of atoms by crossing a quantum phase transition. Furthermore, we show that this system realizes protected quantum memory, and we discuss the implementation of one- and two-qubit gates in this setup.

created: 08-08-2005
A. Micheli, D. Jaksch, J.I. Cirac and P. Zoller,
Many particle entanglement in two-component Bose-Einstein Condensates,
Phys. Rev. A 67, 013607 (2003).
Many particle entanglement in two-component Bose-Einstein Condensates
We investigate schemes to dynamically create many particle entangled states of a two component Bose-Einstein condensate in a very short time proportional to 1/N where N is the number of condensate particles. For small N we compare exact numerical calculations with analytical semiclassical estimates and find very good agreement for N >=50. We also estimate the effect of decoherence on our scheme, study possible scenarios for measuring the entangled states, and investigate experimental imperfections.

created: 08-08-2005
W. Bao and D. Jaksch,
An explicit unconditionally stable numerical method for solving damped nonlinear Schrodinger equations with a focusing nonlinearity,
SIAM J. Numer. Anal. 41, 1406 (2003).
An explicit unconditionally stable numerical method for solving damped nonlinear Schrodinger equations with a focusing nonlinearity
This paper introduces an extension of the time-splitting sine-spectral (TSSP) method for solving damped focusing nonlinear SchrÃ¶dinger equations (NLS). The method is explicit, unconditionally stable and time transversal invariant. Moreover, it preserves the exact decay rate for the normalization of the wave function if linear damping terms are added to the NLS. Extensive numerical tests are presented for cubic focusing nonlinear SchrÃ¶dinger equations in 2d with a linear, cubic or a quintic damping term. Our numerical results show that quintic damping always arrests blowup, while linear or cubic damping can arrest blowup only when the damping parameter $dt$ is larger than a threshold value $delta_{ m th}$. We note that our method can also be applied to solve the 3d Gross-Pitaevskii equation with a quintic damping term to model the dynamics of a collapsing and exploding Bose-Einstein condensate (BEC).

created: 08-08-2005, last modified: 11-08-2006
J.I. Cirac, L.M. Duan, D. Jaksch and P. Zoller,
Quantum information processing with quantum optics,
Ann. Henri Poincare 4, 759 (2003).
Quantum information processing with quantum optics
We review theoretical proposals for implementation of quantum computing and quantum communication with quantum optical methods.

created: 08-08-2005, last modified: 11-07-2006
D. Jaksch and P. Zoller,
Creation of effective magnetic fields in optical lattices: The Hofstadter butterfly for cold neutral atoms,
New J. Phys. 5, 56 (2003).
Creation of effective magnetic fields in optical lattices: The Hofstadter butterfly for cold neutral atoms
We investigate the dynamics of neutral atoms in a 2D optical lattice which traps two distinct internal states of the atoms in different columns. Two Raman lasers are used to coherently transfer atoms from one internal state to the other, thereby causing hopping between the different columns. By adjusting the laser parameters appropriately we can induce a non vanishing phase of particles moving along a closed path on the lattice. This phase is proportional to the enclosed area and we thus simulate a magnetic flux through the lattice. This setup is described by a Hamiltonian identical to the one for electrons on a lattice subject to a magnetic field and thus allows us to study this equivalent situation under very well defined controllable conditions. We consider the limiting case of huge magnetic fields - which is not experimentally accessible for electrons in metals - where a fractal band structure, the Hofstadter butterfly, characterizes the system.

created: 08-08-2005, last modified: 26-11-2007
D. Jaksch, J.I. Cirac and P. Zoller,
Dynamically turning off interactions in a two component condensate,
Phys. Rev. A 65, 33625 (2002).
Dynamically turning off interactions in a two component condensate
We propose a mechanism to change the interaction strengths of a two component condensate. It is shown that the application of pi/2 pulses allows to alter the effective interspecies interaction strength as well as the effective interaction strength between particles of the same kind. This mechanism provides a simple method to transform spatially stable condensates into unstable once and vice versa. It also provides a means to store a squeezed spin state by turning off the interaction for the internal states and thus allows to gain control over many body entangled states.

created: 08-08-2005
T. Calarco, D. Jaksch, J.I. Cirac and P. Zoller,
Controlling dynamical phases in quantum optics,
J. Opt. B 4, 430 (2002).
Controlling dynamical phases in quantum optics
We review and compare several schemes for inducing precisely controlled quantum phases in quantum optical systems. We focus in particular onto conditional dynamical phases, i.e., phases obtained via state- and time-dependent interactions between trapped two-level atoms and ions. We describe different possibilities for the kind of interaction to be exploited, including cold controlled collisions, electrostatic forces, and dipole-dipole interactions.

created: 08-08-2005
D. Jaksch, V. Venturi, J.I. Cirac, C.J. Williams and P. Zoller,
Creation of a molecular condensate by dynamically melting a Mott-insulator,
Phys. Rev. Lett. 89, 040402 (2002).
Creation of a molecular condensate by dynamically melting a Mott-insulator
We propose creation of a molecular Bose-Einstein condensate (BEC) by loading an atomic BEC into an optical lattice and driving it into a Mott insulator (MI) phase with exactly two atoms per site. Molecules in a MI state are then created under well defined conditions by photoassociation with essentially unit efficiency. Finally, the MI is melted and a superfluid state of the molecules is created. We investigate the dynamics of this process photoassociation of tightly trapped atoms.

created: 08-08-2005
M.D. Lukin, M. Fleischhauer, R. Cote, L.M. Duan, D. Jaksch, J.I. Cirac and P. Zoller,
Dipole Blockade and Quantum Information Processing in Mesoscopic Atomic Ensembles,
Phys. Rev. Lett. 87, 037901 (2001).
Dipole Blockade and Quantum Information Processing in Mesoscopic Atomic Ensembles
We describe a technique for manipulating quantum information stored in collective states of mesoscopic ensembles. Quantum processing is accomplished by optical excitation into states with strong dipole-dipole interactions. The resulting dipole blockade can be used to inhibit transitions into all but singly excited collective states. This can be employed for a controlled generation of collective atomic spin states as well as non-classical photonic states and for scalable quantum logic gates. An example involving a cold Rydberg gas is analyzed.

created: 08-08-2005
D. Jaksch, S.A. Gardiner, K. Schulze, J.I. Cirac and P. Zoller,
Uniting Bose-Einstein condensates in optical resonators,
Phys. Rev. Lett. 86, 4733 (2001).
Uniting Bose-Einstein condensates in optical resonators
The relative phase of two initially independent Bose-Einstein condensates can be laser cooled to unite the two condensates by putting them into a ring cavity and coupling them with an internal Josephson junction. First, we show that this phase cooling process already appears within a semiclassical model. We calculate the stationary states, find regions of bistable behavior and suggest a Ramsey-type experiment to measure the build up of phase coherence between the condensates. We also study quantum effects and imperfections of the system.

created: 08-08-2005
T. Calarco, H.-J. Briegel, D. Jaksch, J.I. Cirac and P. Zoller,
Entangling neutral atoms for quantum information processing,
Journal of Modern Optics 47, 2137 (2000).
Entangling neutral atoms for quantum information processing
We review recent proposals for performing entanglement manipulation via cold collisions between neutral atoms. State-dependent, time-varying trapping potentials allow to control the interaction between atoms, so that conditional phase shifts realizing a universal quantum gate can be obtained with high fidelity. We discuss possible physical implementations with existing experimental techniques, for example optical lattices and magnetic micro-traps.

created: 09-08-2005
T. Calarco, H.-J. Briegel, D. Jaksch, J.I. Cirac and P. Zoller,
Quantum computing with trapped particles in microscopic potentials,
Fortschritte der Physik 48, 945 (2000).
Quantum computing with trapped particles in microscopic potentials
We review recent proposals for performing entanglement manipulation via controlled interactions between trapped atoms. State-dependent, time-varying microscopic potentials allow one to obtain with high fidelity a conditional phase shift realizing a universal quantum gate. We discuss possible physical implementations with existing experimental techniques, for example optical lattices and magnetic micro-traps.

created: 09-08-2005
D. Jaksch, J.I. Cirac, P. Zoller, S.L. Rolston, R. Cote and M.D. Lukin,
Fast quantum gates for neutral atoms,
Phys. Rev. Lett. 85, 2208 (2000).
Fast quantum gates for neutral atoms
We propose several schemes for implementing a fast two-qubit quantum gate for neutral atoms with the gate operation time much faster than the time scales associated with the external motion of the atoms in the trapping potential. In our example, the large interaction energy required to perform fast gate operations is provided by the dipole-dipole interaction of atoms excited to low-lying Rydberg states in constant electric fields. A detailed analysis of imperfections of the gate operation is given.

created: 09-08-2005
D. Jaksch, T. Calarco and P. Zoller,
Quantenoptik und Nanotechnologie - auf dem Weg zu einem universellen Quantencomputer,
Physik in unserer Zeit 6, 260 (2000).
Quantenoptik und Nanotechnologie - auf dem Weg zu einem universellen Quantencomputer
Faszinierende Perspektiven fÃ¼r die Kommunikation und die Informationsverarbeitung werden durch die Quantenmechanik erÃ¶ffnet. Beachtliche Fortschritte in einigen fÃ¼hrenden Labors lassen elementare Quantenprozessoren die mit bis zu zehn Quantenbits arbeiten in den nÃ¤chsten Jahren realistisch erscheinen. Obwohl damit noch keine beeindruckenden Rechnungen mÃ¶glich sind werden sich solche Quantenprozessoren doch fÃ¼r wichtige Aufgaben in der Quantenkommunikation einsetzen lassen. Um universell programmierbare Quantenrechner realisieren zu kÃ¶nnen bedarf es der Implementierung von Konzepten zur Quanteninformationsverarbeitung welche sich auf eine groÃŸe Anzahl von Qubits anwenden lassen. Solche skalierbare VorschlÃ¤ge, welche Technologien aus der Quantenoptik und der Nanotechnologie vereinen, kÃ¶nnten sich als mÃ¶glicher Weg zum universell programmierbaren Quantenrechner erweisen.

created: 09-08-2005
S.A. Gardiner, D. Jaksch, R. Dum, J.I. Cirac and P. Zoller,
Nonlinear matter wave dynamics with a chaotic potential,
Phys. Rev. A 62, 023612 (2000).
Nonlinear matter wave dynamics with a chaotic potential
We consider the case of a cubic nonlinear SchrÃ¶dinger equation with an additional chaotic potential, in the sense that such a potential produces chaotic dynamics in classical mechanics. We derive and describe an appropriate semiclassical limit to such a nonlinear SchrÃ¶dinger equation, using a semiclassical interpretation of the Wigner function, and relate this to the hydrodynamic limit of the Gross-Pitaevskii equation used in the context of Bose-Einstein condensation. We investigate a specific example of a Gross-Pitaevskii equation with such a chaotic potential: the one-dimensional delta-kicked harmonic oscillator, and its semiclassical limit. We explore the feasibility of experimental realization of such a system in a Bose-Einstein condensate experiment, giving a concrete proposal of how to implement such a configuration, and considering the problem of condensate depletion.

created: 02-07-2005
T. Calarco, E.A. Hinds, D. Jaksch, J. Schmiedmayer, J.I. Cirac and P. Zoller,
Quantum gates with neutral atoms: Controlling collisional interactions in time dependent traps,
Phys. Rev. A 61, 022304 (2000).
Quantum gates with neutral atoms: Controlling collisional interactions in time dependent traps
We theoretically study specific schemes for performing a fundamental two-qubit quantum gate via controlled atomic collisions by switching microscopic potentials. In particular we calculate the fidelity of a gate operation for a configuration where a potential barrier between two atoms is instantaneously removed and restored after a certain time.Possible implementations could be based on magnetic microtraps or potentials induced by laser light.

created: 02-07-2005
H.-J. Briegel, T. Calarco, D. Jaksch, J.I. Cirac and P. Zoller,
Quantum computing with neutral atoms,
Journal of Modern Optics 47, 415 (2000).
Quantum computing with neutral atoms
We develop a method to entangle neutral atoms using cold controlled collisions. We analyze this method in two particular set-ups: optical lattices and magnetic micro-traps. Both offer the possibility of performing certain multi-particle operations in parallel. Using this fact, we show how to implement efficient quantum error correction and schemes for fault-tolerant computing.

created: 02-07-2005
P. Torma and D. Jaksch,
Pairing of Fermions in Optical Lattices,
Acta Physica Slovaca 49, 605 (1999).
Pairing of Fermions in Optical Lattices
We consider weakly interacting fermionic atoms in optical lattices. We show that the system can be described by the Hubbard model, and solve the BCS gap equations. Cooper-pairing is shown to take place for parameter values which are obtainable for alkali atoms in optical lattices.

created: 02-07-2005
D. Jaksch, H.-J. Briegel, J.I. Cirac, C.W. Gardiner and P. Zoller,
Entanglement of atoms via cold controlled collisions,
Phys. Rev. Lett. 82, 1975 (1999).
Entanglement of atoms via cold controlled collisions
We show that by using cold controlled collisions between two atoms one can achieve conditional dynamics in moving trap potentials. We discuss implementing two qubit quantum-gates and efficient creation of highly entangled states of many atoms in optical lattices.

created: 02-07-2005
D. Jaksch, C. Bruder, C.W. Gardiner, J.I. Cirac and P. Zoller,
Cold bosonic atoms in optical lattices,
Phys. Rev. Lett. 81, 3108 (1998).
Cold bosonic atoms in optical lattices
The dynamics of an ultracold dilute gas of bosonic atoms in an optical lattice can be described by a Bose-Hubbard model where the system parameters are controlled by laser light. We study the continuous (zero temperature) quantum phase transition from the superfluid to the Mott insulator phase induced by varying the depth of the optical potential, where the Mott insulator phase corresponds to a commensurate filling of he lattice (optical crystal). Examples for formation of Mott structures in optical lattices with superimposed harmonic trap, and in optical superlattices are presented.

created: 01-07-2005
D. Jaksch, K.M. Gheri, C.W. Gardiner and P. Zoller,
Quantum Kinetic Theory IV: Intensity and amplitude fluctuations of a Bose-Einstein condensate at finite temperature including trap loss,
Phys. Rev. A 58, 1450 (1998).
Quantum Kinetic Theory IV: Intensity and amplitude fluctuations of a Bose-Einstein condensate at finite temperature including trap loss
We use the quantum kinetic theory to calculate the steady state and the fluctuations of a trapped Bose-Einstein condensate at finite temperature. The system is divided in a condensate and a non-condensate part. A quantum mechanical description based on the number conserving Bogoliubov method is used for describing the condensate part. The non-condensed particles are treated as a classical gas in thermal equilibrium with temperature T and chemical potential Âµ. We find a master equation for the reduced density operator of the Bose-Einstein condensate, calculate the steady state of the system and investigate the effect of one- two- and three particle loss on the condensate. Using linearized Ito equations we find expressions for the intensity fluctuations and the amplitude fluctuations in the condensate. A Lorentzian line shape is found for the intensity correlation function that is characterized by a time constant 1/gamma_I derived in the paper. For the amplitude correlation function we find ballistic behavior for time differences smaller than 1/gamma_I and diffusive behavior for larger time differences.

created: 01-07-2005
D. Jaksch, C.W. Gardiner and P. Zoller,
Quantum Kinetic Theory II: Simulation of the Quantum Boltzmann Master Equation,
Phys. Rev. A 56, 575 (1997).
Quantum Kinetic Theory II: Simulation of the Quantum Boltzmann Master Equation
We present results of simulations of a quantum Boltzmann master equation (QBME) describing the kinetics of a dilute Bose gas confined in a trapping potential in the regime of Bose condensation. The QBME is the simplest version of a quantum kinetic master equations derived in previous work. We consider two cases of trapping potentials: a 3D square well potential with periodic boundary conditions, and an isotropic harmonic oscillator. We discuss the stationary solutions and relaxation to equilibrium. In particular, we calculate particle distribution functions, fluctuations in the occupation numbers, the time between collisions, and the mean occupation numbers of the one-particle states in the regime of onset of Bose condensation.

created: 01-07-2005

Other publications [23]:

C. Noh, S.R. Clark, D. Jaksch and D.G. Angelakis,
Out-of-equilibrium physics in driven dissipative coupled resonator arrays,
Chapter in Quantum Simulations with Photons and Polaritons: Merging Quantum Optics with Condensed Matter Physics; edited by DG Angelakis, Quantum Science and Technology Series, Springer (2017).
Out-of-equilibrium physics in driven dissipative coupled resonator arrays
Coupled resonator arrays have been shown to exhibit interesting many-body physics including Mott and Fractional Hall states of photons. One of the main differences between these photonic quantum simulators and their cold atoms counterparts is in the dissipative nature of their photonic excitations. The natural equilibrium state is where there are no photons left in the cavity. Pumping the system with external drives is therefore necessary to compensate for the losses and realise non-trivial states. The external driving here can easily be tuned to be incoherent, coherent or fully quantum, opening the road for exploration of many body regimes beyond the reach of other approaches. In this chapter, we review some of the physics arising in driven dissipative coupled resonator arrays including photon fermionisation, crystallisation, as well as photonic quantum Hall physics out of equilibrium. We start by briefly describing possible experimental candidates to realise coupled resonator arrays along with the two theoretical models that capture their physics, the Jaynes-Cummings-Hubbard and Bose-Hubbard Hamiltonians. A brief review of the analytical and sophisticated numerical methods required to tackle these systems is included.

created: 03-03-2017
S.R. Clark, A. Cavalleri and D. Jaksch,
Rattling the Cage: Making New Superconductors Using Lasers,
Department of Physics Newsletter 8, 8 (2016).
Rattling the Cage: Making New Superconductors Using Lasers
We describe a recent experiment inducing superconductivity in K3C60 above its critical temperature using short laser pulses.

created: 18-06-2016
D. Jaksch,
Ein Baustein stark korrelierter Materie,
Physik Journal 5, 20 (2015).
Ein Baustein stark korrelierter Materie
Ein Doppeltopfpotential und Lithium-Atome ermoeglichen es, den einfachsten Baustein des Fermi-Hubbard-Modells zu realisieren.

created: 09-05-2015
J.A. Jones and D. Jaksch,
Quantum Information, Computation and Communication,
Cambridge University Press (2012).
Quantum Information, Computation and Communication
Quantum physics allows entirely new forms of computation and cryptography, which could perform tasks currently impossible on classical devices, leading to an explosion of new algorithms, communications protocols and suggestions for physical implementations of all these ideas. As a result, quantum information has made the transition from an exotic research topic to part of mainstream undergraduate courses in physics. Based on years of teaching experience, this textbook builds from simple fundamental concepts to cover the essentials of the field. Aimed at physics undergraduate students with a basic background in quantum mechanics, it guides readers through theory and experiment, introducing all the central concepts without getting caught up in details. Worked examples and exercises make this useful as a self-study text for those who want a brief introduction before starting on more advanced books. #1 Best Seller in Atomic and Molecular Physics on amazon.co.uk in January 2014

created: 28-08-2012
D. Jaksch,
Viewpoint: Mimicking Magnetic Fields in Optical Lattices,
Physics 5, 60 (2012).
Viewpoint: Mimicking Magnetic Fields in Optical Lattices
Two groups use different approaches to realize artificial magnetic fields for atoms trapped in a one-dimensional optical lattice.

created: 30-05-2012, last modified: 28-08-2012
D. Jaksch,
Brennpunkt: Kalte Atome im Quantenkarussell,
Physik Journal 3, 19 (2012).
Brennpunkt: Kalte Atome im Quantenkarussell
In einem optischen Gitter lassen sich mithilfe von Lasern starke kuenstliche, alternierende Magnetfelder erzeugen.

created: 03-03-2012
S. Wall, D. Brida, S.R. Clark, H.P. Ehrke, D. Jaksch, A. Ardavan, S. Bonora, H. Uemura, Y. Takahashi, T. Hasegawa, H. Okamoto, G. Cerullo and A. Cavalleri,
Quantum interference between photo-excited states in a solid-state Mott insulator,
CLEO and QELS (2010).
Quantum interference between photo-excited states in a solid-state Mott insulator
By exciting with sub-10-fs 1.6-µm pulses the quasi-one-dimensional Mott insulator ET-F2TCNQ, we observe prompt collapse of the Mott gap modulated by 24-THz oscillations of the gap, which are assigned to quantum interference between holon-doublon excitations.

created: 28-06-2011
K.F. Reim, J. Nunn, V.O. Lorenz, B.J. Sussman, K.C. Lee, N.K. Langford, D. Jaksch and I.A. Walmsley,
Coherent optical memory with GHz bandwidth,
CLEO and QELS, paper QWH3 (2010).
Coherent optical memory with GHz bandwidth
We demonstrate the coherent storage and retrieval of sub-nanosecond low-intensity light pulses with spectral bandwidths exceeding 1GHz in cesium vapor, using the novel, far off-resonant two-photon Raman memory protocol.

created: 27-06-2011, last modified: 28-06-2011
K.F. Reim, P Bustard, K.C. Lee, J. Nunn, V.O. Lorenz, B.J. Sussman, N.K. Langford, D. Jaksch and I.A. Walmsley,
Applications of Raman Scattering in Quantum Technologies,
XXII International Conference on Raman Spectroscopy 1267, 37 (2010).
Applications of Raman Scattering in Quantum Technologies
The Raman effect couples light to the electronic ground state of matter. This means that it is possible to utilize the long-lived coherence of the material excitation to control, measure and store optical radiation. This, in turn, enables a number of novel applications, including the generation of broadband light, the distribution of quantum entanglement and the storage and retrieval of quantum light for long-distance communications. We describe three applications of Raman scattering to technologies related to the above goals, using a variety of material excitations: molecular vibrations, bulk phonons in a crystalline solid, and atomic spin waves.

created: 14-09-2010
D. Jaksch,
A stimulated atomic response,
Physics 2, 29 (2009).
A stimulated atomic response
A Viewpoint on: Exploring Correlated 1D Bose Gases from the Superfluid to the Mott-Insulator State by Inelastic Light Scattering by D. Clement, N. Fabbri, L. Fallani, C. Fort, and M. Inguscio, Phys. Rev. Lett. 102, 155301 (2009): Inelastic light scattering is used to study correlated phases of one-dimensional Bose gases. This spectroscopic technique can distinguish superfluid and insulating phases and allow identification of the transition from one to the other.

created: 14-04-2009, last modified: 15-04-2009
D. Jaksch,
Bose-Einstein condensates: A peek and a poke,
Nature Physics 4, 906 - 908 (2008).
Bose-Einstein condensates: A peek and a poke
An adapted scanning electron microscope allows the non-destructive measurement and manipulation of Bose–Einstein condensates. The single-atom sensitivity that this technique promises could soon become indispensable in the study of quantum degenerate atomic gases.

created: 03-12-2008
D. Jaksch and P. Zoller,
A magnetic butterfly made of ultracold atoms,
New Journal of Physics, 10th Anniversary Highlights, 13 (2008).
A magnetic butterfly made of ultracold atoms
We give a non-specialist summary of the paper 'Creation of effective magnetic fields in optical lattices: the Hofstadter butterfly for cold neutral atoms' D Jaksch et al 2003 New Journal of physics 5 56

created: 02-04-2008
D. Jaksch,
N&V: Supersolid Simulations,
Nature 442, 147 (2006).
N&V: Supersolid Simulations
Supersolids - substances that are crystalline but also behave as free-flowing superfluids - can exist, according to quantum theory. Models now suggest a route to the clinching experimental evidence.

created: 13-07-2006
D. Jaksch, J.J. Garcia-Ripoll, J.I. Cirac and P. Zoller,
Quantum Computing with Cold Ions and Atoms: Theory,
book chapter in Lectures on Quantum Information (Eds. Bruss, Dagmar / Leuchs, Gerd) (2006).
Quantum Computing with Cold Ions and Atoms: Theory
We review recently developed methods for quantum computing with systems of neutral atoms and ions.

created: 08-08-2005, last modified: 11-08-2006
T. Calarco, D. Jaksch, J.I. Cirac and P. Zoller,
Quantum Computing with Trapped Particles in Microscopic Potentials,
Chapter 11 in Scalable Quantum Computers: Paving the Way to Realization, edited by Prof. Dr. Samuel L. Braunstein, Dr. Hoi Kwong Lo, Pieter Kok (2005).
Quantum Computing with Trapped Particles in Microscopic Potentials
We review recent proposals for performing entanglement manipulation via controlled interactions between trapped atoms. State-dependent time-varying microscopic potentials allow one to obtain with high fidelity a conditional phase shift realizing a universal quantum gate. We discuss possible physical implementations with existing experimental techniques, for example optical lattices and magnetic micro-traps.

created: 28-04-2018
D. Jaksch,
Book review on Bose-Einstein condensation by L. Pitaevskii and S. Stringari,
J. Phys. A: Math. Gen. 36, 9797 (2003).
Book review on Bose-Einstein condensation by L. Pitaevskii and S. Stringari
The Gross-Pitaevskii equation named after one of the authors of the book and its large number of applications for describing the properties of Bose-Einstein condensation (BEC) in trapped weakly interacting atomic gases is the main topic of this book. In total the monograph comprises 18 chapters and is divided into two parts. Part I introduces the notion of BEC and superfluidity in general terms. The most important properties of the ideal and the weakly interacting Bose gas are described and the effects of nonuniformity due to an external potential at zero temperature are studied. The first part is then concluded with a summary of the properties of superfluid 4He. In Part II the authors describe the theoretical aspects of BEC in harmonically trapped weakly interacting atomic gases. A short and rather rudimentary chapter on collisions and trapping of atomic gases which seems to be included for completeness only is followed by a detailed analysis of the ground state, collective excitations, thermodynamics, and vortices as well as mixtures of BECs and the Josephson effect in BEC. Finally, the last three chapters deal with topics of more recent interest like BEC in optical lattices, low dimensional systems, and cold Fermi gases. The book is well written and in fact it provides numerous useful and important relations between the different properties of a BEC and covers most of the aspects of ultracold weakly interacting atomic gases from the point of view of condensed matter physics. The book contains a comprehensive introduction to BEC for physicists new to the field as well as a lot of detail and insight for those already familiar with this area. I therefore recommend it to everyone who is interested in BEC. Very clearly however, the intention of the book is not to provide prospects for applications of BEC in atomic physics, quantum optics or quantum state engineering and therefore the more practically oriented reader might sometimes wonder why exactly an equation is termed useful or important.

created: 08-08-2005, last modified: 11-08-2006
L.M. Duan, W. Duer, J.I. Cirac, D. Jaksch, G. Vidal and P. Zoller,
Quantum computing and quantum communication with atoms,
XVIII ICAP 2002 Proceedings - MIT, Cambridge, MA July 27- August 2 (2002).
Quantum computing and quantum communication with atoms
We review recent theoretical proposals for implementation of quantum computing and quantum communication with atoms. The first example deals with the realization of a universal quantum simulator with atoms and ions. The second example outlines the implementation of a quantum repeater with atomic ensembles.

created: 08-08-2005, last modified: 11-07-2006
H.-J. Briegel, T. Calarco, D. Jaksch, J.I. Cirac and P. Zoller,
Quantum optical systems for implementation of quantum information processes,
Proceedings of the International Conference on Laser Spectroscopy, Innsbruck (Austria) 7-11 June (1999).
Quantum optical systems for implementation of quantum information processes

created: 09-08-2005, last modified: 11-07-2006
T. Calarco, D. Jaksch, J.I. Cirac and P. Zoller,
Ultracold atoms for quantum computation,
Proceedings of the Sixth International Conference on Squeezed States and Uncertainty Relations, Naples (Italy) 24-29 May (1999).
Ultracold atoms for quantum computation

created: 09-08-2005, last modified: 11-07-2006
D. Jaksch,
Bose-Einstein condensation and Applications,
PhD thesis (1999).
Bose-Einstein condensation and Applications
This thesis covers most of my work in the field of Bose-Einstein condensation. It contains several different topics: quantum kinetic theory for describing Bose-gases at finite temperatures, the Bose-Hubbard model and its realization with neutral atoms in an optical lattice, and different ways for implementing quantum computations with neutral atoms in optical lattices and magnetic microtraps. I did all of those works in collaboration with Prof. Peter Zoller, Prof. Crispin Gardiner, and Prof. Ignacio Cirac. I did most of the work at the University of Innsbruck, during a research visit at the Victoria University of Wellington in New Zealand in the beginning of 1998, and while I was staying at the Institute for Theoretical Physics (ITP) in Santa Barbara in the summer of 1998. During my stay in New Zealand I completed the work on quantum kinetic theory. In Santa Barbara I mainly worked on the Bose-Hubbard model. The thesis is divided into four parts. The first part contains a general brief introduction to the field of Bose-Einstein condensation in dilute gases. I also give a list of references which contain all the details. In the second part I first present the basics of quantum kinetic theory, then a publication mostly dealing with the fluctuations of a condensate in its stationary state is reprinted. In the third part, after a short introduction, I present a publication on the Bose-Hubbard model and its realization using neutral atoms in optical lattices. Some details on the Bose-Hubbard model and on optical lattices that were left out in this publication complete the third part of my thesis. In the fourth part, again after an introduction, three publications are presented. The first shows how neutral atoms in optical lattices may be used to perform quantum computations. The second publication demonstrates how to implement two-qubit gates with magnetic microtraps and in the third publication the ideas of the other two publications are extended. It also shows how parallel quantum computing, error correction schemes and fault-tolerant computing can be implemented. However, some of the contents of the third publication is also contained in one of the preceding two publications. Some details left out in the publications complete the fourth part of my thesis. Each chapter of this thesis has its own bibliography. The abbreviation BEC is used for Bose-Einstein condensate as well as for Bose-Einstein condensation.

created: 02-07-2005, last modified: 05-08-2005
H.-J. Briegel, J.I. Cirac, C.W. Gardiner, D. Jaksch and P. Zoller,
Quantum computing in optical lattices, Technical Digest, Summaries of papers presented at the Quantum Electronics and Laser Science Conference,
Postconference Edition. Opt. Soc. America, Wasington, DC, USA, 1999; 283 pp. p.112 (1999).
Quantum computing in optical lattices, Technical Digest, Summaries of papers presented at the Quantum Electronics and Laser Science Conference
We develop a method to entangle neutral atoms using cold controlled collisions. We analyze this method in two particular set-ups: optical lattices and magnetic micro-traps. Both offer the possibility of performing certain multi-particle operations in parallel. Using this fact, we show how to implement efficient quantum error correction and schemes for fault-tolerant computing.

created: 02-07-2005, last modified: 11-07-2006
D. Jaksch and H. Stecher,
The Quantum Boltzmann Master Equation and Sympathetic Cooling,
Proceedings of the 3rd International Meeting of the Young Atom Optics Physicists, University of Pisa (1997).
The Quantum Boltzmann Master Equation and Sympathetic Cooling
We present results of simulations of a quantum Boltzmann master equation (QBME) describing the kinetics of a dilute Bose gas confined in a trapping potential in the regime of Bose condensation. The QBME is the simplest version of a quantum kinetic master equation derived in previous work. We concentrate on the 3D isotropic harmonic oscillator as a trapping potential. In particular, we calculate the time between collisions and the mean occupation numbers of the one-particle states in the regime of onset of Bose-Einstein condensation. Furthermore, we will simulate evaporative cooling and sympathetic cooling using the QBME.

created: 01-07-2005, last modified: 08-08-2005
D. Jaksch,
Kinetik der Bose-Einstein Kondensation (Kinetics of Bose-Einstein Condensation),
Diploma Thesis (1996).
Kinetik der Bose-Einstein Kondensation (Kinetics of Bose-Einstein Condensation)
Seit es erstmals gelungen ist, ein Bosekondensat, bestehend aus schwach wechselwirkenden Atomen, experimentell nachzuweisen, ist auch das Interesse der theoretischen Physiker an der Bose-Einstein Kondensation wieder gestiegen. Einen Beitrag dazu liefert diese Diplomarbeit. In der Einleitung werden kurz die wichtigsten Experimente mit schwach wechselwirkenden Bosegasen wie Na, Li und Cs vorgestellt, auÃŸerdem werden einige weitere Gebiete der Physik in denen BEC eine Rolle spielt genannt, und ein Ãœberblick Ã¼ber aktuelle Arbeiten zum Thema BEC gegeben. Im nÃ¤chsten Kapitel wird zuerst das ideale Bosegas mit Hilfe der Standardverfahren der statistischen Physik behandelt. Als nÃ¤chstes folgt dann die Ableitung der einfachsten Form der Quanten-Boltzmann Mastergleichung fÃ¼r ein, in einer Schachtel eingeschlossenes, schwach wechselwirkendes Bosegas. Am Ende dieses Kapitels wird beschrieben, wie das vorliegende Modell erweitert werden kann, um zu einer realistischeren Beschreibung des Bosegases zu gelangen. Im dritten Kapitel wird erklÃ¤rt, wie die Simulation der zuvor berechneten Quanten-Boltzmann Mastergleichung am Computer implementiert wird. Welche Ergebnisse aus der Simulation zu erwarten sind, und wie diese ausgewertet werden ist im vierten Kapitel enthalten. Im letzten Kapitel werden die aus den Simulationen gewonnenen Ergebnisse prÃ¤sentiert. Auch wird dort das in den Experimenten verwendete VerdampfungskÃ¼hlverfahren mit Hilfe eines sehr einfachen Modells simuliert.

created: 01-07-2005, last modified: 08-08-2005