Frontiers in Quantum Materials Control
The overarching goal of Q-MAC is to exploit materials design, coherent optical methods and multiple theoretical approaches to deterministically control ordered states of strongly correlated electron materials, also referred to as quantum or complex materials. The underlying ideas can be applied to vast number of problems in materials physics, but the stated goal is that of optimizing superconductivity at higher temperatures than achieved so far, possibly even at room temperature.
The proposal starts from research strands that follow challenging but well-establish paths, such as the use of complex-oxide heterostructures and strain engineering at interfaces to modulate the electronic properties. In a second class of investigations, coherent optical control of lattice dynamics with strong field THz transients is proposed to anneal the competing order quenching superconductivity. This builds on our recent discovery of light-induced transient superconductivity in high temperature cuprates, a remarkable process not yet understood or optimized.
We will use a combination of femtosecond optical and x-ray experiments with Free Electron Lasers, together with time dependent real-materials simulations. Perhaps the most ambitious goal will be to develop laser-cooling techniques to reduce quantum phase fluctuations between planes of cuprate superconductors. Finally, we propose to use static and dynamic techniques to engineer new phases of condensed matter, for example by engineering new materials with a single band crossing the Fermi level, to optimize superconductivity.
A unique combination of complementary expertise, from materials design, to coherent and ultrafast optical and x-ray physics, with materials and quantum optics theory, will be key in making true progress in these areas.
| Investigators: | A. Cavalleri, A Georges, D. Jaksch and J.-M. Triscone
|
| | |
| Home page: | http://www.q-mac.eu/
|
| Acronym: | Q-MAC
|
| Funded by: | ERC Synergy Grant, Oxford share Euro 2.3Mio
|
| | |
| Start date: | 2013-10-01
|
| End date: | 2020-09-30
|
| | |
Related Publications
-
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).
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. Buca, C. Booker and D. Jaksch,
Theory of Quantum Synchronization and Limit Cycles under Dissipation,
SciPost Physics 12, 097 (2022).
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
-
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).
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
-
J. Tindall, F. Schlawin, M. Sentef and D. Jaksch,
Lieb Theorem and Maximum Entropy Condensates,
Quantum 5, 610 (2021).
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
-
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).
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).
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
-
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).
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
-
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.
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
-
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).
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
-
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).
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).
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, J.R. Coulthard, D. Jaksch and J. Mur-Petit,
Anomalous spin-charge separation in a driven Hubbard system,
Phys. Rev. Lett. 125, 195301 (2020).
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
-
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).
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
-
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).
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
-
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).
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
-
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).
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
-
M. Medenjak, B. Buca and D. Jaksch,
Isolated Heisenberg magnet as a quantum time crystal,
Phys. Rev. B 102, 041117(R) (2020).
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
-
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).
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
-
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).
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 effective Hamiltonian describing this system for off-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 modification 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 effective 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
-
J. Mur-Petit and R.A. Molina,
Van Hove bound states in the continuum: Localized subradiant states in finite open lattices,
Phys. Rev. B 101, 184306 (2020).
We show that finite lattices with arbitrary boundaries may support large degenerate subspaces, stemming from the underlying translational symmetry of the lattice. When the lattice is coupled to an environment, a potentially large number of these states remains weakly or perfectly uncoupled from the environment, realizing a new kind of bound states in the continuum. These states are strongly localized along particular directions of the lattice, which, in the limit of strong coupling to the environment, leads to spatially localized subradiant states.
created: 23-05-2020
-
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).
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
-
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).
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
-
J. Tindall, C. Sanchez Munoz, B. Buca and D. Jaksch,
Quantum Synchronisation Enabled by Dynamical Symmetries and Dissipation,
New J. Phys. 22, 013026 (2020).
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
-
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).
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
-
B. Buca and D. Jaksch,
Dissipation Induced Nonstationarity in a Quantum Gas,
Phys. Rev. Lett. 123, 260401 (2019).
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
-
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).
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
-
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).
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
-
F. Schlawin and D. Jaksch,
Cavity-mediated unconventional pairing in ultracold fermionic atoms,
Phys. Rev. Lett. 123, 133601 (2019).
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
-
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).
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
-
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).
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
-
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).
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
-
F. Schlawin, A. Cavalleri and D. Jaksch,
Cavity-mediated electron-photon superconductivity,
Phys. Rev. Lett. 122, 133602 (2019).
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).
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
-
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).
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
-
P. Rosson, M. Lubasch, M. Kiffner and D. Jaksch,
Bosonic Fractional Quantum Hall States on a Finite Cylinder,
Phys. Rev. A 99, 033603 (2019).
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
-
M. Kiffner, J.R. Coulthard, F. Schlawin, A. Ardavan and D. Jaksch,
Manipulating quantum materials with quantum light,
Phys. Rev. B 99, 085116 (2019).
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
-
J. Mur-Petit, A. Relano, R.A. Molina and D. Jaksch,
Revealing missing charges with generalised quantum fluctuation relations,
Nature Communications 9, 2006 (2018).
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
-
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).
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
-
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).
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).
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
-
S. Al-Assam, S.R. Clark and D. Jaksch,
The tensor network theory library,
J. Stat. Mech. 2017, 093102 (2017).
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
-
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).
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.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).
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
-
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).
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
-
M. Kiffner, E. OBrien and D. Jaksch,
Topological spin models in Rydberg lattices,
Appl. Phys. B 123, 46 (2017).
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
-
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).
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
-
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).
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
-
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).
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
-
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).
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. 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).
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
-
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).
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.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).
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
-
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).
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
-
S.R. Clark, A. Cavalleri and D. Jaksch,
Rattling the Cage: Making New Superconductors Using Lasers,
Department of Physics Newsletter 8, 8 (2016).
We describe a recent experiment inducing superconductivity in K3C60 above its critical temperature using short laser pulses.
created: 18-06-2016
-
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).
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. 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).
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. 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).
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
-
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).
We use midinfrared pulses with stable carrier-envelope phase offset to drive molecular vibrations in the charge transfer salt ET−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
-
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).
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).
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
-
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).
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
-
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).
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
-
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).
We study the non-equilibrium transport properties of a highly anisotropic two dimensional lattice of spin−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. 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).
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 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
-
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).
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).
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