Frontiers in Quantum Materials Control
The overarching goal of QMAC 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 wellestablish paths, such as the use of complexoxide 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 lightinduced transient superconductivity in high temperature cuprates, a remarkable process not yet understood or optimized.
We will use a combination of femtosecond optical and xray experiments with Free Electron Lasers, together with time dependent realmaterials simulations. Perhaps the most ambitious goal will be to develop lasercooling 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 xray 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.qmac.eu/

Acronym:  QMAC

Funded by:  ERC Synergy Grant, Oxford share Euro 2.3Mio

 
Start date:  20131001

End date:  20200930

 
Related Publications

P. Molignini, C. Leveque, H. Kessler, D. Jaksch, R. Chitra and A.U.J. Lode,
Crystallization via cavityassisted infiniterange interactions,
Phys. Rev. A 106, L011701 (2022).
We study a onedimensional array of bosons with infiniterange interactions mediated by a laserdriven dissipative optical cavity. The cavitymediated infiniterange interactions open up a new pathway to fermionization, hitherto only known for dipolar bosons due to their longrange interactions. In parameter ranges attainable in stateoftheart experiments, we systematically compare observables for bosons and fermions with infiniterange interactions. At large enough laser pump power, many observables, including density distributions in real and momentum space, correlation functions, eigenvalues of the onebody density matrix, and superradiance order parameter, become identical for bosons and fermions. We map out the emergence of this cavityinduced fermionization as a function of pump power and contact interactions. We discover that cavitymediated interactions can compensate a reduction by several orders of magnitude in the strength of the contact interactions needed to trigger fermionization.
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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 semiclassical limits but no comprehensive theory providing a systematic basis for the underlying physical mechanisms has yet been found. Through a complete characterization of the nondecaying 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 antisynchronization of two spin1/2 and discuss synchronization in the FermiHubbard model and its generalisations which are relevant for various fermionic cold atom experiments, including multispecies, multiorbital and higher spin atoms.
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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 photoinduced superconductivity in K3C60,
Nature Physics 17, 611 (2021).
Far and mid infrared optical pulses have been shown to induce nonequilibrium unconventional orders in complex materials, including photoinduced 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 nonequilibrium 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 midinfrared pulses of tunable duration, ranging between one picosecond and one nanosecond. The same superconductinglike 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 photoconductivity is highly unusual for a metal and, when taken together with the transient optical conductivities, it is rather suggestive of metastable lightinduced superconductivity.
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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 bipartite lattices are shown to possess uniform finite offdiagonal longrange order in the thermodynamic limit. For repulsive interactions the induced order corresponds to the formation of a spinwave condensate, whilst for attractive interactions it instead corresponds to the formation of a superconducting, etapaired 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 solidstate photoexcited compounds.
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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 Singleshot Images of Ultracold Atoms via Machine Learning,
Phys. Rev. A 104, L041301 (2021).
Singleshot images are the standard readout of experiments with ultracold atoms  the tarnished looking glass into their manybody physics. The efficient extraction of observables from singleshot 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 twoparticle densities to be accurately obtained from a drastically reduced number of singleshot images. Quantum fluctuations and correlations are directly harnessed to obtain physical observables for bosons in a tilted doublewell potential at an unprecedented accuracy. Strikingly, machine learning also enables a reliable extraction of momentumspace observables from realspace singleshot images and vice versa. This obviates the need for a reconfiguration of the experimental setup between insitu and timeofflight imaging, thus potentially granting an outstanding reduction in resources.
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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 multiphoton microscopy or Heisenberg scaling of the Fisher information in quantum parameter estimation.
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H. Gao, F. Schlawin and D. Jaksch,
Higgs mode stabilization by photoinduced longrange interactions in a superconductor,
Phys. Rev. B 104, L140503 (2021).
We show that lowlying excitations of a 2D BCS superconductor are significantly altered when coupled to an externally driven cavity, which induces controllable longrange attractive interactions between the electrons. We find that they combine nonlinearly with intrinsic local interactions to increase the Bogoliubov quasiparticle excitation energies, thus enlarging the superconducting gap. The longrange nature of the drivencavityinduced 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.
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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 nonperiodic forcing. Such selfoscillators 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 selfoscillation is well understood in classical nonlinear systems and their quantized counterparts, the spontaneous emergence of periodicity in quantum systems without a semiclassical limit is more elusive. Here, we show that this behaviour can emerge within the repeatedinteraction description of open quantum systems. Specifically, we consider a manybody quantum system that undergoes dissipation due to sequential coupling with auxiliary systems at random times. We develop dynamical symmetry conditions that guarantee an oscillatory longtime state in this setting. Our rigorous results are illustrated with specific spin models, which could be implemented in trappedion quantum simulators.
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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 phasespace constraints induced by certain symmetries can, however, prevent this and allow the system to dynamically form robust steady states with offdiagonal longrange 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 longrange particlehole and spinexchange 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 particlehole and spinwave order.
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B. Buca, C. Booker, M. Medenjak and D. Jaksch,
Bethe ansatz approach for dissipation: exact solutions of quantum manybody 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, manybody 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 easyaxis regime despite the presence of loss. Such analytic results have previously been inaccessible for systems of this type.
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Y. Ashida, A. Imamoglu, J. Faist, D. Jaksch, A. Cavalleri and E. Demler,
Quantum Electrodynamic Control of Matter: CavityEnhanced Ferroelectric Phase Transition,
Phys. Rev. X 10, 041027 (2020).
The lightmatter 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 manybody systems via strong classical electromagnetic radiation, leading to a timedependent Hamiltonian for electronic or lattice degrees of freedom. To avoid inevitable heating, pumpprobe setups with ultrashort laser pulses have so far been used to study transient lightinduced 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 lightenhanced electronelectron interactions, we consider a dipolar quantum manybody system embedded in a cavity composed of metal mirrors and formulate a theoretical framework to manipulate its equilibrium properties on the basis of quantum lightmatter 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 lightmatter interaction. This hybridization qualitatively alters the nature of the collective excitations and can be used to selectively control energylevel structures in a wide range of platforms. Most notably, in quantum paraelectrics, we show that the cavityinduced softening of infrared optical phonons enhances the ferroelectric phase in comparison with the bulk materials. Our findings suggest an intriguing possibility of inducing a superradianttype transition via the lightmatter coupling without external pumping. We also discuss possible applications of the cavityinduced modifications in collective excitations to molecular materials and excitonic devices.
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H. Gao, J.R. Coulthard, D. Jaksch and J. MurPetit,
Anomalous spincharge separation in a driven Hubbard system,
Phys. Rev. Lett. 125, 195301 (2020).
Spincharge separation (SCS) is a striking manifestation of strong correlations in lowdimensional 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 halffilling. 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 singleparticle and pairhopping processes.
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C. Booker, B. Buca and D. Jaksch,
Nonstationarity and Dissipative Time Crystals: Spectral Properties and FiniteSize Effects,
New J. Phys. 22, 085007 (2020).
We discuss the emergence of nonstationarity in open quantum manybody systems. This leads us to the definition of dissipative time crystals which display experimentally observable, persistent, timeperiodic 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 twoparticle loss and gain we find a dark Hamiltonian driving oscillations between GHZ states in the longtime limit. Finally, we discuss how the presented examples could be experimentally realized.
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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 ManyBody 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 tworung 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 particlehole excitation pathway. This restriction, which symmetry arguments fail to predict, dictates the transient dynamics of the system, causing the available particlehole degrees of freedom to manifest uniform longrange order. We discuss possible implications of our results for a recent experiment on photoinduced superconductivity in k(BEDTTTF)2Cu[N(CN)2]Br molecules.
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M. Buzzi, D. Nicoletti, M. Fechner, N. TancogneDejean, 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 HighTemperature 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 chargetransfer salt K−(BEDT−TTF)2 Cu[N(CN)2]Br induces a colossal increase in carrier mobility and the opening of a superconductinglike optical gap. Both features track the density of quasiparticles 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 photoexcitation is not observed in the equilibrium superconductor, pointing to a light induced state that is different from that obtained by cooling. Firstprinciple calculations and model Hamiltonian dynamics predict a transient state with longrange pairing correlations, providing a possible physical scenario for photomolecular superconductivity.
created: 27012020, last modified: 10082020

H. Gao, F. Schlawin, M. Buzzi, A. Cavalleri and D. Jaksch,
Photoinduced electron pairing in a driven cavity,
Phys. Rev. Lett. 125, 053602 (2020).
We demonstrate how virtual scattering of laser photons inside a cavity via twophoton processes can induce controllable longrange electron interactions in twodimensional 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, laserinduced 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 twodimensional materials including ABstacked bilayer graphene and the conducting interface between LaAlO3 and SrTiO3.
created: 12032020, last modified: 28072020

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 manybody 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.
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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 Mottinsulator 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 wavefunctions spatially overlap would represent a new and ideal platform to simulate exotic electronic manybody phenomena in the condensed phase. However, this highly excited nonequilibrium system is expected to be so shortlived that it has eluded observation so far. Here, we demonstrate the first step toward its realization by exciting highlying 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 ioncounting 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 BoseEinstein condensate and a Mott insulator.
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H. Gao, J.R. Coulthard, D. Jaksch and J. MurPetit,
Controlling magnetic correlations in a driven Hubbard system far from halffilling,
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 tJ Hamiltonian, which describes the dynamics in doped antiferromagnets and is thought to be relevant to the problem of hightemperature superconductivity in the cuprates. We show analytically that the eﬀective Hamiltonian describing this system for oﬀresonant driving is the tJ 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 nearadiabatic transfer of the system from the ground state of the underlying Hubbard model to the ground state of the eﬀective tJ Hamiltonian. Finally, we report timedependent density matrix renormalization group and exact diagionalization calculations illustrating the control achievable on the dynamics of spinsinglet pairs utilising this technique with current coldatom quantumsimulation technology. These results open new routes to explore the interplay between density and spin in stronglycorrelated fermionic systems through their outofequilibrium dynamics.
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J. MurPetit 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: 23052020

Y Zhang, J. Tindall, J. MurPetit, D. Jaksch and B. Buca,
Stationary State Degeneracy of Open Quantum Systems with NonAbelian Symmetries,
J. Phys. A: Math. Gen. 53, 215304 (2020).
We study the null space degeneracy of open quantum systems with multiple nonAbelian, 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 manybody systems, presenting three illustrative examples: a fullyconnected 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 blockdecomposition of a Liouvillian with nonAbelian 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.
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J. MurPetit, 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 outofequilibrium 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 fewbody 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 outofequilibrium measurements starting from realistic equilibrium states on a fewbody system implementing the Dicke model.
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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 symmetrybased 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 symmetrybreaking perturbations. Using these ideas we identify two central examples where synchronisation arises via this qualitatively new mechanism: a chain of quadratically dephased spin1s and the manybody chargedephased Hubbard model. In both cases, due to their dynamical symmetries, perfect phaselocking occurs throughout the system, regardless of the specific microscopic parameters or initial states. Furthermore, when these systems are perturbed, their nonlinear responses elicit longlived signatures of both phase and frequencylocking.
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P. Rosson, M. Kiffner, J. MurPetit and D. Jaksch,
Characterizing the phase diagram of finitesize dipolar BoseHubbard systems,
Phys. Rev. A 101, 013616 (2020).
We use stateoftheart density matrix renormalization group calculations in the canonical ensemble to determine the phase diagram of the dipolar BoseHubbard 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 nonlocal 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 machinelearning techniques.
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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 twocomponent BoseEinstein 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 meanfield theory. We solve the underlying model in the thermodynamic limit and show that this system is always dynamically unstable—even when meanfield theory predicts stability. Instabilities always occur in higherorder 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.
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C. Sanchez Munoz, B. Buca, J. Tindall, A. GonzalezTudela, D. Jaksch and D. Porras,
Symmetries and conservation laws in quantum trajectories: Dissipative freezing,
Phys. Rev. A 100, 042113 (2019).
In drivendissipative 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.
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F. Schlawin, A.S.D. Dietrich and D. Jaksch,
Optical control of the currentvoltage relation in stacked superconductors,
Phys. Rev. B 100, 134510 (2019).
We simulate the currentvoltage 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 phaselocked 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.
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F. Schlawin and D. Jaksch,
Cavitymediated unconventional pairing in ultracold fermionic atoms,
Phys. Rev. Lett. 123, 133601 (2019).
We investigate longrange 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 twodimensional 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 swave 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 longrange and shortrange interactions.
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M. Kiffner, J.R. Coulthard, F. Schlawin, A. Ardavan and D. Jaksch,
Mott polaritons in cavitycoupled quantum materials,
New J. Phys. 21, 073066 (2019).
We show that strong electronelectron interactions in cavitycoupled quantum materials can enable collectively enhanced lightmatter interactions with ultrastrong effective coupling strengths. As a paradigmatic example we consider a FermiHubbard model coupled to a singlemode cavity and find that resonant electroncavity interactions result in the formation of a quasicontinuum 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 ultrastrong coupling on the singleelectron level. Quantum correlations in the electronic ground state as well as the microscopic nature of the lightmatter interaction enhance the collective lightmatter interaction compared to an ensemble of independent twolevel atoms interacting with a cavity mode.
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J. Tindall, B. Buca, J.R. Coulthard and D. Jaksch,
HeatingInduced LongRange etaPairing 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 longrange correlations between eta pairs. We induce this heating with either dissipation or periodic driving and evolve the system towards a nonequilibrium steady state. This steady state is identical in both cases and displays distanceinvariant offdiagonal 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.
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J. Tangpanitanon, S.R. Clark, V.M. Bastidas, R. Fazio, D. Jaksch and D.G. Angelakis,
Hidden Order in Quantum Manybody Dynamics of DrivenDissipative Nonlinear Photonic Lattices,
Phys. Rev. A 99, 043808 (2019).
We study the dynamics of nonlinear photonic lattices driven by twophoton parametric processes. By means of matrixproduct state based calculations, we show that a quantum manybody state with longrange 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.
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F. Schlawin, A. Cavalleri and D. Jaksch,
Cavitymediated electronphoton superconductivity,
Phys. Rev. Lett. 122, 133602 (2019).
Pairing between fermionic quasiparticles 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 photonmediated 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 electronphoton superconductivity with critical temperatures on the order of 0.5 Kelvin. Because the electronphoton pairing interaction is effectively longrange, both singlet and triplet pairing are possible. Consequently, cavitymediated electronphoton superconductors with nontrivial topology may be induced in 2D materials.
created: 20042018, last modified: 23052019

B. Buca, J. Tindall and D. Jaksch,
Complex coherent quantum manybody dynamics through dissipation,
Nature Communications 10, 1730 (2019).
The assumption that physical systems relax to a stationary state in the longtime 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 decoherencefree 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 manybody system from ever reaching a stationary state. We go beyond dissipative quantum state engineering approaches towards controllable longtime nonstationary dynamics typically associated with macroscopic complex systems. This coherent and oscillatory evolution constitutes a dissipative version of a quantum timecrystal. We discuss the possibility of engineering such complex dynamics with fermionic ultracold atoms in optical lattices.
created: 19042018, last modified: 23052019

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 manybody regimes, the presence of disorder or longrange interactions. Here we propose the simplest scheme to observe discrete time crystal dynamics in a onedimensional driven quantum system of the Ising type with shortrange 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 offresonant 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: 23072018, last modified: 26032019

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 HarperHofstadter 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 singleparticle 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 Laughlinlike behaviour can be either smooth or happens abruptly for some critical interaction strength.
created: 14012019, last modified: 11032019

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 FermiHubbard model and find that the electroncavity coupling enhances the magnetic interaction between the electron spins in the groundstate 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 nextnearestneighbor hopping and mediates a longrange electronelectron interaction between distant sites. We study the groundstate properties with tensor network methods and find that the cavity coupling can induce a phase characterized by a momentumspace pairing effect for electrons.
created: 19062018, last modified: 13022019

J. MurPetit, A. Relano, R.A. Molina and D. Jaksch,
Revealing missing charges with generalised quantum fluctuation relations,
Nature Communications 9, 2006 (2018).
The nonequilibrium dynamics of quantum manybody 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 outofequilibrium 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: 06112017, last modified: 21092019

A.S.D. Dietrich, M. Kiffner and D. Jaksch,
Probing microscopic models for systembath 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 oscillatorbath coupling. We consider a bath with an Ohmic spectral density and a model where the systembath 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 BornMarkov 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 timeaveraged 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: 22032018, last modified: 19072018

J.R. Coulthard, S.R. Clark and D. Jaksch,
Ground state phase diagram of the onedimensional tJ model with pair hopping terms,
Phys. Rev. B 98, 035116 (2018).
The tJ model is a standard model of strongly correlated electrons, often studied in the context of highTc superconductivity. However, most studies of this model neglect threesite terms, which appear at the same order as the superexchange J. As these terms correspond to pairhopping, they are expected to play an important role in the physics of superconductivity when doped sufficiently far from halffilling. In this paper we present a density matrix renormalisation group study of the onedimensional tJ model with the pair hopping terms included. We demonstrate that that these additional terms radically change the onedimensional 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: 26042018, last modified: 13072018

M. Kiffner, D. Jaksch and D. Ceresoli,
A polynomial Ansatz for Normconserving Pseudopotentials,
J. Phys.: Condens. Matter 30, 275501 (2018).
We show that efficient normconserving 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 TroullierMartins method [Phys. Rev. B 43, 1993 (1991)] where pseudopotentials are represented by a polynomial of degree twentytwo. We compare our method to the TroullierMartins 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 TroullierMartins potentials are replaced by the pseudopotentials introduced here.
created: 22032018, last modified: 14062018

S. AlAssam, 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 opensource 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 groundstate and dynamical calculations of onedimensional bosonic lattice system. We also discuss different options for gaining access to the software available at www.tensornetworktheory.org.
created: 10102016, last modified: 13072018

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 centerofmass excitation of the Josephson phase, made possible by the highly nonlinear nature of the lightmatter coupling.
created: 02082017, last modified: 20092017

J.R. Coulthard, S.R. Clark, S. AlAssam, A. Cavalleri and D. Jaksch,
Enhancement of superexchange pairing in the periodicallydriven Hubbard model,
Phys. Rev. B 96, 085104 (2017).
Recent experiments performed on cuprates and alkalidoped 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 BCStype 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 ingap, spatially inhomogeneous, onsite modulations. We demonstrate that such modulations substantially reduce electronic hopping, while simultaneously sustaining superexchange interactions and pair hopping via drivinginduced virtual charge excitations. We calculate realtime dynamics for the onedimensional case, starting from zero and finitetemperature initial states, and we show that enhanced singletpair correlations emerge quickly and robustly in the outofequilibrium manybody state. Our results reveal a fundamental pairing mechanism that might underpin optically induced superconductivity in some strongly correlated quantum materials.
created: 18082016, last modified: 01082017

J. J. MendozaArenas, F.J. GomezRuiz, M. Eckstein, D. Jaksch and S.R. Clark,
Ultrafast control of magnetic relaxation in a periodically driven Hubbard model,
Annals of Physics1700024 (2017).
Motivated by cold atom and ultrafast pumpprobe experiments we study the melting of longrange 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 nonequilibrium dynamical meanfield 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 chargeexcitations transferring energy to the spin sector, while for J0 approximately equal to U it is governed by the dynamics of residual quasiparticles. Here we explore the rich effects strong periodic driving has on this relaxation process spanning three frequency omega regimes:(i) highfrequency omega much larger than J0, (ii) resonant l times omega equal U greater J0 with integer l, and (iii) ingap U greater omega greater J0 away from resonance. In case (i) we can quickly switch the decay from quasiparticle to chargeexcitation 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 chargeexcitation to quasiparticle 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: 17012017, last modified: 19072017

M. Kiffner, E. OBrien and D. Jaksch,
Topological spin models in Rydberg lattices,
Appl. Phys. B 123, 46 (2017).
We show that resonant dipoledipole interactions between Rydberg atoms in a triangular lattice can give rise to artificial magnetic fields for spin excitations. We consider the coherent dipoledipole coupling between
np and ns Rydberg states and derive an effective spin1/2 Hamiltonian for the np excitations. By breaking timereversal symmetry via external fields we engineer complex hopping amplitudes for transitions between two rectangular sublattices. The phase of these hopping amplitudes depends on the direction of the hop. This gives rise to a staggered, artificial magnetic field which induces nontrivial topological effects. We calculate the singleparticle band structure and investigate its Chern numbers as a function of the lattice parameters and the detuning between the two sublattices. We identify extended parameter regimes where the Chern number of the lowest band is C=1 or C=2
created: 17092016, last modified: 10052017

C. Noh, S.R. Clark, D. Jaksch and D.G. Angelakis,
Outofequilibrium 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 manybody
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 nontrivial 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 JaynesCummingsHubbard and BoseHubbard Hamiltonians. A brief review of
the analytical and sophisticated numerical methods required to tackle these systems is included.
created: 03032017

M. Streif, A. Buchleitner, D. Jaksch and J. MurPetit,
Measuring correlations of coldatom systems using multiple quantum probes,
Phys. Rev. A 94, 053634 (2016).
We present a nondestructive method to probe a complex quantum system using multipleimpurity 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 twopoint density correlations, for probesystem contact interactions. We illustrate our findings with analytic and numerical calculations for the BoseHubbard model in the weakly and strongly interacting regimes, under conditions relevant to ongoing experiments in coldatom systems.
created: 11102016, last modified: 28112016

J. Tangpanitanon, V.M. Bastidas, S. AlAssam, 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 onedimensional 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 circuitQED architectures.
created: 15072016, last modified: 19112016

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 alloptical manipulation of plasma waves, which can concentrate electromagnetic energy on subwavelength 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, superconductormetal 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 terahertzphoton devices.
created: 11062016, last modified: 03112016

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 twoelectron state space. We consider large sets of twoatom 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 Manybody Problems with Cold Atoms and Molecule
created: 14072015, last modified: 19102016

J.M. Kreula, S.R. Clark and D. Jaksch,
Nonlinear quantumclassical scheme to simulate nonequilibrium strongly correlated fermionic manybody dynamics,
Scientific Reports 6, 32940 (2016).
We propose a nonlinear, hybrid quantumclassical scheme for simulating nonequilibrium dynamics of strongly correlated fermions described by the Hubbard model in a Bethe lattice in the thermodynamic limit. Our scheme implements nonequilibrium dynamical mean field theory (DMFT) and uses a digital quantum simulator to solve a quantum impurity problem whose parameters are iterated to selfconsistency 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: 29102015, last modified: 15092016

J.M. Kreula, L. Garcia Alvarez, L. Lamata, S.R. Clark, E. Solano and D. Jaksch,
Fewqubit quantumclassical simulation of strongly correlated lattice fermions,
EPJ Quantum Technology 3, 11 (2016).
We study a proofofprinciple example of the recently proposed hybrid quantumclassical simulation of strongly correlated fermion models in the thermodynamic limit. In a twosite dynamical meanfield theory (DMFT) approach we reduce the Hubbard model to an effective impurity model subject to selfconsistency conditions. The resulting minimal twosite representation of the nonlinear 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 nearfuture technology.
created: 16062016, last modified: 13082016

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 BoseEinstein 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 occupationdependent parameters to capture the effects of strong impurityimpurity interactions. These include both a repulsive direct interaction and an attractive effective interaction mediated by the BoseEinstein 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: 04012016, last modified: 19062016

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: 18062016

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 wellunderstood 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 TasakiCrooks theorem for nonequilibrium work distributions. We provide examples for a coldatom system, in which our thermometry protocol may retain accuracy and precision at subnanoKelvin temperatures.
created: 29102015, last modified: 25052016

J. J. MendozaArenas, S.R. Clark, S. Felicetti, G. Romero, E. Solano, D.G. Angelakis and D. Jaksch,
Beyond meanfield bistability in drivendissipative lattices: bunchingantibunching 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 twolevel systems. A commonly used meanfield ansatz, in which spatial correlations are neglected, predicts a bistable behavior with a sharp shift between low and highdensity states. In contrast onedimensional matrix product methods reveal these effects to be artifacts of the meanfield approach, with both disappearing once correlations are taken fully into account. Instead, a bunchingantibunching 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: 29102015, last modified: 26022016

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 lightinduced superconductivity in K3C60 at high temperature,
Nature 530, 461464 (2016).
The nonequilibrium 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 superconductinglike optical properties at temperatures far greater than the superconducting transition temperature, Tc. This effect was accompanied by the disruption of competing chargedensitywave 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 midinfrared 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 nonequilibrium hightemperature superconductivity, we propose this as a possible explanation of our results.
created: 26022016

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,
THzFrequency Modulation of the Hubbard U in an Organic Mott Insulator,
Phys. Rev. Lett. 115, 187401 (2015).
We use midinfrared pulses with stable carrierenvelope phase offset to drive molecular vibrations in the charge transfer salt ET&#8722;F2TCNQ, a prototypical onedimensional 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 onsite 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: 04092014, last modified: 29102015

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 largeamplitude infraredactive vibrations in a LaAlO3 substrate we induce magnetic order melting in a NdNiO3 film across a heterointerface. Femtosecond resonant soft Xray 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: 06072015, last modified: 26082015

P.L. Giscard, K. Lui, S.J. Thwaite and D. Jaksch,
An Exact Formulation of the TimeOrdered Exponential using PathSums,
Journal of Mathematical Physics 56, 053503 (2015).
We present the pathsum formulation for the timeordered exponential of a timedependent matrix. The pathsum formulation gives the timeordered exponential as a branched continued fraction of finite depth and breadth. The terms of the pathsum have an elementary interpretation as selfavoiding walks and selfavoiding polygons on a graph. Our result is based on a representation of the timeordered 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 superexponential decay bound for the magnitude of the entries of the timeordered exponential of sparse matrices. We give explicit results for matrices with commonly encountered sparse structures.
created: 24042015, last modified: 11052015

J. J. MendozaArenas, 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 onedimensional 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 largescale matrix product simulations of the equation of motion of the system, we calculate its steady state in the longtime 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: 18112014, last modified: 24042015

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 caxis vibrational mode. The scheme exploits the vibration as a transducer making the Josephson plasma frequencies timedependent. We show how modulation at the difference frequency between the intra and interbilayer plasmon substantially suppresses interbilayer phase fluctuations, responsible for switching caxis transport from a superconducting to resistive state. Our calculations indicate that this may provide a viable mechanism for stabilizing nonequilibrium 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: 18112014, last modified: 31032015

J. J. MendozaArenas, M.T. Mitchinson, S.R. Clark, J. Prior, D. Jaksch and M.B. Plenio,
Transport enhancement from incoherent coupling between onedimensional quantum conductors,
New J. Phys. 16, 053016 (2014).
We study the nonequilibrium 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 onedimensional 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: 08052014, last modified: 09052014

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,
PressureDependent Relaxation in the Photoexcited Mott Insulator ETF2TCNQ: Influence of Hopping and Correlations on Quasiparticle Recombination Rates,
Phys. Rev. Lett. 112, 117801 (2014).
We measure the ultrafast recombination of photoexcited quasiparticles (holondoublon pairs) in the one dimensional Mott insulator ETF2TCNQ 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 timedependent extended Hubbard Hamiltonian suggests that the competition between local recombination and delocalization of the MottHubbard exciton dictates the efficiency of the recombination.
created: 19032014

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 midinfrared 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 (HubbardU) in Mottinsulator time dependent. Midinfrared optical pulses excite localized molecular vibrations in ETF2TCNQ, a prototypical onedimensional Mottinsulator. A broadband ultrafast probe interrogates the resulting optical spectrum between THz and visible frequencies. A redshifted chargetransfer resonance is observed, consistent with a timeaveraged reduction of the
electronic correlation strength U. Secondly, a sideband manifold inside of the Mottgap appears, resulting from a periodically modulated U. The response is compared to computations based on a quantummodulated dynamic Hubbard model. Heuristic fitting suggests asymmetric holondoublon
coupling to the molecules and that electron doubleoccupancies strongly squeeze the vibrational mode.
created: 31072012, last modified: 22012014

J. J. MendozaArenas, S. AlAssam, 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 spin1/2 Heisenberg chain with homogeneous magnetic field, incoherently driven out of equilibrium by reservoirs at the boundaries. We focus on the effect of bulk dephasing (energydissipative) processes in different parameter regimes of the system. The nonequilibrium 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 stronglyinteracting regime, as evidenced by the heatcurrent scaling with the system size. When bulk dephasing takes place in the system, diffusive transport is induced in the weaklyinteracting regime, with the heat current monotonically decreasing with the dephasing rate. In contrast, in the stronglyinteracting regime, the heat current can be significantly enhanced by dephasing for systems of small size.
created: 27032013, last modified: 16072013

J. J. MendozaArenas, T. Grujic, D. Jaksch and S.R. Clark,
Dephasing enhanced transport in nonequilibrium stronglycorrelated 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 singleparticle interference effects. However, it is not yet clear whether dephasing can enhance transport in an interacting manybody system. Here we address this question by analysing the transport properties of a boundary driven spinless fermion chain with nearestneighbour interactions subject to bulk dephasing. The manybody nonequilibrium 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 darkstates 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 stronglycorrelated particles.
created: 22032013, last modified: 24062013