M. Will, T. A. Cochran, E. Rosenberg, Jobst, N. M. Eassa, P. Roushan,M. Knap, A. Gammon-Smith, F. Pollmann, Nature 645 346
Out-of-equilibrium phases in many-body systems constitute a new paradigmin quantum matter - they exhibit dynamical properties that may otherwise be forbidden by equilibrium thermodynamics. Among these non-equilibrium phases are periodically driven (Floquet) systems, which are generically difficult to simulate classically because of their high entanglement. Here we realize a Floquet topologically ordered state theoretically proposed earlier, on an array of superconducting qubits.
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T. A. Cochran et al., Nature 642 315
Lattice gauge theories (LGTs) can be used to understand a wide range of phenomena, from elementary particle scattering in high-energy physics to effective descriptions of many-body interactions in materials. Studying dynamical properties of emergent phases can be challenging, as it requires solving many-body problems that are generally beyond perturbative limits. Here we investigate the dynamics of local excitations in a Z2LGT using a two-dimensional lattice of superconducting qubits.
Guilherme Zambon, Gerardo Adesso, Phys. Rev. Lett. 134 200401
Quantum thermodynamics studies how quantum systems and operations may be exploited as sources ofwork to perform useful thermodynamic tasks. In real-world conditions, the evolution of open quantumsystems typically displays memory effects, resulting in a non-Markovian dynamics. The associatedinformation backflow has been observed to provide advantage in certain thermodynamic tasks. However, ageneral operational connection between non-Markovianity and thermodynamics in the quantum regime hasremained elusive. Here, we analyze the role of non-Markovianity in the central task of extracting work viathermal operations from general multitime quantum processes, as described by process tensors.
Kay Brandner, Phys. Rev. Lett. 134 037101
Memory effects are ubiquitous in small-scale systems. They emerge from interactions between accessible and inaccessible degrees of freedom and give rise to evolution equations that are nonlocalin time. If the characteristic timescales of accessible and inaccessible degrees of freedom are sharply separated, locality can be restored through the standard Markov approximation. Here, we show that this approach can be rigorously extended to a well-defined weak-memory regime, where the relevant timescales can be of comparable order of magnitude.
Kay Brandner, Keiji Saito, Phys. Rev. Lett. 135 046302
We derive a universal thermodynamic uncertainty relation for fermionic coherent transport, whichbounds the total rate of entropy production in terms of the mean and fluctuations of a single particle current. This bound holds for any multiterminal geometry and arbitrary chemical and thermal biases, as long as no external magnetic fields are applied. It can further be saturated in two-terminal settings with boxcar-shaped transmission functions and reduces to its classical counterpart in linear response.
Robert Mattes, Igor Lesanovsky, Federico Carollo, Phys. Rev. Lett. 134 070402
Enhanced experimental capabilities to control nonlocal and power-law decaying interactions arecurrently fueling intense research in the domain of quantum many-body physics. Compared to theircounterparts with short-ranged interactions, long-range interacting systems display novel physics, such asnonlinear light cones for the propagation of information or inequivalent thermodynamic ensembles. In thiswork, we consider generic long-range open quantum systems in arbitrary dimensions and focus on the so-called strong long-range regime. We prove that in the thermodynamic limit local properties, captured byreduced quantum states, are described by an emergent noninteracting theory.
Luke Causer, Mari Carmen Bañuls, Juan P. Garrahan,Phys. Rev. B 111 L060303
We study the nonequilibrium dynamics of the dissipative quantum East model via numerical tensor networks.We use matrix product states to represent evolution under quantum-jump unravellings for sizes beyond thoseaccessible to exact diagonalization. This allows us to demonstrate that dynamical heterogeneity accompaniesslow relaxation, in analogy with what is seen in classical glassy systems.
Stephen Powell, Sukla Pal, Phys. Rev. Lett. 134 256701
We present a dynamic scaling theory to describe relaxation dynamics following a magnetic-field quench near an unconventional phase transition in the magnetic material spin ice. Starting from a microscopic model, we derive an effective description for the critical dynamics in terms of the seeding and growth of string excitations, and use this to find scaling forms in terms of time, reduced temperature, and monopole fugacity.
Arash Jafarizadeh, Frank Pollmann, Adam Gammon-Smith,Phys. Rev. Research 7 043018
The simulation of quantum many-body systems, relevant for quantum chemistry and condensed matter physics, is one of the most promising applications of near-term quantum computers before fault-tolerance. However, since the vast majority of quantum computing technologies are built around qubits and discrete gate-based operations, the translation of the physical problem into this framework is a crucial step. This translation will often be device specific, and a suboptimal implementation will be punished by the exponential compounding of errors on real devices. The importance of an efficient mapping is already revealed for models of spinful fermions in two or three dimensions, which naturally arise when the relevant physics relates toelectrons. Using the most direct and well-known mapping, the Jordan-Wigner transformation, leads to a nonlocal representation of local degrees of freedom, and necessities efficient decompositions of nonlocal unitary gates into a sequence of hardware accessible local gates. In this paper, we provide a step-by-step recipe for simulatingthe paradigmatic two-dimensional Fermi-Hubbard model on a quantum computer using only local operations.
Albert Cabot, Federico Carollo, Igor Lesanovsky, Phys. Rev. Lett. 132 050801
A boundary time crystal is a quantum many-body system whose dynamics is governed by the competition between coherent driving and collective dissipation. It is composed of N two-level systems and features a transition between a stationary phase and an oscillatory one. The fact that the system is openallows one to continuously monitor its quantum trajectories and to analyze their dependence on parameterchanges. This enables the realization of a sensing device whose performance we investigate as a function ofthe monitoring time T and of the system size N.
Dongsheng Ding, Zhengyang Bai, Zongkai Liu, Baosen Shi, Guangcan Guo, Weibin Li, C. Stuart Adams, Sci. Adv. 10 eadl5893
It is challenging to probe ergodicity breaking trends of a quantum many-body system when dissipation inevitably damages quantum coherence originated from coherent coupling and dispersive two-body interactions. Rydberg atoms provide a test bed to detect emergent exotic many-body phases and nonergodic dynamics where thestrong Rydberg atom interaction competes with and overtakes dissipative effects even at room temperature.Here, we report experimental evidence of a transition from ergodic toward ergodic breaking dynamics in driven-dissipative Rydberg atomic gases.
Bruno Bertini, Cecilia De Fazio, Juan P. Garrahan, Katja Klobas, Phys. Rev. Lett. 132 120402
We study the nonequilibrium dynamics of the Floquet quantum East model (a Trotterized version of the kinetically constrained quantum East spin chain) at its “deterministic point,” where evolution is defined interms of CNOT permutation gates. We solve exactly the thermalization dynamics for a broad class of initialproduct states by means of “space evolution.”
Kohdai Kuroiwa, Ryuji Takagi, Gerardo Adesso, Hayata Yamasaki, Phys. Rev. Lett. 132 150201
Identifying what quantum-mechanical properties are useful to untap a superior performance in quantumtechnologies is a pivotal question. Quantum resource theories provide a unified framework to analyze andunderstand such properties, as successfully demonstrated for entanglement and coherence. While these areexamples of convex resources, for which quantum advantages can always be identified, many physicalresources are described by a nonconvex set of free states and their interpretation has so far remained elusive.Here we address the fundamental question of the usefulness of quantum resources without convexityassumption, by providing two operational interpretations of the generalized robustness measure in generalresource theories.
Xiao-Qiang Shao, Shi-Lei Su, Lin Li, Rejish Nath, Jin-Hui Wu, Weibin Li,Appl. Phys. Rev. 11 031320
Dense atom ensembles with Rydberg excitations display intriguing collective effects mediated by their strong, long-range dipole-dipole interactions. These collective effects, often modeled using Rydberg superatoms, have gained significant attention across various fields due to their potential applications in quantum information processing and quantum optics. In this review article, we delve into the theoretical foundations of Rydberg interactions and explore experimental techniques for their manipulation and detection.
Sean Greenaway, Adam Smith, Florian Mintert, Daniel Malz, Quantum 8 1263
We experimentally assess the suitability of transmon qubits with fixed frequencies and fixed interactions for the realization of analogue quantum simulations of spin systems. We test a set of necessary criteria for this goalon a commercial quantum processor using full quantum process tomography and more efficient Hamiltonian tomography. Significant single qubit errors at low amplitudes are identi-fied as a limiting factor preventing the realization of analogue simulations on currently avail-able devices.
Luke Causer, Felix Jung, Asimpunya Mitra, Frank Pollmann, Adam Gammon-Smith, Phys. Rev. Research 6 033062
The advent of near-term digital quantum computers could offer us an exciting opportunity to investigate quantum many-body phenomena beyond that of classical computing. To make the best use of the hardware available, it is paramount that we have methods that accurately simulate Hamiltonian dynamics for limited circuit depths. In this paper, we propose a method to classically optimize unitary brickwall circuits to approximate quantum time evolution operators.
Federico Girotti, Alfred Godley, Mădălin Guţă, J. Phys. A: Math. Theor. 57 245304
We revisit the problem of estimating an unknown parameter of a pure quantum state, and investigate ‘null-measurement’ strategies in which the experimenter aims to measure in a basis that contains a vector close to the true system state.Such strategies are known to approach the quantum Fisher information for models where the quantum Cramér-Rao bound (QCRB) is achievable but adetailed adaptive strategy for achieving the bound in the multi-copy setting has been lacking.
George-Bakewell-Smith, Federico Girotti, Mădălin Guţă, Juan P. Garrahan, Phys. Rev. Lett. 131 197101
Thermodynamic uncertainty relations (TURs) are general lower bounds on the size of fluctuations of dynamical observables. They have important consequences, one being that the precision of estimation of a current is limited by the amount of entropy production. Here, we prove the existence of general upper bounds on the size of fluctuations of any linear combination of fluxes (including all time-integrated currents or dynamical activities) for continuous-time Markov chains.
Gabriele Perfetto, Federico Carollo, Juan P. Garrahan, Igor Lesanovsky, Phys. Rev. Lett. 130 210402
We consider the quantum nonequilibrium dynamics of systems where fermionic particles coherently hop on a one-dimensional lattice and are subject to dissipative processes analogous to those of classical reaction-diffusion models. Particles can either annihilate in pairs or coagulate upon contact, and possibly also branch. In classical settings, the interplay between these processes and particle diffusion leads to critical dynamics as well as to absorbing-state phase transitions. Here, we analyze the impact of coherent hopping and of quantum superposition, focusing on the so-called reaction-limited regime.
Nico Kirchner, Darragh Millar, Babatunde M. Ayeni, Adam Smith, Joost K. Slingerland, Frank Pollmann, Phys. Rev. B. 107 195129
Two-dimensional systems such as quantum spin liquids or fractional quantum Hall systems exhibit anyonic excitations that possess more general statistics than bosons or fermions. This exotic statistics makes it challenging to solve even a many-body system of non-interacting anyons. We introduce an algorithm that allows to simulate anyonic tight-binding Hamiltonians on two-dimensional lattices. The algorithm is directly derived from the low energy topological quantum field theory and is suited for general Abelian and non-Abelian anyon models. As concrete examples, we apply the algorithm to study the energy level spacing statistics,which reveals level repulsion for freesemions, Fibonacci anyons, and Ising anyons.
Nikita Astrakhantsev, Sheng-Hsuan Lin, Frank Pollmann, Adam Smith, Phys. Rev. Research 5 033187
Simulating time evolution of generic quantum many-body systems using classical numerical approaches has an exponentially growing cost either with evolution time or with the system size. In this work we present a polynomially scaling hybrid quantum-classical algorithm for time evolving a one-dimensional uniform systemin the thermodynamic limit. This algorithm uses a layered uniform sequential quantum circuit as a variational Ansatz to represent infinite translation-invariant quantum states. We show numerically that this Ansatz requires anumber of parameters polynomial in the simulation time for a given accuracy.
Marcel Cech, Igor Lesanovsky, Federico Carollo, Phys. Rev. Lett. 131 120401
Quantum computers have recently become available as noisy intermediate-scale quantum devices. Already these machines yield a useful environment for research on quantum systems and dynamics. Building on this opportunity, we investigate open-system dynamics that are simulated on a quantum computer by coupling a system of interest to an ancilla.
Benjamin Yadin, Benjamin Morris, Kay Brandner, Phys. Rev. Res. 5 033018
Quantum systems of indistinguishable particles are commonly described using the formalism of second quantization, which relies on the assumption that any admissible quantum state must be either symmetric or antisymmetric under particle permutations. Coherence-induced many-body effects such as superradiance, however, can arise even in systems whose constituents are not fundamentally indistinguishable as long as all relevant dynamical observables are permutation-invariant. Such systems are not confined to symmetric or antisymmetric states and therefore require a different theoretical approach. Focusing on noninteracting systems, here we combine tools from representation theory and thermodynamically consistent master equations to develop such a framework.
Luke Causer, Mari Carmen Bañuls, Juan P. Garrahan, Phys. Rev. Lett. 128 090605
Recent work has shown the effectiveness of tensor network methods for computing large deviation functions in constrained stochastic models in the infinite time limit. Here we show that these methods can also be used to study the statistics of dynamical observables at arbitrary finite time.
Trystan Surawy-Stepney, Jonas Kahn, Richard Kueng, Madalin Guţă,Quantum 6 844
We propose and investigate a new method of quantum process tomography (QPT) which we call projected least squares (PLS). In short, PLS consists of first computing the least-squares estimator of the Choi matrix of an unknown channel, and subsequently projecting it onto the convex set of Choi matrices.
Chris Nill, Kay Brandner, Beatriz Olmos, Federico Carollo, Igor Lesanovsky,Phys. Rev. Lett. 129 243202
When atoms are excited to high-lying Rydberg states they interact strongly with dipolar forces. The resulting state-dependent level shifts allow us to study many-body systems displaying intriguing nonequilibrium phenomena, such as constrained spin systems, and are at the heart of numerous technological applications, e.g., in quantum simulation and computation platforms. Here, we show that these interactions also have a significant impact on dissipative effects caused by the inevitable coupling of Rydberg atoms to the surrounding electromagnetic field.
Federico Girotti , Merlijn van Horssen, Raffaella Carbone, Mădălin Guţă, J. Math. Phys. 63 062202
The theory of quantum jump trajectories provides a new framework for understanding dynamical phase transitions in open systems. A candidate for such transitions is the atom maser, which for certain parameters exhibits strong intermittency in the atom detection counts and has a bistable stationary state. Although previous numerical results suggested that the “free energy” may not be a smooth function, we show that the atom detection counts satisfy a large deviations principle and, therefore, we deal with a phase crossover rather than a genuine phase transition. We argue, however, that the latter occurs in the limit of an infinite pumping rate. As a corollary, we obtain the central limit theorem for the counting process.
E. K. Twyeffort Irish, A. D. Armour, Phys. Rev. Lett. 129 183603
The crossover from quantum to semiclassical behavior in the seminal Rabi model of light-matter interaction still, surprisingly, lacks a complete and rigorous understanding. A formalism for deriving the semiclassical model directly from the quantum Hamiltonian is developed here.
Yu-Jie Liu, Kirill Shtengel, Adam Smith, Frank Pollmann, PRX Quantum 3 040315
The finding of physical realizations of topologically ordered states in experimental settings, from condensed matter to artificial quantum systems, has been the main challenge en route to utilizing their unconventional properties. We show how to realize a large class of topologically ordered states and simulate their quasiparticle excitations on a digital quantum computer. To achieve this, we design a set of linear-depth quantum circuits to generate ground states of general string-net models together with unitary open-string operators to simulate the creation and braiding of Abelian and non-Abelian anyons. We show that the Abelian (non-Abelian) unitary string operators can be implemented with a constant- (linear-) depth quantum circuit. Our scheme allows us to directly probe characteristic topological properties, including topological entanglement entropy, braiding statistics, and fusion channels of anyons. Moreover,this set of efficiently prepared topologically ordered states has potential applications in the development of fault-tolerant quantum computers.
Federico Carollo, Antonio Lasanta, Igor Lesanovsky, Phys. Rev. Lett. 127 060401
Ergodicity breaking and slow relaxation are intriguing aspects of nonequilibrium dynamics both in classical and quantum settings. These phenomena are typically associated with phase transitions, e.g., the emergence of metastable regimes near a first-order transition or scaling dynamics in the vicinity of critical points. Despite being of fundamental interest the associated divergent timescales are a hindrance when trying to explore steady-state properties. Here we show that the relaxation dynamics of Markovian open quantum systems can be accelerated exponentially by devising an optimal unitary transformation that is applied to the quantum system immediately before the actual dynamics.
Benjamin Yadin, Benjamin Morris, Gerardo Adesso, Nat. Commun. 12 1471
The classical Gibbs paradox concerns the entropy change upon mixing two gases. Whetheran observer assigns an entropy increase to the process depends on their ability to distinguishthe gases. A resolution is that an “ignorant” observer, who cannot distinguish the gases, hasno way of extracting work by mixing them. Moving the thought experiment into the quantumrealm, we reveal new and surprising behaviour: the ignorant observer can extract work frommixing different gases, even if the gases cannot be directly distinguished.
Filippo M. Gambetta, Chi Zhang, Markus Hennrich, Igor Lesanovsky, Weibin Li, Phys. Rev. Lett. 126 233404
Conical intersections between electronic potential energy surfaces are paradigmatic for the study ofnonadiabatic processes in the excited states of large molecules. However, since the corresponding dynamicsoccurs on a femtosecond timescale, their investigation remains challenging and requires ultrafastspectroscopy techniques. We demonstrate that trapped Rydberg ions are a platform to engineer conicalintersections and to simulate their ensuing dynamics on larger length scales and timescales of the order ofnanometers and microseconds, respectively; all this in a highly controllable system.
K. J. Satzinger et al., Science 374 1237-1241
The discovery of topological order has revised the understanding of quantum matter and provided thetheoretical foundation for many quantum error-correcting codes. Realizing topologically ordered states has proven to be challenging in both condensed matter and synthetic quantum systems. We prepared the ground state of the toric code Hamiltonian using an efficient quantum circuit on a superconductingquantum processor. We measured a topological entanglement entropy near the expected value of –ln2 and simulated anyon interferometry to extract the braiding statistics of the emergent excitations.
D. Cattiaux. I. Golokolenov, S. Kumar, M. Sillanpää, L. Mercier de Lépinay,R. R. Gazizulin, X. Zhou. A. D. Armour, O. Bourgeois, A. Fefferman, E. Collin,Nat Commun. 12 6182
The nature of the quantum-to-classical crossover remains one of the most challenging open question of Science to date. In this respect, moving objects play a specific role. Pioneering experiments over the last few years have begun exploring quantum behaviour of micron-sized mechanical systems, either by passively cooling single GHz modes, or by adapting lasercooling techniques developed in atomic physics to cool specific low-frequency modes far below the temperature of their surroundings. Here instead we describe a very different approach, passive cooling of a whole micromechanical system down to 500 μK, reducing the average number of quanta in the fundamental vibrational mode at 15 MHz to just 0.3 (with even lower values expected for higher harmonics); the challenge being to be still able to detect the motion without disturbing the system noticeably.
Nicola Pancotti, Giacomo Giudice, J. Ignacio Cirac, Juan P. Garrahan, Mari Carmen Bañul, Phys. Rev. X 10 021051
We study in detail the properties of the quantum East model, an interacting quantum spin chain inspired by simple kinetically constrained models of classical glasses. Through a combination of analytics, exact diagonalization, and tensor-network methods, we show the existence of a transition, from a fast to a slow thermalization regime, which manifests itself throughout the spectrum.
Chi Zhang, Fabian Pokorny, Weibin Li, Gerard Higgins, Andreas Pöschl, Igor Lesanovsky, Markus Hennrich, Nature 580 345-349
Generating quantum entanglement in large systems on timescales much shorter than the coherence time is key to powerful quantum simulation and computation. Trapped ions are among the most accurately controlled and best isolated quantum systems with low-error entanglement gates operated within tens of microseconds using the vibrational motion of few-ion crystals. To exceed the level of complexity tractable by classical computers the main challenge is to realize fast entanglement operations in crystals made up of many ions. The strong dipole–dipole interactions in polar molecule and Rydberg atom systems allow much faster entangling gates, yet stable state-independent confinement comparable with trapped ions needs to be demonstrated in these systems. Here we combine the benefits of these approaches: we report a two-ion entangling gate with 700-nanosecond gate time that uses the strong dipolar interaction between trapped Rydberg ions, which we use to produce a Bell state with 78 per cent fidelity.
Benjamin Morris, Benjamin Yadin, Matteo Fadel, Tilman Zibold,Philipp Treutlein, Gerardo Adesso, Phys. Rev. X 10 041012
The existence of fundamentally identical particles represents a foundational distinction between classicaland quantum mechanics. Because of their exchange symmetry, identical particles can appear to beentangled—another uniquely quantum phenomenon with far-reaching practical implications. However, along-standing debate has questioned whether identical particle entanglement is physical or merely a mathematical artifact. In this work, we provide such particle entanglement with a consistent theoreticaldescription as a quantum resource in processes frequently encountered in optical and cold atomic systems.
M. Guţǎ, J. Kahn, R. Kueng, J. A .Tropp, J. Phys. A: Math. Theor. 53 204001
Projected least squares is an intuitive and numerically cheap technique for quantum state tomography: compute the least-squares estimator and project it onto the space of states. The main result of this paper equips this point estimator with rigorous, non-asymptotic convergence guarantees expressed interms of the trace distance.
Zhengyang Bai, Charles S. Adams, Guoxiang Huang, Weibin Li,Phys. Rev. Lett. 125 263605
We study dispersive optical nonlinearities of short pulses propagating in high number density, warm atomic vapors where the laser resonantly excites atoms to Rydberg P states via a single-photon transition. Three different regimes of the light-atom interaction, dominated by either Doppler broadening, Rydberg atom interactions, or decay due to thermal collisions between ground state and Rydberg atoms, are found.
D. Cattiaux, X. Zhou, S. Kumar, I. Golokolenov, R. R. Gazizulin, A. Luck, L. Mercier de Lépinay, M. Sillanpää, A. D. Armour, A. Fefferman, E. Collin, Phys. Rev. Research 2 033480
We explore the nonlinear dynamics of a cavity optomechanical system. Our realization consisting of a drumhead nanoelectromechanical resonator (NEMS) coupled to a microwave cavity allows for a nearly idealplatform to study the nonlinearities arising purely due to radiation-pressure physics. Experiments are performed under a strong microwave Stokes pumping which triggers mechanical self-sustained oscillations. We analyze the results in the framework of an extended nonlinear optomechanical theory and demonstrate that quadraticand cubic coupling terms in the opto-mechanical Hamiltonian have to be considered.
F. M. Gambetta, F. Carollo, M. Marcuzzi, J. P. Garrahan, I. Lesanovsky,Phys. Rev. Lett. 122 015701
We establish a link between metastability and a discrete time-crystalline phase in a periodically driven open quantum system. The mechanism we highlight requires neither the system to display any microscopic symmetry nor the presence of disorder, but relies instead on the emergence of a metastable regime.
Ryuji Takagi, Bartosz Regula, Kaifeng Bu, Zi-Wen Liu, Gerardo Adesso,Phys. Rev. Lett. 122 140402
One of the central problems in the study of quantum resource theories is to provide a given resource withan operational meaning, characterizing physical tasks in which the resource can give an explicit advantage over all resourceless states. We show that this can always be accomplished for all convex resource theories.We establish in particular that any resource state enables an advantage in a channel discrimination task, allowing for a strictly greater success probability than any state without the given resource.
Federico Carollo, Edward Gillman, Hendrik Weimer, Igor Lesanovsky, Phys. Rev. Lett. 123 100604
The contact process is a paradigmatic classical stochastic system displaying critical behavior even in one dimension. It features a nonequilibrium phase transition into an absorbing state that has been widely investigated and shown to belong to the directed percolation universality class. When the same process isconsidered in a quantum setting, much less is known. So far, mainly semiclassical studies have been conducted and the nature of the transition in low dimensions is still a matter of debate. Also, from anumerical point of view, from which the system may look fairly simple - especially in one dimension - results are lacking. In particular, the presence of the absorbing state poses a substantial challenge, whichappears to affect the reliability of algorithms targeting directly the steady state. Here we perform real-time numerical simulations of the open dynamics of the quantum contact process and shed light on the existence and on the nature of an absorbing state phase transition in one dimension.
Anirudh Acharya, Theodore Kypraios, Mădălin Guţă, J. Phys. A: Math. Theor. 52 234001
As quantum tomography is becoming a key component of the quantum engineering toolbox, there is a need for a deeper understanding of the multitude of estimation methods available. Here we investigate and compare several such methods: maximum likelihood, least squares, generalised least squares, positive least squares, thresholded least squares and projected least squares. The common thread of the analysis is that each estimator projects the measurement data onto a parameter space with respect to a specific metric, thus allowingus to study the relationships between different estimators.
Mattia Mantovani, Andrew D. Armour, Wolfgang Belzig, Gianluca Rastelli,Phys. Rev. B 99 045442
We study the dynamical multistability of a solid-state single-atom laser implemented in a quantum-dot spinvalve. The system is formed by a resonator that interacts with a two-level system in a dot in contact with two ferromagnetic leads of antiparallel polarization. We show that a spin-polarized current provides high-efficiency pumping leading to regimes of multistable lasing, in which the Fock distribution of the oscillator displays a multipeaked distribution.
Zhihao Lan, Merlijn van Horssen, Stephen Powell, Juan P. Garrahan,Phys. Rev. Lett. 121 040603
One of the general mechanisms that give rise to the slow cooperative relaxation characteristic of classical glasses is the presence of kinetic constraints in the dynamics. Here we show that dynamical constraints can similarly lead to slow thermalization and metastability in translationally invariant quantum many-body systems.
Matthew Levitt, Mădălin Guţă, Hendra I. Nurdin, Automatica 90 255-262
We investigate system identification for general quantum linear systems in the situation where the input field is prepared as stationary (squeezed) quantum noise. In this regime the output field is characterised by the power spectrum, which encodes covariance of the output state. We address which parameters can be identified from the power spectrum and how to construct a system realisation from the power spectrum.
Tom Oakes, Stephen Powell, Claudio Castelnovo, Austen Lamacraft,Juan P. Garrahan, Phys. Rev. B 98 064302
We study the connection between the phase behavior of quantum dimers and the dynamics of classical stochasticdimers. At the so-called Rokhsar-Kivelson (RK) point a quantum dimer Hamiltonian is equivalent to the Markov generator of the dynamics of classical dimers. A less well understood fact is that away from the RK point thequantum-classical connection persists: in this case the Hamiltonian corresponds to a nonstochastic “tilted” operatorthat encodes the statistics of time-integrated observables of the classical stochastic problem. This implies a directrelation between the phase behavior of quantum dimers and properties of ensembles of stochastic trajectories of classical dimers.
C.G. Wade, M. Marcuzzi, E. Levi, J.M. Kondo, I. Lesanovsky, C.S. Adams, K.J. Weatherill, Nat. Commun. 9 3567
There are few demonstrated examples of phase transitions that may be driven directly by terahertz frequency electric fields, and those that are known require field strengths exceeding 1MV/cm. Here we report a non-equilibrium phase transition driven by a weak, continuous-wave terahertz electric field. The system consists of room temperature caesium vapour under continuous optical excitation to a high-lying Rydberg state, which is resonantly coupled to a nearby level by the terahertz electric field.
Dynamics of many-body quantum synchronisation
C. Davis-Tilley, C. K. Teoh, A. D. Armour, New J. Phys. 20 113002
We analyse the properties of the synchronisation transition in a many-body system consisting of quantum van der Pol oscillators with all-to-all coupling using a self-consistent mean-field method. We find that the synchronised state, which the system can access for oscillator couplings above acritical value, is characterised not just by a lower phase uncertainty than the corresponding unsynchronised state, but also a higher number uncertainty.
Kay Brandner, Ville F. Maisi, Jukka P. Pekola, Juan P. Garrahan, Christian Flindt, Phys. Rev. Lett. 118 180601
Statistical physics provides the concepts and methods to explain the phase behavior of interacting many-body systems. Investigations of Lee-Yang zeros - complex singularities of the free energy in systems of finite size - have led to a unified understanding of equilibrium phase transitions. The ideas of Lee and Yang, however, are not restricted to equilibrium phenomena. Recently, Lee-Yang zeros have been used to characterize nonequilibrium processes such as dynamical phase transitions in quantum systems after a quench or dynamic order-disorder transitions in glasses. Here, we experimentally realize a scheme for determining Lee-Yang zeros in such nonequilibrium settings.
Gerard Higgins, Weibin Li, Fabian Pokorny, Chi Zhang, Florian Kress, Christine Maier, Johannes Haag, Quentin Bodart, Igor Lesanovsky, Markus Hennrich, Phys. Rev. X 7 021038
Trapped Rydberg ions are a promising new system for quantum information processing. They have the potential to join the precise quantum operations of trapped ions and the strong, long-range interactions between Rydberg atoms. Combining the two systems is not at all straightforward. Rydberg atoms are severely affected by electric fields which may cause Stark shifts and field ionization, while electric fieldsare used to trap ions. Thus, a thorough understanding of the physical properties of Rydberg ions due to the trapping electric fields is essential for future applications. Here, we report the observation of two fundamental trap effects. First, we investigate the interaction of the Rydberg electron with the trapping electric quadrupole fields which leads to Floquet sidebands in the excitation spectra. Second, we report onthe modified trapping potential in the Rydberg state compared to the ground state that results from thestrong polarizability of the Rydberg ion.
Alexander Streltsov, Gerardo Adesso, Martin B. Plenio, Rev. Mod. Phys. 89 041003
The coherent superposition of states, in combination with the quantization of observables, representsone of the most fundamental features that mark the departure of quantum mechanics from the classical realm. Quantum coherence in many-body systems embodies the essence of entanglement and is anessential ingredient for a plethora of physical phenomena in quantum optics, quantum information, solid state physics, and nanoscale thermodynamics. In recent years, research on the presence and functional role of quantum coherence in biological systems has also attracted considerable interest. Despite the fundamental importance of quantum coherence, the development of a rigorous theory of quantum coherence as a physical resource has been initiated only recently. This Colloquium discusses and reviews the development of this rapidly growing research field that encompasses the characterization, quantification, manipulation, dynamical evolution, and operational application ofquantum coherence.
Madalin Guţă, Jukka Kiukas, J. Math. Phys. 58 052201
This paper deals with the problem of identifying and estimating dynamical parameters of continuous-time Markovian quantum open systems, in the input-output formalism. First, we characterise the space of identifiable parameters for ergodic dynamics, assuming full access to the output state for arbitrarily long times, and show that the equivalence classes of undistinguishable parameters are orbits of a Lie group acting on the space of dynamical parameters. Second, we define an information geometric structure on this space, including a principal bundle given by the action of the group, as well as a compatible connection, and a Riemannian metric based on the quantum Fisher information of the output.
Zhihao Lan, Stephen Powell, Phys. Rev. B 96 115140
We use exact diagonalization to study the eigenstate thermalization hypothesis (ETH) in the quantum dimermodel on the square and triangular lattices. Due to the nonergodicity of the local plaquette-flip dynamics, the Hilbert space, which consists of highly constrained close-packed dimer configurations, splits into sectors characterized by topological invariants.
Katarzyna Macieszczak, Mădălin Guţă, Igor Lesanovsky, Juan P. Garrahan,Phys. Rev. Lett. 116 240404
By generalizing concepts from classical stochastic dynamics, we establish the basis for a theory of metastability in Markovian open quantum systems. Partial relaxation into long-lived metastable states - distinct from the asymptotic stationary state - is a manifestation of a separation of time scales due to asplitting in the spectrum of the generator of the dynamics. We show here how to exploit this spectral structure to obtain a low dimensional approximation to the dynamics in terms of motion in a manifold of metastable states constructed from the low-lying eigenmatrices of the generator.
Matteo Marcuzzi, Michael Buchhold, Sebastian Diehl, Igor Lesanovsky,Phys. Rev. Lett. 116 245701
Stochastic processes with absorbing states feature examples of nonequilibrium universal phenomena. While the classical regime has been thoroughly investigated in the past, relatively little is known about the behavior of these nonequilibrium systems in the presence of quantum fluctuations. Here, we theoretically address such a scenario in an open quantum spin model which, in its classical limit, undergoes a directed percolation phase transition.
The University of NottinghamUniversity Park Nottingham, NG7 2RD