Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session D01: Open Quantum Systems IFocus

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Sponsoring Units: DAMOP Chair: Igor Lesanovsky Room: 103 
Monday, March 2, 2020 2:30PM  3:06PM 
D01.00001: Discovering feedback strategies for open quantum systems via deep reinforcement learning Invited Speaker: Florian Marquardt Recent rapid advances in deep neural networks are helping to revolutionize science and technology. In this talk, I will describe how neural networks can discover from scratch feedback strategies to help control open quantum systems, by exploiting the toolbox of reinforcement learning. A fewqubit quantum memory can be protected against decoherence via quantum error correction strategies that have been autonomously constructed in this way. Additional illustrative examples from our recent research include reinforcement learning applied to systems from the domain of cavity quantum electrodynamics and to arrays of coupled modes. 
Monday, March 2, 2020 3:06PM  3:18PM 
D01.00002: Integrability Meets Dissipation: Critical Dynamics of Open Driven Systems Daniel Paz, Mohammad Maghrebi Driven quantum systems coupled to an environment are generally nonintegrable and typically exhibit relaxational dynamics. We investigate the paradigmatic open Dicke model which describes collective lightmatter interactions subject to dissipation. In a certain limit (at large detuning), this model is governed by an effective drivendissipative Ising model in a transverse field, which is integrable in the absence of dissipation. In the limit of weak dissipation though, integrability is only weakly broken. We show that, in this regime, the system undergoes a dynamical crossover from relaxational dynamics to underdamped critical dynamics, each described by a distinct dynamical exponent. We identify these critical behaviors with the infiniterange classical (stochastic) and quantum (unitary) Ising models at finite temperature, respectively. These results are obtained through a nonequilibrium quantumtoclassical mapping in addition to an efficient numerical analysis that exploits the permutation symmetry. 
Monday, March 2, 2020 3:18PM  3:30PM 
D01.00003: Dissipative generation of highly entangled states of light and matter CatalinMihai Halati, Ameneh Sheikhan, Helmut Ritsch, Corinna Kollath We investigate the full quantum evolution of ultracold interacting bosonic atoms confined to a chain geometry and coupled to the field of an optical cavity. Extending the timedependent matrix product state techniques to capture the global coupling to the cavity mode and the open nature of the cavity, we fully include the lightatom entanglement. We examine the long time behavior of the system beyond the meanfield elimination of the cavity field. We show that in the selforganized phase the steady state consists in a mixture of the meanfield predicted density wave states and coherent states with lower photon number, with a large entanglement between the atomic and photonic degrees of freedom. In the regime of large dissipation strengths we develop a variant of the manybody adiabatic elimination technique and obtain a highly entangled steady state with a fully mixed atomic sector. We observe numerically the crossover from the density wave state towards the fully mixed state. 
Monday, March 2, 2020 3:30PM  3:42PM 
D01.00004: Boundary dissipation in Ising CFT umar javed, Michael Kolodrubetz Boundary conformal field theory (CFT) is one of few exact analytical tools for the study of quantum nonequilibrium systems, yet so far has been primarily restricted to closed Hamiltonian systems. We study one of the simplest CFTs, namely thequantum critical Ising model with boundary dephasing along the direction of the most relevant boundary operator. Naïve scaling arguments suggest that such dissipation should be marginal. Numerically for large systems, we find that boundary spin in the presence of dissipation, with or without boundary field, depolarizes and reaches a nonequilibrium steady state (NESS). In the presence of RGrelevant boundary term, scaling functions appear unmodified, suggesting that the dissipation is marginally irrelevant in that case. In the absence of boundary field, we study modifications of the Zeno effect by Keldysh RG and numerical calculations of the twotime correlation function of the boundary spin. Finally, we comment on potential application to other CFTs 
Monday, March 2, 2020 3:42PM  3:54PM 
D01.00005: On the nature of the nonequilibrium phase transition in the nonMarkovian driven Dicke model Rex Lundgren, Alexey V. Gorshkov, Mohammad Maghrebi The Dicke model exhibits a phase transition to a superradiant phase with a macroscopic population of photons and is realized in multiple settings in open quantum systems. In this work, we study a variant of the Dicke model where the cavity mode is lossy due to the coupling to a Markovian environment while the atomic mode is coupled to a colored bath. We find a simple effective theory for this model allowing us to derive analytical expressions for various critical exponents, including those, such as the dynamical critical exponent, that have not been previously considered. We find excellent agreement with previous numerical results when the nonMarkovian bath is at zero temperature; however, contrary to these studies, our lowfrequency approach reveals that the same exponents govern the critical behavior when the colored bath is at finite temperature unless the chemical potential is zero. Furthermore, we show that the superradiant phase transition is classical in nature, while it is genuinely nonequilibrium. Finally, we consider finitesize effects at the phase transition and identify the finitesize scaling exponents, unlocking a rich behavior in both statics and dynamics of the photonic and atomic observables. 
Monday, March 2, 2020 3:54PM  4:06PM 
D01.00006: Anomalous exceptional point and nonMarkovian Purcell effect at threshold in 1D open quantum systems Savannah Garmon, Gonzalo Ordonez, Naomichi Hatano We show that when a quantum emitter is coupled near threshold to a generic 1D continuum with a van Hove singularity in the density of states, a characteristic spectral configuration appears involving a bound state, a resonance state and an antiresonance state, as well as several exceptional points (EPs). At one EP appearing below the threshold, the resonance and antiresonance states coalesce while the bound state instead experiences an avoided crossing. Meanwhile, if one considers the limit in which the coupling g vanishes, all three states converge on the continuum threshold itself. For small g values the eigenvalue and norm of each of these states can be expanded in a Puiseux expansion in terms of powers of g^{2/3}, which suggests a third order EP occurs at the threshold. However, in the actual g ––> 0 limit, only two discrete states in fact coalesce as the system can be reduced to a 2x2 Jordan block; the third state instead merges with the continuum. We further demonstrate the influence of the EP on nonMarkovian dynamics characterizing the relaxation process of the quantum emitter in the vicinity of the threshold. 
Monday, March 2, 2020 4:06PM  4:18PM 
D01.00007: NonMarkovian collective emission from macroscopically separated emitters Kanupriya Sinha, Pierre Meystre, Pablo Solano We study the collective radiative decay of a system of two twolevel emitters coupled to a onedimensional waveguide in a regime where their separation is comparable to the coherence length of a spontaneously emitted photon. The electromagnetic field propagating in the cavitylike geometry formed by the emitters exerts a retarded backaction on the system leading to strongly nonMarkovian dynamics. The collective spontaneous emission rate of the emitters exhibits an enhancement or inhibition beyond the usual Dicke super and subradiance due to a selfconsistent coherent timedelayed feedback. 
Monday, March 2, 2020 4:18PM  4:30PM 
D01.00008: Analysis of matterwave emission dynamics and polariton formation in a quantumemitter array coupled to a band structure Alfonso Lanuza, Michael A Stewart, Joonhyuk Kwon, Youngshin Kim, Dominik Schneble Recent progress on a spontaneous emitter for atomic matter waves [1] has enabled studies of exotic emission phenomena in bandgap materials. Here we present analytic solutions for the single excitation dynamics in a 1D sinusoidal lattice potential and a finite or infinite array of emitters. The calculated phenomenology ranges from nonMarkovian decay into bound states for the case of one emitter, to the emergence of an additional band structure of polaritons for the case of infinite emitters. 
Monday, March 2, 2020 4:30PM  4:42PM 
D01.00009: Generalized Theory of Pseudomodes for Exact Descriptions of NonMarkovian Quantum Processes Graeme Pleasance, Barry M Garraway, Francesco Petruccione In this talk we develop a general approach to analyzing the nonMarkovian behavior of an open quantum system in a setting where the interaction is modelled by a generalized class of spectral density function. By introducing an auxiliary model in which the environment is replaced by a set of discrete bosonic modes exhibiting Markovian dissipative interactions, we prove the validity of the conditions allowing for a nonMarkovian open system dynamics to be amended to a Markovian description, where the dynamics in the latter is governed by an exact master equation of Lindblad form. Initially we apply our result to obtain a generalization of the pseudomode method [1] in cases where the spectral density function has a Lorentzian structure. For many other types of spectral density function, we extend this result to show that an open system dynamics may be modelled physically using discrete modes which admit a nonHermitian coupling to the system, and for such cases determine the equivalent master equation to no longer be of Lindblad form. For applications involving two discrete modes, we demonstrate how to convert between pathological and Lindblad forms of the master equation via [1]. 
Monday, March 2, 2020 4:42PM  4:54PM 
D01.00010: Stabilization of Fractional Quantum Hall States of Light: Effects of Fractionalization Pavel Kurilovich, Jose Lebreuilly, Vladislav Kurilovich, Steven Girvin Recently, the possibility of realizing stronglycorrelated states of matter with photons has started to become an experimental reality. In combination with artificial gauge fields this paves the way to simulation of fractional quantum Hall states with light. A major hindrance to such simulations is the inevitable dissipation of photons into the environment which creates holes in the correlated state. The leakage of light can be counteracted by a stabilization scheme in which lost particles are irreversibly refilled. We investigate the efficiency of such a scheme for the preparation of a photonic Laughlin state at halffilling. We explore the limitations imposed by the ability of holes in the Laughlin state to fractionalize into several spatially separated quasiholes. Quasiholes correspond to the absence of a fraction of a particle and thus cannot be refilled efficiently. We find that the fractionalization drastically restricts the steadystate fidelity of the Laughlin state and leads to a long relaxation time in the system. 
Monday, March 2, 2020 4:54PM  5:06PM 
D01.00011: Transport through a quantum critical system: A thermodynamically consistent approach Christopher Wächtler, Gernot Schaller Quantum phase transitions are striking phenomena of manybody systems at low temperatures. The current experimental feasibilities enable us to bring such critical systems out of equilibrium in a controlled manner. Due to the vanishing energy gap above the ground state [1], appropriate methods have to be developed to study the dynamics and thermodynamical applications. We show for a class of critical systems connected to several nonMarkovian heat baths that by combining the reaction coordinate mapping [2] and a polaron technique [3] it is possible to find analytic expressions of the reduced system dynamics in the vicinity of quantum critical points, which are consistent with the laws of thermodynamics. As an example we consider the LipkinMeshkovGlick model in a transport setup, where the underlying phase transition manifests itself in the heat transfer statistics. 
Monday, March 2, 2020 5:06PM  5:18PM 
D01.00012: Deconstructing Effective NonHermitian Dynamics in Quadratic Bosonic Hamiltonians Vincent Flynn, Emilio Cobanera, Lorenza Viola Unlike their fermionic counterparts, the dynamics of Hermitian, bosonic, quadratic Hamiltonians are governed by a generally nonHermitian Bogoliubov deGennes Hamiltonian. This effective nonHermiticity gives rise to two distinct dynamical phases: one with bounded evolution of observables in time, and one without. We elucidate the physical manifestations of the transitions between these two dynamical phases. We show how a generalized notion of PT symmetry may be used to classify the mechanisms by which this transition can occur. By combining this understanding with tools from Krein stability theory, we derive an indicator of dynamical phase boundaries inspired by the notion of phase rigidity in nonHermitian quantum systems. As an example, we fully characterize the dynamical phase diagram of a bosonic analogue to the KitaevMajorana chain under a wide class of boundary conditions, and further establish a connection between phasedependent transport properties and the onset of instability. We discuss potential applications of our techniques to quadratic Lindblad dynamics. 
Monday, March 2, 2020 5:18PM  5:30PM 
D01.00013: NonHermitian Linear Response Theory Lei Pan, Xin Chen, Yu Chen, Hui Zhai Linear response theory lies at the heart of quantum manybody physics because it builds up connections between the dynamical response to an external probe and correlation functions at equilibrium. Here we consider the dynamical response of a Hermitian system to a nonHermitian probe, and we develop a nonHermitian linear response theory that can also relate this dynamical response to equilibrium properties. As an application of our theory, we consider the realtime dynamics of momentum distribution induced by onebody and twobody dissipations. We find that, for many cases, the dynamics of momentum occupation and the width of momentum distribution obey the same universal function, governed by the singleparticle spectral function. We also find that, for critical state with no welldefined quasiparticles, the dynamics are slower than normal state and our theory provides a model independent way to extract the critical exponent. We apply our results to analyze recent experiment on the BoseHubbard model and find surprising good agreement between theory and experiment. We also propose to further verify our theory by carrying out a similar experiment on a onedimensional Luttinger liquid. 
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