Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session P27: NonEquilibrium Physics in AMO Systems I: Quenches and ThermalizationFocus

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Sponsoring Units: DAMOP DCOMP Chair: Nikolai Sinitsyn, Los Alamos National Laboratory Room: LACC 404B 
Wednesday, March 7, 2018 2:30PM  3:06PM 
P27.00001: Recurrences in an isolated quantum manybody system Invited Speaker: Joerg Schmiedmayer The evolution of an isolated quantum system is unitary. If the system becomes large and its constituents interact one is not able to follow the evolution of the complex many body eigenstates. A system out of equilibrium it will ‘relax’ for practical observables [1]. The steady state reached can be described by a Generalized Gibbs Ensemble [2]. In our present study [3] we ask the question if, under which conditions the initial coherence can be revived. 
Wednesday, March 7, 2018 3:06PM  3:18PM 
P27.00002: Prethermalization: Quantum and dynamical universality in quench dynamics Mohammad F. Maghrebi A longlived prethermal state may emerge upon a sudden quench of a quantum system. In this talk, I consider a quantum quench of an initial critical state, and show that the resulting prethermal state exhibits a genuinely quantum and dynamical universal behavior. Specifically, I consider a scenario where the “speed of light” characterizing the propagation of local perturbations is suddenly quenched at criticality. I also briefly discuss how the system approaches the prethermal state in a universal way described by a new exponent that characterizes a kind of quantum aging. 
Wednesday, March 7, 2018 3:18PM  3:30PM 
P27.00003: Why Quasiequilibrium, Quasistationary and Prethermalization phases are related  Relaxation in strongly nonlinear systems Surajit Sen, Michelle Przedborski, Rahul Kashyap, Tyler Barrett, Guo Deng In 2004, we introduced the concept of the quasiequilibrium state in strongly nonlinear classical many particle systems. Later in the same year, Berges et al introduced a similar state and named it prethermalization. In recent experiments, this state, also called the quasistationary state, has been reported. We will discuss the properties of the quasiequilibrium phase for the FermiPastaUlamTsingou and Hertz systems and how equipartitioning can set in from the quasiequilibrium state. 
Wednesday, March 7, 2018 3:30PM  3:42PM 
P27.00004: Probing quantum phase transitions via quench dynamics in integrable and nearlyintegrable systems Paraj Titum, Joseph Iosue, Zhexuan Gong, Alexey Gorshkov The study of quantum phase transitions requires the preparation of a manybody system near its ground state, a task challenging for many experimental systems. The measurement of quench dynamics, on the other hand, is a routine practice now in most cold atomic platforms. Here we show that quintessential ingredients of quantum phase transitions can be probed directly with quench dynamics in many integrable and nearintegrable systems. As a paradigmatic example, we study a global quench in a transversefield Ising model with nearest and nextnearestneighbor interactions. When the model is integrable, we find nonanalytic behavior of a dynamical order parameter, which reflects the corresponding ground state quantum phase transition. This behavior persists at finite times in the presence of a weak integrability breaking perturbation. This nonanalyticity can be readily observed in experimental platforms such as trapped ions or Rydberg atoms. 
Wednesday, March 7, 2018 3:42PM  3:54PM 
P27.00005: Approximate Counterdiabatic Driving Protocols for Nonintegrable Quantum Systems Mohit Pandey, Dries Sels, David Campbell Noise and decoherence caused by the environment are two major challenges in applying adiabatic protocols to quantum technologies. Counterdiabatic (CD) driving protocols, which are also known as "shortcutstoadiabaticity," provide powerful alternatives for controlling a quantum system. These protocols allow one to change Hamiltonian parameters rapidly while still mimicking adiabatic dynamics. They have been shown to work well for a wide variety of systems, but it is exponentially hard to find exact CD protocols for nonintegrable quantum manybody systems. We study a method to develop approximate CD protocols which avoids exponential sensitivity to perturbations of the Hamiltonian. Our finitesize scaling of CD Hamiltonians reveals remarkable differences between integrable and nonintegrable quantum systems. We identify numerically different scaling regimes and show how they arise from the eigenstate thermalization hypothesis. 
Wednesday, March 7, 2018 3:54PM  4:06PM 
P27.00006: Quantum Joule expansion for the onedimensional Bose Hubbard model Jin Zhang, Yannick Meurice, Shanwen Tsai We study the quantum Joule expansion for a Bose Hubbard model in a one dimensional chain initially at unity filling. Particles are first confined to the left half of the system by an infinite barrier in the middle. At t = 0, the barrier is removed and particles begin to expand to the other half of the system. We consider both pure and thermal initial states. We observe revivals in small systems and these are quickly pushed to long times as the size of the system is increased. As an example, we study a system with 10 sites by exact diagonalization. Particle density and entanglement entropy equilibrate quickly during the expansion for comparable values for the hopping amplitude J and onsite repulsion U. We argue that the diagonal ensemble of the density matrix is similar to the canonical ensemble with an effective temperature. This effective temperature can be negative if the energy of the initial system is large enough. The finite size effects of this effective temperature are also studied by finite temperature density matrix renormalization group. 
Wednesday, March 7, 2018 4:06PM  4:18PM 
P27.00007: Nonadiabatic preparation of critical ground states using superluminal quenches Kartiek Agarwal, Ravindra Bhatt, Shivaji Sondhi We show that a space and timedependent quench protocol allows faster preparation of the ground state of critical theories than adiabatic protocols. Specifically, assuming the system initially resides in the ground state of a corresponding massive model, we show that a superluminallymoving `front' that locally quenches the mass, leaves behind it (in space) a state arbitrarily close to the ground state of the gapless model. The protocol takes time O(L) to produce the ground state of a system of size L^{d} (in d spatial dimensions), while a fully adiabatic protocol requires time O(L^{2}) to produce a state with exponential accuracy in L. We present exact results for such quenches in free theories of bosons and fermions, and discuss implications for systems with interactions and ultraviolet features. Finally, we discuss results for such (and related) quenches in arbitrary conformal field theories. 
Wednesday, March 7, 2018 4:18PM  4:30PM 
P27.00008: A Toy Model for Thermalization in Systems with Manybody Localization Phase Transition Oles Shtanko, Iman Marvian, Seth Lloyd The transition from ergodic to manybody localized (MBL) phase remains a challenge from both analytical and numerical perspective. The system thermalization involves an exponential number of degrees of freedom encoded in manybody quantum correlations. This leads to the enormous computational complexity of ergodic phase description and limits the ability of numerical simulations. To address this challenge, we propose a phenomenological model for thermalization of generic manybody systems with local manybody interactions. The method allows reducing the number of degrees of freedom from exponential to polynomial in system size. We construct an effective Hamiltonian and show numerically that the model exhibits a transition from WignerDyson to Poisson level statistics with an increase of the disorder strength. Also, we propose a method to derive the growth the bipartite quantum correlations in the model after a quench. Finally, we apply the results to MBL phase transition problem of interacting spinless fermions in the onedimensional lattice. 
Wednesday, March 7, 2018 4:30PM  4:42PM 
P27.00009: Microscopic justification of the eigenstate thermalization hypothesis (ETH) Nils Abeling, Stefan Kehrein The ETH explains how isolated quantum manybody systems thermalize by proposing that each energy eigenstate is already thermal [1]. ETH has been shown to be essential to the understanding of quantum chaos and implies various important thermodynamic relations [2]. 
Wednesday, March 7, 2018 4:42PM  4:54PM 
P27.00010: Nonergodic Quantum Dynamics in Highly Excited States of a Kinematically Constrained Rydberg Chain Christopher Turner, Alexios Michailidis, Maksym Serbyn, Dmitry Abanin, Zlatko Papic Motivated by recent experimental advances in Rydberg atom quantum simulators, we study the possibility of quantum coherent manybody dynamics far from equilibrium. We identify a class of models which contain an extensive number of highlyexcited energy eigenstates with low entanglement entropy, which are responsible for the persistent oscillations observed in recent experiments. We develop a novel nonperturbative forwardscattering method which quantitatively describes the structure of these eigenstates and predicts the frequency of the oscillations. Remarkably the existence of these states does not rely on Many Body Localisation and occurs despite almost all of the other eigenstates being thermal. Our study opens the way towards understanding the unusual case of quantum coherent states in otherwise thermalising manybody systems. 
Wednesday, March 7, 2018 4:54PM  5:06PM 
P27.00011: Quantum Quench in a Model Ising Magnet Matthew Libersky, Thomas Rosenbaum, Daniel Silevitch The KibbleZurek (KZ) mechanism predicts the defect density as a function of time after a quench into an ordered state. Extending this formalism from classical to quantum phase transitions (QPTs) entails a quantum tuning parameter that can be varied rapidly compared to the system’s equilibration times. To this end, we study the model transverse Ising magnet LiHoF_{4}, which undergoes a ferromagnetparamagnet QPT as a function of transverse magnetic field. Pumping with Sband microwaves can saturate the nuclear spin levels, suppressing an effective rescaling of the electronic spin at temperatures below 0.4 K. This should move the phase boundary through a fixed transverse field at frequencies comparable to the 100 kHz singlespin attempt frequency. Using a compact highQ resonator, we report on initial measurements of the ordering behavior following a quantum quench in the linear and nonlinear response regimes. 
Wednesday, March 7, 2018 5:06PM  5:18PM 
P27.00012: The level statistics of random spin ½ XY chain model Wai pang Sze, Xiaohui Li, Tai Kai Ng Different level spacing distributions such as Poisson distribution and Gaussian orthogonal ensemble(GOE) have been used to characterize manybody localization in random quantum systems. For simple quantum manybody systems, the energy level spacing can be obtained by exact diagonalization studies[1, 2]. However, because of heavy computation cost, not all eigenstates can be taken into account in exact diagonalization studies of manybody quantum systems with even moderate sizes. In this talk, we investigate a spin 1/2 random xy chain model where all eigenstates can be obtained up to rather large system size. we find that for high enough disorder, the energy level distribution approaches a Poisson distribution, in agreement with previous results. However, there exists a gapped region where neither Poisson nor GOE are found in weak disorder. 
Wednesday, March 7, 2018 5:18PM  5:30PM 
P27.00013: Probing quantum dynamics using outoftimeordered measurements Pranjal Bordia, Fabien Alet, Pavan Hosur Recently, out of timeordered correlators (OTOCs), i.e., correlators of the form 〈W(t)V(0)W(t)V(0)〉 where W and V are local operators, have been used extensively to study quantum dynamics in various quantum statistical phases, such as chaotic, manybodylocalized and delocalized integrable systems. These measures capture subtle quantum correlations that are missed by ordinary timeordered or retarded correlators. However, measuring OTOCs experimentally requires creating two identical copies of the system, applying the outoftimeordered operator on one of them, and taking the overlap of the resultant state with the second copy. This procedure becomes prohibitively difficult for any reasonably large system. In this work, we propose an alternate quantity that we dub the outoftimeordered measurement (OTOM) for probing quantum dynamics. The OTOM is closely related to the OTOC and hence, inherits many of its properties. However, it can be measured experimentally with a single copy of the system. Thus, it overcomes the experimental challenge of scalability that the OTOC faces, and makes it possible to study correlations in quantum dynamics that are out of reach experimentally using OTOCs. 
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