Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session E19: Thermalization and ManyBody Localization in Small Quantum SystemsInvited

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Sponsoring Units: DAMOP Chair: Dries Sels, Boston University Room: 278279 
Tuesday, March 14, 2017 8:00AM  8:36AM 
E19.00001: Quantum thermalization through entanglement Invited Speaker: Adam Kaufman Understanding how an isolated manybody state thermalizes and develops entropy is foundational to quantum statistical mechanics, yet appears antithetical to notions that we have about entropy. An evolving quantum state can develop observables that agree with thermal ensembles, yet the unitarity of quantum evolution preserves the purity of this full quantum state in time. Hence, a pure, and in this sense, zero entropy quantum state can dynamically become seemingly entropic and thermal. In this talk, I will describe our experimental studies of thermalization in a verifiably pure manybody state, and how the entropy induced by entanglement facilitates thermalization. I will describe our experimental method for measuring quantum purity, and thereby entanglement entropy, through the interference of two copies of a manybody state. By comparing the entanglement entropy we measure to the thermal entropy expected from an ensemble, I will illustrate how thermalization is manifest locally within a globally pure quantum state, and how these observations are related to the Eigenstate Thermalization Hypothesis. [Preview Abstract] 
Tuesday, March 14, 2017 8:36AM  9:12AM 
E19.00002: Ergodic dynamics and thermalization in an isolated quantum system Invited Speaker: Charles Neill Statistical mechanics is founded on the assumption that all accessible configurations of a system are equally likely. This requires dynamics that explore all states over time, known as ergodic dynamics. In isolated quantum systems, however, the occurrence of ergodic behavior has remained an outstanding question. Here, we demonstrate ergodic dynamics in a small quantum system consisting of only three superconducting qubits. The qubits undergo a sequence of rotations and interactions and we measure the evolution of the density matrix. Maps of the entanglement entropy show that the full system can act like a reservoir for individual qubits, increasing their entropy through entanglement. Surprisingly, these maps bear a strong resemblance to the phase space dynamics in the classical limit; classically, chaotic motion coincides with higher entanglement entropy. We further show that in regions of high entropy the full multiqubit system undergoes ergodic dynamics. Our work illustrates how controllable quantum systems can investigate fundamental questions in nonequilibrium thermodynamics. [Preview Abstract] 
Tuesday, March 14, 2017 9:12AM  9:48AM 
E19.00003: Quantum thermalization and manybody Anderson localization Invited Speaker: David Huse A standard assumption in statistical mechanics is that the internal dynamics of a closed, interacting system of many degrees of freedom will bring the system to thermal equilibrium in the limit of long time. For many systems this is indeed true and that is the process of quantum thermalization, whereby the system is able to act as a ``bath'' for itself and thus under its own unitary quantum dynamics bring its subsystems to thermal equilibrium. But there are other systems that fail to be a bath for themselves due to being manybody Anderson localized. In such systems, there can be a novel dynamic quantum phase transition between the thermal phase that does thermalize and the manybody localized (MBL) phase that does not. This is a thermodynamic phase transition, but not in the usual sense of that phrase: in the thermal phase equilibrium thermodynamics does emerge at long time from the system's dynamics, while in the MBL phase equilibrium thermodynamics fails to emerge. I will review some recent developments in this fascinating topic. [Preview Abstract] 
Tuesday, March 14, 2017 9:48AM  10:24AM 
E19.00004: Anomalous Thermalization Invited Speaker: David J. Luitz It is commonly believed that quantum isolated systems satisfying the eigenstate thermalization hypothesis (ETH) are diffusive. We show that this assumption is too restrictive since there are systems that are asymptotically in a thermal state, yet exhibit anomalous, subdiffusive thermalization. Such systems satisfy a modified version of the ETH ansatz and we derive a general connection between the scaling of the variance of the offdiagonal matrix elements of local operators, written in the eigenbasis of the Hamiltonian, and the dynamical exponent. We find that for subdiffusively thermalizing systems the variance scales more slowly with system size than expected for diffusive systems. We corroborate our findings by numerically studying the distribution of the coefficients of the eigenfunctions and the diagonal and offdiagonal matrix elements of local operators of the random field Heisenberg chain, which has anomalous transport in its thermal phase. Surprisingly, this system also has nonGaussian distributions of the eigenfunctions, thus, directly violating Berry’s conjecture. [1] David J. Luitz and Yevgeny Bar Lev, Phys. Rev. Lett. 117, 170404 (2016) [2] David J. Luitz, Phys. Rev. B 93, 134201 (2016) [Preview Abstract] 
Tuesday, March 14, 2017 10:24AM  11:00AM 
E19.00005: ManyBody Localization Through the Lens of Ultracold Atoms Invited Speaker: Pranjal Bordia Manybody localized (MBL) quantum systems show a drastic disregard for the Eigenstate Thermalization Hypothesis (ETH), giving rise to a fundamentally new dynamical quantum manybody phase. While a multitude of theoretical studies have been very successful in backing the existence of MBL in onedimension, much less so can be said about the transformation from the ETH respecting phase to MBL and about dynamics in higher dimensions, with many outstanding challenging questions.\\ \\To confront such cases, I will describe our efforts in creating and probing MBL with ultracold atoms in optical lattices in both one and two dimensions. In particular, I will focus on recently obtained results on the observation of slowrelaxation arising due to rare, configurational Griffithstype effects in both one and two dimensions. Further, by studying the relaxation dynamics of a farfromequilibrium state, we find evidence for MBL in quasiperiodic potentials in both one and two dimensions. Our results demonstrate how controlled quantum simulators can explore fundamental questions about quantum statistical mechanics in genuinely novel regimes, often not accessible to classical computations. [Preview Abstract] 
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