Bulletin of the American Physical Society
47th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 61, Number 8
Monday–Friday, May 23–27, 2016; Providence, Rhode Island
Session B5: Many-Body Localization and Disorder |
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Chair: Elizabeth Goldschmidt, Army Research Lab Room: 551AB |
Tuesday, May 24, 2016 10:30AM - 10:42AM |
B5.00001: Observing a self-thermalizing many-body state Alexander Lukin, Eric Tai, Philipp Preiss, Matthew Rispoli, Schittko Robert, Adam Kaufman, Markus Greiner There is a clear intuition for the dynamics of a classical many-body system that is suddenly displaced from thermal equilibrium: Unless there are conserved quantities, the system re-thermalizes and reaches a new equilibrium distribution constrained by only a few thermodynamic variables. In contrast, an isolated quantum many-body system subject to a sudden perturbation undergoes unitary evolution. The dynamics is reversible and preserves memory of the microscopic details of the initial state. Yet, the long-time behavior of local observables in quenched, non-integrable systems is very well described by thermal ensembles. This thermalization within globally pure quantum states is mediated by the growth of entanglement entropy, which takes on the role of thermodynamic entropy. We use recently developed methods to study the global and local quantum purity in the dynamics of quenched Bose-Hubbard systems. We observe a rapid growth and saturation of the entanglement entropy, during which the full system remains verifiably pure. Using number-resolved measurements in a quantum gas microscope, we show that local observables thermalize in agreement with the Eigenstate Thermalization Hypothesis, and we detect a near-volume law in the entanglement entropy. [Preview Abstract] |
Tuesday, May 24, 2016 10:42AM - 10:54AM |
B5.00002: Trotterized Optical Lattice as a 3D Disordered Hamiltonian M. E. W. Reed, Z. S. Smith, A. Dewan, S. L. Rolston We discuss our Trotterized implementation of a disordered Hamiltonian that maintains isotropy in two of three dimensions through the use of a high NA pulsed accordion lattice. The optical potential and atomic cloud can be characterized in situ using the same 1 $\mu m$ imaging system with every shot, allowing a precise comparison between models of the sliding phase and our particular approximations of its Hamiltonian. [Preview Abstract] |
Tuesday, May 24, 2016 10:54AM - 11:06AM |
B5.00003: Powerlaw Decays and Thermalization in Isolated Many-Body Quantum Systems Marco Tavora, E.J. Torres-Herrera, Lea Santos We propose a new criterion for thermalization in isolated many-body quantum systems. It is based on the powerlaw behavior of the survival probability at long times. The value of the powerlaw exponent depends on the shape and filling of the energy distribution of the initial state. Exponents larger than or equal to 2 correspond to ergodic filling and consequent thermalization. We show that the algebraic behavior, which occurs in both integrable and chaotic systems, may be caused by bounds in the spectrum or by the presence of correlations between the eigenstates of the Hamiltonian. Numerical and analytical results as well as comparisons with existing rigorous mathematical derivations are presented. Our focus are on initial states that can be prepared experimentally using cold atoms in optical lattices. [Preview Abstract] |
Tuesday, May 24, 2016 11:06AM - 11:18AM |
B5.00004: Dynamics of isolated quantum systems: many-body localization and thermalization E. Jonathan Torres-Herrera, Marco Tavora, Lea F. Santos We show that the transition to a many-body localized phase and the onset of thermalization can be inferred from the analysis of the dynamics of isolated quantum systems taken out of equilibrium abruptly. The systems considered are described by one-dimensional spin-1/2 models with static random magnetic fields and by power-law band random matrices. We find that the short-time decay of the survival probability of the initial state is faster than exponential for sufficiently strong perturbations. This initial evolution does not depend on whether the system is integrable or chaotic, disordered or clean. At long-times, the dynamics necessarily slows down and shows a power-law behavior. The value of the power-law exponent indicates whether the system will reach thermal equilibrium or not. We present how the properties of the spectrum, structure of the initial state, and number of particles that interact simultaneously affect the value of the power-law exponent. We also compare the results for the survival probability with those for few-body observables. [Preview Abstract] |
Tuesday, May 24, 2016 11:18AM - 11:30AM |
B5.00005: Double occupancies in a disordered, atomic Mott insulator Philip Russ, Laura Wadleigh, Brian DeMarco Understanding the interplay between disorder and interactions in quantum systems is not only of fundamental interest but has practical relevance, such as in the field of materials engineering. A complete understanding for the combination of these ingredients remains elusive. We explore this problem in a new regime by trapping an ultracold strongly interacting atomic Bose gas in a 3D optical disordered lattice. We measure how the fraction of doubly occupied sites is affected by the addition of disorder and compare our observations to a simple site-decoupled model. By varying the entropy of the gas, the more complex problem of finite temperature is also investigated. [Preview Abstract] |
Tuesday, May 24, 2016 11:30AM - 11:42AM |
B5.00006: Adiabatic dynamics with classical noise in optical lattice Guanglei Xu, Andrew Daley The technique of adiabatic state preparation is an interesting potential tool for the realisation of sensitive many-body states with ultra-cold atoms at low temperatures. However, questions remain regarding the influence of classical noise in these adiabatic dynamics. We investigate such dynamics in a situation where a level dressing scheme can make amplitude noise in an optical lattice proportional to the Hamiltonian, leading to a quantum Zeno effect for non-adiabatic transitions. We compute the dynamics using stochastic many-body Schr\"{o}dinger equation and master equation approaches. Taking the examples of 1D Bose-Hubbard model from Mott insulator phase to superfluid phase and comparing with analytical calculations for a two-level system, we demonstrate that when the total time for the process is limited, properly transformed noise can lead to an increased final fidelity in the state preparation. We consider the dynamics also in the presence of imperfections, studying the resulting heating and dephasing for the many-body states, and identifying optimal regimes for future experiments. [Preview Abstract] |
Tuesday, May 24, 2016 11:42AM - 11:54AM |
B5.00007: Disordered quantum walks in a momentum-space lattice Fangzhao An, Eric Meier, Bryce Gadway Anderson's theorem says that in one dimension the transport of quantum particles is strongly suppressed by small amounts of random disorder. This picture changes drastically for correlated or time-varying disorder, allowing for ballistic or diffusive transport. To study disordered transport in a versatile way, we experimentally engineer arbitrary tight binding models for cold atoms based on their momentum-space evolution in a driven optical lattice. We first study the dynamics of atoms undergoing quantum walks in quasiperiodic disorder (Aubry-Andr\'{e} model) at the single-site level, directly observing coherent delocalization for weak disorder and a transition to Anderson localization for strong disorder. Second, we study the effects of annealed disorder, or time-varying disorder that mimics coupling to a thermal bath. For increasing ``temperature,'' we observe a crossover from ballistic quantum spreading to classical diffusion. Finally, we present results from applying binary disorder in the random dimer model, which is predicted to violate Anderson localization in one dimension. The techniques we present open a path towards studying transport in arbitrary types of correlated disorder, as well as studying the influence of disorder on topological systems. [Preview Abstract] |
Tuesday, May 24, 2016 11:54AM - 12:06PM |
B5.00008: Quantum emulation of quasiperiodic systems Ruwan Senaratne, Zachary Geiger, Kurt Fujiwara, Kevin Singh, Shankari Rajagopal, David Weld Tunable quasiperiodic optical traps can enable quantum emulation of electronic phenomena in quasicrystals.~ A 1D bichromatic lattice or a Gaussian beam intersecting a 2D square lattice in a direct analogy of the "cut-and-project" construction can be used to create tunable 1D quasiperiodic potentials for cold neutral atoms.~ We report on progress towards the observation of singular continuous diffraction patterns, fractal energy spectra, and Bloch oscillations in these synthetic quasicrystals. We will also discuss the existence of edge states which can be topologically pumped across the lattice by varying a phasonic parameter. [Preview Abstract] |
Tuesday, May 24, 2016 12:06PM - 12:18PM |
B5.00009: Approximate Wannier functions using discrete variable representation for asymmetric optical lattices Saurabh Paul, Eite Tiesinga We propose a numerical method using discrete variable representation (DVR) for constructing real-valued approximate Wannier functions localized in a unit cell for both symmetric and asymmetric periodic potentials in the context of optical lattices. For a symmetric lattice with inversion symmetry, we construct Wannier functions for the lowest two bands as eigen states of the position operators. To ensure that the Wannier functions are real valued, we numerically obtain the band structure and real-valued eigen states using a uniform Fourier grid DVR. We then show by a comparison of tunneling energies, that the Wannier functions are accurate to better than ten significant digits when using double-precision arithmetic. The calculations are performed for a periodic double-well optical lattice having double-wells per unit cell with tunable asymmetry along the $x$ axis and a single sinusoidal potential along the perpendicular directions. Localized functions at the two potential minima within each unit cell are similarly constructed, but using a superposition of solutions from the two lowest bands. We finally use these localized basis functions to determine the two-body interaction energies in the Bose-Hubbbard (BH) model, and show the dependence of the BH model on lattice asymmetry. [Preview Abstract] |
Tuesday, May 24, 2016 12:18PM - 12:30PM |
B5.00010: Friction Effects in Atom-Surface Interactions Ulrich D. Jentschura Atom-surface interactions both have a conservative as well as a dissipative component. A treatment of the dissipative terms requires the use of the fluctuation-dissipation theorem, and the calculation of the imaginary part of the polarizability of an atom is required. The paper will review the recent resolution of a long-standing question regarding the low-frequency asymptotics of the imaginary part, as summarized in [Phys. Rev. Lett. 114 (2015) 043001] and [Eur. Phys. J. D 69 (2015) 118], together with K. Pachucki, G. Lach and M. De Kieviet. The surprising conclusion is that the precise form of the imaginary part for low driving frequency is dominated by a so-called quantum electrodynamic loop correction, where "correction" here is not to be taken literally. The one-loop QED term dominates over the tree-level contribution! The findings drastically alter theoretical predictions for non-contact friction, and blackbody friction (due to the atom's interaction with a bath of thermal photons). The basic principle behind the calculation and the methods of nonrelativistic quantum electrodynamics will be discussed. [Preview Abstract] |
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