Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session M29: Quench dynamics |
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Sponsoring Units: DAMOP Chair: Marcos Rigol, Penn State Room: 603 |
Wednesday, March 5, 2014 11:15AM - 11:27AM |
M29.00001: Quantum quenches of cold-atom gases in optical lattices: the influence of Anderson localization Chris Hooley, Jorge Quintanilla, Vito Scarola We consider the following kind of non-equilibrium experiment. An ultracold fluid of fermions is prepared in a potential consisting of three parts: an optical lattice; a short-range-correlated disorder potential of finite strength; and a shallow harmonic trapping potential. After the fluid has equilibrated, the minimum of the harmonic potential is suddenly ``jumped'' to the side by a finite distance, $d$. The observables of interest are the subsequent evolution of the density distribution and phase correlations in the fluid. This kind of experiment is theoretically interesting because it contains two energy-dependent length scales: the localization length of the single-particle orbitals due to the disorder potential, $\xi$; and the ``Bragg localization length'' of the single-particle orbitals due to the combined effect of the harmonic trap and optical lattice, $l_B$. We present numerical results on the evolution of the density distributions and phase correlations in such cases, for a range of strengths of the disorder. In addition, we provide an approximate analytical framework for understanding our results in terms of the relative size of the length scales $\xi$ and $l_B$ at the Fermi energy. Possibilities for further work are also discussed. [Preview Abstract] |
Wednesday, March 5, 2014 11:27AM - 11:39AM |
M29.00002: Quantum quenches and work distributions in ultra-low-density systems Yulia Shchadilova, Pedro Ribeiro, Masudul Haque In our contribution we present results on quantum quenches in systems with a fixed number of particles in a large volume, the situation accessible in cold atom experiments. We show that the typical differences between local and global quenches present in systems with regular thermodynamic limit are lacking in this low-density limit. In particular, we show that local and global quenches can have power-law work distributions (edge singularities) typically associated with only local quenches for finite-density systems. We show that this regime allows for large edge singularity exponents beyond that allowed by the constraints of the usual thermodynamic limit (e.g., by Anderson's orthogonality catastrophe). This large-exponent singularity has observable consequences in the time evolution, leading to a distinct intermediate power-law regime in time. We demonstrate these results using local quantum quenches in a low-density Kondo-like system, and additionally through global and local quenches in Bose-Hubbard, Aubry-Andre, and hard-core boson systems in the low-density regime.\\[4pt] [1] Y.E. Shchadilova, P. Ribeiro, M. Haque, Quantum quenches and work distributions in ultra low density systems, arXiv:1303.4103, (2013). [Preview Abstract] |
Wednesday, March 5, 2014 11:39AM - 11:51AM |
M29.00003: Quantum quench from classical evolution: the fate of a soliton Fabio Franchini, Andrey Gromov, Manas Kulkarni, Andrea Trombettoni In a quantum quench, one prepares a system in an eigenstate of a given Hamiltonian, and then lets it evolve after suddenly changing a control parameter of the Hamiltonian. By observing this evolution, one tries to understand whether and how a quantum system reaches a (thermal) equilibrium. Normally, the initial state is taken to be the ground state: we propose a different experimentally feasible protocol, in which the system is prepared in an excited state corresponding to a collective solitonic excitation. If we are interested only in the single particle density, the time evolution can be reduced to the study of a semi-classical non-linear differential equation. We study both integrable and non-integrable systems, in a confining (parabolic) potential and on a ring. The short time dynamics is universal, while the long time configuration depends on the system. [Preview Abstract] |
Wednesday, March 5, 2014 11:51AM - 12:03PM |
M29.00004: Correlations after a quantum quench in the Bose Hubbard model Matthew Fitzpatrick, Malcolm Kennett Recent experimental advances that allow for the atomic resolution of dynamics for cold atoms in optical lattices call for theory to describe these dynamics. We use the Schwinger-Keldysh technique to study time and space dependent correlations after a quantum quench in the Bose Hubbard model. We focus on the case of time dependent hopping and use a real-time action that allows for the description of both the superfluid and Mott insulating phases to obtain dynamical equations for these correlations. We relate our results to recent experiments. [Preview Abstract] |
Wednesday, March 5, 2014 12:03PM - 12:15PM |
M29.00005: Particle-hole pair excitations in Mott insulator quench dynamics Khan Mahmud, Lei Jiang, Philip Johnson, Eite Tiesinga We investigate the dynamics of strongly interacting bosons in an optical lattice in a quantum quench scenario where we start from a Mott insulator state and suddenly raise the lattice depth. Despite the nature of short-range coherence in the Mott state, we find that the coherence visibility exhibits collapse and revival oscillations which could be observed in experiments. The quasi-momentum distribution oscillates between a maximum occupation at $k=0$ (during revivals) and $k=\pi$ (during collapse). We show that the $k=\pi$ revivals are caused by the presence of particle-hole pair excitations on top of a constant Mott background. We further show that similar effects are found in other lattice models such as with fermions and Bose-Fermi mixtures. We provide a general framework and point to a new avenue to probe strongly correlated many-body states going beyond the superfluid paradigm of collapse and revivals. [Preview Abstract] |
Wednesday, March 5, 2014 12:15PM - 12:27PM |
M29.00006: Bloch oscillations and quench dynamics of interacting bosons in an optical lattice Eite Tiesinga, Khan Mahmud, Lei Jiang, Phillip Johnson We study the dynamics of interacting superfluid bosons in a one dimensional vertical optical lattice after a sudden increase of the lattice potential depth. We show that this system can be exploited to investigate the effects of strong interactions on Bloch oscillations. We perform theoretical modelling of this system, identify experimental challenges and explore a new regime of Bloch oscillations characterized by interaction-induced matter-wave collapse and revivals. In addition, we study three dephasing mechanisms: effective three-body interactions, finite value of tunneling, and a background harmonic potential. We also find that the center of mass motion in the presence of finite tunneling goes through collapse and revivals, giving an example of quantum transport where interaction-induced revivals are important. We quantify the effects of residual harmonic trapping on the momentum distribution dynamics and show the effects of interactions on the temporal Talbot effect. Finally, we analyze the prospects and challenges exploiting Bloch oscillations of cold atoms in the mean-field regime for precision measurement of the gravitational acceleration $g$. [Preview Abstract] |
Wednesday, March 5, 2014 12:27PM - 12:39PM |
M29.00007: Quantum ratchets, the orbital Josephson effect, and chaos in Bose-Einstein condensates Lincoln D. Carr, Martin Heimsoth, Charles E. Creffield, Fernando Sols In a system of ac-driven condensed bosons we study a new type of Josephson effect occurring between states sharing the same region of space and the same internal atom structure. We first develop a technique to calculate the long-time dynamics of a driven interacting many-body system. For resonant frequencies, this dynamics can be shown to derive from an effective time-independent Hamiltonian which is expressed in terms of standard creation and annihilation operators. Within the subspace of resonant states, and if the undriven states are plane waves, a locally repulsive interaction between bosons translates into an effective attraction. We apply the method to study the effect of interactions on the coherent ratchet current of an asymmetrically driven boson system. We find a wealth of dynamical regimes which includes Rabi oscillations, self-trapping and chaotic behavior. In the latter case, a full quantum many-body calculation deviates from the mean-field results by predicting large quantum fluctuations of the relative particle number. Moreover, we find that chaos and entanglement, as defined by a variety of widely used and accepted measures, are overlapping but distinct notions. [Preview Abstract] |
Wednesday, March 5, 2014 12:39PM - 12:51PM |
M29.00008: Many-body Bloch oscillations Masud Haque We consider Bloch oscillations of interacting quantum particles in a one-dimensional lattice subject to a linear potential gradient (a tilt). For hard-core bosons and for free fermions, we show perfectly periodic behavior of density and momentum distributions. The oscillations can be predominantly position oscillations, or predominantly width oscillations, depending on the initial state. We show how the periodic behavior is modified for weak and strong interactions. [Preview Abstract] |
Wednesday, March 5, 2014 12:51PM - 1:03PM |
M29.00009: Quench dynamics of a strongly interacting resonant Bose gas Xiao Yin, Leo Radzihovsky We explore the dynamics of a Bose gas following its quench to a strongly interacting regime near a Feshbach resonance. Within a self-consistent Bogoliubov analysis we find that after the initial condensate-quasiparticle Rabi oscillations, at long time scales the gas is characterized by a nonequilibrium steady-state momentum distribution function, with depletion, condensate density and contact that deviate strongly from their corresponding equilibrium values. These are in a qualitative agreement with recent experiments on $^{85}$Rb by Makotyn. Our analysis also suggests that for sufficiently deep quenches close to the resonance the nonequilibrium state undergoes a phase transition to a fully depleted state, characterized by a vanishing condensate density. [Preview Abstract] |
Wednesday, March 5, 2014 1:03PM - 1:15PM |
M29.00010: Many-body dynamics of a BEC quenched to unitarity John Corson, Andrew Sykes, Jose D'Incao, Andrew Koller, Chris Greene, Ana Maria Rey, Kaden Hazzard, John Bohn The dynamics of a dilute BEC quenched to unitarity are studied using a variational ansatz for the many-body quantum state. Despite the resonant atom-atom interactions, the condensate does not deplete instantaneously, and this allows for a self-consistent mean-field-like description of the system at short (but experimentally-accessible) times. At infinite scattering length and zero temperature, the dynamics are found to scale universally with the number density, as reported in the experiment of Makotyn et al, arXiv1308.3696. We predict the time evolution of observables such as the momentum distribution $n_k(t)$, the contact $C(t)$, and the density $n_m(t)$ of Feshbach molecules generated by the interaction quench. We observe a saturation of large-momentum populations on a time scale that is consistent with recent measurements. [Preview Abstract] |
Wednesday, March 5, 2014 1:15PM - 1:27PM |
M29.00011: Quenching to unitarity: Quantum dynamics in a 3D Bose gas Andrew Sykes, John Corson, Jose D'Incao, Andrew Koller, John Bohn, Ana Maria Rey, Kaden Hazzard, Chris Greene We study the dynamics of a zero temperature Bose condensate following a sudden quench of the scattering length from noninteracting to unitarity (infinite scattering length). In this talk we discuss how a qualitative understanding of the dynamics can be built up by understanding few-body physics under the same dynamical scenario. We calculate the coherent evolution of the momentum distribution, particularly focusing on the time dependence of the contact. By comparing the results to a many-body mean-field calculation, we gauge the qualitative and quantitative accuracy of this approach. We then discuss the results of a three-body calculation, in which loss dynamics occurs due to three-body recombination. One the key results of this work indicates that loss dynamics takes place over a much longer timescale than the coherent dynamics. This exciting result supports the idea that meta-stable degenerate unitary Bose gases may be experimentally observable in such a non-equilibrium scenario. [Preview Abstract] |
Wednesday, March 5, 2014 1:27PM - 1:39PM |
M29.00012: Equilibrating Dynamics in Quenched Bose Gases Kathy Levin, Adam Rancon Recent interaction quench experiments in cold bosonic gases are challenging our understanding of out-of-equilibrium dynamics of quantum systems. In particular, the cross-over between short-time (strongly out-of-equilibrium) and long-time equilibration (to a (meta-)stable state) is a complicated problem that needs to be addressed in order to understand the multiple time scales (associated with oscillations, equilibration, etc.), and their momentum dependence in these experiments. In this talk, we present a model that simulates the out-of-equilibrium dynamics of a condensed Bose gas, which importantly allows for the ultimate equilibration of the system via a coupling to a bath [Phys. Rev. A (88) 031601 (2013)]. We show why (as in quench experiments) large k, high energy states equilibrate more rapidly than those at small k. In this context we discuss the implications for calculations and measurements of the Tan contact. We finally address the question of how the intermediate time dynamics can, in principle, reflect the presence or absence of a condensate. [Preview Abstract] |
Wednesday, March 5, 2014 1:39PM - 1:51PM |
M29.00013: Dynamics of phase separation and coarsening in binary Bose-Einstein condensates Johannes Hofmann, Stefan Natu Cold quantum gases provide an ideal testbed to study the out-of-equilibrium dynamics of quantum systems. We consider the nonequilibrium dynamics of a coupled binary mixture of Bose-Einstein condensates. Depending on the coupling between the two components, the system can exist in either a miscible or a phase-separated ground state, which are separated by a quantum phase transition. We present results on the dynamics of domain formation and coarsening after a quench across this phase boundary. [Preview Abstract] |
Wednesday, March 5, 2014 1:51PM - 2:03PM |
M29.00014: Small Quench Dynamics as an Investigative Tool for Cold Atom Systems Sunil Yeshwanth, Marcos Rigol, Lorenzo Campos Venuti Finite one-dimensional systems of bosons or fermions described by the Hubbard model can be realized using cold atoms confined in an optical lattice. The ground states of these systems are often characterized by a coexistence of phases when a non-homogeneous trapping potential is applied. We propose to analyze this phase coexistence by studying the out-of-equlibrium dynamics following a sudden quench. In particular we show that the temporal variance of the local densities is able to spot the boundaries between the different phases. The feasibility of this approach is demonstrated for several Hamiltonians using numerical simulations. We first consider an integrable system, hardcore bosons confined by quadratic or quartic trapping potentials, where Mott and superfluid phases are coexistent. We also analyze a non-integrable system, a $t-VV'$ model which has a charge density wave phase coexisting with a superfluid one when subjected to a quadratic confining potential. We find that the temporal variance is more effective than other standard indicators of phase boundaries such as the local compressibility or density fluctuations. [Preview Abstract] |
Wednesday, March 5, 2014 2:03PM - 2:15PM |
M29.00015: Quantum quenches in 1D Bose Gases: Glimmers of Quantum KAM Robert Konik, Giuseppe Brandino, J.-S. Caux Using a numerical renormalization group based on exploiting an underlying exactly solvable non-relativistic theory, we study the out-of-equilibrium dynamics of a 1D Bose gas (as described by the Lieb-Liniger model) released from a parabolic trap into a weak cosine potential. The presence of the cosine potential breaks integrability and leads the formerly conserved charges of Lieb-Liniger to be time dependent. We however argue that from these charges we are able to construct approximately conserved quantities despite the presence of the cosine term. How good the time invariance of these quasi-conserved quantities is can be related to the strength of the post-quench cosine potential and the width of this perturbation in Fourier space. This gives then an analog in a quantum example to the classical KAM theorem. [Preview Abstract] |
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