Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session F13: Non-Equilibrium Physics with Ultracold Atoms IIIFocus Session
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Sponsoring Units: DAMOP Chair: Lukas Sieberer, University of California, Berkekely Room: 272 |
Tuesday, March 14, 2017 11:15AM - 11:51AM |
F13.00001: Observation of a dynamical phase transition in the non-equilibrium dynamics of ultracold quantum gases in driven optical lattices Invited Speaker: Christof Weitenberg Ultracold atoms are a versatile system to emulate solid-state physics including the fascinating phenomena of gauge fields and topological band structures. By circular driving of a hexagonal optical lattice, we engineer the Berry curvature of the Bloch bands and realize a Haldane-like model. We have developed a full momentum-resolved state tomography of the Bloch states, which allows measuring the distribution of Berry curvature and obtaining the Chern number [1]. Furthermore, we study the time-evolution of the many-body wavefunction after a sudden quench of the lattice parameters and observe the appearance, movement, and annihilation of dynamical vortices in reciprocal space. We identify them as the Fisher zeros in the Loschmidt amplitude and define them as a dynamical equivalent of an order parameter, which suddenly changes its value at critical evolution times [2]. Our measurements constitute the first observation of a so-called dynamical phase transition and address the intriguing question of the relation between this phenomenon and the equilibrium phase transition in the system. [1] Flaeschner et al., Science 352, 1091 (2016). [2] Flaeschner et al., arXiv:1608.05616 (2016). [Preview Abstract] |
Tuesday, March 14, 2017 11:51AM - 12:03PM |
F13.00002: Observation of discrete time-crystalline order in a disordered dipolar many-body system Soonwon Choi, Joonhee Choi, Renate Landig, Georg Kucsko, Hengyun Zhou, Junichi Isoya, Fedor Jelezko, Shinobu Onoda, Hitoshi Sumiya, Vedika Khemani, Curt von Keyserlingk, Norman Yao, Eugene Demler, Mikhail Lukin The interplay of periodic driving, disorder, and strong interactions has recently been predicted to result in exotic ``time-crystalline'' phases, which spontaneously break the discrete time-translation symmetry of the underlying drive. Here, we report the experimental observation of such discrete time-crystalline order in a driven, disordered ensemble of $\sim 10^6$ dipolar spin impurities in diamond at room-temperature [1]. We observe long-lived temporal correlations at integer multiples of the fundamental driving period, experimentally identify the phase boundary and find that the temporal order is protected by strong interactions; this order is remarkably stable against perturbations, even in the presence of slow thermalization [2]. We provide a theoretical description of approximate Floquet eigenstates of the system based on product state ansatz and predict the phase boundary, which is in qualitative agreement with our observations. [1] S. Choi et al, arXiv:1610.08057 [2] G. Kucsko et al, arXiv:1609.08216 [Preview Abstract] |
Tuesday, March 14, 2017 12:03PM - 12:15PM |
F13.00003: Long-Range Pre-Thermal Time Crystals Francisco Machado, Dominic V. Else, Chetan Nayak, Norman Yao Driven quantum systems have recently enabled the realization of a discrete time crystal --- an intrinsically out-of-equilibrium phase of matter that spontaneously breaks time translation symmetry. One strategy to prevent the drive-induced, runaway heating of the time crystal phase is the presence of strong disorder leading to many-body localization. A simpler disorder-less approach is to work in the pre-thermal regime where time crystalline order can persist to long times, before ultimately being destroyed by thermalization. In this talk, we will consider the interplay between long-range interactions, dimensionality, and pre-thermal time-translation symmetry breaking. As an example, we will consider the phase diagram of a 1D long-range pre-thermal time crystal. [Preview Abstract] |
Tuesday, March 14, 2017 12:15PM - 12:27PM |
F13.00004: Abstract Withdrawn
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Tuesday, March 14, 2017 12:27PM - 12:39PM |
F13.00005: Numerically Shaking Bosonic Condensates: Successes and Breakdowns of Floquet-Band Engineering Brandon Anderson, Logan Clark, Jennifer Crawford, Andreas Glatz, Igor Aronson, Peter Scherpelz, Cheng Chin, Kathyrn Levin Here we numerically study homogeneous Bose condensates subjected to a periodically driven lattice, as was performed in recent experiments [1,2]. Making no assumptions about Floquet bandstructure, we show where and when lattice shaking leads to the domain formation anticipated by the Floquet picture. This occurs abruptly at a critical shaking amplitude and is consistent with a (dynamical) quantum critical phase transition. In the weak interaction limit, for fast and slow ramp rates, we find that the transition is second order and we present clear evidence for Kibble-Zurek scaling. Detailed comparison with recent experiments shows very good agreement [1,2]. [1] C. V. Parker, L.-C. Ha, C. Chin Nat. Phys. 9, 769-774 (2013) [2] L. W. Clark, L. Feng, C. Chin, Science 354, 6312 (2016) [Preview Abstract] |
Tuesday, March 14, 2017 12:39PM - 12:51PM |
F13.00006: Floquet prethermalization and regimes of heating in a periodically driven, interacting quantum system Simon Weidinger, Michael Knap We study the regimes of heating in the periodically driven $O(N)$-model, which represents a generic model for interacting quantum many-body systems. By computing the absorbed energy with a non-equilibrium Keldysh Green's function approach, we establish three dynamical regimes: at short times a single-particle dominated regime, at intermediate times a stable Floquet prethermal regime in which the system ceases to absorb, and at parametrically late times a thermalizing regime. Our simulations suggest that in the thermalizing regime the absorbed energy grows algebraically in time with an the exponent that approaches the universal value of $1/2$, and is thus significantly slower than linear Joule heating. Our results demonstrate the parametric stability of prethermal states in a generic many-body system driven at frequencies that are comparable to its microscopic scales. This paves the way for realizing exotic quantum phases, such as time crystals or interacting topological phases, in the prethermal regime of interacting Floquet systems. [Preview Abstract] |
Tuesday, March 14, 2017 12:51PM - 1:03PM |
F13.00007: Keldysh approach to periodically driven systems with fermionic bath: non-equilibrium steady state, proximity effect, and interaction instabilities Dong E. Liu, Alex Levchenko, Roman M. Lutchyn We study properties of a periodically driven system coupled to a thermal bath. As a nontrivial example, we consider periodically driven metallic system coupled to a superconducting bath. The effect of the superconductor on the driven system is two-fold: it (a) modifies density of states in the metal via the proximity effect and (b) acts as a thermal bath for light-excited quasi-particles. Using Keldysh formalism, we calculate, nonpertubatively in the system-bath coupling, the steady-state properties of the system and obtain non-equilibrium distribution function. The latter allows one to calculate observable quantities which can be spectroscopically measured in tunneling experiments. A more interesting question is: Can interactions generate instabilities (e.g. BCS, Stoner's, charge density-wave, et al.) for dissipative Floquet systems. If the driving potential do not change the structure in momentum space, we then developed an RG processes, where we can integrate out the excitations in the momentum space but still keep the structures in the frequency space invariant. Based on this approach, we study BCS instability and transition temperature for 2D dissipative periodically driven systems with interaction. [Preview Abstract] |
Tuesday, March 14, 2017 1:03PM - 1:15PM |
F13.00008: Semi-classical approach to transitionless quantum driving: Explicitness and Locality Benjamin Loewe, Rafael Hipolito, Paul M. Goldbart Berry has shown [1] that, via a reverse engineering strategy, non-adiabatic transitions in time-dependent quantum systems can be stifled through the introduction of a specific auxiliary hamiltonian. This hamiltonian comes, however, expressed as a formal sum of outer products of the original instantaneous eigenstates and their time-derivatives. Generically, how to create such an operator in the laboratory is thus not evident. Furthermore, the operator may be non- local. By following a semi-classical approach, we obtain a recipe that yields the auxiliary hamiltonian explicitly in terms of the fundamental operators of the system (e.g., position and momentum). By using this formalism, we are able to ascertain criteria for the locality of the auxiliary hamiltonian, and also to determine its exact form in certain special cases. [1] Berry, M. V. Transitionless quantum driving. J. Phys. A 42, 365303 (2009) [Preview Abstract] |
Tuesday, March 14, 2017 1:15PM - 1:27PM |
F13.00009: Partial breakdown of quantum thermalization in a Hubbard-like model James R. Garrison, Ryan V. Mishmash, Matthew P. A. Fisher We study the possible breakdown of quantum thermalization in a model of itinerant electrons on a one-dimensional chain without disorder, with both spin and charge degrees of freedom. The eigenstates of this model exhibit peculiar properties in the entanglement entropy, the apparent scaling of which is modified from a ``volume law'' to an ``area law'' after performing a partial, site-wise measurement on the system. These properties and others suggest that this model realizes a new, non-thermal phase of matter, known as a quantum disentangled liquid (QDL). The putative existence of this phase has striking implications for the foundations of quantum statistical mechanics. [Preview Abstract] |
Tuesday, March 14, 2017 1:27PM - 1:39PM |
F13.00010: Effective adiabatic guiding of quantum systems: Continuous tracking vs.\ stroboscopic hopping Rafael Hipolito, Paul Goldbart Time-dependent Hamiltonians generally induce transitions between their corresponding instantaneous eigenstates. In cases where parameters in the Hamiltonian change slowly enough (compared with intrinsic dynamical timescales), the adiabatic theorem tells us that transitions are strongly suppressed. When parameters change more quickly, Berry has shown that the addition of a specific term $H_1$ to the Hamiltonian suppresses transitions completely, thus recovering transition- less driving. We discuss a convenient reformulation of Berry's approach in which $H_1$ is given in terms of an integral formula involving only operator quantities, including the nonabelian extension required for degenerate systems. We show that the integral formula is well suited to many-body problems and approximation schemes. Finally, we address a complementary issue: how to hop to an instantaneous eigenstate without necessarily tracking it. To do this, we construct a variational approach to seeking paths in parameter space that optimize the overlap between the time-evolved state and a given instantaneous eigenstate. This approach has the advantage that one can limit the types of operators appearing in the Hamiltonian which is useful when limiting the search to local or readily applicable operators. [Preview Abstract] |
Tuesday, March 14, 2017 1:39PM - 1:51PM |
F13.00011: Non-equilibrium bosonic transport through local manipulations in closed and open quantum systems Chen-Yen Lai, CHIH-CHUN CHIEN In cold atom systems, driving neutral atom through the system by using particle reservoir can be a challenging task. Here, we address an issue on tuning local potentials dynamically as controllable particle source and sink. In equilibrium, a deep potential can collect many bosons locally as a faithful sink, which indicates the usefulness in adiabatic limit. However, the sudden quenched of local potential shows low efficiency of attracting bosons into it, and this lack of efficiency is a consequence of the energy conservation in the isolated systems. Under different interactions and quenched potential depth, an averse response is observed where a deeper quenched potential results in less bosons in the sink. By considering additional reservoir, the system-environment couplings extend the theoretical description to open quantum systems. Several system-environment couplings are discussed, and we found a Lindblad operator corresponding to local cooling processes which can significantly improve the effectiveness of the dynamical emerged sink. (arXiv:1609.00468 to be appeared in Sci. Rep.) [Preview Abstract] |
Tuesday, March 14, 2017 1:51PM - 2:03PM |
F13.00012: Thermalization of Periodically Driven Interacting systems at Finite Size Paraj Titum, Karthik Seetharam, Gil Refael Conventional wisdom suggests that the fate of closed interacting driven (Floquet) systems is quite bleak - a featureless maximal entropy state characterized by an infinite temperature. Efforts to thwart this uninteresting fixed point include adding sufficient disorder to possibly realize a Floquet many-body localized phase or more recently, for clean systems, work in a narrow region of drive frequencies that leads to glassy non-thermal behavior at long time. Here we show that in clean systems, specifically due to finite size, the Floquet eigenstates can exhibit non-thermal behavior. We consider a 1d system of spinless fermions with nearest neighbor interacations where the interaction term is driven. Interestingly, even with no static component of the interaction (only static hopping), the quasienergy spectrum contains gaps and a significant fraction of the Floquet eigenstates, at all quasienergies, have non-thermal average doublon correlations. We show how this behavior scales with system size. [Preview Abstract] |
Tuesday, March 14, 2017 2:03PM - 2:15PM |
F13.00013: Operator entanglement entropy of the time evolution operator in chaotic systems Tianci Zhou, David Luitz We study the growth of the operator entanglement entropy (EE) of the time evolution operator in chaotic, many-body localized and Floquet systems. In the random field Heisenberg model we find a universal power law growth of the operator EE at weak disorder, a logarithmic growth at strong disorder, and extensive saturation values in both cases. In a Floquet spin model, the saturation value after an initial linear growth is identical to the value of a random unitary operator (the Page value). We then map the operator EE to a global quench problem evolved with a similar parent-Hamiltonian in an enlarged Hilbert space with the same chaotic, MBL and Floquet properties. The scaling and saturation properties reflect the spreading of the state EE of the corresponding time evolution. We conclude that the EE of the evolution operator should characterize the propagation of information in these systems. [Preview Abstract] |
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