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: NonEquilibrium Physics with Ultracold Atoms IIIFocus

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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 nonequilibrium dynamics of ultracold quantum gases in driven optical lattices Invited Speaker: Christof Weitenberg Ultracold atoms are a versatile system to emulate solidstate 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 Haldanelike model. We have developed a full momentumresolved state tomography of the Bloch states, which allows measuring the distribution of Berry curvature and obtaining the Chern number [1]. Furthermore, we study the timeevolution of the manybody 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 socalled 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 timecrystalline order in a disordered dipolar manybody 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 ``timecrystalline'' phases, which spontaneously break the discrete timetranslation symmetry of the underlying drive. Here, we report the experimental observation of such discrete timecrystalline order in a driven, disordered ensemble of $\sim 10^6$ dipolar spin impurities in diamond at roomtemperature [1]. We observe longlived 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: LongRange PreThermal Time Crystals Francisco Machado, Dominic V. Else, Chetan Nayak, Norman Yao 
Tuesday, March 14, 2017 12:15PM  12:27PM 
F13.00004: Abstract Withdrawn Periodic driving provides a powerful experimental tool to engineer isolated, synthetic quantum systems. However, the lack of energy conservation and heating effects means that nontrivial manybody states in periodically driven (Floquet) systems are only metastable. Therefore it is necessary to find strategies for preparing longlived manybody states in Floquet systems. We develop a theoretical framework for describing the dynamical preparation of states in such systems by a slow ramp of the drive. We find that dynamics is well approximated by the initial state evolving under a slowly varying effective Hamiltonian $H_{\rm eff}^{(s)}(t)$, provided the ramp speed $s\gg t_*^{1}\sim e^{{C\frac{\omega}{J}}}$, the inverse of the characteristic heating timescale in the Floquet system. Thus, the system effectively undergoes a slow quench from $H_0$ to the effective Hamiltonian of the unramped Floquet system. We also consider the passage of the slow quench through a quantum critical point, and obtain an optimal ramp speed $s_*$ by minimizing the energy absorbed due to both the drive and the ramp. Our results bridge the gap between the numerous proposals to obtain interesting systems via Floquet engineering, and the actual preparation of such systems in their effective ground states. 
Tuesday, March 14, 2017 12:27PM  12:39PM 
F13.00005: Numerically Shaking Bosonic Condensates: Successes and Breakdowns of FloquetBand 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 KibbleZurek scaling. Detailed comparison with recent experiments shows very good agreement [1,2]. [1] C. V. Parker, L.C. Ha, C. Chin Nat. Phys. 9, 769774 (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 manybody systems. By computing the absorbed energy with a nonequilibrium Keldysh Green's function approach, we establish three dynamical regimes: at short times a singleparticle 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 manybody 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: nonequilibrium 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 twofold: it (a) modifies density of states in the metal via the proximity effect and (b) acts as a thermal bath for lightexcited quasiparticles. Using Keldysh formalism, we calculate, nonpertubatively in the systembath coupling, the steadystate properties of the system and obtain nonequilibrium 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 densitywave, 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: Semiclassical 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, nonadiabatic transitions in timedependent 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 timederivatives. Generically, how to create such an operator in the laboratory is thus not evident. Furthermore, the operator may be non local. By following a semiclassical 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 Hubbardlike 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 onedimensional 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, sitewise measurement on the system. These properties and others suggest that this model realizes a new, nonthermal 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 Timedependent 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 manybody 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 timeevolved 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: Nonequilibrium bosonic transport through local manipulations in closed and open quantum systems ChenYen Lai, CHIHCHUN 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 systemenvironment couplings extend the theoretical description to open quantum systems. Several systemenvironment 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 manybody localized phase or more recently, for clean systems, work in a narrow region of drive frequencies that leads to glassy nonthermal behavior at long time. Here we show that in clean systems, specifically due to finite size, the Floquet eigenstates can exhibit nonthermal 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 nonthermal 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, manybody 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 parentHamiltonian 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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