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 T8: Quench Dynamics in Degenerate Gases |
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Chair: Donald Fahey, Joint Quantum Institute Room: 555AB |
Friday, May 27, 2016 8:00AM - 8:12AM |
T8.00001: Studying quench dynamics in an ultracold quantum gas by near-field interferometry Bodhaditya Santra, Christian Baals, Ralf Labouvie, Herwig Ott The effect of interferometric self-imaging in the near-field, also known as Talbot effect, has been exploited in many areas of research since its discovery in the 19th century. In our experiment the temporal Talbot effect is used to measure the coherence length of a matter-wave field. A Bose-Einstein condensate of Rb-87 is loaded adiabatically into a 1D or a 3D optical lattice. Subsequently, the lattice potential is switched off for a short time and then on again. After a holdtime the momentum distribution is obtained by time-of-flight absorption imaging where the width of the central peak serves as a measure of coherence. For a superfluid this width shows oscillations where the period corresponds to the Talbot time. In the Mott-insulating regime these oscillations disappear but can be restored by quenching the system to the superfluid regime before the pulse is applied. With increasing waiting time between the quench and the pulse the coherence length increases which can directly be seen by the appearance of oscillations in the measured signal. [Preview Abstract] |
Friday, May 27, 2016 8:12AM - 8:24AM |
T8.00002: Quench-induced correlation waves, and quantum grenades John Corson, John Bohn We investigate the wave packet dynamics of a pair of particles that undergoes a rapid change of scattering length. Such quenches have recently become experimentally feasible with fast magnetic-field ramps and optical switching in the vicinity of a Feshbach resonance. The short-range interactions are modelled in the zero-range limit, where the quench is accomplished by switching the boundary condition of the wave function at vanishing particle separation. This generates a correlation wave that propagates rapidly to nonzero particle separations. We have derived universal, analytic results for this process that lead to a simple phase-space picture of quench-induced scattering. Intuitively, the strength of the correlation wave relates to the initial contact of the system. A natural consequence is that the waves are significant when the quench dissociates, at least partially, a bound state. These waves can propagate with high energy from one lattice site to another, potentially triggering highly non-equilibrium dynamics. [Preview Abstract] |
Friday, May 27, 2016 8:24AM - 8:36AM |
T8.00003: Out of equilibrium spatio-temporal correlations in the Bose-Hubbard model Malcolm Kennett, Matthew Fitzpatrick The Bose-Hubbard model (BHM) provides a model system to study quench dynamics across a quantum phase transition. Theoretically, it has proven challenging to study spatio-temporal correlations in the BHM in dimensions higher than one. We use the Schwinger-Keldysh technique and a strong-coupling expansion to develop a two-particle irreducible formalism to allow us to study spatio-temporal correlations in both the superfluid (SF) and Mott-insulating (MI) regimes during a quantum quench for dimensions higher than one. We obtain equations of motion for both the superfluid order parameter and two-point correlation functions and present numerical results for the evolution of two-time correlation functions. We relate our results to experiments on cold atoms in optical lattices. [Preview Abstract] |
Friday, May 27, 2016 8:36AM - 8:48AM |
T8.00004: Quench dynamics of a Bose gas under synthetic spin-orbit coupling Tian-Shu Deng, Wei Zhang, Wei Yi, Guang-Can Guo We study the quench dynamics of a Bose-Einstein condensate under a Raman-asssited synthetic spin-orbit coupling. To model the dynamical process, we adopt a self-consistent Bogoliubov approach, which is equivalent to applying the time-dependent Bogoliubov-de-Gennes equations. We investigate the dynamics of the condensate fraction as well as the momentum distribution of the Bose gas following a sudden change of system parameters. Typically, the system evolves into a steady state in the long-time limit, which features a stationary condensate fraction and an oscillating momentum distribution. The condensate fraction of the steady state depends on the quench parameter. We investigate how different quench parameters such as the inter- and intra-species interactions and the spin-orbit-coupling parameters affect the condensate fraction in the steady state. Furthermore, we find that the oscillatory momentum distribution in the long-time limit can be described by a generalized Gibbs ensemble with two branches of momentum-dependent Gibbs temperatures. Our study is relevant to the experimental investigation of dynamical processes in a spin-orbit coupled Bose-Einstein condensate. [Preview Abstract] |
Friday, May 27, 2016 8:48AM - 9:00AM |
T8.00005: Collapse Dynamics of an Attractive Box-Trapped Bose-Einstein Condensate Christoph Eigen, Alexander Gaunt, Nir Navon, Zoran Hadzibabic, Robert Smith We study the collapse dynamics of an attractive Bose-Einstein condensate confined in an optical box potential. After initiating the collapse (by suddenly changing the interaction to sufficiently negative) the wave-function shrinks in an accelerating manner. At some point (the collapse time), there is a sudden loss of atoms due to three-body recombination and an almost simultaneous emission of a shell of atoms with excess kinetic energy leaving the remnant condensate. We find that the collapse time, which we observe to vary over two orders of magnitude, can be expressed as a universal function of atom number, interaction strength and box size. Furthermore, we measure how the energy of the emitted shell and the remnant condensate atom number vary across this parameter space. In certain finely tuned conditions we observe a striking and unexplained bifurcation of possible outcomes. [Preview Abstract] |
Friday, May 27, 2016 9:00AM - 9:12AM |
T8.00006: Spin models for two-site resonant tunnelling dynamics of bosons in a tilted optical lattice Anton Buyskikh, David Pekker, Andrew Daley We study the non-equilibrium dynamics of a one dimensional tilted Bose-Hubbard model, beginning from unit filling in the Mott insulator regime. Studying a quench to the resonance point for tunnelling of the particles over two sites, we show how in the presence of a superlattice, a spin model emerges involving two subchains described by an Ising model that are then coupled by interaction terms. Using this model, we study the behaviour of the system near the quantum critical point in the vicinity of the tunnelling resonance, especially looking at the out-of-equilibrium dynamics after the quench. We compare the dephasing of local observables corresponding to the number of doubly occupied sites, which were measured in recent experiments, to the dynamics expected in the presence of noise and decoherence. These results should be directly measurable in experiments, and provide a diagnostic tool for investigating decoherence in such out-of-equilibrium dynamics. [Preview Abstract] |
Friday, May 27, 2016 9:12AM - 9:24AM |
T8.00007: Mode-coupling of interaction quenched ultracold bosons in periodically driven lattices Simeon Mistakidis, Peter Schmelcher The out-of-equilibrium dynamics of interaction quenched finite ultracold bosonic ensembles in periodically driven one-dimensional optical lattices is investigated. As a first attempt a brief analysis of the dynamics caused exclusively by the periodically driven lattice is presented and the induced low-lying modes are introduced. It is shown that the periodic driving enforces the bosons in the outer wells to exhibit out-of-phase dipole-like modes, while in the central well the cloud experiences a local-breathing mode. The dynamical behavior of the system is investigated with respect to the driving frequency, revealing a resonant-like behavior of the intra-well dynamics. Subsequently, we drive the system to a highly non-equilibrium state by performing an interaction quench upon the periodically driven lattice. This protocol gives rise to admixtures of excitations in the outer wells, an enhanced breathing in the center and an amplification of the tunneling dynamics. As a result (of the quench) the system experiences multiple resonances between the inter- and intra-well dynamics at different quench amplitudes. Finally, our study reveals that the position of the resonances can be adjusted e.g. via the driving frequency or the atom number manifesting their many-body nature. [Preview Abstract] |
Friday, May 27, 2016 9:24AM - 9:36AM |
T8.00008: Detecting the BCS pairing amplitude via a sudden lattice ramp in a honeycomb lattice Eite Tiesinga, Marlon Nuske, Ludwig Mathey We determine the exact time evolution of an initial Bardeen-Cooper-Schrieffer (BCS) state of ultra-cold atoms in a hexagonal optical lattice. The dynamical evolution is triggered by ramping the lattice potential up, such that the interaction strength $U_f$ is much larger than the hopping amplitude $J_f$. The quench initiates collective oscillations with frequency $|U_f|/(2\pi)$ in the momentum occupation numbers and imprints an oscillating phase with the same frequency on the order parameter $\Delta$. The latter is not reproduced by treating the time evolution in mean-field theory. The momentum density-density or noise correlation functions oscillate at frequency $|U_f|/(2\pi)$ as well as its second harmonic. For a very deep lattice, with negligible tunneling energy, the oscillations of momentum occupation numbers are undamped. Non-zero tunneling after the quench leads to dephasing of the different momentum modes and a subsequent damping of the oscillations. This occurs even for a finite-temperature initial BCS state, but not for a non-interacting Fermi gas. We therefore propose to use this dephasing to detect a BCS state. Finally, we predict that the noise correlation functions in a honeycomb lattice will develop strong anti-correlations near the Dirac point. [Preview Abstract] |
Friday, May 27, 2016 9:36AM - 9:48AM |
T8.00009: Interaction quantum quenches in the one-dimensional Fermi-Hubbard model Fabian Heidrich-Meisner, Andreas Bauer, Florian Dorfner, Luis Riegger, Giuliano Orso We discuss the nonequilibrium dynamics in two interaction quantum quenches in the one-dimensional Fermi-Hubbard model. First, we study the decay of the N{\'e}el state as a function of interaction strength [1]. We observe a fast charge dynamics over which double occupancies are built up, while the long-time decay of the staggered moment is controlled by spin excitations, corroborated by the analysis of the entanglement dynamics. Second, we investigate the formation of Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) correlations in a spin-imbalanced system in quenches from the noninteracting case to attractive interactions [2]. Even though the quench puts the system at a finite energy density, peaks at the characteristic FFLO quasimomenta are visible in the quasi-momentum distribution function, albeit with an exponential decay of s-wave pairing correlations. We also discuss the imprinting of FFLO correlations onto repulsively bound pairs and their rapid decay in ramps. [1] Bauer, Dorfner, Heidrich-Meisner, Phys. Rev. A 91, 053628 (2015) [2] Riegger, Orso, Heidrich-Meisner, Phys. Rev. A 91, 043623 (2015) [Preview Abstract] |
Friday, May 27, 2016 9:48AM - 10:00AM |
T8.00010: Finite temperature quenches of fermions in an optical lattice Ian G. White, Randall G. Hulet, Kaden R. A. Hazzard Although interaction quenches are known to drive interesting dynamics, most prior work has focused on quenches initiated from states that are well below the system's ordering temperature. Motivated by experiments with ultracold fermions in optical lattices, which currently are outside of this regime, we study interaction quenches in the Fermi-Hubbard model that start from finite-temperature initial states. We show that interesting dynamics occurs even under these conditions. A particularly important scenario is quenching to non-interacting systems, which despite its simplicity has been the focus of recent work as a prototype for integrability and prethermalization. In the limit where the temperature $T$ is much greater than the tunneling $t$, we find that there is transient growth of short-ranged correlations. However, the steady state created in this case is essentially trivial: it is equivalent to an equilibrium $T/t = \infty$ state. We find more interesting steady states for large, but finite, $T/t$. We calculate the associated experimental observables by combining a high-$T$ expansion of the interacting initial state with the exact calculation of the non-interacting dynamics. [Preview Abstract] |
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