Bulletin of the American Physical Society
48th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 62, Number 8
Monday–Friday, June 5–9, 2017; Sacramento, California
Session N9: Excitations in Degenerate Qantum Gases |
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Room: 315 |
Thursday, June 8, 2017 10:30AM - 10:42AM |
N9.00001: Can a supersonically expanding Bose-Einstein Condensates be used to study cosmological inflation? Swarnav Banik, Stephen Eckel, Avinash Kumar, Ted Jacobson, Ian Spielman, Gretchen Campbell The massive scale of the universe makes the experimental study of cosmological inflation difficult. This has led to an interest in developing analogous systems using table top experiments. Here, we present the basic features of an expanding universe by drawing parallels with an expanding toroidal Bose Einstein Condensate (BEC) of~$^{\mathrm{23}}$Na~atoms. The toroidal BEC serves as the background vacuum and phonons are the analogue to photons in the expanding universe. We study the dynamics of phonons in both non-expanding and expanding condensates and measure dissipation using the structure factor. We demonstrate red shifting of phonons and quasi-particle production similar to pre-heating after the inflation of universe.~At the end of expansion, we also observe spontaneous non-zero winding numbers in the ring. Using Monte-Carlo simulations, we predict the widths of the resulting winding number distribution, which agree well with our experimental findings. [Preview Abstract] |
Thursday, June 8, 2017 10:42AM - 10:54AM |
N9.00002: Nonequilibrium quantum dynamics of partial symmetry breaking for a vortex state of ultracold bosons in a ring trap Xinxin Zhao, Marie A. McLain, Javier Vijande, Albert Ferrando, Lincoln D. Carr, Miguel A. Garcia One common subject for an isolated system is the memory of the system's initial conditions after a parameter change, such as a quantum quench. We investigate a vortex in a Bose-Einstein condensate on an optical ring lattice in response to partial symmetry breaking. Bosons are originally trapped in a discrete ring trap with six sites and periodic boundary conditions, whose six-fold rotational symmetry is suddenly broken but retains a three-fold rotational symmetry. During real time evolution, no critical behavior is manifested in the system's microscopic and macroscopic features, fidelity and total current. Instead, a critical point at which the system forgets its initial symmetry state is well characterized by a new measurement, symmetry memory. Similar critical phenomena are equally discovered in larger systems, which makes it pervasive in this type of partial symmetry breaking. Further studies uncover a physical understanding of the two typical trends of critical symmetry breaking strength with the help of a newly identified energy gap in the low-lying excited states identified by its discrete rotational symmetry properties. [Preview Abstract] |
Thursday, June 8, 2017 10:54AM - 11:06AM |
N9.00003: Dissipative hydrodynamics in a quantum-fluid piston shock Maren Mossman, Mark Hoefer, P. G. Kevrekidis, Peter Engels Dilute-gas Bose-Einstein condensates are effective systems for modelling and analyzing quantum hydrodynamic behavior. Recently, much emphasis has been placed on the study of quantum turbulence in these systems. We discuss theoretical, numerical and experimental results of a prototypical piston shock experiment in which a repulsive barrier is driven through a Bose-Einstein condensate. We show that under appropriate conditions the behavior is that of a dissipative rather than that of a dispersive system. Effective dissipation can be generated by the emergence of a turbulent bulge in the BEC. Experimental results are accompanied by detailed numerical simulations for the parameters of the experiment. Current status and future directions of the experiment will be discussed. [Preview Abstract] |
Thursday, June 8, 2017 11:06AM - 11:18AM |
N9.00004: Atom loss in a matter-wave soliton train Rick Mukherjee, Kaden R. A. Hazzard Solitons are localized perturbations that propagate and collide without distortion, which appear in integrable models. One such notable model is the Gross-Pitaevskii equation realized in Bose-Einstein condensates (BEC), which allow one to control the model parameters and tunably break integrability. One approach to forming solitons in a BEC is to sweep the interaction strength to a negative value, forming a train of $\sim 10$ matter-wave solitons . Even though this was realized in a BEC over a decade ago, questions remain about details of their formation and decay. For example, which properties are manifestations of the physics of single solitons, and which arise from inter-soliton interactions? Recent experiments at Rice University find surprising behavior in the atom loss after an interaction quench. To understand the atom loss mechanism better, we numerically solve the Gross-Piteaevskii equation including dissipation. We compare these results to simpler analytic models that include the effects of dissipation in simplified manners, for example including the effect of dissipation by a rate equation. We find that some aspects of the non-trivial behavior of the number of atoms can be captured by simple models of single soliton physics. [Preview Abstract] |
Thursday, June 8, 2017 11:18AM - 11:30AM |
N9.00005: Damping of Collective Oscillations in a Box Trap Nick Proukakis, Kean Loon Lee, Eugene Zaremba, Patrik Turzak, Chris Eigen, Alex Gaunt, Rob Smith, Zoran Hadzibabic, Nir Navon We model numerically the lowest-lying collective mode of a Bose gas in a box trap excited by a kick in the potential, as in a recent experiment. Our analysis is performed at finite temperatures (below the critical region), based on the so-called “ZNG” model, in which the condensate is described by a dissipative Gross-Pitaevskii equation which is itself self-consistently coupled to a dynamical thermal cloud described by a quantum Boltzmann equation – a model which has proven most successful in describing damping observed in harmonic traps. For typical parameters probed far from the hydrodynamic region, we find a single oscillation -- whose frequency agrees well with experiments -- with the thermal cloud rapidly damping out higher frequency modes primarily through self-consistent dynamical mean-field coupling. Our results are confirmed by an independent analysis with the stochastic projected Gross-Pitaevskii equation. Intuitively, we find damping in a box trap to depend much more weakly on temperature than in harmonic traps, in broad agreement with experimental data. [Preview Abstract] |
Thursday, June 8, 2017 11:30AM - 11:42AM |
N9.00006: Dynamics and Interaction of Quantized Vortex Lines in Trapped Bose-Einstein Condensates Franco Dalfovo, Simone Serafini, Elena Iseni, Tom Bienaim\'e, Russell N. Bisset, Giacomo Lamporesi, Gabriele Ferrari, Luca Galantucci, Carlo F. Barenghi We report experimental and numerical observations of the dynamics and the interaction of 3D quantum vortex filaments in a cigar-shaped atomic Bose--Einstein condensate. Vortices are spontaneously created by the Kibble-Zurek mechanism by quenching the system across the BEC transition. We then use an innovative imaging technique which exploits self-interference effects of out-coupled atoms in order to extract both the position and orientation of vortex lines from a temporal sequence of absorption images. We combine experiments and numerical Gross-Pitaevskii simulations to study the interaction between two vortices approaching at various relative speeds and angles. We show that the interaction between vortex lines in a finite system is rather different from the one in infinite uniform superfluids. In particular, the presence of boundaries induce new effects, such as rebounds, double reconnections, and ejections. These processes may play an important role in the dynamics of trapped condensates in multi-vortex and turbulent-like configurations, and, on a wider perspective, they can represent novel keys for better understanding the behavior of superfluids near boundaries. [Preview Abstract] |
Thursday, June 8, 2017 11:42AM - 11:54AM |
N9.00007: Spatiotemporal optical vortices Nihal Jhajj, Ilia Larkin, Eric Rosenthal, Sina Zahedpour, Jared Wahlstrand, Howard Milchberg We present the first experimental evidence, supported by theory and simulation, of spatiotemporal optical vortices (STOVs). A STOV is an optical vortex with phase and energy circulation~\textit{in a spatiotemporal plane}. Depending on the sign of the material dispersion, the local electromagnetic energy flow is saddle or spiral about the STOV. STOVs are shown to be a fundamental element of the nonlinear collapse and subsequent propagation of short optical pulses in material media. STOVs conserve topological charge, constraining their birth, evolution, and annihilation. We measure a self-generated STOV consisting of a ring-shaped null in the electromagnetic field about which the phase is spiral, forming a dynamic torus that is concentric with and tracks the propagating pulse. Our results, here obtained for optical pulse collapse and filamentation in air, are generalizable to a broad class of nonlinearly propagating waves. [Preview Abstract] |
Thursday, June 8, 2017 11:54AM - 12:06PM |
N9.00008: Quantum turbulence in a ``racetrack'' atomtronic circuit Mark Edwards, Benjamin Eller, Oletunde Oladehin, Charles Clark We have studied the flow produced by stirring an ultracold atomtronic system consisting of a gaseous Bose--Einstein condensate (BEC) confined in a ``racetrack'' potential. The BEC is assumed to be strongly confined in a horizontal plane by a vertical harmonic trap and, within this plane, subjected to an arbitrary two--dimensional potential. The racetrack potential is made up of two straight parallel channels connected on both ends by semicircular channels of the same width and (energy) depth as the straightaways. The Gross--Pitaevskii equation was used to simulate the behavior of the BEC in this potential when stirred by rotating paddles of various shapes including ellipses and rectangles. The rich variety of topological excitations produced during the stirring was studied by looking at the optical density, momentum distribution, velocity field and the vorticity. The momentum spectrum was studied for the development and presence of scalings indicative of quantum turbulence. Here we also report the type and number of excitations and effect of racetrack shape on their behavior. [Preview Abstract] |
Thursday, June 8, 2017 12:06PM - 12:18PM |
N9.00009: Thermally activated phase slips of a Bose-Einstein condensate in a ring trap Masaya Kunimi, Ippei Danshita Recently, the NIST group has experimentally measured the lifetime of the superflow of Bose-Einstein condensates in ring traps and found that it significantly depends on the temperature [1]. If the superflow decays dominantly due to thermally activated phase slips(TAPS), the lifetime is expected to obey the Arrhenius law. They argued that the measured lifetime is inconsistent with the Arrhenius law. However, their estimation of the energy barrier, which determines a dominant contribution to the temperature dependence of the lifetime, is not quantitatively accurate so that more profound theoretical analyses are needed in order to examine the possibility of the superflow decay via TAPS. In this work, we quantitatively calculate the lifetime of the superflow due to TAPS by the Kramers formula combined with the mean-filed theory [2,3]. Recently, this formalism has been successfully applied to explaining the experiments of the damping of dipole oscillations of 1D Bose gases in optical lattices [4], in terms of TAPS [3]. We will compare our results with the NIST experiment. [1] A. Kumar, et al., arXiv:1608.02894. [2] J. S. Langer and V. Ambegaokar, Phys. Rev. {\bf164}, 498 (1967). [3] M. Kunimi and I. Danshita, arXiv:1610.08982. [4] L. Tanzi et al., Sci. Rep. {\bf6}, 25965 (2016). [Preview Abstract] |
(Author Not Attending)
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N9.00010: Signatures of two-step impurity mediated vortex lattice melting in Bose-Einstein condensate Bishwajyoti Dey We study impurity mediated vortex lattice melting in a rotating two-dimensional Bose-Einstein condensate (BEC). Impurities are introduced either through a protocol in which vortex lattice is produced in an impurity potential or first creating the vortex lattice in the absence of random pinning and then cranking up the impurity potential. These two protocols have obvious relation with the two commonly known protocols of creating vortex lattice in a type-II superconductor: zero field cooling protocol and the field cooling protocol respectively. Time-splitting Crank-Nicolson method has been used to numerically simulate the vortex lattice dynamics. It is shown that the vortex lattice follows a two-step melting via loss of positional and orientational order. This vortex lattice melting process in BEC closely mimics the recently observed two-step melting of vortex matter in weakly pinned type-II superconductor Co-intercalated NbSe$_{\mathrm{2}}$. Also, using numerical perturbation analysis, we compare between the states obtained in two protocols and show that the vortex lattice states are metastable and more disordered when impurities are introduced after the formation of an ordered vortex lattice. [Preview Abstract] |
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