Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session B43: BEC in Trapped Atomic Gases |
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Sponsoring Units: DAMOP Chair: Henry Glyde, University of Delaware Room: Baltimore Convention Center 346 |
Monday, March 13, 2006 11:15AM - 11:27AM |
B43.00001: Superfluidity without Symmetry-Breaking: The Time-Dependent Hartree-Fock Approximation H.A. Fertig, Chang-hua Zhang We develop a time-dependent Hartree-Fock approximation that is appropriate for Bose-condensed systems. In order to explicitly capture the exchange energy for interactions between the condensate and single- particle excitations, we work with an ensemble with a fixed condensate particle number, so that there is no breaking of gauge symmetry in our approach. Defining a ``depletion Green's function'' allows the construction of condensate and depletion particle densities from eigenstates of a single time-dependent Hamiltonian, guaranteeing that our approach is a conserving approximation. We show that its application to the infinite uniform system produces the expected superfluid mode, and discuss the structure of the density response function. [Preview Abstract] |
Monday, March 13, 2006 11:27AM - 11:39AM |
B43.00002: Number-of-Particle Fluctuations and Stability of Bose-Condensed Systems Chang-hua Zhang In this paper we show that a normal total number-of-particle fluctuation can be obtained consistently from the static thermodynamic relation and dynamic compressibility sum rule. In models using the broken $U(1)$ gauge symmetry, in order to keep the consistency between statics and dynamics, it is important to identify the equilibrium state of the system with which the density response function is calculated, so that the condensate particle number $N_0$, the number of thermal depletion particles $\tilde{N}$, and the number of non-condensate particles $N_{nc}$ can be unambiguously defined. We also show that the chemical potential determined from the Hugenholtz-Pines theorem should be consistent with that determined from the equilibrium equation of state. The $N^{4/3}$ anomalous fluctuation of the number of non- condensate particles is an intrinsic feature of the broken $U(1) $ gauge symmetry. However, this anomalous fluctuation does not imply the instability of the system. Using the random phase approximation, which preserves the $U(1)$ gauge symmetry, such an anomalous fluctuation of the number of non-condensate particles is completely absent. [Preview Abstract] |
Monday, March 13, 2006 11:39AM - 11:51AM |
B43.00003: Measuring Correlations by Single Atom Detection in Quantum Degenerate Gases Anton \"{O}ttl, Stephan Ritter, Tobias Donner, Thomas Bourdel, Michael K\"{o}hl, Tilman Esslinger We observe single atoms by means of cavity QED detection in a quantum degenerate gas. Starting from a magnetically trapped $^{87}$Rb BEC of 5 million atoms we coherently output couple a continuous atom laser by means of a local radio-frequency induced spin-flip into a magnetically untrapped hyperfine state. These atoms propagate freely downward due to gravity and enter the detector which is placed 36\,mm below the BEC. Our single atom detector consists of an ultra-high finesse (${\mathcal F}=3\cdot10^{5}$) optical cavity in the strong coupling limit of cavity QED. We record the transmission of a weak resonant probe laser through the cavity with a single photon counter and detect single atom transits by their significant reduction of transmission. We measured the second order correlation function of an atom laser beam in a Hanbury Brown \& Twiss type experiment. Analyzing the correlations in the arrival time of individual atoms reveals the second order coherence of the atom laser. The high quantum efficiency and fast response time of our single atom detector enables us to extract the full counting statistic of atomic beams. We verified a poissonian distribution for the atom laser whereas a pseudo-thermal atomic beam showed a bunching behavior corresponding to a Bose-Einstein statistical distribution function. [Preview Abstract] |
Monday, March 13, 2006 11:51AM - 12:03PM |
B43.00004: Casimir-like drag force in a slow-moving superfluid David Roberts, Yves Pomeau It is widely accepted that a superfluid flow exhibits a critical velocity~below which there is no dissipation.~However, the often-neglected zero-temperature quantum fluctuations have implications for the existence of~this critical velocity. The drag force on an object created by the scattering of these quantum fluctuations in a three-dimensional, weakly ~interacting Bose-Einstein condensate is discussed.~ A non-zero force at low velocities is found to exist for two specific experimentally realizable examples, which suggests that the effective critical velocity in these systems is zero. [Preview Abstract] |
Monday, March 13, 2006 12:03PM - 12:15PM |
B43.00005: Berry Phase Effect on Semiclassical Dynamics of Bogoliubov Quasiparticles Chuanwei Zhang, Artem Dudarev, Qian Niu We develop a semiclassical theory for Bogoliubov quasiparticles in a superfluid by following the center of mass motion of a quasiparticle wavepacket. Berry phase arises when the underlying condensate moves, invalidating the usual canonical relation between the mechanical momentum and position variables. The equations of motion become non-canonical, and the quantization rule and the density of states are also modified. We study quasiparticles in a condensate with a vortex to show explicitly the Berry phase effects and their experimental observation. [Preview Abstract] |
Monday, March 13, 2006 12:15PM - 12:27PM |
B43.00006: Collective modes of a spherical shell condensate Courtney Lannert, Tzu-Chieh Wei, Smitha Vishveshwara Dilute ultracold bosonic atoms in three-dimensional optical lattices with a harmonic confining potential are expected to exhibit inhomogeneous phases -- in particular phases containing superfluid regions confined to a spherical shell. It may also be possible to confine superfluids in this geometry using a special trap designed for this purpose. We explore the low-energy collective modes of a superfluid confined to a thin spherical shell analytically. In particular, in the limit of strong interactions we find two breathing modes with frequencies distinct from the spherical condensate breathing mode. [Preview Abstract] |
Monday, March 13, 2006 12:27PM - 12:39PM |
B43.00007: Expansion of spherical shell condensates Smitha Vishveshwara, Tzu-Chieh Wei, Courtney Lannert Bose-condensed atoms in traps of novel geometries can show dramatic features in time-of-flight measurements that are absent in conventional spherical trap settings. The specific case of a trap creating a spherical condensate shell is presented. The dynamics of such a shell can yield a significant accumulation of mass at the center upon release of the trapping potential. Morever, the expanded cloud can undergo self- interference and exhibit the associated interference fringes. These features are substantiated by numerical simulations and studied for a range of interaction strength between constituent atoms. [Preview Abstract] |
Monday, March 13, 2006 12:39PM - 12:51PM |
B43.00008: Bose-Einstein condensation and superfluidity in finite sized systems. Ali Shams, Henry Glyde We present results on the relationship between the superfluid fraction and the condensate fraction in finite sized systems of hard-core bosons. The systems are investigated using path integral Monte Carlo methods over a wide range of densities. While it is well known that in strongly interacting bose systems like liquid helium the condensate fraction is much smaller than the superfluid fraction, our results indicate that for finite sized systems and weak interaction, condensate fraction can actually exceed superfluid fraction. This is especially the case as we get closer to the superfluid transition temperature. Condensation in an interacting system can be suppressed both through increased correlation effects and decreased exchange effects. Our investigation also sheds light on the relative importance of these two depletion mechanisms. [Preview Abstract] |
Monday, March 13, 2006 12:51PM - 1:03PM |
B43.00009: Bose Einstein Condensation in a Box Trap: A Monte Carlo Study Chongshan Zhang, Kwangsik Nho, D. P. Landau Using the path-integral Monte Carlo method, we have investigated Bose-Einstein condensates in an optical box trap which has been produced very recently \footnote{T. P. Meyrath, F. Schreck, J. L. Hanssen, C.-S. Chuu, and M. G. Raizen, Phys. Rev. A {\bf 71}, 041604 (2005)}. We have used the same geometry as the experiment, and approximated the trapping potential with Gaussian functions at the boundary in order to mimic such a trap. The two-body interaction was described by a hard-sphere potential whose radius equals the $s$-wave scattering length. We have studied the temperture dependence of the equilibrium properties such as the total energy, the total density profiles, and the superfluid fraction. In addition, we have calculated the equilibrium properties as a function of the strength of the two-body interaction. We will compare our simulation results with those for harmonic and hard wall traps. \\ \\ $^*$Research supported by NASA [Preview Abstract] |
Monday, March 13, 2006 1:03PM - 1:15PM |
B43.00010: Transistor like behavior of a Bose Einstein condensate in triple well potential James Stickney, Dana Anderson, Alex Zozulya The availability of cold atom-based analogs of building blocks of electronic circuits, especially with the ability to integrate them into larger devices, may be extremely valuable in applications. We demonstrate that a device based on a BEC in a triple well potential can show behavior similar to a field effect transistor. The left well is coupled only to the middle and is equivalent to the source in a transistor, the middle is coupled to both the left and right wells and is the equivalent of a gate, and the right well is the drain. The potential is designed so that the chemical potential of the atoms in the empty middle well is sufficiently smaller than in the left or right wells to block tunneling. A small number of atoms placed in the middle well switches the device and enables strong tunneling from the left to the right. This tunneling, controlled by the middle well, is due the atom-atom interactions which increase the chemical potential in the middle well and remove the energy mismatch. We show that the number of atoms tunneling into the right well can be much larger than the number of controlling atoms in the middle well. The three well structure demonstrates both absolute and differential gain. Estimates of the switching time and parameters for the potential are presented. [Preview Abstract] |
Monday, March 13, 2006 1:15PM - 1:27PM |
B43.00011: Phase ordering kinetics of a spinor condensate Subroto Mukerjee, Cenke Xu, Joel Moore Bose-Einstein condensates of atoms with nonzero total spin show order parameters and gapless excitations that are distinct from both ordinary magnetic states and superfluid helium. In light of recent experiments on the development of magnetic order in this system, we construct a model dynamics (in the spirit of dynamical critical phenomena) that is consistent with the equilibrium physics of the system and reproduces all the relevant gapless modes of the magnetized condensate.Although the parameters in this time-dependent Ginzburg-Landau are fixed by quantities that are in principle measurable, there are several parameter-independent predictions that can be directly compared to existing experiments: we focus in this talk on the phase-ordering kinetics of magnetic and superfluid order in 2D condensates in an axial field, for comparison with the experiments of Higbie {\em et. al}(cond-mat/0502517). [Preview Abstract] |
Monday, March 13, 2006 1:27PM - 1:39PM |
B43.00012: The three-body recombination of a condensed Bose gas near a Feshbach resonance Lan Yin We study the three-body recombination rate of a homogeneous dilute Bose gas with a Feshbach resonance at zero temperature. The ground state and excitations of this system are obtained. The three-body recombination in the ground state is due to the break-up of an atom pair in the quantum depletion and the formation of a molecule by an atom from the broken pair and an atom from the condensate. The rate of this process is in good agreement with the sodium experiment in a wide range of magnetic fields. The three-body recombination at finite temperatures is also discussed. [Preview Abstract] |
Monday, March 13, 2006 1:39PM - 1:51PM |
B43.00013: The Vortex Phase Qubit: Generating Arbitrary, Counter-Rotating, Coherent Superpositions in Bose-Einstein Condensates via Optical Angular Momentum Beams Kishor Kapale, Jonathan Dowling We propose a scheme for generation of arbitrary coherent superposition of vortex states in Bose-Einstein condensates (BEC) using the orbital angular momentum (OAM) states of light. We devise a scheme to generate coherent superpositions of two counter-rotating OAM states of light using known experimental techniques. We show that a specially designed Raman scheme allows transfer of the optical vortex superposition state onto an initially non-rotating BEC. This creates an arbitrary and coherent superposition of a vortex and anti-vortex pair in the BEC. The ideas presented here could be extended to generate entangled vortex states, design memories for the OAM states of light, and perform other quantum information tasks. [Preview Abstract] |
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