Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session S36: Trapped Atoms in Optical Lattices |
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Sponsoring Units: DAMOP Chair: James Williams, NIST-Gaithersburg Room: LACC 510 |
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S36.00001: Bragg Spectroscopy of Excitations of a Quantum Bose Gas in a Lattice Xu Du, Emek Yesilada, Changhyun Ryu, Shoupu Wan, Daniel Heinzen We have measured the excitation spectrum of a quantum degenerate Bose gas in an optical lattice with Bragg spectroscopy. We begin each cycle of the experiment by producing a magnetically trapped $^{87}$Rb Bose condensate. We then superimpose a three-dimensional optical lattice of cubic symmetry onto the condensate. We turn the lattice potential on adiabatically, so that the gas temperature remains very close to zero. This provides an experimental realization of the Bose-Hubbard model, which exhibits a quantum phase transition between a superfluid and an insulating state. We find that in the superfluid state, the resonant excitation energy in the phonon-like regime decreases with increasing lattice strength. In the insulating regime, we observe the appearance of a sharp increase in the excitation rate at non-zero frequencies, which we interpret as a measurement of the gap in the insulating state of the gas. [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S36.00002: Calculation of collective modes of atoms in an optical lattice Sara Bergkvist, Emil Lundh, Patrik Henelius, Anders Rosengren The excitation spectrum of bosons in an optical lattice with a confining potential has been studied using quantum Monte Carlo. Moments of the strength function have been calculated from the ground state expectation values. From these moments information about the collective behavior of the atoms has been extracted. The response of the system to two different perturbations has been examined. [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S36.00003: Single-Particle Excitations of a Bose Gas in an Optical Lattice Near the Mott Transition Satoru Konabe, Tetsuro Nikuni, Masaaki Nakamura We study single-particle excitations of a Bose gas in an optical lattice near the Mott transition. We derive the excitation spectra in both Mott insulator phase and superfluid phase. The characteristic feature in the Mott insulator phase is the existence of an energy gap between the particle and hole excitations. We show that this energy gap can be directly probed by an output coupling experiment. Applying the general expression for the output current derived by Luxat and Griffin to the Mott insulator phase, we find that the energy spectrum of the momentum-resolved output current exhibits two characteristic peaks corresponding to the particle and hole excitations. Thus, it can be used to detect the transition point from the Mott insulator to superfluid phase where the energy gap disappears. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S36.00004: Surface Waves at the Mott Insulator-Superfluid interface for confined BEC Eros Mariani, Ady Stern Bose Einstein Condensates in optical lattices under an external confinement are expected to form Superfluid and Mott-Insulating domains [1] reminiscent of the quantum phase transitions for the infinite system [2]. The experimentally observed transition [3] is presently ascribed to relative shrinking/widening of the different domains. The interfaces between the two phases and their excitations therefore play a crucial role in the time evolution of the system under an external perturbation as well as in its thermodynamic properties. In this work we derive the dispersion relation of the surface waves at the interfaces between Mott-Insulating and Superfluid phases. We then calculate their contribution to the heat capacity of the system and show how its low temperature scaling allows a direct experimental test of the existence and properties of Mott-Superfluid domains [4]. [1] D. Jaksch et al. , Phys. Rev. Lett. \textbf{81}, 3108 (1998) [2] M. P. A. Fisher et al. , Phys. Rev. B \textbf{40}, 546 (1989) [3] M. Greiner et al. , Nature \textbf{415}, 39 (2002) [4] E. Mariani and A. Stern, to be submitted to Phys. Rev. Lett. (2004) [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S36.00005: Inhomogeneous Mott phases of bosons in optical lattices Courtney Lannert, Brian DeMarco, Smitha Vishveshwara, Tzu-Chieh Wei The feasibility of creating a spatially-inhomogeneous phase of neutral bosonic atoms (such as Rubidium) in which multiple Mott-insulating states coexist in a shell structure is discussed and analyzed. In the set-up of trapped, tightly confined bosons in an optical lattice, we derive the conditions on experimental parameters, such as number of bosons and the curvature of the trapping potential, for creating an onion-like structure of any prescribed number of Mott shells with different occupation numbers. We discuss the stability of such a structure with an eye toward experimental conditions necessary for measuring the structure with spectroscopic probes. The analyses ought to give a controlled means of achieving the Mott groundstate observed in previous experiments, and give some constraints for how successfully these states can be employed for quantum computing. [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S36.00006: Spectroscopic Probe of Inhomogeneous Mott Phases Smitha Vishveshwara, Brian DeMarco, Courtney Lannert, Tzu-Chieh Wei The feasibility of creating a spatially-inhomogeneous phase of neutral bosonic atoms (such as Rubidium) in which multiple Mott-insulating states coexist in a shell structure is discussed and analyzed. In the set-up of trapped, tightly confined bosons in an optical lattice, we derive the conditions on experimental parameters, such as number of bosons and the curvature of the trapping potential, for creating an onion-like structure of any prescribed number of Mott shells with different occupation numbers. We discuss the stability of such a structure with an eye toward experimental conditions necessary for measuring the structure with spectroscopic probes. The analyses ought to give a controlled means of achieving the Mott groundstate observed in previous experiments, and give some constraints for how successfully these states can be employed for quantum computing. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 3:54PM |
S36.00007: Atomic pair statistics and adiabatic realization of the Mott state in an optical lattice Guido Pupillo, Carl J. Williams, Nikolay V. Prokof'ev In a series of recent experiments, several groups demonstrated the experimental realization of a Mott insulator state, created by loading a trapped atomic Bose-Einstein condensate into an optical lattice. A superfluid-insulator transition is then induced by varying the intensity of lattice laser beams. When the average filling of lattice sites at the trap center is of order one, double occupancy of lattice sites in the Mott state is both consequence of zero-temperature mixing of high energy basis states into the ground state and of finite temperature population of high energy states. Finite temperature may be due to imperfect adiabaticity while increasing the intensity of the lattice laser beams. In this talk we discuss the distribution of atomic pairs in the trapped Mott insulator relevant to current experiments and suggest that statistics of detection of atomic pairs may be used to bound the temperature of the trapped atoms for energies well below the lattice level spacing. We discuss time scales for adiabatic realization of the Mott state in the trap. [Preview Abstract] |
Wednesday, March 23, 2005 3:54PM - 4:06PM |
S36.00008: Evidence for Phase Variance Oscillations in a Bose-Einstein Condensate and Applications to Precision Interferometry Ari Tuchman, Chad Orzel, Anatoli Polkovnikov, Mark Kasevich We report the dynamic restoration of phase coherence after generating an array of highly squeezed number states loaded from a Bose-Einstein Condensate (BEC) into an optical lattice. We induce oscillations in the phase variance of the array by rapidly reducing the intensity of the lattice. This sequence projects the number squeezed array onto a superfluid groundstate, inducing phase variance oscillations as the quantum state evolves. It is critical to recognize that the experimental signature, of oscillations in the interference contrast, can be nearly reproduced by driving semiclassical excitations. However, by comparing data both with the semiclassical GPE where a coherent state array with identical initial phases is time evolved and with a model which accounts for an initial state with large quantum fluctuations, we find evidence supporting the quantum mechanical nature of these oscillations. We further discuss applications of these phase variance oscillations to precision interferometry. [Preview Abstract] |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S36.00009: Role of quantum fluctuations in the dissipative dynamics of a 1D Bose gas in an optical lattice Ana Maria Rey, Julio Gea-Banacloche, Guido Pupillo, Carl J. Williams, Charles W. Clark We will present a theoretical treatment[1] of the surprisingly large damping observed recently in a experiment done at NIST [2] where the transport properties of a harmonically trapped 1D Bose gas in a periodic (optical lattice) potential were studied by observing small amplitude dipole oscillations. In the absence of the lattice these oscillations are expected to be undamped (generalized Kohn's theorem), however, large damping of the dipole mode was observed in the experiment for very weak optical lattices and very small cloud displacements. We will show that the observed damping can be derived from a model whose main ingredients are (a) a large noncondensate fraction that arises as a direct consequence of the enhanced effective on-site interaction due to the tight transverse confinement, (b) the fact that a non-negligible part of it occupies high-momentum states and is therefore affected by dynamical instabilities, and (c) the interaction of the condensate atoms with the random field created by these noncondensate atoms when their equilibrium state is perturbed. We find good agreement between the model and the experimental results. [1] Julio Gea-Banacloche \textit{et al. }cond-mat/0410677. [2] C. D. Fertig, K. \textit{et al.}cond-mat/0410491. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S36.00010: Effect of Quantum Fluctuations on the Dipolar Motion of Bose-Einstein Condensates in Optical Lattices Daw-Wei Wang, A. Polkovnikov We reexamine dipolar motion of condensate atoms in one-dimensional optical lattices and harmonic magnetic traps including quantum fluctuations within the truncated Wigner approximation. In the strong tunneling limit we reproduce the mean field results with a sharp dynamical transition at the critical displacement. When the tunneling is reduced, on the contrary, strong quantum fluctuations lead to finite damping of condensate oscillations even at infinitesimal displacement. We show that there is a smooth crossover between the chaotic classical transition at finite displacement and the superfluid-to-insulator phase transition at zero displacement. We further analyze the time dependence of the density fluctuations and of the coherence of the condensate and find several nontrivial dynamical effects, which can be observed in the present experimental conditions.Many of our prediction has been observed recently by C. D. Fertig et al, in cond-mat/0410491. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S36.00011: Superfluid Insulator transition of two species ultracold bosons in an optical lattice K. Sengupta, M-C Cha, A. Issacson, S.M. Girvin We analyze the superfluid-insulator (SI) transition for a two-species, ultracold bosonic atoms confined in an optical lattice for odd filling at commensurate densities. We find that in contrast to the even-filling case, the SI transition, for most experimentally accessible parameter ranges, occurs either a) with complete depopulation of one species or b) with simultaneous onset of superfluidity for both species or c) with superfluidity of one species and Mott insulator of another species. The analysis consists of a analytical mean-field study of the SI transition using a variational wave function and numerical treatments which includes canonical transformation method and a quantum Monte Carlo study. The numerical studies allow us to study the effect of quantum fluctuations on the SI transition and point out the domain of applicability of the mean-field theory. [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S36.00012: $^{87}$Rb in a double well optical lattice Ian Spielman, Chad Fertig, Johnny Huckans, James Porto, William Phillips Bose condensed alkali gasses present an ideal venue for the study of weakly interacting, phase coherent quantum phenomena. The subsequent application of an optical lattice can controllably increase the importance of interactions. Such a strongly interacting, yet tunable, system is interesting of its own right; consider for example the recent direct observation of a Mott insulator-superfluid transition. More practically, however, these strong interactions coupled with long coherence times, and easily tuned parameters suggest this system as a potential quantum information processor. In this talk, we first introduce a scheme for 2 qbit gates between pairs of $^{87}$Rb atoms in a tunable double-well optical lattice. An external phase sets the barrier between wells and can be varied externally; thus, atoms in each well can be controllably interacted. Here we present preliminary experimental results demonstrating loading of the individual lattice sites, and comment on the effects of changing the barrier potential with the loaded atoms. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:06PM |
S36.00013: Quantum walks with ultracold atoms in optical lattices Stuart van der Lee, David Feder The behavior of several ultracold atoms (bosons or fermions) undergoing a quantum walk in a one-dimensional optical lattice is investigated numerically. Both discrete and continuous time quantum walks are implemented, the latter within the context of a tight-binding model. Because the quantum statistics place constraints on the overlap between different many-particle states, the Hamiltonian generates a one-particle quantum walk on a graph with vertices of higher degree. The results will be used to make predictions for experiments with ultracold atoms in optical lattices, as well as to explore fundamental issues related to quantum information, such as graph covering and the role of entanglement. [Preview Abstract] |
Wednesday, March 23, 2005 5:06PM - 5:18PM |
S36.00014: Dynamics of Bose condensates in an optical lattice with a basis Wen-Chin Wu, Chou-Chun Huang Dynamics of atomic Bose-Einstein condensates in an optical lattice with a basis is investigated. For a 1D optical lattice of two types of potential barrier within a unit cell, similar to the case of a crystal lattice with two-atom basis in a unit cell, acoustic as well as optical phonons can propagate along the lattice of atom clouds. These are in addition to in-phase and out-of-phase collective excitations of the condensates. The dispersions of phonons depend crucially on the relative size of two tunneling amplitudes ($J_1$ and $J_2$) across the two barriers and the ratio of $J_1,J_2$ to the repulsion U between the atoms. Using a variational method, the effect of condensate breathing modes on the phonons is studied in details. The dynamic structure factor of the system is also studied. [Preview Abstract] |
Wednesday, March 23, 2005 5:18PM - 5:30PM |
S36.00015: Band Effects in Optical Lattices Containing Cold Atoms Vito Scarola, Sankar Das Sarma The possibility of manipulating band structure to engineer novel, many-body ground states of cold bosons in optical lattices is discussed. Effective Hamiltonians are derived using the realistic band structure of one and two dimensional systems. We analyze the mean field phase diagram and stability of these models in experimentally relevant parameter regimes. [Preview Abstract] |
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