Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session P32: Bosons in Optical Lattices |
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Sponsoring Units: DAMOP Chair: Lincoln Carr, Colorado School of Mines Room: Colorado Convention Center 402 |
Wednesday, March 7, 2007 11:15AM - 11:27AM |
P32.00001: Paired phases of bosons in optical lattices Stephen Powell, Subir Sachdev We describe the conditions under which bosons in optical lattices can form paired condensates, focusing on the case of bosons with spin. We show that the ground state of such a system, with sufficiently strong spin-dependent interactions, is a \textit{spin-singlet condensate}, which preserves spin-rotation symmetry. We then consider the gapped single-particle excitations across the phase transition from the insulator, and show that they have nontrivial scaling behavior, determined by coupling to the critical pair modes. [Preview Abstract] |
Wednesday, March 7, 2007 11:27AM - 11:39AM |
P32.00002: Edge States in Cold Atom Optical Lattices Vito Scarola, Sankar Das Sarma We argue that edge state response to external potentials applied to trapped insulators in cold atom optical lattices offer a unique probe of bulk physics. As an example we study the trapped Bose-Hubbard model using Gutzwiller mean-field theory. We calculate the response of Mott insulator edge states to external potentials. We show that the response leads to observables which may be extracted from time of flight measurements. [Preview Abstract] |
Wednesday, March 7, 2007 11:39AM - 11:51AM |
P32.00003: Superfluid to Mott Transition in the Bose Hubbard Model: Evidence for New Modes Naoki Kawashima, Yasuyuki Kato, Chiara Menotti, Nandini Trivedi Using a combination of methods (mean-field theory, fluctuations within random phase approximation, and quantum Monte Carlo simulations), we determine the nature of the phases of the Bose Hubbard model. In addition to the sound mode, we find evidence for extra gapped modes in the correlated superfluid phase from the location of the poles of the Green function. We also calculate the effect of thermal and quantum fluctuations on the condensate fraction and compare with recent experiments in optical lattices. In particular, we have obtained the superfluid density and the order parameter independently which agree with each other deep in the condensate phase but disagree in the critical region. We also calculate the Green's function as a function of the distance and the imaginary time separation, from which we estimate the excitation gap of the boson quasi particles. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P32.00004: Lattice with a Twist: A Helical Waveguide for Ultracold Matter M. Bhattacharya The behavior of matter is governed by the geometry of the potential it experiences. We consider the construction of optical potentials with helical symmetry, which can confine cold atoms and molecules. Microparticles have been experimentally confined in similar potentials [1]. Using two counter-propagating Laguerre-Gaussian beams we show that this simple chiral system realizes a superlattice of helical waveguides for ultracold matter and allows experimental control of their number, helicity, radius, pitch as well as strength and aspect ratio of confinement. In the simplest nontrivial case the potential has double-helical symmetry, similar to DNA. In general the behavior of massive particles in a helical potential is expected to be rich due to the periodic modulation of their motion along the lattice; negative group velocities and effective masses are expected. Effects such as spin squeezing and Berry's phase are also possible. A helical waveguide can provide a phase hologram for atom-waves, and perhaps support geometrically bound states. We will also address the curious possibility of simulating atom transport in carbon nanotubes. \newline [1] M. P. MacDonald \textit{et al.} Opt.Commun. \textbf{201},21(2002). [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P32.00005: Tunneling resonances and entanglement dynamics of ultracold bosons in a tilted two-well potential Dimitri Dounas-Frazer, Ann Hermundstad, Lincoln Carr We study the quantum sloshing of ultracold bosons in a tilted double-well potential via exact diagonalization of the two-mode Bose-Hubbard Hamiltonian. Tunneling is extremely sensitive to a small potential difference between wells, or tilt. However, when the barrier is high, atom-atom interactions can compensate the tilt and produce a tunneling resonance [1, 2].~ At resonance, tunneling times on the order of 10-100 ms are possible. Furthermore, tunneling resonances constitute a dynamic scheme for creating robust few-atom entangled states in the presence of many bosons. \newline [1] D. R. Dounas-Frazer and L. D. Carr, e-print: quant-ph/0610166 (2006). \newline [2] D. R. Dounas-Frazer, A. M. Hermundstad, and L. D. Carr, e-print: quant-ph/0609119 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P32.00006: Cavity QED determination of atomic number statistics in optical lattices. Wenzhou Chen, Dominic Meiser, Pierre Meystre We study the reflection of two counter-propagating modes of the light field in a ring resonator by ultracold atoms either in the Mott insulator state or in the superfluid state of an optical lattice. We obtain exact numerical results for a simple two-well model and carry out statistical calculations appropriate for the full lattice case. We find that the dynamics of the reflected light strongly depends on both the lattice spacing and the state of the matter-wave field. Depending on the lattice spacing, the light field is sensitive to various density-density correlation functions of the atoms. The light field and the atoms become strongly entangled if the latter are in a superfluid state, in which case the photon statistics typically exhibit complicated multimodal structures. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P32.00007: Bose-Einstein Condensates in Optical Lattices: Resonantly Enhanced Tunneling and Nonlinear Effects Alessandro Zenesini, Carlo Sias, Lignier Hans, Yeshpal Singh, Donatella Ciampini, Sandro Wimberger, Riccardo Mannella, Oliver Morsch, Arimondo Ennio In our experiments we study the tunneling between different sites of a periodic potential in the presence of an external force. As a consequence of Wannier-Stark localization of atomic wavefunctions inside the single lattice sites, \emph{Resonantly Enhanced Tunneling (RET)} occurs when the spacing between energy levels in a potential well is equal to the field-induced energy shift between different wells. These resonances are an important modification to the smooth Landau-Zener formula. We observed \emph{RET} using Bose-Einstein condensates in accelerated optical lattice potentials. We have perfect control over the parameters of this system: the depth of the lattice $U_0$, the recoil energy $E_{rec}$ and the peak density $n_0$ in the dipole trap. The latter determines the nonlinear interaction energy of the system, which allowed us to study the behavior of condensates in different regimes of the nonlinearity. In the linear case, as predicted in the Wannier-Stark solution, we observed \emph{RET} and we verified the dependence between the positions of the resonances and the lattice depth for tunneling between $1^{st}$, $2^{nd}$ and $3^{rd}$ neighboring sites. In the nonlinear regime, we observed a suppression of the resonances for increasing nonlinearity, in agreement with numerical simulations. [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P32.00008: Phase diagram for ultracold bosons in double-well optical lattices Ippei Danshita, James E. Williams, Carlos Sa de Melo, Charles W. Clark We study the superfluid-Mott insulator transition of bosons in double-well optical lattices. Applying a mean-field approximation to the Bose-Hubbard Hamiltonian, we obtain the zero-temperature phase diagram and find that there exist the half-integer-filling and integer-filling Mott insulator domains in the phase diagram. We show that the half-integer-filling Mott insulator phase is stabilized as the intra-well hopping energy increases. We also calculate the phase diagram by employing the time evolving block decimation (TEBD) algorithm and compare the results obtained from the mean-field approximation with those from the TEBD. [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P32.00009: On-site number statistics of ultracold lattice bosons Evgeny Kozik, Barbara Capogrosso-Sansone, Nikolay Prokof'ev, Boris Svistunov We study on-site occupation number fluctuations in a system of interacting bosons in an optical lattice. The ground-state distribution is obtained analytically in the limiting cases of strong and weak interaction, and by means of exact Monte Carlo simulations in the strongly correlated regime. As the interaction is increased, the distribution evolves from Poissonian in the non-interacting gas to a sharply peaked distribution in the Mott-insulator (MI) regime. In the special case of large occupation numbers, we demonstrate analytically and check numerically that there exists a wide interval of interaction strength, in which the on-site number fluctuations remain Gaussian and are gradually squeezed until they are of order unity near the superfluid (SF)-MI transition. Recently, the on-site number statistics were studied experimentally in a wide range of lattice potential depths [Phys. Rev. Lett. \textbf{96}, 090401 (2006)]. In our simulations, we are able to directly reproduce experimental conditions using temperature as the only free parameter. Pronounced temperature dependence suggests that measurements of on-site atom number fluctuations can be employed as a reliable method of thermometry in both SF and MI regimes. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P32.00010: Phases in an anisotropic two-dimensional optical lattice Sara Bergkvist, Anders Rosengren, Robert Saers, Emil Lundh, Magnus Rehn, Anders Kastberg We have studied the effects of anisotropy on a two-dimensional optical lattice using quantum Monte Carlo simulations. For finite lengths, such a system undergoes a one-dimensional quantum phase transition to a 1D Mott insulator of decoupled chains. Time of flight pictures and other measurable observables are calculated for a specified experimental setup. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P32.00011: Mach-Zehnder Interference of Boson Flavor States in the Excited Band of a 2D Optical Lattice John Challis, Steven Girvin, Leonid Levitov Bosons promoted to the first excited Bloch band of an optical lattice have two important properties: they are metastable, having lifetimes long compared to the nearest neighbor hopping rate, and they carry a ``flavor'' quantum number which controls the direction of highly anisotropic hopping in the lattice \footnote{A Isacsson and SM Girvin. Phys. Rev. A 72, 053604 (2005) }. For a 2D optical lattice where the laser beams are not quite perpendicular, there is a small energy which causes the flavors to mix. The two flavor states can be treated as a two-level system with an avoided crossing, with the relative intensity of the two laser beams serving as a tuning parameter controlling the energy difference between the two flavors. When the tuning parameter is varied sinusoidally around some nonzero offset, the avoided crossing acts like a beam splitter in a Mach-Zehnder interferometer \footnote{WD Oliver et al. Science Vol. 310, Issue 5754, pp. 1653-1657(2005)}. Since this offset is momentum dependent, the rate of flavor change varies throughout the Brillouin Zone. This fact leads to interesting time-dependent momentum distributions which should be readily observable experimentally by free expansion of the bosons. [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P32.00012: Exploring the correspondence principle with spinor condensates: from quantum Bloch oscillations to classical Bogoliubov excitations Reinhold Walser, Carsten Wei{\ss}, Oliver Crasser, Wolfgang Schleich By tuning the relative strength between single and two-body energies in a spinorial F=1 Bose-Einstein condensate (e.g. $^{87}$Rb), we can effectively control the dynamics of the macroscopic Fock-state [1,2,3]. We will study the static as well as dynamic aspects of this few mode quantum system and illustrate the ``classical'' as well as quantum aspects of this system, which can be realized in deep optical lattices.\\ \noindent [1] M-S. Chang {\em et al.}, Coherent spinor dynamics in a spin-1 Bose-Einstein condensate, Nature Phys., {\bf 1}, 111 (2005). \\ \noindent [2] J. Kronj{\"a}ger {\em et al.}, Evolution of a spinor condensate: coherent dynamics, dephasing and revivals, Phys. Rev. A, {\bf 72}, 063619 (2005).\\ \noindent [3] A. Widera {\em et al.}, Precision measurements of spin-dependent interaction strength for spin-1 and spin-2 $^{87}$Rb atoms, NJP, {\bf 8}, 152, (2006). [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P32.00013: Dispersive shock waves in optical lattices Shu Jia, Wenjie Wan, Jason Fleischer We study dispersive, superfluid-like shock waves in optical lattices. Compared with the homogeneous case, the presence of a periodic potential inhibits shock propagation, both via Peierls-Nabarro trapping forces and through Bragg reflections. Photonic experiments are performed in SBN photorefractive crystals, using optical induction to create nonlinear waveguide arrays. By applying defocusing (repulsive) nonlinearity, we directly observe the nonlinear properties of shock waves as a function of the intensity of lattice, $i.e.$ the depth of the potential wells. Direct comparisons are made between a plane-wave background, suitable for homogeneous systems, and Bloch-wave backgrounds, which are more appropriate for arrays. Nonlinear coupling between transmission bands is demonstrated, with numerical simulations showing excellent agreement with experimental results. Similarities between photonic systems and cold atom systems in periodic potentials will be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 1:51PM - 2:03PM |
P32.00014: Evolution of Hard-Core Bosons in a Time-Dependent Trap Aditya Raghavan, Marcos Rigol, Stephan Haas We present a study of the time evolution of hard-core bosons (HCB) in a one-dimensional, time-varying optical trap. Previous results have shown that one-dimensional HCBs can form superfluid and Mott-insulator phases. Using numerical techniques in the Bose-Hubbard model, we explore different types of time variations, such as sinusoidally varying trap curvature, using either initial configurations (filling {\&} trap curvature) of a superfluid or a Mott-insulator. ~When the curvature of the optical trap is suddenly increased, we observe a ``melting'' of the Mott-insulator. The approximate numerical technique used to study time-varying traps is discussed. [Preview Abstract] |
Wednesday, March 7, 2007 2:03PM - 2:15PM |
P32.00015: Quantum phases and phase transitions in bosonic mixtures induced by non-s-wave Feschbach resonances in optical lattices Anatoly Kuklov Feschbach resonance at finite angular momentum in a mixture of distinguishable bosons in optical lattice (OL) can induce quantum phase transitions (QPTs) into states which break OL symmetries and time reversal. In particular, a two-component mixture, with one component being superfluid and the other Mott insulator, can undergo QPT into, e.g., p-wave condensate characterized by lines of zeros, spontaneous currents and by strong quantum depletion \footnote[2]{A.B.Kuklov, PRL {\bf 97}, 110405(2006)}. The ground state is sensitive to rotation of OL. Analogously, a featureless two-component Mott insulator can undergo QPT into the insulator with broken lattice symmetries. While impossible for an absolute ground state, such effect can be realized in the context of metastable phases generic for atomic traps and OLs as long as there is a large energy difference between the resonance and true molecular ground state. A condition for such transition is that the closed-open channels coupling exceeds the onsite excitation energy in the regime of weak tunneling between sites. Standard imaging techniques can be used to identify such phases. [Preview Abstract] |
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