Superfluid and Dissipative Dynamics of a Bose-Einstein Condensate in a Periodic Optical Potential

Abstract

We create Bose-Einstein condensates of 87 Rb in a static magnetic trap with a superimposed blue-detuned 1D optical lattice. By displacing the magnetic trap center we are able to control the condensate evolution. We observe a change in the frequency of the center-of-mass oscillation in the harmonic trapping potential, in analogy with an increase in effective mass. For fluid velocities greater than a local speed of sound, we observe the onset of dissipative processes up to full removal of the superfluid component. A parallel simulation study visualizes the dynamics of the BEC and accounts for the main features of the observed behavior. 03.75.Fi, 32.80.Pj, 67.57.De Bose-Einstein condensates (BEC) in dilute atomic gases are macroscopic quantum systems which can be manipulated by a variety of experimental techniques [1]. The current development of such techniques is opening up a wealth of possibilities to explore new physics, e.g., in non-linear atom optics [2], and to study various aspects of superfluid behavior in the precisely controllable context of atomic physics [3].Atoms confined in a periodic potential share some properties with systems of electrons in crystals. Effects known from solid state physics, like Bloch oscillations and Wannier-Stark ladders, have been observed by exposing cold atoms to the dipole potential of far detuned optical lattices [4]. Macroscopic quantum interference has been observed in an experiment on a BEC confined to the antinodes of a far detuned optical lattice [5]. Bragg diffraction from a condensate has been induced in moving optical lattices [6]. This has been used, e.g., as an atom-laser outcoupler [7] and as a tool for spectroscopy of the momentum in BEC's [8]. Applications of BEC's in periodic potentials range from matter-wave transport [9] to interferometry [5] and quantum computing [10]. The question of the stability of the BEC during the evolution in optical potentials is crucial for these applications and has been addressed in theoretical works [11].In this Letter we report on some novel aspects of superfluidity in BEC's by studying their center-of-mass oscillations inside the harmonic potential of a magnetic trap in presence of a one-dimensional (1D) optical lattice. We identify different dynamical regimes by varying the initial displacement of the BEC from the bottom of the trap. For small displacements the BEC performs undamped oscillations in the harmonic potential and feels the periodic potential only through a shift in the oscillation frequency. At larger displacements we observe the onset of dissipative processes appearing through a damping in the oscillations. We can describe the experimental results in terms of an inhomogeneous superfluid having a density-dependent critical velocity. In parallel we report numerical studies of the Gross-Pitaevskii equation (GPE), which capture the main features of the observed dynamics.In our experimental setup [12] we now produce BEC's of 87 Rb atoms in the (F=1,m F = −1) state. The fundamental frequencies of our Ioffe-type magne...

Superfluid current disruption in a chain of weakly coupled Bose–Einstein condensates

“…We also observe the appearance of an intermediate regime where no oscillations are visible while the relative phase coherence is not completely lost. In a previous experiment we have investigated the regime of small periodic potential height, showing that in this regime and for large atom numbers, the mechanism for superfluidity breakdown was in agreement with predictions based on the onset of the Landau instability [12]. In the present experiment we explore the dynamics of the system in the case of a large periodic potential height, where a modulational instability is expected to play a fundamental role [8,9].…”

Bose-Einstein condensates in optical lattices: Band-gap structure and solitons

“…[2,3,4,5,6,7,8]). Coupling between the BEC droplets localized in weakly interacting minima of a standing light wave creates a reconfigurable matter-wave structure that can be easily manipulated by varying the strength of the periodic potential and its well spacing.…”

Fast frictionless expansion of an optical lattice

“…Let us now discuss dipole oscillation. In the experiment for 3-D BEC [36], the trap in the presence of the optical lattice was suddenly shifted. It was observed that the subsequent oscillations of the BEC are undamped if the initial displacement is small, but become dissipative if the initial displacement exceeds a critical value.…”

“…Due to strong quantum fluctuations, mean-field theories fail for the description of the system. Consider an experiment like the ones in [36,37] to study dipole oscillation for an accordion lattice. The amplitude of the small dipole oscillation increases with Λ without leading instability since the lattice is expanding (5).…”