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Physical Review A

Ballistic expansion of trapped thermal atoms

Li You, Murray Holland

Published Jan 1, 1996• 30 citations
Publisher

Abstract

We study the time evolution of the distribution function for an ensemble of classical particles in a harmonic trap when the oscillation frequencies are varied with time. We compare our results with the near circular density distributions observed for the thermal part of the expanded cloud in a recent experiment demonstrating the Bose-Einstein condensation of an alkali-metal vapor. The shape and extent of this noncondensed component characterize the temperature of the original cloud.

DOI
10.1103/physreva.53.r1

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We study the time evolution of the distribution function for an ensemble of classical particles in a harmonic trap when the oscillation frequencies are varied with time. We compare our results with the near circular density distributions observed for the thermal part of the expanded cloud in a recent experiment demonstrating the Bose-Einstein condensation of an alkali-metal vapor. The shape and extent of this noncondensed component characterize the temperature of the original cloud.
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We study the temporal expansion of an ultracold Bose gas in two-dimensional square optical lattices. The gas is described by the Bose-Hubbard model deep in the superfluid regime, with initially all bosons condensed in the central site of the lattice. We use the previously developed nonequilibrium propagator method for capturing the time evolution of an interacting bosonic system, where the many-body Hamiltonian is represented in an appropriate local basis and the corresponding field operators are separated into the classical [Bose-Einstein condensate (BEC)] part and quantum mechanical fluctuations. After a quench, i.e., after a sudden switch of the lattice nearest-neighbor hopping, the expanding bosonic cloud separates spatially into a fast ballistic forerunner and a slowly expanding central part controlled by self-trapping. We show that the forerunner expansion is driven by the coherent dynamics of the BEC and that its velocity is consistent with the Lieb-Robinson bound. For smaller lattices we analyze how quasiparticle collisions lead to enhanced condensate depletion and oscillation damping.