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arXiv:cond-mat/0701420

Visibility of a Bose-condensed gas released from an optical lattice at finite temperatures

Fabrice Gerbier, Simon Foelling, Artur Widera, Immanuel Bloch

Published Jan 17, 2007• 4 citations
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Abstract

In response to a recent manuscript [cond-mat/0609685] on the analysis of interference patterns produced by ultracold atoms released from an optical lattice, we point out that in the presence of a Bose-Einstein condensate the interference pattern can be strongly modified by interaction effects and the presence of a harmonic trap superimposed on the lattice potential. Our results show that the visibility of the interference pattern is significant only if a sizeable condensate fraction is present in the trap.

DOI
10.48550/arxiv.cond-mat/0701420

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In response to a recent manuscript [cond-mat/0609685] on the analysis of interference patterns produced by ultracold atoms released from an optical lattice, we point out that in the presence of a Bose-Einstein condensate the interference pattern can be strongly modified by interaction effects and the presence of a harmonic trap superimposed on the lattice potential. Our results show that the visibility of the interference pattern is significant only if a sizeable condensate fraction is present in the trap.
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In current experiments with cold quantum gases in periodic potentials, interference fringe contrast is typically the easiest signal in which to look for effects of nontrivial many-body dynamics. In order better to calibrate such measurements, we analyze the background effect of thermal decoherence as it occurs in the absence of dynamical interparticle interactions. We study the effect of optical lattice potentials, as experimentally applied, on the condensed fraction of a noninteracting Bose gas in local thermal equilibrium at finite temperatures. We show that the experimentally observed decrease of the condensate fraction in the presence of the lattice can be attributed, up to a threshold lattice height, purely to ideal-gas thermodynamics; conversely, we confirm that sharper decreases in first-order coherence observed in stronger lattices are indeed attributable to many-body physics. Our results also suggest that the fringe visibility ``kinks'' observed in Gerbier et al., Phys. Rev. Lett. 95, 050404 (2005) may be explained in terms of the competition between increasing lattice strength and increasing mean gas density, as the Gaussian profile of the red-detuned lattice lasers also increases the effective strength of the harmonic trap.
We discuss the experimental signal for the Bose-Einstein condensation of cold atoms in an optical lattice at finite temperature. Instead of using the visibility of the interference pattern via time-of-flight imaging, we show that the momentum-space density profile in the first Brillouin zone, in particular its bimodal distribution, provides an unambiguous signal for the Bose-Einstein condensation. We confirm this point with detailed calculations of the change in the atomic momentum distribution across the condensation phase transition, taking into account both the global trapping potential and the atomic interaction effects.
Abstract We have studied the interference of degenerate quantum gases in a vertical optical lattice. The coherence of the atoms leads to an interference pattern when the atoms are released from the lattice. This has been shown for a Bose‐Einstein condensate in early experiments. Here we demonstrate that also for fermions an interference pattern can be observed provided that the momentum distribution is smaller than the recoil momentum of the lattice. Special attention is given to the role of interactions which wash out the interference pattern for a condensate trapped in the lattice but do not affect a spin polarized Fermi gas, where collisions at ultra cold temperatures are forbidden. Comparing the interference of the two quantum gases we find a clear superiority of fermions for trapped atom interferometry.
For the Bose-condensed gas in a combined potential consisting of an axially-symmetric harmonic magnetic trap and one-dimensional (1D) optical lattice, using the mean-field Gross-Pitaevskii (G-P) equation and the propagator method, we study the formation of matter-wave interference upon releasing the condensate from the combined potential. The analytical result of the wave function for matter wave interference at any time t is given, and the relation between the interference fringe period and the optical lattice period is obtained. The interference pattern of Bose-condensed gas in the optical lattices is analogous to the multiple slit interference pattern in optics, and coincides with the results of experiment performed by Andrews et al. and Peil et al. and the numerical solution of the G-P equation by Sadhan K. Adhikari.