Finite temperature coherence of the ideal Bose gas in an optical lattice

TL;DR

This paper analyzes how thermal decoherence affects the coherence properties of a non-interacting Bose gas in an optical lattice at finite temperatures, clarifying the role of ideal gas thermodynamics versus many-body effects.

Contribution

It provides a detailed analysis of thermal decoherence effects on a non-interacting Bose gas in optical lattices, distinguishing between thermodynamic and many-body contributions to coherence loss.

Findings

The decrease in condensate fraction can be explained by ideal gas thermodynamics up to a certain lattice height.

Sharper coherence decreases in stronger lattices are due to many-body physics.

Fringe visibility 'kinks' may result from the interplay of lattice strength and gas density.

Abstract

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 non-trivial many-body dynamics. In order better to calibrate such measurements, we analyse 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 non-interacting 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 F.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.