Spatial coherence and density correlations of trapped Bose gases

TL;DR

This paper investigates the spatial coherence and density correlations in trapped Bose gases, highlighting how these correlations depend on temperature, position, and atomic interactions, and discussing implications for recent experiments.

Contribution

It provides a detailed analysis of first and second order coherence in trapped Bose gases, emphasizing the local homogeneity approximation and the influence of interactions and collective excitations.

Findings

Correlations are well-described by a locally homogeneous gas model.

Atomic interactions and clustering tendencies significantly influence second order correlations.

Recent experiments' interpretations of coherence measurements need revision due to weaker relations than previously assumed.

Abstract

We study first and second order coherence of trapped dilute Bose gases using appropriate correlation functions. Special attention is given to the discussion of second order or density correlations. Except for a small region around the surface of a Bose-Einstein condensate the correlations can be accurately described as those of a locally homogeneous gas with a spatially varying chemical potential. The degrees of first and second order coherence are therefore functions of temperature, chemical potential, and position. The second order correlation function is governed both by the tendency of bosonic atoms to cluster and by a strong repulsion at small distances due to atomic interactions. In present experiments both effects are of comparable magnitude. Below the critical temperature the range of the bosonic correlation is affected by the presence of collective quasi-particle excitations. The results of some recent experiments on second and third order coherence are discussed. It is shown that the relation between the measured quantities and the correlation functions is much weaker than previously assumed.