An Empirical Formula for the Measurement of Spatial Coherence of Trapped Bose Gases

An empirical formula, which demonstrates the interference fringe visibility as a function of the slit separation and the temperature of thermal atoms, is obtained by analysing a recent experiment Bloch et al. Nature 403 (2000) 166 that refers to the spatial correlation function of a trapped Bose gas below the critical temperature. We find that the decay rate of the coherence function for the non-condensed component is of about two orders larger than that of the pure condensed component. The changes of the interference fringe visibility versus the falling time for different temperatures and slit separations are also discussed.