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Canonical ensemble of an interacting Bose gas: stochastic matter fields and their coherence

We present a novel quantum stochastic evolution equation for a matter field describing the canonical state of a weakly interacting ultracold Bose gas. In the ideal gas limit our approach is exact. This numerically very stable equation suppresses high-energy fluctuations exponentially, which enables us to describe condensed and thermal atoms within the same formalism. We present applications to ground state occupation and fluctuations, density profile of ground state and thermal cloud, and ground state number statistics. Our main aim are spatial coherence properties which we investigate through the determination of interference contrast and spatial density correlations. Parameters are taken from actual experiments [1]. [1] S. Hofferberth et al., Nature Physics 4, 489 (2008).