An exact stochastic field method for the interacting Bose gas at thermal equilibrium

We present a new exact method to numerically compute the thermodynamical properties of an interacting Bose gas in the canonical ensemble. As in our previous paper (Carusotto I, Castin Y and Dalibard J 2001 Phys. Rev. A 63 023606), we write the density operator ρ as an average of Hartree dyadics |N:ϕ₁⟩⟨N:ϕ₂| and we find stochastic evolution equations for the wavefunctions ϕ_1,2 such that the exact imaginary-time evolution of ρ is recovered after averaging over noise. In this way, the thermal equilibrium density operator can be obtained for any temperature T. The method is then applied to study the thermodynamical properties of a homogeneous one-dimensional N-boson system: although Bose-Einstein condensation cannot occur in the thermodynamical limit, a macroscopic occupation of the lowest mode of a finite system is observed at sufficiently low temperatures. If k_BT>>µ, the main effect of interactions is to suppress density fluctuations and to reduce their correlation length. Different effects such as a spatial antibunching of the atoms are predicted for the opposite k_BT⩽µ regime. Our exact stochastic calculations have been compared with existing approximate theories.