4TH STATISTICAL ENSEMBLE FOR THE BOSE-EINSTEIN CONDENSATE

We study the Bose-Einstein condensate (BEG) with a fixed number of par ticles. On the basis of conventional statistical mechanics we introduc e the, so called, Maxwell's demon ensemble, where only particle transf er (without energy exchange) is allowed. We show that this new ensembl e can be used for the microcanonical description of the system. We app ly our formalism to the case of BEC in a harmonic trap and give the an alytic expressions for the ground state fluctuations. We compare these expressions with the exact numerical results obtained for relatively small condensates.