Interaction Effects on Number Fluctuations in a Bose-Einstein Condensate   of Light
  We investigate the effect of interactions on condensate-number fluctuations in Bose-Einstein condensates. For a contact interaction we variationally obtain the equilibrium probability distribution for the number of particles in the condensate. To facilitate comparison with experiment, we also calculate the zero-time delay autocorrelation function $g^{(2)}(0)$ for different strengths of the interaction. Finally, we focus on the case of a condensate of photons and discuss possible mechanisms for the interaction. 
E.C.I. van der Wurff, A.-W. de Leeuw, R.A. Duine, and H.T.C. Stoof
cond-mat.quant-gas