Interference effects between two initially independent Bose-condensed   gases
  When two initially independent Bose-condensed gases are allowed to overlap, we investigate the density expectation value of the whole system by using the second quantization method. In the presence of interatomic interaction, based on the exact expression of the density expectation value, it is found that there is a nonzero interference term in the density expectation value of the whole system. The evolution of the density expectation value is shown for different coupling constants. The present work shows clearly that there is an interaction-induced coherence process between two initially independent condensates. 
Hongwei Xiong, Shujuan Liu