Generation of macroscopic pair-correlated atomic beams by four-wave   mixing in Bose-Einstein condensates
  By colliding two Bose-Einstein condensates we have observed strong bosonic stimulation of the elastic scattering process. When a weak input beam was applied as a seed, it was amplified by a factor of 20. This large gain atomic four-wave mixing resulted in the generation of two macroscopically occupied pair-correlated atomic beams. 
J.M. Vogels, K. Xu, W. Ketterle