Having it Both Ways: Distinguishable Yet Phase-Coherent Mixtures of   Bose-Einstein Condensates
  We have begun a series of experiments on mixed bosonic quantum fluids. Our system is mixed Bose-Einstein condensates in dilute Rb-87. By simultaneously trapping the atoms in two different hyperfine states, we are able to study the dynamics of component separation and of the relative quantum phase of two interpenetrating condensates. Population can be converted from one state to the other at a rate that is sensitive to the relative phase. 
E. A. Cornell, D. S. Hall, M. R. Matthews, and C. E. Wieman