Collective oscillations of two colliding Bose-Einstein condensates
  Two 87Rb condensates (F=2, m_f=2 and m_f=1) are produced in highly displaced harmonic traps and the collective dynamical behaviour is investigated. The mutual interaction between the two condensates is evidenced in the center-of-mass oscillations as a frequency shift of 6.4(3)%. Calculations based on a mean-field theory well describe the observed effects of periodical collisions both on the center-of-mass motion and on the shape oscillations. 
P. Maddaloni, M. Modugno, C. Fort, F. Minardi, and M. Inguscio