Dynamical response of a Bose-Einstein condensate to a discontinuous   change in internal state
  A two-photon transition is used to convert an arbitrary fraction of the 87Rb atoms in a |F=1,m_f=-1> condensate to the |F=2,m_f=1> state. Transferring the entire population imposes a discontinuous change on the condensate's mean-field repulsion, which leaves a residual ringing in the condensate width. A calculation based on Gross-Pitaevskii theory agrees well with the observed behavior, and from the comparison we obtain the ratio of the intraspecies scattering lengths for the two states, a_|1,-1> / a_|2,1> = 1.062(12). 
M.R. Matthews, D.S. Hall, D.S. Jin, J.R. Ensher, C.E. Wieman, and E.A.   Cornell, F. Dalfovo, C. Minniti, and S. Stringari