Quadrupole Oscillation of a Single-Vortex Condensate: Evidence for   Kelvin Modes
  We study the two transverse quadrupole modes of a cigar-shaped Bose-Einstein condensate with a single centered vortex. We show that the counter-rotating mode is more strongly damped than in the absence of a vortex, whereas the co-rotating mode is not affected appreciably by the vortex. We interpret this result as a decay of the counter-rotating quadrupole mode into two excitations of the vortex line, the so-called Kelvin modes. This is supported by direct observation of the wiggling vortex line. 
V. Bretin, P. Rosenbusch, F. Chevy, G.V. Shlyapnikov, and J. Dalibard