Excitation spectroscopy of vortex states in dilute Bose-condensed gases
  We apply linear-response analysis to the Gross-Pitaevskii equation to obtain the excitation frequencies of a Bose-Einstein condensate in a vortex state, and apply it to a system of Rubidium atoms confined in a time-averaged orbiting potential trap. The excitation frequencies of a vortex differ significantly from those of the ground state, and may therefore be used to obtain a spectroscopic signature of the presence of a vortex state. 
R. J. Dodd, K. Burnett, Mark Edwards, C. W. Clark