  We report on measurements of the excitation spectrum of a strongly
interacting Bose-Einstein condensate (BEC). A magnetic-field Feshbach resonance
is used to tune atom-atom interactions in the condensate and to reach a regime
where quantum depletion and beyond mean-field corrections to the condensate
chemical potential are significant. We use two-photon Bragg spectroscopy to
probe the condensate excitation spectrum; our results demonstrate the onset of
beyond mean-field effects in a gaseous BEC.
