Bragg Spectroscopy of a Strongly Interacting $^{85}$Rb Bose-Einstein Condensate

We report on measurements of the large-momentum excitation spectrum of a strongly interacting Bose-Einstein condensate (BEC). Using a magnetic-field Feshbach resonance to tune atom-atom interactions in the condensate, we reach a regime where quantum depletion of the ground state and beyond mean-field corrections to the condensate chemical potential are significant. The Bragg resonance line shift due to strong interactions was found to be significantly less than that predicted by a mean-field theory, and demonstrates the onset of beyond mean-field effects in a gaseous BEC as Ref[1]. \newline \newline [1] E.~K. Irish, Phys. Rev. Lett. {\bf 99}, 173601 (2007).