Bragg spectroscopy of a strongly interacting BEC

The ability to tune the scattering length in ultracold atomic gases has enabled access to strongly interacting Bose-Einstein condensates (BECs). Particularly intriguing physics is expected to occur when the scattering length is increased to on order of the interatomic spacing; in this case many-body interactions are important. The capability to probe the excitation spectrum of the BEC will be important in understanding this experimental regime. We will report initial Bragg spectra of a strongly interacting BEC. Using a Feshbach resonance, we vary the scattering length in a $^{85}$Rb BEC to study the Bragg spectra over a wide range of interaction strength.