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

Bragg spectroscopy of a weakly interacting Bose-Einstein condensate (BEC) in the low-k limit has shown phonon-like dispersion (J. Steinhauer {\it et al.}, PRL {\bf 88}, 120407 (2002)). We report on Bragg spectroscopy of a strongly interacting BEC where we expect a divergence from the Bogoliubov spectrum (S. Papp {\it et al.}, PRL {\bf 101}, 135301 (2008)). We access the strongly interacting regime via a magnetic-field Feshbach resonance. Recent efforts focus on low-momentum excitations, and for this work we have implemented a shot-noise limited photon detection scheme. Additionally, we are working with a new, spherical trap that will allow us to interrogate the atoms in the strongly interacting regime for longer times than was possible in our previous work.