  We demonstrate a photon-counting technique for detecting Bragg excitation of
an ultracold gas of atoms. By measuring the response of the light field to the
atoms, we derive a signal independent of traditional time-of-flight
atom-imaging techniques. With heterodyne detection we achieve photon shot-noise
limited detection of the amplification or depletion of one of the Bragg laser
beams. Photon counting for Bragg spectroscopy will be useful for strongly
interacting gases where atom-imaging detection fails. In addition, this
technique provides the ability to resolve the evolution of excitations as a
function of pulse duration.
