Coherent frequency shift of atomic matter waves

We demonstrate how Bragg diffraction of atomic matter waves at a time-modulated thick standing light wave can be used to coherently shift the de Broglie frequency of the diffracted atoms. The coherent frequency shift is experimentally confirmed by interferometric superposition of modulated and unmodulated atoms resulting in time-dependent interference fringes. Our frequency shifter for atomic matter waves is a generalization of an acousto-optic frequency shifter for photons.