Probing an ultracold atomic crystal with matter waves*

We explore the scattering of matter waves from ultracold atoms held in an optical lattice. By ``shining'' a one-dimensional Bose gas onto an atomic Mott insulator (target), we observe Bragg diffraction peaks that reveal the target's crystalline structure. We find a systematic dependence of the Bragg peak intensity on the degree of atom localization, and recover a transition to coherent momentum and energy exchange (``Newton's cradle'') in the limit of free target atoms. Neutral-atom diffraction can serve as a novel experimental technique for probing atomic many-body systems.