Cavity QED Detection of Interfering Matter Waves
  We observe the build-up of a matter wave interference pattern from single atom detection events in a double-slit experiment. The interference arises from two overlapping atom laser beams extracted from a Rubidium Bose-Einstein condensate. Our detector is a high-finesse optical cavity which realizes the quantum measurement of the presence of an atom and thereby projects delocalized atoms into a state with zero or one atom in the resonator. The experiment reveals simultaneously the granular and the wave nature of matter. 
T. Bourdel, T. Donner, S. Ritter, A. \"Ottl, M. K\"ohl, and T.   Esslinger