From Single to Multiple-Photon Decoherence in an Atom Interferometer
  We measure the decoherence of a spatially separated atomic superposition due to spontaneous photon scattering. We observe a qualitative change in decoherence versus separation as the number of scattered photons increases, and verify quantitatively the decoherence rate constant in the many-photon limit. Our results illustrate an evolution of decoherence consistent with general models developed for a broad class of decoherence phenomenon. 
David A. Kokorowski, Alex D. Cronin, Tony D. Roberts, and David E.   Pritchard