Coherence of light scattering by atom wavepacket*

We study the scattering of photons from an array of atomic wavepackets in free space and discuss the results in terms of atom-photon entanglement and which-way information. Using ultracold atoms released from an optical lattice, we realize a Gedanken experiment that interferes individual photon scattering off of Heisenberg uncertainty-limited wavepackets. We unify the free-space and trapped-atom pictures by measuring the light scattered during wavepacket expansion and show the coherence properties of the scattered light are independent of the presence of the trap. Our experiment demonstrates the potential of using atomic Mott insulators to create single-atom wavepackets for fundamental studies, as well as using incoherent scattering of light to probe the structure of such Mott insulator.