  We experimentally demonstrate coherent light scattering from an atomic Mott
insulator in a two-dimensional lattice. The far-field diffraction pattern of
small clouds of a few hundred atoms was imaged while simultaneously laser
cooling the atoms with the probe beams. We describe the position of the
diffraction peaks and the scaling of the peak parameters by a simple analytic
model. In contrast to Bragg scattering, scattering from a single plane yields
diffraction peaks for any incidence angle. We demonstrate the feasibility of
detecting spin correlations via light scattering by artificially creating a
one-dimensional antiferromagnetic order as a density wave and observing the
appearance of additional diffraction peaks.
