  We demonstrate single site addressability in a two-dimensional optical
lattice with 600 nm lattice spacing. After loading a Bose-Einstein condensate
in the lattice potential we use a focused electron beam to remove atoms from
selected sites. The patterned structure is subsequently imaged by means of
scanning electron microscopy. This technique allows us to create arbitrary
patterns of mesoscopic atomic ensembles. We find that the patterns are
remarkably stable against tunneling diffusion. Such micro-engineered quantum
gases are a versatile resource for applications in quantum simulation, quantum
optics and quantum information processing with neutral atoms.
