  No imaging apparatus can produce perfect images: spatial resolution is
limited by the Rayleigh diffraction bound that is a consequence of the imager's
finite spatial extent. We show some N-photon strategies that permit resolution
of details that are smaller than this bound, attaining either a 1/sqrt(N)
enhancement (standard quantum limit) or a 1/N enhancement (Heisenberg limit)
over standard techniques. In the incoherent imaging regime, the methods
presented are loss resistant, because they can be implemented with
classical-state light sources. Our results may be of importance in many
applications: microscopy, telescopy, lithography, metrology, etc.

Quantum Physics