The Rayleigh diffraction bound sets the minimum separation for two point objects to be distinguishable in a conventional imaging system. We demonstrate sub-Rayleigh resolution by scanning a focused beam—in an arbitrary, object-covering pattern that is unknown to the imager—and using $N$-photon photodetection implemented with a single-photon avalanche detector array. Experiments show resolution improvement by a factor $\sim (N-{\overline{N}}_{\max }{)}^{1/2}$ beyond the Rayleigh bound, where ${\overline{N}}_{\max }$ is the maximum average detected photon number in the image, in good agreement with theory.

• Received 3 July 2010

DOI:https://doi.org/10.1103/PhysRevLett.105.163602