  The optical diffraction limit, formulated by Abbe 140 years ago, imposes a
bound on imaging resolution in classical optics. Over the last twenty years,
many theoretical schemes have been presented for overcoming the diffraction
barrier in optical imaging using quantum properties of light. An experimental
realization of sub-diffraction limited quantum imaging has, however, remained
elusive. Here, we demonstrate a quantum imaging method taking advantage of
non-classical light naturally produced in fluorescence microscopy due to photon
antibunching, a fundamentally quantum phenomenon prohibiting simultaneous
emission of multiple photons. Using a photon counting digital camera, we detect
antibunching-induced second and third order intensity correlations and perform
sub-diffraction limited quantum imaging in a standard wide-field fluorescence
microscope.
