Breaking the diffraction limit using entanglement based microscopy

The resolution of optical scanning microscopes is limited by the spot size of the scanning beam. It has been proposed by Boto(1) et al. and Nasr(2) et al. that momentum entangled photons can be focused more tightly than classical light. Thus they should enable one to break the diffractive resolution limit. We report experiments measuring the spatial distribution of strong focussed momentum entangled photon pairs at a center wavelength of 812 nm from a spontaneous parametric down conversion source. Using a single gold stripe deposited on a glass surface as test object we observe a two-photon spot size as small as 194 nm, thereby beating the diffraction limit even for conventional two-photon microscopy.