Separation estimation of two incoherent point sources using sum frequency generation.

In the context of separation estimation between two incoherent point sources, it has recently been shown that an optimal measurement strategy, which saturates the quantum Cramer-Rao bound, involves the use of spatial mode demultiplexing method (SPADE). To realize mode selective measurement required for SPADE, we propose a new approach based on sum frequency generation (SFG). The conversion of infrared light coming from two incoherent point sources is performed in a periodically-poled lithium niobate (PPLN) crystal by mean of a spatially shaped pump laser. By analyzing the converted images obtained with pump beams shaped as Hermite-Gaussian (HG) modes, we demonstrate the mode-sorting capabilities of this system. Our experiment, shows that our measurement method can estimate separations in sub-Rayleigh regime with improved accuracy compared to the traditional direct imaging method.