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.
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