Spatially entangled pairs of photons from lithium niobate nonlocal metasurfaces

We predict and demonstrate experimentally, for the first time to our knowledge, the generation of spatially entangled photon pairs through spontaneous parametric down-conversion from a metasurface incorporating a nonlinear thin film of lithium niobate covered by a silica meta-grating. We identify the spatial entanglement of the photon pairs through violation of the classical Cauchy-Schwartz inequality. Simultaneously, the photon-pair rate is strongly enhanced by 450 times compared to unpatterned films.