Fig. 1. Spatial entanglement and disentanglement of a two-photon state at a metasurface.

a If a single photon pair with orthogonal linear polarization is inserted into the metasurface, it is divided into its circular polarization components. Since the inserted quantum state is a two-photon NOON state in the circular polarization basis, both photons will always choose the same output channel, and spatial entanglement is obtained. b The situation reverses for the insertion of a spatially entangled NOON state into the metasurface. The quantum interference leads to a projection back to the original orthogonal linear polarization states, and the photon pair is spatially disentangled