Polarization-entangled photon pairs from lithium niobate metasurfaces

Flat optics has emerged as a promising platform for ultrathin polarization-entangled sources via spontaneous parametric downconversion in subwavelength-thick nonlinear films or metasurfaces. However, previous schemes typically required multiplexing two films or two meta-structures with different orientations. Here, we propose a nonlinear metasurface composed of a single silica grating atop a lithium niobate film to generate polarization-entangled photon pairs. By engineering the angular dispersion of metasurface resonances, adjusting the pump frequency, and leveraging the polarization correlated transverse phase matching, the |HH〉 and |VV〉 states are simultaneously enhanced by over 2000 times, while the |HV〉 and |VH〉 states remain similar when compared to the pure film. Therefore, the degree of polarization entanglement is significantly improved. Furthermore, by tuning the pump polarization and film orientation, and applying spatial filtering, the concurrence of the two-photon state is optimized to near one, meaning maximal entanglement. Such an ultrathin and spatial-separated-entangled photon-pair source will be useful in realizing miniaturized quantum photonic systems for various applications.