  Optical metasurfaces open new avenues for precise wavefront control of light
for integrated quantum technology. Here, we demonstrate a hybrid integrated
quantum photonic system that is capable to entangle and disentangle two-photon
spin states at a dielectric metasurface. By interfering single-photon pairs at
a nanostructured dielectric metasurface, a path-entangled two-photon NOON state
with circular polarization is generated that exhibits a quantum HOM
interference visibility of 86 $\pm$ 4%. Furthermore, we demonstrate
nonclassicality and phase sensitivity in a metasurface-based interferometer
with a fringe visibility of 86.8 $\pm$ 1.1 % in the coincidence counts. This
high visibility proves the metasurface-induced path entanglement inside the
interferometer. Our findings provide a promising way to hybrid-integrated
quantum technology with high-dimensional functionalities in various
applications like imaging, sensing, and computing.
