  Polarization-entangled photon pair sources are essential for diverse quantum
technologies, such as quantum communication, computation, and imaging. However,
the generation of complex polarization-entangled quantum states has long been
constrained by the available nonlinear susceptibility tensor of natural
nonlinear crystals, necessitating a cumbersome and intricate setup for
additional coherent superposition or post-selection. In this study, we
introduce and experimentally demonstrate a nanoscale polarization-entangled
photon pair source utilizing an artificially-engineered metamaterial platform.
This platform is based on a plasmonic metasurface that is strongly coupled to
an epsilon-near-zero (ENZ) material. By precisely engineering resonances at
both pump and signal/idler wavelengths, and leveraging the field enhancement
provided by the ENZ effect, the photon pair generation efficiency of the
68-nm-thick metasurface is significantly boosted. More notably, the ENZ
metasurface platform facilitates versatile manipulation of the system's
anisotropic second-order nonlinear susceptibility tensor, enabling direct
control over the polarization states of the photon pairs, which leads to the
generation of a polarization-entangled Bell state without the need for
additional components. Our approach opens a new avenue for the simultaneous
photon pair generation and quantum state engineering in a compact platform.
