Chiral entangled photon pair generation with metasurfaces

We theoretically and experimentally demonstrate compact plasmonic metasurfaces to efficiently generate entangled and correlated single-photon pairs with high Spontaneous Parametric Down-Conversion (SPDC) generation rates mainly due to the improved efficiency of the classical counterpart of Sum Frequency Generation (SFG) nonlinear optical process. We also present chiral single-photon emission based on new quasi-bound state in the continuum (q-BIC) plasmonic metasurface cavity designs. Nonclassical light emission exhibiting circular polarization can further enhance the encoding capacity of photonic qubits.The presented metasurface designs are ultrathin, support efficient free space outcoupling of chiral and achiral classical and quantum radiation, and operate at room temperature. Hence, they offer the potential to realize compact and integrable quantum sources of circularly polarized entangled single-photon pairs.