Chiral spontaneous photon-pair generation with plasmonic bound states in the continuum

Spontaneous parametric down conversion (SPDC) has proven to be a robust and prominent method for creating non-classical light sources of entangled single-photon pairs. However, such sources suffer from low efficiency due to the inherent weakness of the SPDC process. Moreover, there is no control of polarization in the generated photons. Circularly polarized SPDC-based sources with an efficient chiral response are currently missing, which severely precludes the realization of a wide range of practical applications in quantum computing and communications. The current work solves these inherent problems by demonstrating a novel chiral plasmonic metasurface that leverages a strong quasi-bound state in the continuum (qBIC) resonance, which significantly increases the SPDC photon generation efficiency and realizes circularly polarized (chiral) single-photon pairs. The presented metasurface design is ultrathin, supports efficient free space outcoupling of chiral classical and quantum radiation, and operates at room temperature. Hence, it offers the potential to realize a compact and integrable quantum source of circularly polarized entangled single-photon pairs.