Chiral Single Photon Emission with Bound States in the Continuum Plasmonic Metasurfaces

Single-photon sources can be realized by spontaneous parametric down-conversion (SPDC), where a pair of entangled single photons is generated. However, SPDC is an inherently weak process and lacks control in the polarization of the generated photons. Increasing the efficiency and controlling the polarization of SPDC-based sources is critical for their practical use in quantum communications and computing. Here, we demonstrate a new chiral plasmonic metasurface leveraging a resonant quasi-bound state in the continuum (qBIC) response in reflection that significantly enhances single-photon pair generation rates and circularly polarizes the produced non-classical (quantum) light. The ultrathin design consists of a lithium niobate (LiNbO₃) crystal overlaid by a chiral silver grating made of tilted nanorods responsible for the qBIC chiral response. When excited by sources of polarity and wavelength matching to the metasurface resonant conditions, a significant enhancement of the electric field can occur within the LiNbO₃ layer. With two incident circular polarized sources, the nonlinear sum frequency generation (SFG) efficiency is increased drastically over the LiNbO₃ layer alone leading to a substantially boosted rate of single photon-pairs created by SPDC due to the quantum-classical correspondence principle. The design presented here is ultrathin, relies on efficient free space coupling of chiral radiation, and operates in room temperature, resulting in a new quantum source of chiral single-photon pairs that can be easily integrated into a wide range of applications in quantum photonics.