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Here we demonstrate the efficient photon-pair generation via spontaneousparametric down conversion from a semiconductor metasurface supporting dualquasi-bound states in the continuum (quasi-BICs). In a simple metasurfacedesign composed of AlGaAs ellipse nano-cyclinders, the two high-$Q$ quasi-BICresonances that coincide with the generated signal and idler frequenciessignificantly boost the local electric field. This leads to a substantialenhancement in the reverse classical nonlinear process of sum frequencygeneration and subsequently the remarkable high generation rate of photon pairsunder the quantum-classical correspondence principle. Within a narrowbandwavelength regime around the quasi-BIC resonances, the rate of pair productionis enhanced up to $\sim10^{4}$ Hz, two orders of magnitude larger than that inthe Mie resonant AlGaAs nanoantennas. Moreover, the photon pair emission ismainly concentrated in the normal direction with respect to the metasurface,and shows tunable rate with the $Q$ factor by engineering the rotation angle ofnano-cylinders. The presented work enables nanoscale sources of high-qualityentangled photons which will find applications in advanced quantum imaging andcommunications.

Efficient photon-pair generation empowered by dual quasi-bound states in the continuum