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
LiNbO3layer. With two incident circular polarized sources, the nonlinear sum frequency generation (SFG) efficiency is increased drastically over the
LiNbO3layer 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.
Spontaneous parametric down-conversion (SPDC) is a fundamental quantum optical technique for generating entangled photon pairs, which serves as the foundation for various quantum applications, including quantum cryptography, quantum simulation, quantum metrology, and the exploration of fundamental physical laws in quantum mechanics. Nevertheless, SPDC is plagued by low efficiency due to the extremely weak nature of this spontaneous quantum nonlinear process. In this work, we design a simple dielectric metasurface structure composed of lithium niobate (LN) defect disks that can support dual quasi-bound states in the continuum (quasi-BICs) in the near-infrared range. By aligning the high-Q quasi-BIC resonances with the generated signal and idler frequencies, our numerical simulation results demonstrate that the local electric field in the nonlinear medium reaches a substantial enhancement of up to two orders of magnitude, resulting in a significant improvement in the efficiency of the classical nonlinear optical sum frequency generation (SFG) and its inverse SPDC quantum process. Our findings pave the way for the development of high-quality compact nanosources of entangled photon pairs with potential applications spanning across cutting-edge fields such as quantum communications, computing, and imaging that can potentially revolutionize the field of quantum technology.
Here we demonstrate the efficient photon-pair generation via spontaneous parametric down conversion from a semiconductor metasurface supporting dual quasibound states in the continuum (quasi-BICs). In a simple metasurface composed of AlGaAs elliptical nanocyclinders, the two high-
Q quasi-BIC resonances that coincide with the generated signal and idler frequencies significantly boost the local electric field. This leads to a substantial enhancement in the reverse classical nonlinear process of sum-frequency generation and subsequently the remarkably high generation rate of photon pairs under the quantum-classical correspondence principle. Within a narrowband wavelength regime around the quasi-BIC resonances, the rate of pair production is enhanced up to $\ensuremath{\sim}{10}^{4}$ Hz, two orders of magnitude larger than that in the Mie resonant AlGaAs nanoantennas. Moreover, the photon pair emission is mainly concentrated in the normal direction with respect to the metasurface, and shows a tunable rate with the
Q factor by engineering the rotation angle of nanocylinders. The presented work enables nanoscale sources of high-quality entangled photons which will find applications in advanced quantum imaging and communications.