Enhanced entangled photon-pair generation rate with dual quasi-bound states in the continuum metasurfaces

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.