Advanced Multi‐Objective Optimization of Nonlinear Metasurface Unlocks Quantum Light Generation

ABSTRACT The design of nonlinear metasurfaces for quantum applications has traditionally focused on classical figures of merit such as tuning resonances at the pump and target frequencies. While these approaches can enhance nonlinear efficiency, they do not guarantee the precise phase control needed for generating pure quantum state. In this work, we present a hybrid quantum‐classical inverse design framework that directly incorporates quantum metrics into the optimization process. By leveraging an advanced multiobjective optimization, we optimized both entanglement fidelity and the spontaneous parametric down conversion (SPDC) rate, ensuring that high brightness and quantum purity are achieved simultaneously. In general, classical methods that rely on bound states in the continuum (BIC) modes or sharp resonances, can improve the SPDC rate but are often sensitive and difficult to control across the polarization channels leading to low quantum purity, uncontrolled correlations, and reduced entanglement fidelity. In contrast, our approach demonstrates that reliable, easy‐to‐fabricate metasurfaces can achieve superior quantum performance without these constraints. When applied to a [100] AlGaAs nanohole metasurface, our numerical simulations predict a Bell‐state fidelity of 0.989 combined with a collected SPDC rate of 55 Hz/mW, integrated over a wide angular emission range. Our optimized design represents a robust architecture for compact, high‐purity sources of entangled photon pairs.