Enhanced and Switchable Generation of Two Polarization Bell States from a Single Metasurface

Polarization Bell states are essential resources for various quantum technologies. The most common approach for generating polarization Bell states is through the spontaneous parametric down‐conversion (SPDC) in nonlinear crystals, or more recently, in ultrathin nonlinear metasurfaces. However, both platforms typically require multiplexing two SPDC processes, each producing one of the two orthogonal polarization states whose superposition results in the Bell states. Here, a Bell state source is proposed from a nonlinear metasurface by integrating a 2D and symmetrical SiO 2 nano‐grating on a LiNbO 3 film. This metasurface supports multiple high‐Q guided mode resonances and enables enhanced generation of polarization Bell states without the need of multiplexing. Furthermore, the same metasurface enables switchable generation of two distinct Bell states with high fidelity simply by switching the pump wavelength‐a capability not achievable with conventional crystal sources. This work paves an effective way toward ultrathin, multifunctional entangled photon sources, which are essential for the development of miniaturized, scalable quantum photonic systems.