Polarization entanglement enabled by orthogonally stacked van der Waals NbOCl₂ crystals

Polarization entanglement holds significant importance for photonic quantum technologies. Recently emerging subwavelength nonlinear quantum light sources, e.g., GaP and LiNbO₃ thin films, benefiting from the relaxed phase-matching constraints and volume confinement, have shown intriguing properties, such as high-dimensional hyperentanglement and robust entanglement anti-degradation. Van der Waals (vdW) NbOCl₂ crystal, with strong optical nonlinearities, has emerged as a potential candidate for ultrathin quantum light sources. However, polarization entanglement is inaccessible in the NbOCl₂ crystal due to its unfavorable nonlinear susceptibility tensor. Here, by leveraging the twist-stacking degree of freedom inherently in vdW systems, we showcase the preparation of polarization entanglement and quantum Bell states.