Nonlinear quantum light source with van der Waals ferroelectric NbOX₂ (X = Br, I)

Two-dimensional (2D) materials have been demonstrated as promising candidates for ultracompact entangled photon sources and on-chip integrated quantum photonic devices. Ferroelectric van der Waals materials have recently emerged as promising ultrathin sources of entangled photons via nonlinear optical processes. Here, the authors compare spontaneous parametric down conversion in NbOBr₂ and NbOI₂, demonstrating near infrared polarization entangled photon generation and linking performance to crystal structure and optical properties. Our results extend the wavelength range of NbOX₂-based SPDC sources and demonstrate NbOI₂ is especially suitable for near-infrared SPDC. Polarization entangled state (concurrence of 0.90 ± 0.05) can also be directly generated from NbOI₂. Besides adding new candidates to the SPDC material library, our work provides new insight for the future generation of integrated quantum materials.