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.