Air-Stable Bright Entangled Photon-Pair Source from Graphene-Encapsulated van der Waals Ferroelectric NbOI2.

van der Waals (vdW) ferroelectrics are emerging nonlinear photonic materials that combine large second-order susceptibility χ(2) with heterostructure compatibility, offering an attractive route toward miniaturized spontaneous parametric down-conversion (SPDC) sources. Practical vdW SPDC, however, remains constrained by low brightness and poor stability in air under continuous irradiation, where ambient exposure and pump-induced heating cause material degradation. Here we demonstrate a bright, air-stable SPDC source based on ferroelectric NbOI2 enabled by graphene encapsulation. Graphene provides robust environmental protection and suppresses pump-induced degradation by enhancing heat dissipation. We report an absolute photon-pair generation rate of 258 Hz and a normalized brightness of 19 900 Hz mW-1 mm-1. Leveraging this stabilized platform, we further generate polarization-entangled photon pairs from graphene-encapsulated 90°-twisted bilayer NbOI2, with 94% fidelity to a maximally entangled Bell state. These results establish graphene-encapsulated NbOI2 as a practical vdW ferroelectric SPDC platform for integrated quantum photonic sources.