Air-stable bright entangled photon-pair source from graphene-encapsulated van der Waals ferroelectric NbOI2

TL;DR

This paper presents an air-stable, high-brightness entangled photon-pair source using graphene-encapsulated ferroelectric NbOI2, overcoming environmental stability issues of previous van der Waals quantum light sources.

Contribution

It introduces a novel graphene encapsulation technique that enhances stability and heat dissipation in vdW ferroelectric SPDC sources, enabling practical on-chip quantum photonics.

Findings

Record photon-pair generation rate of 258 Hz

Normalized brightness of 19,900 Hz/(mW·mm)

94% fidelity in polarization entangled photon pairs

Abstract

Van der Waals (vdW) ferroelectrics are emerging nonlinear photonic materials that combine large second-order susceptibility \c{hi}(2) with heterostructure compatibility, offering an attractive route toward miniaturized spontaneous parametric down-conversion (SPDC) sources. However, vdW SPDC sources operating under continuous irradiation in air remain limited in low brightness and poor operational stability, as oxygen and moisture exposure, together with pump-induced heating, lead to material degradation and permanent damage. Here we demonstrate an air-stable, bright SPDC source based on ferroelectric NbOI2 enabled by graphene encapsulation. Graphene provides robust environmental protection and can effectively supress pump induced degradation by enhancing heat dissipation. We report a record photon-pair generation absolute rate of 258 Hz and a normalized brightness of 19,900 Hz/(mW.mm). Leveraging this stabilized platform, we further generate polarization entangled photon pairs with 94% fidelity with respect to the maximally entangled Bell states from graphene-encapsulated 90{\deg} twisted bilayer NbOI2. Our results establish a practical and air-stable vdW ferroelectric SPDC platform that overcomes key limitations of existing vdW quantum light sources and provides a viable pathway toward scalable, integrated entangled photon sources for on chip quantum photonics.