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Nano Letters

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

Mayank Joshi, Meng-Ting Jiang, Yu Xing, Yuerui Lu, Jie Zhao, Ping Koy Lam, Syed M. Assad, Xuezhi Ma, Young‐Wook Cho

Published Aug 3, 2026• 0 citations
Publisher

Abstract

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.

DOI
10.1021/acs.nanolett.6c02170

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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.
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° 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.
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Miniaturized quantum light sources that operate directly in optical fibers are an attractive platform for optical quantum technologies. However, most miniaturized spontaneous parametric down- conversion (SPDC) sources still rely on objective-lens-based free-space pumping and collection, which limits compactness, robustness, and direct compatibility with fiber-based systems. Here we demonstrate a lens-free in-line SPDC photon-pair source by integrating a van der Waals NbOI2 flake directly onto the end facet of an optical fiber. In this configuration, the generated photon- pairs are efficiently collected into optical fibers, eliminating the need for bulk free-space collection optics. Despite the limited numerical aperture of the single-mode fiber, efficient photon-pair collection with high purity, characterized by a coincidence-to-accidental ratio of up to ~4600, is achieved in an ultracompact configuration. These results establish van der Waals ferroelectric materials as a promising platform for fiber-integrated quantum light sources and provide a pathway toward compact, alignment-free quantum photonic devices.
Nanoscale quantum light sources are essential building blocks for integrated quantum photonic systems. Here, we report a wavelength-scale entangled-photon source based on van der Waals-engineered NbOBr2 and benchmark its performance for telecom-wavelength quantum light generation. By exploiting the material's second-order nonlinearity, we generate quantum-correlated photon pairs via spontaneous parametric down-conversion. We then use a 90° twisted stacking to induce quantum interference in photon-pair generation, yielding polarization-entangled photons. This approach enables tunability of the quantum optical state via control of the excitation laser polarization. We experimentally obtain entanglement fidelities exceeding 95% for Bell states, along with a high coincidence-to-accidental ratio of ∼335 and a brightness approximately 1 order of magnitude higher than recently reported telecom sources based on transition metal dichalcogenide two-dimensional materials. These results establish twisted van der Waals engineering as a powerful platform for highly tunable, high-brightness quantum light sources at telecom wavelengths.
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