Abstract 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 NbOBr 2 and NbOI 2 , demonstrating near infrared polarization entangled photon generation and linking performance to crystal structure and optical properties. Our results extend the wavelength range of NbOX 2 -based SPDC sources and demonstrate NbOI 2 is especially suitable for near-infrared SPDC. Polarization entangled state (concurrence of 0.90 ± 0.05) can also be directly generated from NbOI 2 . Besides adding new candidates to the SPDC material library, our work provides new insight for the future generation of integrated quantum materials.
Abstract 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 NbOBr 2 and NbOI 2 , demonstrating near infrared polarization entangled photon generation and linking performance to crystal structure and optical properties. Our results extend the wavelength range of NbOX 2 -based SPDC sources and demonstrate NbOI 2 is especially suitable for near-infrared SPDC. Polarization entangled state (concurrence of 0.90 ± 0.05) can also be directly generated from NbOI 2 . Besides adding new candidates to the SPDC material library, our work provides new insight for the future generation of integrated quantum materials.
The realization of quantum light sources is crucial for advancing quantum technologies. Two‐dimensional van der Waals (vdW) materials offer a compelling platform for ultracompact entangled photon‐pair sources due to their strong optical nonlinearities and relaxed phase‐matching constraints. Here, the generation of entangled photon pairs from vdW NbOI 2 flakes on Au film via spontaneous parametric down‐conversion (SPDC) process is demonstrated. The observed inverse relationship between second‐order correlation function and pump power and the linear relationship between coincidence rate and pump power confirm the characteristic features of SPDC process. Furthermore, the measured polarization‐dependent coincidence rate shows the anisotropic entangled photon‐pair emission in NbOI 2 crystal. The dependence of coincidence rate on flake thickness is characterized by considering both optical interference and the phase‐matching effects. The demonstrated results open new avenues for the development of on‐chip integrated quantum photonic devices for applications in quantum state engineering, quantum communication, and quantum information processing.
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.
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 $\sim$335, and a brightness approximately one order of magnitude higher than recently reported telecom sources based on transition metal dichalcogenide (TMD) 2D materials. These results establish twisted van der Waals engineering as a powerful platform for highly tunable, high-brightness quantum light sources at telecom wavelengths.
Abstract Ultracompact entangled photon sources are pivotal to miniaturized quantum photonic devices. Van der Waals (vdW) nonlinear crystals promise efficient photon-pair generation and on-chip monolithic integration with nanophotonic circuitry. However, it remains challenging to generate maximally entangled Bell states of photon pairs with high purity, generation rate, and fidelity required for practical applications. Here, we realize a polarization-entangled photon-pair source based on spontaneous parametric down conversion in an ultrathin rhombohedral tungsten disulfide (3R-WS 2 ) crystal. This vdW entangled photonic source exhibits a high photon-pair purity with a coincidence-to-accidental ratio of above 800, a generation rate of 31 Hz, and two maximally polarization-entangled Bell states with fidelities exceeding 0.93 and entanglement degree over 0.97. These results stem from scalable optical nonlinearity, enhanced second-order susceptibility by electronic transitions, and a well-defined symmetry-enabled selection rule inherent in 3R-WS 2 . Our polarization entangled photon source can be integrated with photonic structures for generating more complex entangled states, thus paving an avenue for advanced quantum photonic systems toward computation and metrology.
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.