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Advanced Science

Efficient Polarization‐Entangled Photon‐Pair Generation by a Fiber‐In‐Line van der Waals Material

Jungseok Choi, Seongju Ha, Seungjae Lim, Joohyeon Ahn, Jaekyoung Kim, Jong Hyuk Yim, Nam Hun Park, Youngdong Yoo, Jae‐Ung Lee, Hee Su Park, Sang‐Min Lee, Dong‐Il Yeom

Published Jul 17, 2026• 0 citations

Abstract

ABSTRACT The recent emergence of van der Waals (vdW) materials with exceptional optical nonlinearity has driven advances in next‐generation nonlinear photonic devices beyond classical optics. While integrating these materials into fiber‐optic platforms offers unprecedented functionalities in quantum systems, realizing telecom‐band in‐line photon‐pair sources based on highly nonlinear vdW materials remains challenging. Here, we report photon‐pair generation centered at telecom wavelength via spontaneous parametric down‐conversion in a fiber‐integrated device incorporating a ferroelectric vdW crystal, SnP 2 S 6 . Benefiting from its giant optical nonlinearity and relaxed phase‐matching across near‐visible to telecom‐band wavelengths, a remarkable nonlinear conversion efficiency of 0.07% is achieved in a few‐micrometer‐thick SnP 2 S 6 film. Moreover, the fiber‐integrated SnP 2 S 6 device delivers high‐quality photon pairs at telecom wavelengths, achieving a detected coincidence rate of 102 counts/s. The coincidence‐to‐accidental ratio reached up to 55 662, which is much larger than that of previous vdW‐material‐based sources. Furthermore, the device generates polarization‐entangled photon pairs, which are verified via quantum state tomography to exhibit fidelities of 0.97 and concurrences of 0.95. This demonstration establishes a scalable, robust fiber‐integrated platform that combines the versatility of vdW materials, paving the way toward low‐loss, functionalized quantum photonic systems for networking, sensing, and computing.

DOI: 10.1002/advs.76712

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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.

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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 NbOBr

_2

, 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

^{\circ}

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.

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.

We present a tunable, single-mode-optical-fiber-based source of polarization entangled photon pairs for the near-infrared telecommunication band that is deployable in standard infrastructure. The photon pairs are generated via spontaneous parametric down-conversion (SPDC) in a submicron-scale thin film of the inversion-broken rhombohedral polytype of the transition metal dichalcogenide molybdenum disulfide (3R-MoS

_2

), located between two fiber connectors. By exploiting the intrinsic symmetries of the second-order nonlinear susceptibility tensor of 3R-MoS

_2

, this hybrid approach offers control over the generated two-photon polarization state through the incident pump polarization. Most notably, two of the four maximally entangled Bell states, as well as fully co-polarized pairs can be produced. This represents a substantial improvement in terms of tunability and simplicity over established fiber-integrated sources, which require additional optical elements, precise alignment, or careful engineering of design parameters. Additionally, a nonlinear drop in the background photoluminescence signal of 3R-MoS

_2

is observed at low pump powers, allowing us to reach a coincidences-to-accidentals ratio (CAR) of

^{\circ}

, the highest value recorded for SPDC in van der Waals materials to date.