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APL Engineering Physics

High-brightness fiber-based Sagnac source of entangled photon pairs for multiplexed quantum networks

Tess Troisi, Yoann Pelet, Romain Dalidet, Grégory Sauder, Olivier Alibart, Sébastien Tanzilli, Anthony Martin

Published Jul 1, 2026• 1 citation
Publisher

Abstract

A fully fibered source of entangled photon pairs based on a nonlinear Sagnac interferometer is reported. Operating at telecom wavelengths, the source relies exclusively on standard fiber-optic components and periodically poled lithium niobate waveguides, resulting in a compact, robust, and field-deployable architecture. The generation stage supports both polarization and energy-time entanglement without modification, enabling versatile operation depending on the targeted application. Broadband spontaneous parametric down-conversion allows dense wavelength-division multiplexing over the telecom C and L bands. A lumped device brightness of about 104 pairs s−1 nm−1 mW−2 is reported over several standard 100-GHz ITU channel pairs, together with high entanglement quality. Polarization and energy-time encodings are characterized through state tomography and two-photon interference measurements, yielding fidelities, purities, and visibilities exceeding 96% over multiple wavelength channels. The stability and reproducibility of the source are further evaluated through long-duration operation in a network environment. These results demonstrate that the proposed Sagnac source constitutes a practical and scalable building block for future plug-and-play quantum communication and quantum networking platforms.

DOI
10.1063/5.0328026

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A fully fibered source of entangled photon pairs based on a nonlinear Sagnac interferometer is reported. Operating at telecom wavelengths, the source relies exclusively on standard fiber-optic components and periodically poled lithium niobate waveguides, resulting in a compact, robust, and field-deployable architecture. The generation stage supports both polarization and energy-time entanglement without modification, enabling versatile operation depending on the targeted application. Broadband spontaneous parametric down-conversion allows dense wavelength-division multiplexing over the telecom C and L bands. A lumped device brightness of about 104 pairs s−1 nm−1 mW−2 is reported over several standard 100-GHz ITU channel pairs, together with high entanglement quality. Polarization and energy-time encodings are characterized through state tomography and two-photon interference measurements, yielding fidelities, purities, and visibilities exceeding 96% over multiple wavelength channels. The stability and reproducibility of the source are further evaluated through long-duration operation in a network environment. These results demonstrate that the proposed Sagnac source constitutes a practical and scalable building block for future plug-and-play quantum communication and quantum networking platforms.
A fully fibered source of entangled photon pairs based on a nonlinear Sagnac interferometer is reported. Operating at telecom wavelengths, the source relies exclusively on standard fiber-optic components and periodically poled lithium niobate (PPLN) waveguides, resulting in a compact, robust, and field-deployable architecture. The generation stage supports both polarization and energy-time entanglement without modification, enabling versatile operation depending on the targeted application. Broadband spontaneous parametric down-conversion allows dense wavelength-division multiplexing over the telecom C and L bands. High normalized brightness (10.3 kpairs/s/nm/mW2^22) is achieved on a standard 100 GHz ITU channel pair, together with high entanglement quality. Polarization and energy-time encodings are characterized through state tomography and two-photon interference measurements, yielding fidelities, purities, and visibilities exceeding 96 % over multiple wavelength channels. The stability and reproducibility of the source are further evaluated through long-duration operation in a network environment. These results demonstrate that the proposed Sagnac source constitutes a practical and scalable building block for future plug-and-play quantum communication and quantum networking platforms.
We demonstrate a compact, practical all-fiber source of non-degenerate telecom-band polarization-entangled photon pairs assembled from off-the-shelf components. The compact fiber-integrated architecture is designed to be affordable, straightforward to assemble and align, and compatible with existing optical-fiber infrastructure. Photon pairs are generated through type-0 spontaneous parametric down-conversion in a pigtailed periodically poled lithium-niobate waveguide embedded in a Sagnac interferometer. At 0.5 mW pump power, quantum-state tomography yields a Bell-state fidelity FΦ−=0.850F_{Φ^-}=0.850FΦ−​=0.850, purity P=0.836P=0.836P=0.836, and concurrence C=0.782C=0.782C=0.782. A separate CHSH measurement gives S=2.3301±0.0075S=2.3301\pm0.0075S=2.3301±0.0075. The measured Bell-state fidelity is sufficient for the reconstructed polarization state to support qubit teleportation above the classical fidelity limit under ideal protocol operations. Analysis of the density matrix shows that approximately 96.7% of the population remains in the desired ∣HH⟩|HH\rangle∣HH⟩-∣VV⟩|VV\rangle∣VV⟩ subspace, with a normalized coherence of approximately 0.86. A residual relative-phase offset indicates that local phase compensation could increase the Bell-state fidelity to approximately 0.8865. We further characterize the brightness--quality trade-off and obtain a corrected pair-rate slope of (0.918±0.040)(0.918\pm0.040)(0.918±0.040) Mcps mW−1^{-1}−1. The broadband emission and fiber interfaces offer a route to multiuser entanglement distribution through off-the-shelf multichannel wavelength-division multiplexers with passbands matched to the signal and idler wavelengths.
We report the demonstration of a bulk, intrinsically phase-stable source of polarization- and time-energy-entangled photon pairs at 810nm and 1550nm, directly coupled into single-mode optical fibers. This highly non-degenerate wavelength combination is well suited for hybrid quantum communication networks, enabling low-loss transmission in optical fibers at 1550nm while maintaining efficient free-space propagation and detection at 810nm. The source is based on spontaneous parametric down-conversion in a periodically poled lithium niobate crystal embedded in a polarization Sagnac interferometer, providing inherent stability and dual-degree-of-freedom entanglement. We measure a spectral brightness of B = 4800 pair/s/mW/GHz, with fiber coupling efficiencies exceeding 0.48 at both wavelengths. The entanglement quality is characterized by high-visibility two-photon interference, yielding net visibilities of 0.995 in the polarization basis and 0.991 in the energy-time basis. These performances demonstrate a compact and robust entanglement source compatible with hybrid fiber/free-space quantum key distribution architectures, and suitable for future ground-to-satellite quantum communication links.
The scalability of quantum communication networks requires compact, fiber-integrated, easy-to-deploy, and efficient wavelength-division-multiplexed (WDM) sources for multi-user secure key distribution. Here, we demonstrate such a multi-channel source of polarization-entangled photon pairs in the low-loss telecom C-band based on type-0 spontaneous parametric downconversion in a fiber-coupled Zn-indiffused MgO-doped periodically poled lithium-niobate (MgO:PPLN) ridge waveguide in the Sagnac configuration. The source can be easily tuned to generate the |Φ+⟩ or |Φ−⟩ Bell state in 14 channel pairs of the International Telecommunication Union dense WDM (100-GHz spacing) grid around 1550-nm with a raw fidelity of ≳89%, with a maximum value of ≳94%. The raw concurrence is ≳0.8 for both Bell states in all channel pairs, and the observed S-parameter (>2.56 ± 0.04 in all 14-channel pairs) shows a strong violation of CHSH-Bell’s inequality. The source’s suitability for long-distance entanglement transmission is also demonstrated by the successful transfer of entangled photons up to 100 km while maintaining fidelity >85% and quantum bit error rate <9%. The effect of polarization mode dispersion on entanglement distribution among remote users is also studied in detail. All these performance metrics are measured using conventional room-temperature semiconductor-based single-photon avalanche detectors, and these are the best reported with these detectors. Our highly flexible source can support up to ∼40 user pairs to communicate simultaneously, and it can be easily deployed into the current metro-area fiber-optic telecom infrastructure to form a complete WDM-based quantum communication network.
Combining a Ti-diffusion periodically poled lithium niobate (PPLN) waveguide with a Sagnac interferometer, two opposite directions type-II spontaneous parametric down conversions (SPDC) occur coherently and yield a high brightness, high stability polarization entanglement source. The source produces degenerate photon pairs at 1540.4 nm with a brightness of B = (1.36 ± 0.03) × 106 pairs/(s⋅nm⋅mW). We perform quantum state tomography to reconstruct the density matrix of the output state and obtain a fidelity of F = 0.983 ± 0.001. The high brightness and phase stability of our waveguide source enable a wide range of quantum information experiments operating at a low pump power as well as hold the advantage in mass production which can promote the practical applications of quantum technologies.
Quantum networks based on wavelength-multiplexed entanglement enable parallel distribution of quantum correlations, increasing channel capacity for secure communication and distributed quantum information processing. However, broadband integrated sources capable of generating multipartite entanglement beyond photon pairs remain scarce. Here we report on-chip generation of telecom-band four-photon entanglement in a periodically poled thin-film lithium niobate on insulator (LNOI) waveguide. Type-0 spontaneous parametric down-conversion provides a phase-matching bandwidth exceeding 200 nm, enabling spectrally separable generation of multi-photon entanglement across the telecom band. The generated photons are encoded in time bins for robust fiber compatibility, and a coherent interface enabling reversible conversion between time-bin and polarization degrees of freedom allows complete quantum state tomography. We measure two-photon entanglement with a brightness of 6.7 MHz/mW/nm and a fidelity of 0.874±0.0020.874 \pm 0.0020.874±0.002. At a pump power of 0.08 mW, the four-photon state exhibits a fourfold coincidence rate of 1 Hz and a fidelity of 0.74±0.010.74 \pm 0.010.74±0.01, representing a threefold improvement over previous integrated platforms. Our results establish LNOI as a scalable platform for broadband multi-photon entanglement and provide a practical route toward dense wavelength-multiplexed quantum networks.