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arXiv:2407.00802 · Optics Express

Realizing a compact, high-fidelity, telecom-wavelength source of multipartite entangled photons

Laura dos Santos Martins, Nicolas Laurent-Puig, Pascal Lefebvre, Simon Neves, Eleni Diamanti

Published Oct 6, 2025• 5 citations
arXivHTMLPDF

Abstract

Multipartite entangled states are an essential building block for advanced quantum networking applications. Realizing such tasks in practice puts stringent requirements on the characteristics of the states in terms of fidelity and generation rate, along with a desired compatibility with telecommunication network deployment. Here, we demonstrate a photonic platform design capable of producing high-fidelity Greenberger-Horne-Zeilinger (GHZ) states, at telecom wavelength and in a compact and scalable configuration. Our source relies on spontaneous parametric down-conversion in a layered Sagnac interferometer, which only requires a single nonlinear crystal. This enables the generation of highly indistinguishable photon pairs, followed by entanglement fusion to create four-qubit polarization-entangled GHZ states with fidelity up to (94.73±0.21)% with respect to the ideal state, at a rate of 1.7 Hz. We provide a complete characterization of our source and highlight its suitability for practical quantum network applications.

DOI
10.1364/oe.571310

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Multipartite entangled states are an essential building block for advanced quantum networking applications. Realizing such tasks in practice puts stringent requirements on the characteristics of the states in terms of fidelity and generation rate, along with a desired compatibility with telecommunication network deployment. Here, we demonstrate a photonic platform design capable of producing high-fidelity Greenberger-Horne-Zeilinger (GHZ) states, at telecom wavelength and in a compact and scalable configuration. Our source relies on spontaneous parametric down-conversion in a layered Sagnac interferometer, which only requires a single nonlinear crystal. This enables the generation of highly indistinguishable photon pairs, followed by entanglement fusion to create four-qubit polarization-entangled GHZ states with fidelity up to (94.73±0.21)% with respect to the ideal state, at a rate of 1.7 Hz. We provide a complete characterization of our source and highlight its suitability for practical quantum network applications.
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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.
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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.
Fiber-based entangled photon sources are a key resource in emerging quantum technologies due to compactness, long-term stability and alignment-free operation. Fast control of the quantum state properties is also necessary in practical scenarios, where a universal source can be employed with different temporal and spectral configurations. Here, we report on a flexible source design that produces high-quality entanglement from continuous-wave up to GHz-pulsed operation modes. Our approach uses off-the-shelf optical components in a Sagnac configuration to generate polarization-entangled photon pairs at telecom wavelength. This design further allows phase modulation above MHz speeds with high fidelity. Together with its 60 nm spectral bandwidth, the proposed system is entirely suitable for wavelength-multiplexed and reconfigurable quantum networks, where state modulation or active bandwidth allocation is required.