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arXiv:2503.07198 · Physical Review Applied

Entanglement distribution over metropolitan fiber using an on-chip broadband polarization-entangled photon source

Zi-Heng Jiang, Yikai Chen, Wenhan Yan, Chi Lu, Wenjun Wen, Yu-Yang An, Leizhen Chen, Yuchen Liu, Hua-Ying Liu, Zhenda Xie, Yanqing Lu, Shining N. Zhu, Xiao‐Song Ma

Published Sep 16, 2025• 7 citations
arXivHTMLPDF

Abstract

Entangled photon pairs are of crucial importance in quantum networks. For the future demands of large-scale and secure quantum communication, integrated photon sources are highly effective solutions. Here, we report entanglement distribution over 30 km in a metropolitan area using an on-chip broadband silicon nanowire biphoton polarization-entangled source based on a silicon-on-insulator platform. This source generates a continuous spectrum spanning the entire C band (4.5 THz), achieving locally detected coincidence counts of about 154 kHz within a 100-GHz bandwidth, making it suitable for long-distance entanglement distribution among multiple users. By combining this source with quantum entanglement, enhanced by high-precision clock synchronization that achieves a standard deviation of 56.8 ps over 600 s, we observe a violation of the Clauser-Horne-Shimony-Holt inequality by 27.8 standard deviations. Our results showcase the potential of silicon photonic technology as a scalable and practical platform for quantum technologies.

DOI
10.1103/5c5v-cw8c

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Entangled photon pairs are of crucial importance in quantum networks. For the future demands of large-scale and secure quantum communication, integrated photon sources are highly effective solutions. Here, we report entanglement distribution over 30 km in a metropolitan area using an on-chip broadband silicon nanowire biphoton polarization-entangled source based on a silicon-on-insulator platform. This source generates a continuous spectrum spanning the entire C band (4.5 THz), achieving locally detected coincidence counts of about 154 kHz within a 100-GHz bandwidth, making it suitable for long-distance entanglement distribution among multiple users. By combining this source with quantum entanglement, enhanced by high-precision clock synchronization that achieves a standard deviation of 56.8 ps over 600 s, we observe a violation of the Clauser-Horne-Shimony-Holt inequality by 27.8 standard deviations. Our results showcase the potential of silicon photonic technology as a scalable and practical platform for quantum technologies.
Transmitting entangled states over long distances is crucial for developing quantum networks. Previous demonstrations using satellites or fibers relied on photon pairs generated from bulk crystal arrangements. Polarization entanglement distribution based on complementary metal-oxide-semiconductor (CMOS)-compatible silicon chips has long been restricted to lab-scale demonstrations spanning only a few meters due to the difficulty of achieving sufficient off-chip brightness. Here, we report a silicon chip platform that provides an off-chip entangled photon pair brightness ranging from 8,000 to 460,000 pairs per second, exceeding previous reports by 3 orders of magnitude. The entanglement fidelity reaches 99.85(6)% and 97.90(3)%, respectively. After addressing key challenges in long-distance entanglement distribution over deployed fiber, including phase drift and chromatic dispersion, entangled photons were successfully distributed over 155 km (66 dB loss). These results demonstrate that CMOS-compatible silicon chips can perform competitively with bulk crystal sources and represent an important step toward scalable, chip-based quantum networks.
Abstract Transmitting an entangled state over an extended distance is crucial for the development of quantum networks. Previous demonstrations of transmitting entangled photons over long distance using satellites or fibers have use entangled photon pairs generated from bulk crystal arrangements. An alternative approach would be to generate photon pairs using silicon-on-insulator (SOI) chips. Despite numerous proof-of-concept studies, no long range distribution has been achieved using this platform because of the challenge of getting sufficient off-chip brightness. We report a SOI platform that provides an off-chip entangled photon pair brightness of between 8,000 to 460,000 pairs per second. This exceeds previous reports by three orders of magnitude in brightness. The entanglement fidelity is 99.85(6)% and 97.90(3)% respectively. Measuring one photon locally, and transmitting the other over 93 km of deployed fiber (link loss of 40 dB), achieves a count rate of 132 pairs per second with an entanglement fidelity of 93.3(3)%, after solving the additional challenges of chromatic dispersion. The source can be pumped harder to enable transmission of entangled photons over 155 km of deployed fiber (link loss of 66 dB) at a rate of 0.7 pairs per second, with an entanglement fidelity of 87.6(5)%. These results demonstrate that SOI nanophotonic chips can perform competitively with bulk crystal sources and represent an important step toward building quantum networks using integrated nanophotonic platforms.
Long-range quantum entanglement is essential for building large-scale quantum networks and unconditionally secure cryptographic systems based on quantum key distribution (QKD). While photonic integrated circuits offer a highly scalable platform, the fragility of phase coherence between spatial modes has prevented the distribution of path-encoded entanglement over long distances. Here, we report chip-to-chip distribution of path-encoded entangled states over 80 km between fully integrated silicon photonic transmitter and receiver chips. Telecom-band entangled photon pairs are generated via spontaneous four-wave mixing in on-chip spiral waveguides and distributed between chips over a dual-core, actively stabilized fiber link. Upon distribution, we measure a Bell state fidelity of 85.7±0.2%85.7 \pm 0.2 \%85.7±0.2%. Implementing the BBM92 protocol with the same source, we obtain a secure key rate of 2.03 bit/s in the infinite-key regime. These results establish silicon photonic chips as a viable platform for long-distance path-encoded entanglement-based quantum key distribution, paving the way toward scalable, device-independent quantum networks.
Abstract Quantum communication networks enable applications ranging from highly secure communication to clock synchronization and distributed quantum computing. Miniaturized, flexible, and cost-efficient resources will be key elements for ensuring the scalability of such networks as they progress towards large-scale deployed infrastructures. Here, we bring these elements together by combining an on-chip, telecom-wavelength, broadband entangled photon source with industry-grade flexible-grid wavelength division multiplexing techniques, to demonstrate reconfigurable entanglement distribution between up to 8 users in a resource-optimized quantum network topology. As a benchmark application we use quantum key distribution, and show low error and high secret key generation rates across several frequency channels, over both symmetric and asymmetric metropolitan-distance optical fibered links and including finite-size effects. By adapting the bandwidth allocation to specific network constraints, we also illustrate the flexible networking capability of our configuration. Together with the potential of our semiconductor source for distributing secret keys over a 60 nm bandwidth with commercial multiplexing technology, these results offer a promising route to the deployment of scalable quantum network architectures.
Leveraging the properties of quantum entanglement and squeezing, quantum clock synchronization offers significant advantages in improving precision and security. For scalable quantum clock synchronization networks, developing an accurate time deviation analysis model is essential to characterize long-term timing stability and enable reliable deployment in real-world systems. This paper proposes a synchronization stability analysis model that establishes the theoretically achievable time deviation based on the Cramér-Rao lower bound. We experimentally validate this model using a round-trip quantum clock synchronization protocol over 50 km of fiber, employing an integrated silicon-photonic chip that generates frequency-entangled photon pairs via four-wave mixing. Results show a synchronization accuracy of 15.08 ps and a time deviation of 901 fs at an averaging time of 10,240 seconds, while our model analysis shows a standard deviation of 12.21 ps. This work provides a fundamental framework for building robust, large-scale quantum networks.