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Quantum Communications and Quantum Imaging

Silicon chip-based polarization entanglement distribution through ultra-low-loss fiber: splicing and polarization control

Jinyi Du, En Teng Lim, Xingjian Zhang, George F. R. Chen, Hong Wei Gao, Dawn T. H. Tan, Alexander Ling

Published Sep 18, 2025• 1 citation
Publisher

Abstract

Entanglement distribution is critical for quantum networks, enabling secure communication and distributed quantum computing. We demonstrate long-distance polarization-entangled photon pair distribution using a silicon nanophotonic chip and ultra-low-loss (ULL) fiber. The polarization-entangled photons from the silicon chip achieve a fidelity of 97.90% with a raw observed entangled photon rate of 460,000 pairs/s. The chip is efficiently coupled to optical fiber with losses as low as 0.64dB per facet. Leveraging ULL fiber (0.16dB/km), we extend the entanglement distribution distance to over 300km, surpassing the previous record of 155km using deployed fiber. Key advancements include optimized splicing (sub-0.01dB loss) and active polarization correction, ensuring stable entanglement over long distances. This work highlights the potential of silicon photonics and ULL fiber for scalable and practical quantum communication networks.

DOI
10.1117/12.3062267

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Entanglement distribution is critical for quantum networks, enabling secure communication and distributed quantum computing. We demonstrate long-distance polarization-entangled photon pair distribution using a silicon nanophotonic chip and ultra-low-loss (ULL) fiber. The polarization-entangled photons from the silicon chip achieve a fidelity of 97.90% with a raw observed entangled photon rate of 460,000 pairs/s. The chip is efficiently coupled to optical fiber with losses as low as 0.64dB per facet. Leveraging ULL fiber (0.16dB/km), we extend the entanglement distribution distance to over 300km, surpassing the previous record of 155km using deployed fiber. Key advancements include optimized splicing (sub-0.01dB loss) and active polarization correction, ensuring stable entanglement over long distances. This work highlights the potential of silicon photonics and ULL fiber for scalable and practical quantum communication 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.
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.
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.