Polarization-entangled photon source on a silicon chip for ultra-low-loss fiber transmission

Entanglement distribution is key for quantum networks, enabling secure communication and quantum computing. Silicon photonics, compatible with CMOS technology, offers compactness, scalability, and cost-effective manufacturing. While satellite-based and free-space links achieve long-distance distribution, fiber-based links are more practical for urban environments but often rely on bulky crystal sources, limiting scalability. Chip-based sources face challenges like low off-chip photon brightness and high collection losses. This work demonstrates a silicon nanophotonic chip generating off-chip raw brightness of 460 kcps with 97.90% fidelity. A novel edge coupler ensures stable coupling losses of 0.64 dB per facet. Using a Sagnac interferometer, Bell states with 99.85% fidelity and 8 kcps were detected. Tests over 155 km of metropolitan fibers (66 dB loss) achieved 0.7 cps with 87.6% fidelity. Ultra-low-loss fibers (0.16 dB/km) extended the feasible distance to over 300 km, with splicing losses of 0.01 dB per facet and minimal polarization mode dispersion. These results validate silicon chip-based entangled photon sources as scalable and competitive for long-distance quantum networks.