Entanglement distribution over long distances is essential for many quantum communication schemes like quantum teleportation, some variants of quantum key distribution, or implementations of a quantum internet. Previous long-distance entanglement distributions were realized with dispersion-shifted fibers and sophisticated superconducting detectors as well as suffered from low pair rates. This work demonstrates entanglement distribution with a high rate over 50 km in a standard optical fiber which is likely already deployed in current metropolitan areas as part of their existing telecommunication infrastructure. 


Our approach is implemented by carefully designing a bright and highly non-degenerate polarization-entangled photon-pair source. The wavelengths of the photon pair are optimized for long-distance fiber transmission and high detection efficiency using the semiconductor avalanche single-photon diodes without the need for a cryogenic cooling system. The highly non-degeneracy in wavelength leads to a sub-nanometer intrinsic linewidth for the photon pairs, allowing the photon pairs to stay tightly time corrected after long-distance transmission without a lossy spectral filtering process. These features result in a high rate (>10000 pairs/second) and high entanglement quality (visibility > 97%) even after distributing the entanglement over 50 km fiber.

We believe this work is a significant step forward for entanglement-based quantum key distribution integrated into the existing metropolitan fiber network. Moreover, our quantum key distribution signal photons (around 1310 nm) could co-propagate with classical optical traffics in the C band (around 1550 nm) without interference.