Continuous entanglement distribution over a transnational 248 km fibre link

Entanglement is the basis of many quantum applications. The technically most mature of them, quantum key distribution, harnesses quantum correlations of entangled photons to produce cryptographic keys of provably unbreakable security. A key challenge in this context is the establishment of continuously working, reliable long-distance distributions of entanglement. However, connections via satellites don't allow for interruption-free operation, and deployed fibre implementations have so far been limited to less than 100 km by losses, a few hours of duty time, or use trusted nodes. Here, we present a continuously working international link between Austria and Slovakia, directly distributing polarization-entangled photon pairs via 248 km of deployed telecommunication fibre. Despite 79 dB loss, we measure stable pair rates of 9 s$^{-1}$ over an exemplary operation time of 110 hours. We mitigate multi-pair detections with strict temporal filtering, enabled by nonlocal compensation of chromatic dispersion. Fully automatized active polarization stabilization keeps the entangled state's visibility at 86 % for altogether 82 hours, producing 403 kbit of quantum-secure key at a rate of 1.4 bits/s. Our work paves the way for low-maintenance, ultra-stable quantum communication over long distances, independent of cloud coverage and time of day, thus constituting an important step towards the quantum internet.Comment: 23 pages, 6 figure