@article{62860,
  abstract     = {{<jats:p>
                    The Quantum Internet, a network of quantum-enabled infrastructure, represents the next frontier in telecommunications, promising capabilities that cannot be attained by classical counterparts. A crucial step in realizing such large-scale quantum networks is the integration of entanglement distribution within existing telecommunication infrastructure. Here, we demonstrate a real-world scalable quantum networking testbed deployed within Deutsche Telekom’s metropolitan fibers in Berlin. Using commercially available quantum devices and standard add-drop multiplexing hardware, we distributed polarization-entangled photon pairs over dynamically selectable looped fiber paths ranging from 10 m to 60 km and showed entanglement distribution over up to approximately 100 km. Quantum signals, transmitted at 1324 nm (O-band), coexist with conventional bidirectional C-band traffic without dedicated fibers or infrastructure changes. Active stabilization of the polarization enables robust long-term performance, achieving entanglement Bell-state fidelity bounds between 85% and 99% and Clauser–Horne–Shimony–Holt parameter
                    <jats:italic>S</jats:italic>
                    -values between 2.36 and 2.74 during continuous multiday operation. By achieving a high-fidelity entanglement distribution with less than 1.5% downtime, we confirm the feasibility of hybrid quantum-classical networks under real-world conditions at the metropolitan scale. These results establish deployment benchmarks and provide a practical roadmap for telecom operators to integrate quantum capabilities.
                  </jats:p>}},
  author       = {{Sena, Matheus and Flament, Mael and Andrewski, Shane and Caltzidis, Ioannis and Bigagli, Niccolò and Rieser, Thomas and Bello Portmann, Gabriel and Sekelsky, Rourke and Braun, Ralf-Peter and Craddock, Alexander N. and Schulz, Maximilian and Jöns, Klaus and Ritter, Michaela and Geitz, Marc and Holschke, Oliver and Namazi, Mehdi}},
  issn         = {{1943-0620}},
  journal      = {{Journal of Optical Communications and Networking}},
  number       = {{12}},
  publisher    = {{Optica Publishing Group}},
  title        = {{{High-fidelity quantum entanglement distribution in metropolitan fiber networks with co-propagating classical traffic}}},
  doi          = {{10.1364/jocn.575396}},
  volume       = {{17}},
  year         = {{2025}},
}