Deployed trusted-node quantum key distribution over 300 km with a multi-core fiber access link

2026-06-04

AI Summary

This work addresses the challenges of long-distance quantum key distribution (QKD) in real-world fiber networks—namely heterogeneous infrastructure, high channel loss, and coexistence with classical optical signals—by constructing a 303-km hybrid link comprising 270 km of deployed single-mode fiber and 33 km of multicore fiber within a trusted-node architecture. For the first time, dynamic switching of QKD channels is demonstrated over a live network incorporating multicore fiber, while simultaneously co-propagating Ethernet traffic and broadband noise. By integrating a commercial QKD system with external superconducting nanowire single-photon detectors, stable key distribution is achieved, enabling secure image transmission via one-time-pad encryption. The experiment further reveals the impact of key rate fluctuations on application-layer performance, thereby validating the feasibility of deploying QKD in reconfigurable hybrid quantum-classical networks.

Abstract

Quantum key distribution (QKD) is increasingly considered for deployment in realistic communication networks, where long distances, heterogeneous fiber infrastructure, and coexistence with classical traffic present substantial challenges. Here, we demonstrate trusted-node QKD between Link\"oping University and the Stockholm hub of the Swedish national quantum communication infrastructure over 270 km of deployed single-mode fiber, extended by a 33 km multi-core fiber (MCF) segment emulating a metropolitan access link, for a total distance of 303 km. The two sub-links use commercial QKD systems whose receivers are interfaced with external superconducting nanowire single-photon detectors, enabling operation at losses beyond those supported by standard internal gated-mode detectors. We operate the link while actively switching the QKD channel between two MCF cores, with co-propagating Ethernet traffic and injected broadband optical noise in the other cores. The results demonstrate the integration of commercial QKD into demanding, dynamically reconfigurable fiber infrastructure relevant to future hybrid quantum-classical networks. Finally, using the generated secret keys, we illustrate how limited and time-varying QKD throughput affects one-time-pad-protected image transmission: image fidelity depends strongly on the available QKD-generated key budget and the choice of compression algorithm, highlighting application-level challenges for QKD-based encryption in realistic scenarios.

Problem

• quantum key distribution
• trusted-node
• multi-core fiber
• coexistence with classical traffic
• realistic communication networks

Innovation

• trusted-node QKD
• multi-core fiber
• superconducting nanowire single-photon detector
• coexistence with classical traffic
• deployed fiber network