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arXiv:2606.06107

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

Martin Clason, Joakim Argillander, Didrik Bergström, Daniel Spegel-Lexne, Giulio Foletto, Ashraf El Hassan, Mohamed Bourennane, Onur Günlü, Katia Gallo, Rui Lin, Guilherme B. Xavier

quant-phcs.ITcs.LGeess.IVphysics.opticsSubmitted Jun 4, 2026• Announced Jun 5, 2026• 0 citations
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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 Linköping 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.

Comments
11 pages, 4 figures

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Quantum key distribution (QKD) is a secure communication method for sharing symmetric cryptographic keys based on the principles of quantum physics. Its integration into the fiber-optic network infrastructure is important for ensuring privacy in optical communications. Multi-core fibers (MCFs), the likely building blocks of future high-capacity optical networks, offer new opportunities for such integration. Here, we experimentally demonstrate, for the first time, the coexistence of discrete-variable QKD and high-throughput classical communication in the C-band over a field-deployed MCF with industry standard cladding diameter of 125 μm. Specifically, we demonstrate successful secure-key establishment in one core of a 25.2-km uncoupled-core MCF, while simultaneously loading the remaining three cores with full C-band counter-propagating classical traffic at an aggregate net rate of 110.8 Tb/s. By proposing and experimentally validating an improved analytical model for inter-core spontaneous Raman scattering noise, we find that this configuration is optimal for our deployed MCF link as it is immune to four-wave mixing, that becomes relevant when the quantum and classical signals are propagating in the same direction. Our findings make an important step forward in demonstrating the integration of QKD and classical transmission in uncoupled-core multi-core fibers for next-generation optical communication networks.
Integrating quantum key distribution (QKD) with classical optical networks is essential for the large-scale deployment of quantum-secure communication. However, the nonlinear impairments induced by classical signals pose significant challenges to co-propagation systems based on single-mode fiber. Although hollow-core fiber (HCF) offers low nonlinearity and low transmission loss, non-uniform attenuation and residual nonlinear effects still degrade the performance of the coexistence transmission system. To address these issues, we construct a co-propagation system based on HCF and propose a quantum-classical joint optimization (QCJO) scheme that effectively minimizes nonlinear noise and enhances the overall robustness of the system. In addition, we demonstrate the co-propagation of a commercial QKD system and an optical transmission network with 40 Tbps (50×800 Gbps ) classical communication capacity over a 101.6 km HCF link. The result represents the recorded classical capacity in fiber-based quantum-classical coexistence transmission and the secure key rate of 9.56 kbps satisfies the fundamental requirements for secure encryption. This work marks significant progress toward massive quantum network deployment and lays a foundation for inherent-security and high-capacity telecommunication infrastructures.
Current technological progress is driving Quantum Key Distribution towards a commercial and worldwide scale expansion. Its capability to deliver secure communication regardless of the computational power of the attackers will be a fundamental feature in the next generations of telecommunication networks. Nevertheless, demonstrations of QKD implementation in a real operating scenario and their coexistence with the classical telecom infrastructure are of fundamental importance for reliable exploitation. Here we present a Quantum Key Distribution application implemented over a classical fiber-based infrastructure. We exploit a 50 MHz source at 1550 nm, a single 13 km-long fiber cable for both the quantum and the classical channel, and a simplified receiver scheme with just one single-photon detector. In this way, we achieve an error rate of approximately 2% and a secret key rate of about 1.7 kbps, thus demonstrating the feasibility of low-cost and ready-to-use Quantum Key Distribution systems compatible with standard classical infrastructure.