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.

翻译：量子密钥分发（QKD）正越来越多地被考虑部署于实际通信网络中，而长距离传输、异构光纤基础设施以及与经典通信流共存等问题构成了重大挑战。本文演示了瑞典国家量子通信基础设施中林雪平大学与斯德哥尔摩节点之间基于可信节点的QKD：通过270公里已部署单模光纤，并辅以33公里模拟城域接入链路的多芯光纤（MCF）段，实现了总距离303公里的QKD传输。两个子链路采用商用QKD系统，其接收端与外部超导纳米线单光子探测器连接，从而能够在标准内部门控模式探测器无法支持的损耗条件下运行。我们在主动将QKD信道在两根MCF纤芯间切换的同时，在其他纤芯中传输以太网流量并注入宽带光学噪声，成功实现链路运行。结果证明了将商用QKD集成到要求严苛且可动态重构的光纤基础设施中的可行性，这对未来混合量子-经典网络具有重要参考价值。最后，利用生成的密钥，我们展示了受限且时变的QKD吞吐量如何影响一次性密码本保护的图像传输：图像保真度强烈依赖于可用的QKD生成密钥预算及压缩算法的选择，揭示了实际场景中基于QKD加密的应用层面挑战。