Quantum-key-distribution (QKD) networks are gaining importance and it has become necessary to analyze the most appropriate methods for their long-distance interconnection. In this paper, four different methods of interconnecting remote QKD networks are proposed. The methods are used to link three different QKD testbeds in Europe, located in Berlin, Madrid, and Poznan. Although long-distance QKD links are only emulated, the used methods can serve as a blueprint for a secure interconnection of distant QKD networks in the future. Specifically, the presented approaches combine, in a transparent way, different fiber and satellite physical media, as well as common standards of key-delivery interfaces. The testbed interconnections are designed to increase the security by utilizing multipath techniques and multiple hybridizations of QKD and post quantum cryptography (PQC) algorithms.

翻译：量子密钥分发（QKD）网络的重要性日益凸显，分析其长距离互联的最优方法已成为迫切需求。本文提出四种远程QKD网络互联的不同方法，并利用这些方法成功连接了位于柏林、马德里和波兹南的三个欧洲QKD测试床。尽管长距离QKD链路仅为模拟实现，但所采用的方法可为未来远距离QKD网络的安全互联提供蓝图参考。具体而言，所提出的方案以透明方式融合了不同光纤与卫星物理介质，以及密钥交付接口的通用标准。测试床互联设计通过利用多路径技术与QKD及后量子密码（PQC）算法的多重混合机制，显著提升了安全性。