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 methods used can serve as a blueprint for the 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.

Within the Madrid Quantum Communication Infrastructure (MadQCI), a cloud-like, quantum-enabled network scenario has been commissioned to promote the growth of the quantum technology scientific community. This scenario is designed to provide both quantum communication primitives and quantum-enabled services to potential end users. This work focuses on exposing these quantum services in a user-friendly manner by abstracting the underlying technical complexity, letting end users operate without prior knowledge of implementation details. To this end, multiple quantum services—the SD-QKD software stack, QRNG, Quantum-Safe TLS, and Quantum-Safe IPsec as a Service—are offered following the cloud “anything as a service” (XaaS) model. The delivery of quantum-enabled services is therefore researched using an applied and transferable cloud-based paradigm.

A monitor and control framework for quantum-key-distribution (QKD) networks equipped with switching capabilities was developed. On the one hand, this framework provides real-time visibility into operational metrics. Specifically, it extracts essential data, such as the switching capabilities of QKD modules, the number of keys stored in buffer queues of the QKD links, and the respective key generation and consumption rates along these links. On the other hand, this framework allows software-defined networking (SDN) applications to operate on the collected information and address the cryptographic needs of the network. The SDN applications dynamically adapt the configuration of the switched network to align with its changing demands, e.g., prioritizing key availability on critical paths, responding to link failures, or reallocating generation capacity to prevent bottlenecks. This contribution demonstrates that the combination of switched QKD, centralized control, and global optimization strategies enables efficient, policy-driven operation of QKD networks. The cryptographic resources are allocated to maximize performance and resilience while remaining aligned with the specific policies set by network administrators.