Rafael J. Vicente

During its fourth iteration, the Madrid Quantum Network (MadQCI) gathered up to 26 quantum key distribution (QKD) modules in 9 nodes connected by approximately 110 kilometres of fibre optic pairs. These nodes were part of two domains that had to be connected by an border link. It also hosted a co-located network testing a QKD-switched network. The deployment allowed quantum resources of different nature to be distributed and multiple use cases to be tested. This article details a single node in the network, called Norte, to exemplify the network deployment as a whole.

Current quantum key distribution (QKD) networks focus almost exclusively on transporting secret keys with the highest possible rate. Consequently, they are built as mostly fixed, ad hoc, logically, and physically isolated infrastructures designed to avoid any penalty to the quantum channel. This architecture is neither scalable nor cost-effective and future, real-world deployments will differ considerably. The structure of the MadQCI QKD network presented here is based on disaggregated components and modern paradigms especially designed for flexibility, upgradability, and facilitating the integration of QKD in the security and telecommunications-networks ecosystem. These underlying ideas have been tested by deploying many QKD systems from several manufacturers in a real-world, multi-tenant telecommunications network, installed in production facilities and sharing the infrastructure with commercial traffic. Different technologies have been used in different links to address the variety of situations and needs that arise in real networks, exploring a wide range of possibilities. Finally, a set of realistic use cases have been implemented to demonstrate the validity and performance of the network. The testing took place during a period close to three years, where most of the nodes were continuously active.