  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.
