Building the Practical Foundations of a European Quantum Communication Infrastructure

Today, quantum networks have made Quantum Key Distribution (QKD) the standard method for securely distributing encryption keys among multiple parties. In this study, we present an approach to implementing quantum networks, validated through on-field implementations. Utilizing a weak coherent pulse source, an optimized BB84 protocol (efficient BB84), and optical fiber channels, we took the initial steps within the European Quantum Communication Infrastructure (EuroQCI) initiative. Our method successfully established a quantum network linking the cities of Trieste (Italy), Rijeka (Croatia), and Ljubljana (Slovenia)-the first to connect three different countries. A second quantum link between Malta and Sicily (Italy) was established for technological characterization, while a multi-core optical fiber in downtown L’Aquila (Italy) facilitated the first QKD process involving high-dimensional quantum states in a real-world scenario.