Linking QKD testbeds across Europe

TL;DR

The paper investigates interconnecting European metropolitan QKD testbeds (Berlin, Madrid, Poznan) using emulated long-distance links and four border-node methods that combine QKD with PQC. It demonstrates SDN-enabled cross-domain key forwarding, application-layer integration, and multi-path diversity across fiber and satellite media, providing a blueprint for future pan-European quantum-safe networks. Experimental demonstrations show consistent key-delivery at 256 bps for Methods 1–3, with Method 4 achieving up to 4 kbits/s through space-ground diversity, illustrating the practical viability of hybrid QKD-PQC interconnects. The work highlights how parallel hybridization reduces shared side-channel risks and offers scalable, standards-aligned interfaces (ETSI GS QKD 004/014/015) to advance secure interconnections among metropolitan networks ahead of mature long-distance QKD deployment.

Abstract

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.

Figures (11)

• Figure S1: Connection of the quantum testbeds in Madrid (left), Berlin (middle), and Poznan (right) with emulated, long-distance QKD links. The key exchange is indicated by the curved blue lines, which connect dedicated QKD gateway nodes (blue circles) in each testbed. The other QKD nodes in the respective testbeds are indicated by orange circles.
• Figure S2: Dark-fiber topology in the Berlin metropolitan-area testbed (top left), deployed three-layer architecture (bottom left), rack hosting QKD modules, servers, HSMs, and encryptors (right). For more details the reader is referred to Berlin-testbed.
• Figure S3: PSNC testbed together with the dark fiber topology in the Poznań metropolitan area (top left), deployed layer architecture for both metro and backbone networks - POZMAN and PIONIER (center left), racks hosting QKD equipment and encryptors (right), additionally connected trusted-node configuration of the long-distance QKD link between Poznań and Warsaw (bottom) NLPQT.
• Figure S4: Madrid Network – topology (top left); Madrid Network – functional diagram (bottom left); Quijote Node (right) For more details the reader is referred to martin2023madqci.
• Figure S5: The disjoint network is realized by a "space link" via the Iridium network and a "ground link" via the public internet. The network connects the gateway nodes of the Madrid, Berlin, and Poznan QKD testbeds. The Munich Research Center of Huawei in Germnay serves as a pseudo-internal node of the Madrid network.