Efficient-bb84 Quantum Key Distribution Network with Optical Switches Enables Scalable, Robust Communication

Quantum key distribution (QKD) promises fundamentally secure communication, and researchers are now extending this technology beyond simple connections to create practical, multi-user networks. Alberto De Toni, Edoardo Bortolozzo, and Alessandro Emanuele, working at the University of Padova and ThinkQuantum s. r. l., lead a team that demonstrates a functioning QKD network operating in a real metropolitan environment. The team utilises an optimised version of the BB84 protocol, known as efficient-BB84, alongside optical switching technology, to achieve scalable and cost-effective secure communication. This work represents a significant step towards building robust, future-proof quantum communication systems capable of widespread deployment, showcasing the potential of QKD to move beyond laboratory demonstrations and into practical application.

Quantum Key Distribution (QKD) represents a leading technology for enabling information-theoretic secure communication, with protocols such as BB84 and its variants already deployed in practical field implementations. As QKD evolves from point-to-point links to multi-node networks, scalability and cost-effectiveness become central challenges. Efficient-BB84 protocols and optical switching techniques enable flexible, scalable, and cost-efficient integration of QKD into existing infrastructures, as demonstrated by the development of an active QKD network.

Padua QKD Network Deployment and Testing

This work details the deployment and testing of a Quantum Key Distribution (QKD) network in Padua, Italy, demonstrating a practical, real-world implementation focused on performance, stability, and integration with existing network infrastructure. Utilizing existing fiber infrastructure and the Quky platform from ThinkQuantum, the network connects multiple nodes, enabling secure key exchange at 1550nm with discrete-variable QKD and decoy states for enhanced security. The network incorporates a qubit-based synchronization method for precise timing and integrates with Software Defined Networking (SDN) for dynamic control and management. Advanced polarization tracking and compensation mitigate fiber impairments, while the system minimizes vulnerabilities related to the QKD source and improves practicality through stable operation without frequent calibration.

Fast synchronization is achieved through an efficient qubit-based method, and hybrid encoding schemes combining time-bin and polarization states are explored. Leveraging SDN for dynamic network configuration, key distribution, and security management, the network uses the Secure Key Integration Protocol (SKIP) for secure key handling and integrates with MACsec (IEEE 802. 1AE) for layer 2 security. Experiments demonstrate a key rate of several Mbps over a distance of 10km, maintaining a low Quantum Bit Error Rate (QBER) indicating high-quality key generation. The network demonstrated stable operation over extended periods and successfully integrated with existing network infrastructure, adhering to relevant ETSI standards for QKD orchestration and control interfaces, and considering ITU-T recommendations for network control and management.

Production QKD Network Achieves High Key Rates

This work demonstrates a functional and stable Quantum Key Distribution (QKD) network operating in a production environment, achieving robust key generation and paving the way for future secure communication systems. Experiments reveal a consistent Secret Key Rate (SKR) exceeding 3 kB/s on all links, demonstrating substantial capacity for supporting numerous simultaneous secure connections, with each connection consuming only 32 bytes of key material per minute, highlighting the network’s scalability. The team measured processing times for batches of 500 kB of sifted key detections, averaging approximately 6 minutes per block. Analysis revealed two distinct distributions; initial blocks, incorporating base alignment, took longer, while subsequent blocks exhibited more consistent and shorter processing times.

Data collected over two months shows the Raw Key Rate (RKR) and SKR for each link, with connections demonstrating greater stability compared to links exhibiting higher variance and less predictable losses. Detailed measurements of Quantum Bit Error Rate (QBER) in both the X and Z bases were collected, providing a comprehensive view of link quality. Analysis of hourly data revealed negligible daily trends in key rates, suggesting minimal correlation with external factors. The network consistently generated keys, proving its reliability and positioning it to meet future traffic demands, establishing a clear path for expansion and increased security in communications.

Metropolitan Quantum Key Distribution Network Demonstrated

The VenQCI network represents a significant advance in quantum communication, demonstrating the practical operation of a four-node quantum key distribution system in a metropolitan environment. Researchers successfully deployed and operated the network for two months, achieving stable and continuous secret key generation rates suitable for high-throughput infrastructures, integrating efficient-BB84 protocols, optical switching, and standardized interfaces. This achievement extends the potential of QKD beyond point-to-point links, establishing a model for a networked quantum utility. The network’s node-agent-driven switching mechanism reduces deployment costs and enhances resilience by allowing service continuity during fiber maintenance. Validated end-to-end system integration, including minute-scale rekeying using QKD-derived keys within a 100 Gbps MPLS backbone, provides a practical blueprint for metropolitan-scale quantum-secured services and a foundation for larger, interoperable quantum networks.