Long-term analysis of efficient-BB84 4-node network with optical switches in metropolitan environment

TL;DR

The paper addresses scaling quantum-secure communications by validating a metropolitan multi-node QKD network (VenQCI) that combines efficient-BB84 with optical switching. It presents a production deployment using ThinkQuantum QuKy devices with iPOGNAC polarization encoding and Qubit4Sync synchronization, achieving sustained $SKR>3\ \mathrm{kB/s}$ per link over a two-month period and enabling dynamic, loss-tolerant link switching. End-to-end integration is demonstrated, including MACsec re-keying at $32\ \mathrm{B}/\mathrm{min}$ over a $100\ \mathrm{Gbps}$ MPLS backbone and a SKIP-based interface for cross-domain key exchange, aligned with ETSI/QKD standards. The work provides a practical blueprint for large-scale, interoperable quantum-secured networks and outlines a path toward a national Italian QCI backbone with standardized, vendor-agnostic interfaces.

Abstract

Quantum Key Distribution (QKD) is 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. Among the approaches to address these issues, efficient-BB84 has shown durable and reliable performances, while optical switching techniques enable flexible, scalable, and cost-efficient integration of QKD into existing infrastructures. In this work, we present an active QKD network in a production environment, employing efficient-BB84 and optical switching, orchestrated in a coordinated manner, emphasizing their potential to support robust, future-proof quantum-secure communication systems.

Figures (9)

• Figure 1: QuKy by ThinkQuantum s.r.l. thinkquantum
• Figure 2: The VenQCI network: VSIX – CavPD (Padova toll booth) – CavVE (Mestre toll booth) – VEGA. Aerial view provided by https://earth.google.com.
• Figure 3: Scheme of the VenQCI network.
• Figure 4: For each link, probability distribution of the time required to accumulate and process 500 kB of sifted key. In each graph are present two peaks, the one on the right corresponding to the first block after the switching (including the initial base alignment time), and the one on the left for the following blocks, which skip this phase (inside the colored area).
• Figure 5: RKR and SKR of each link distinguished by line type. The line represents the mean taken on a daily basis.