Entanglement-based QKD network links four cities in Slovakia with real-time keys

Entanglement‑Based QKD Goes Live Across Four Slovak Cities

Researchers at the Slovak Academy of Sciences have turned a laboratory‑grade quantum key distribution (QKD) experiment into a field‑tested service. Using a single broadband entangled‑photon source in Trnava, they have linked Bratislava, Trnava, Nitra and Trenčín with a fully meshed sub‑network that operates without trusted nodes for three of the cities and uses Trnava as a relay for the fourth. The system follows the ETSI GS QKD 014 standard, enabling real‑time key delivery through a defined API.

Real‑Time Key Generation and Performance

• Key rate: 13.5 bit s⁻¹ over a 109 km link (32.1 dB loss) and 4.3 bit s⁻¹ on a shorter, indirect link.
• Continuous operation: Keys are distilled, error‑corrected and handed off to applications as they are generated, rather than being post‑processed after a run.
• Hardware: A PolCo polarization‑compensation module (Quantum Optics Jena GmbH) actively stabilises each fiber channel, keeping the quantum bit error rate low.

Architecture and Scalability

The network’s novelty lies in its passive wavelength‑routing matrix, which assigns independent wavelength channels to each user pair and verifies connectivity without active switching. This deterministic design simplifies scaling: adding a new node only requires extending the matrix and routing additional wavelength channels. The three‑city core operates completely free of trusted nodes, while the fourth city connects through a trusted‑node‑free relay, demonstrating a hybrid approach that balances security and reach.

Security and Standards

Key distillation follows the full finite‑key security analysis, including coincidence detection, sifting, error detection, verification and privacy amplification. Keys are 256 bits long, stored in pools of up to 30 000 per link, and delivered via the ETSI GS QKD 014 interface, paving the way for interoperability with future European Quantum Communication Infrastructure (EuroQCI) deployments.

Outlook

The authors note that replacing the current detect‑and‑discard scheme with full error correction could boost the finite‑key rate by a factor of 4–6, potentially enabling positive key rates on even longer links (e.g., 142.9 km). The work represents a concrete step from proof‑of‑concept demonstrations toward a commercially viable quantum‑secure communications layer across Europe.

--- The study is available as a pre‑print on arXiv (https://arxiv.org/abs/2611.17265v1).