Researchers at the Slovak Academy of Sciences have deployed a four-city entanglement-based quantum key distribution (QKD) network in western Slovakia, delivering real-time keys over deployed telecommunication fiber. The network achieves a 13.5 bit per second key rate over 109 kilometers of fiber after complete post-processing, a figure indicating practical usability beyond typical QKD demonstrations. A novel matrix formalism for passive wavelength routing underpins a fully meshed sub-network, enabling scalable architectures that avoid reliance on trusted nodes, and all links operate simultaneously using the ETSI GS QKD 014 protocol.

Entanglement Network Deployment: Four-City Slovakia Infrastructure

The system, part of the larger European Quantum Communication Infrastructure (EuroQCI) launched in 2019, interconnects Bratislava, Trnava, Nitra and Trenčín, demonstrating a scalable architecture for quantum key distribution. The network’s design prioritizes minimizing trusted components; the three-city core operates without trusted nodes, while Trenčín connects through the central node in Trnava.

This configuration allows for potential expansion to full four-node connectivity by using Trnava as a key relay, creating a hybrid architecture that balances security and reach. This network implementation represents a commercial evaluation solution developed within the EuroQCI ecosystem, designed to assess the feasibility of widespread quantum communication.

The network uses a single broadband entangled-photon-pair source in Trnava, which routes conjugate-wavelength channels to each node, facilitating the distribution of entanglement across the four cities. The team’s work differs from earlier approaches, such as that of Wengerowsky et al., who pioneered a fully connected four-user network using wavelength demultiplexing and multiplexing, by assigning independent wavelength channels to each user pair.

The current deployment instead focuses on a deterministic design of a passive quantum network provider, offering greater control and predictability. The SkQCI network is not merely a demonstration of entanglement distribution; it is a fully operational system with a complete key distillation and delivery chain running in real time, signifying a move towards practical quantum communication infrastructure.

Replacing detect-and-discard with error correction would increase the finite-key rate by a factor of 4.1-6.6, and yield a positive finite-key rate on the 142.9 km link where the implemented scheme yields none. The system’s ability to deliver keys in real time, while adhering to industry standards and maintaining robust security, represents an advancement in the field.

Real-Time Key Delivery via ETSI GS QKD 014 Interface

The network’s key distillation and delivery pipeline operates continuously, a departure from many demonstrations that estimate secure key rates from post-experiment data processing. Real-time key generation and delivery, while previously achieved in trusted-node networks using prepare-and-measure protocols, had not been demonstrated across a multi-city entanglement-based network operating simultaneously until now. The system’s adherence to the ETSI GS QKD 014 interface signals a move toward standardized key delivery, facilitating interoperability with existing and future quantum infrastructure.

This interface allows for the consumption of distilled keys through a defined application programming interface, with pools capable of storing 30,000 keys per link. A key component enabling this real-time operation is the PolCo module, manufactured by Quantum Optics Jena GmbH, which dynamically compensates for polarization drifts induced by fiber propagation.

The module uses electronically driven liquid crystal cells to independently transform the polarization state of each port, continuously monitoring the quantum bit error rate (QBER) and applying real-time electrical corrections to minimize it. Unlike signal photons, idler photons are routed directly to a multiplexing module, streamlining the entanglement distribution process. The security of the generated keys was benchmarked against finite-key security bounds, an essential step for assessing practical usability.

The post-processing software detailed in the study includes stages for coincidence detection, sifting, error detection, error verification and privacy amplification, culminating in the generation of 256-bit keys. The pipeline assesses disclosure blocks for errors, discarding them if detected and updating the QBER accordingly. The network’s architecture features a fully meshed sub-network of three cities, with a fourth node connected through the central node, all operating simultaneously.

Key Rate Performance: 13.5 bit/s over 109 km Links

Real-time key distillation required shorter integration times, necessitating a key rate sufficient for practical use on each link of the deployed network. The system achieved 13.5 bit/s over 109 km, and 4.3 bit/s over 32.1 dB of accumulated loss between two cities without a direct fiber link. Characterization of the optical fiber links using an optical time-domain reflectometer revealed total losses and lengths, contributing to a more comprehensive understanding of the technological maturity of QKD.

The network’s design minimized wavelength load at each node, illustrating the trade-off between key generation rate and the number of nodes, as formalized in a companion work. The team also introduced a matrix formalism for passive wavelength routing, which verifies the connectivity of a given quantum network processor and provides the basis for resource laws derived in their related research. The use of this standardized interface signifies a move beyond purely experimental setups toward practical, deployable quantum communication systems.

Coincidence histograms, generated with and without a dispersion compensation module, revealed the impact of chromatic dispersion on signal quality, with Gaussian fits used to analyze the data. The analysis of key rate and coincidences as a function of the coincidence-time window further refined the system’s performance parameters. These detailed measurements and analyses contribute to a deeper understanding of the practical challenges and potential solutions for building robust and secure entanglement-based quantum communication networks.

Finite-Key Security Benchmarking of Privacy Amplification

Benchmarking against finite-key security bounds revealed the implemented heuristic privacy amplification can, in some instances, exceed those bounds, a finding that challenges conventional assumptions about key length requirements for secure communication. The analysis focused on short blocks, indicating a potential for optimization in key management strategies. The network’s post-processing software incorporates a 16-bit cyclic redundancy check for each frame, exchanging tags to identify and discard erroneous frames, a process that introduces a leakage of 16 bits per 256 kept bits.

The team also measured the impact of adding a user to an existing mesh network, further refining the system’s scalability. The analysis of these data points to the importance of balancing the trade-offs between key rate, link distance and the computational overhead of privacy amplification and error correction.

The researchers declare no conflicts of interest, and data supporting the results are available upon request, suggesting a commitment to transparency and collaboration within the quantum communication community. The network’s success in establishing a trusted-node-free sub-network and delivering keys in real time positions it as a key component in the development of secure quantum communication networks across Europe.

👉 More information
🗞 A deployed four-city entanglement-based QKD network with real-time key delivery, benchmarked against finite-key security bounds
✍️ Gilberto Borges, Saeid Salari, Ekta Panwar, Samgeeth Puliyil, Kartik Kakade, Peter Rapčan, Mario Ziman and Djeylan Aktas
🧠 ArXiv: https://arxiv.org/abs/2611.17265v1

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