Abstract In this paper, we present an inter-city Quantum Key Distribution network testbed spanning tens of kilometers within the Central Bohemian Region. The testbed operates under harsh conditions, leveraging a robust fiber-optic infrastructure. Specifically, the quantum key distribution devices using two different key generating protocols underwent rigorous testing during winter months. The experiment assesses the performance and stability of quantum key distribution devices in a communication channel under real-world conditions. A quantum channel utilized for this experiment was established along the railway tracks provided by the Czech Railway Administration. Our findings contribute to advancing secure quantum communication networks by addressing key rate stability challenges. An upper bound for relative secret key rate deviation is introduced to support link design. Moreover, it has been experimentally verified that the measured key rate demonstrated robust stability despite its quantum nature and strong influence from outside conditions.
Abstract In this paper, we present an inter-city Quantum Key Distribution network testbed spanning tens of kilometers within the Central Bohemian Region. The testbed operates under harsh conditions, leveraging a robust fiber-optic infrastructure. Specifically, the quantum key distribution devices using two different key generating protocols underwent rigorous testing during winter months. The experiment assesses the performance and stability of quantum key distribution devices in a communication channel under real-world conditions. A quantum channel utilized for this experiment was established along the railway tracks provided by the Czech Railway Administration. Our findings contribute to advancing secure quantum communication networks by addressing key rate stability challenges. An upper bound for relative secret key rate deviation is introduced to support link design. Moreover, it has been experimentally verified that the measured key rate demonstrated robust stability despite its quantum nature and strong influence from outside conditions.
Quantum Key Distribution (QKD) exploits the principles of quantum mechanics to enable information-theoretic security, offering significant advantages over classical cryptographic approaches. Despite extensive experimental demonstrations worldwide, many implementations remain confined to laboratory environments that may be impractical for large-scale deployment. This thesis addresses the critical challenge of making QKD viable within real-world telecommunications networks. Current field trials often prioritise metrics such as distance and key rate at the expense of practicality, employing ultra-low-loss fibres, custom components, and dedicated dark channels. These constraints could hinder integration with existing infrastructure. To overcome these limitations, this work introduces UKQNtel—a testbed designed to incorporate commercial-grade equipment and multiplex quantum and classical traffic. UKQNtel represents the first field trial of a coherent one-way QKD system integrated with a commercial-grade encrypted Dense Wavelength Division Multiplexing (DWDM) network, delivering 500 Gbps of encrypted data over 121 km of real-world fibre. The network comprises four trusted-node links, each achieving performance metrics suitable for practical deployment. Complementing the experimental work, novel simulations were developed to predict QKD performance using only fundamental parameters such as distance, wavelength, and optical power; marking a clear improvement over models that require broad assumptions or empirical measurements of an existing QKD link. This simulation develops new tools for simulating non-linear effects such as Raman scattering and four-wave mixing, as well as many other aspects of QKD noise, loss, and detection. Validation against UKQNtel measurements showed strong agreement in attenuation modelling and link-level performance. While secret key rate estimates demonstrate the need to account for additional losses, the model provides a robust foundation for network planning and scalability analysis. This research demonstrates a clear pathway toward practical QKD deployment, combining experimental integration with predictive modelling applicable to larger, more complex networks. The results underscore QKD’s potential to become a cornerstone of future secure communications.
Quantum Key Distribution (QKD) is a promising tool for secure communication in the near future. In combination with one-time-pad technique, it provides an unconditionally secure communication channel (meaning that it is secure against an adversary, even with unlimited computational power), at least in principle. However, a major implementation challenge for some schemes is the reliable creation, transportation, and measurement of entangled photon pairs over long-distance fiber networks. Our project aims to explore the possibilities for distributing quantum information on an existing network infrastructure while measuring the effects of real-world conditions. We characterized a commercial source of entangled photons. We measured its spectrum, brightness (1.6±0.3)×104 pairs/s/μW, and performed quantum state tomography (QST) to reconstruct the density matrix of the quantum state. Our implementation focuses on an all-fiber solution, which would enable a simplified QKD implementation. In laboratory conditions, we achieved the visibility equal to (0.957 ± 0.004) as a mean in both bases with a coincidence rate of (275 ± 4) counts/s and successfully ran QKD protocol with secret key rate of (86 ± 1) bits/s and average quantum bit error rate (QBER) of (4.8 ± 0.7) %.
As the limitations of classical cryptographic systems become increasingly evident in the face of advancing quantum algorithms, Quantum Key Distribution (QKD) emerges as a vital paradigm for securing information in next-generation networks. This paper explores the theoretical foundations and protocol architectures underpinning QKD, including BB84, E91, and newer hybrid approaches designed for scalable integration into existing infrastructures. Performance metrics such as key generation rate, error thresholds, and resistance to various attack models are critically examined across both fiber-optic and free-space implementations. Furthermore, the study delves into real-world deployment challenges—ranging from synchronization errors and quantum channel noise to cost-efficiency and interoperability with classical systems. By offering a comparative analysis and proposing optimization strategies, this work aims to pave the way for robust quantum-secure communication protocols suited for large-scale, heterogeneous network environments.
In-field demonstrations in real-world scenarios boost the development of quantum key distribution (QKD) towards its integration in existing infrastructures. Although QKD devices are already adopted outside the laboratories, current field implementations still suffer from high costs and low performances, preventing this technology from a large-scale deployment in telecommunication networks. Here, we present the developing of a cost-effective setup for QKD field trials, implemented for testing QKD performances in two different scenarios: (i) using an installed fiber link the metropolitan area of Florence (Italy); (ii) using a submarine optical fiber channel between Malta and Sicily.
This contribution focuses on experimental verification of the QKD system deployment in a multi-domain network environment managed by Czech and Polish National Research and Educational Network (NREN) operators. We demonstrate full functionality of such a solution for transmission of secret keys in boundary conditions, and with this we open up new possibilities for further use of extremely secure communication between two neighboring network entities, and the services built upon it. Moreover, we have shared the cross-border link among strong QKD service channels, accurate time, and classical data channels together with weak quantum channel to reduce the total number of optical fibers needed for transmission. To our knowledge, this is the first shared cross-border QKD transmission in the region of Central and Eastern Europe (CEE).
Quantum key distribution (QKD) has reached a commercially viable stage, with several companies offering hardware systems designed for operational deployment. Evaluating the performance of commercial QKD devices under real-world deployment conditions is essential for users seeking to understand the practical limitations and operational reliability of these systems. In this paper, we present a long-term performance analysis of ID Quantique's Clavis XGR deployed within the Hermes Quantum Network, in Brazil. Our study provides a detailed characterization of key operational metrics, such as secret key rate, quantum bit error rate (QBER), visibility, and detection counts, mapping their behavior over extended periods of continuous operation. We analyze the system's stability across two distinct optical links: a 40.3 km indoor spooled fiber and a 3.5 km outdoor deployed underground fiber. Monitored under both unregulated tropical ambient fluctuations and actively controlled thermal stress, our results demonstrate excellent overall baseline resilience, with the system maintaining visibility above 97% and QBER below 1% on average. These findings provide practical insights into the expected behavior and thermal bottlenecks of commercial QKD systems in field deployments, particularly in tropical climates, helping to inform realistic expectations for operational quantum-safe infrastructures.