Ing. Jiří Weiss

We present a framework for empirical characterization and prediction of quantum key storage depletion using Markov Chain Monte Carlo simulation in Quantum Key Distribution systems. The framework leverages operational data from QKD systems implementing BB84, T12, and COW protocols measured over various observation periods to establish initial model parameters. We identify three distinct operational regimes of quantum key storage by analysis of measured data. Through comprehensive analysis of depletion rates as a function of storage capacity and key consumption-to-generation ratio, we characterize system-specific operating points exhibiting efficient reduction of depletion rate relative to storage size. The simulation provides predictions across varying system conditions. Model validation establishes practical validity boundaries. This research provides experimentalists with quantitative methods for storage optimization, proactive reliability assessment, and informed planning regarding transitions between quantum, post-quantum, and classical cryptographic systems. The developed framework supports strategic planning for quantum key distribution system deployment, addressing fundamental requirements for maintaining continuous operation.

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.

This paper presents a model for path-based growing network with preferential attachment motivated by the deployment of quantum key distribution networks. The model is based on a network constructed from path segments of nodes on average to mimic real-world quantum key distribution network architectures. Using continuum formalism and the rate equation method, we derive degree exponent, exact degree distributions and demonstrate properties similar to random networks. The theoretical framework incorporates preferential attachment with variable crossover rates and strategic shortcuts, the satellite links. The approach is validated through extensive simulations implemented in Python. Key findings reveal that network robustness, measured by critical fraction for giant component loss, increases with crossover rate and number of satellite links but decreases with segment length. Average distance scales logarithmically with network size, directly impacting secret key consumption during relaying processes in quantum key distribution networks. While preferential attachment enhances connectivity, the model network does not achieve ultra-small world properties of scale-free networks that would minimize key consumption, providing insights for designing cost-effective quantum communication infrastructures.

In this letter, we present a Fiber-Bragg Grating (FBG) interrogator combining wavelength and time division multiplexing to increase the number of interrogation up to 800 independent gratings. The unit consists of a super luminescent light emitting diode, a variable electro-optical attenuator and a spectrometer. The wavelength range is 85 nm spanning through the S, C, and L optical communication bands. High variability is assured by independent settings of time delay and optional signal enhancement for each individual time window with simultaneous wavelength multiplexing. The variability of the setup is advantageous for such an application as in civil and mechanical engineering where large numbers of gratings or groups of gratings are used and allows to measure a wide range of setups of FBGs.