Quantum cryptography: Theoretical foundations and practical implementations for the protection of critical infrastructure

The purpose of the article was to theoretically determine harmonised criteria for the applicability of quantum key distribution for the protection of critical infrastructure, taking into account the long-term risk of decryption by adversaries possessing quantum computational resources and traffic archiving capabilities. The methodology relied on a theoretical systems analysis of three Quantum Key Distribution subsystems – sources, detectors, and channels (fibre/free space) – complemented by simplified models of the link budget and bit error probability. A concise scenario analysis was conducted for banking networks, energy systems, and government communications, taking into account scalability, compatibility, and cost. The main results showed that laser intensity stability of 1-2% and higher detector efficiencies – 60-70% for Avalanche Photodiodes and 80-90% for Superconducting Nanowire Single-Photon Detectors – extended practical distances and reduced errors. Without trusted nodes in fibre, 150-200 km was achievable; for urban Free-Space Optics lines, 5-20 km was optimal, while longer distances required network segmentation or satellite segments. Architecturally, it was justified that: for banks – rings with trusted nodes and Free-Space Optics reserve; for energy systems – dual fibre channels “control centre-substation” with a local key cache; for government communications – segmented domains with interagency gateways and satellite redundancy. In all scenarios, a Quantum Key Distribution + Post-Quantum Cryptography hybrid with short key rotation and operational countermeasures against attacks on detectors and channels was appropriate, confirming practical suitability for urban and regional networks. The practical significance lay in providing engineering teams in banking, energy, and government communications with ready-made architectural profiles – from rings and dual fibre channels to segmented domains – with Key Management System/Hardware Security Module integration, definition of Service Level Objective/Service Level Agreement, and phased roadmaps. For regulators and operators, this formed a basis for updating requirements and audits, as well as for planning redundancy (Free-Space Optics/satellite) and Total Cost of Ownership categories