Study of the Vulnerability of Neutral Optical Filters Used in Quantum Key Distribution Systems against Laser Damage Attack

Quantum key distribution systems are accessible to an eavesdropper; i.e., the eavesdropper can attack not only quantum states but also the equipment. In the former case, vulnerabilities are removed by fundamental constraints (e.g., no-cloning theorem) or theoretical foundations for their removal exist. In the latter case, vulnerability is determined by the practical implementation of equipment. An attack with laser damage of optical components called laser damage attack can allow the eavesdropper to reduce the attenuation of optical elements and compromise distributed keys. In this work, a method based on neutral optical filters has been described for protection from laser damage attack against optical components. A scheme imitating actions of the eavesdropper in the process of laser damage attack has been presented. An approach has been proposed to calculate the parameters of optical elements after the attack. Processes occurring in samples exposed to intense laser radiation have been analyzed. The analysis of experimental data has indicated the stability and constraints of neutral optical filters used in quantum key distribution systems for protection from laser damage attack.