Feasibility of bright illumination attack using 830 nm coherent light on the practical quantum key distribution systems

Bright illumination attack (BIA) is considered one of the most serious eavesdropping attacks on quantum key distribution (QKD) systems. Various countermeasures have been proposed to eliminate this threat. Most of them focus on bright light at the telecom wavelength of 1550 nm, often involving power monitors with a sensitivity range from 900 to 1700 nm. However, BIA at other wavelengths—typically shorter than 900 nm—could also pose a threat. The light in this wavelength range has been overlooked because avalanche photodiodes (APDs) in QKD systems have low detection efficiency for communication. However, Indium Phosphide (InP) layers, commonly used as the avalanche-amplification layer and substrate in APDs, can absorb light at these shorter wavelengths, potentially causing blinding due to photocurrent. Such attacks cannot be detected by power monitors designed for telecom wavelengths. In this study, we experimentally investigated the feasibility of BIA on practical QKD systems using the 830-nm continuous-wave light. We found clear evidence of APD blinding, which reduces quantum efficiency. This finding suggests that the security certification of commercial QKD systems should consider potential attacks using near-infrared light.