Effect of light injection on the security of practical quantum key distribution

Quantum key distribution (QKD) based on the fundamental laws of quantum physics can allow the distribution of secure keys between distant users. However, imperfections in realistic devices may lead to potential security risks, which must be accurately characterized and considered in practical security analysis. High-speed optical modulators, being one of the core components of practical QKD systems, can be used to prepare the required quantum states. Here, we find that optical modulators based on LiNbO{}₃$, including phase modulators and intensity modulators, are vulnerable to photorefractive effects caused by external light injection. By changing the power of external light, eavesdroppers can control the intensities of the prepared states, posing a potential threat to the security of QKD. We have experimentally demonstrated the influence of light injection on LiNbO{}₃$-based optical modulators and analyzed the security risks caused by the potential light-injection attack, along with the corresponding countermeasures.