Effect of femtosecond-pulse-injection silicon photonic modulators on the security of quantum key distribution

Integrated photonics plays a pivotal role in quantum communication devices, offering exceptional robustness, scalability, and performance. The growing maturity of silicon photonics technology offers significant potential for miniaturization and cost-effective implementation of quantum key distribution (QKD). While the practical security of lithium niobate modulators in QKD systems has been extensively examined, the adoption of a new modulation mechanism by silicon photonic modulators necessitates a thorough investigation into their impact on the security of QKD systems. For the first time, we reveal that the two-photon absorption effect in silicon photonic intensity modulators will introduce vulnerabilities, enabling an eavesdropper (Eve) to execute malicious attacks on QKD systems. Through experimental demonstration, we show that by externally injecting femtosecond pulses, Eve can manipulate the intensities of decoy states sent by legitimate users. Based on the experimental results, we find that in the presence of the attack, legitimate users consistently overestimate the secret key rate predicted by the security proof. Our work represents an initial exploration into a new security aspect for the design and standardization of integrated QKD systems.