Hacking the Quantum Key Distribution System by Exploiting the Avalanche-Transition Region of Single-Photon Detectors

Avalanche-photodiode-based single-photon detectors (SPDs), as crucial and practical components, are widely used in quantum key distribution (QKD) systems. For effective detection, most of these SPDs are operated in the gated mode, in which the gate is added to obtain high avalanche gain and is removed to quench the avalanche. The avalanche-transition region (ATR) is certain to exist in the process of adding and removing the gate. We first experimentally investigate the characteristic of the ATR, including in a commercial SPD and a high-speed SPD, and then propose an ATR attack to control the detector. In the experiment on hacking the plug-and-play QKD system, Eve introduces less than 0.5% quantum bit error rate and almost leaves no traces of her presence, including the photocurrent and afterpulse probability. We finally give possible countermeasures against this attack.