In a study published in Optica, the research group led by academician GUO Guangcan from CAS Key Laboratory of Quantum Information of University of Science and Technology of China of the Chinese Academy of Sciences proposed and demonstrated an effective countermeasure against detector-control attacks. This countermeasure is general and can be easily applied to the existing systems to improve their practical security.

Quantum key distribution (QKD) is one of the most promising applications of quantum information. It promises to achieve the unconditional security in communication without making assumptions on the computational power of the potential eavesdropper. Unfortunately, real-life realizations of QKD often have imperfections, which may be exploited by the eavesdropper to learn the distributed key without being detected.

Based on imperfections in the single photon detectors (SPD), detector control attacks have been successfully launched on several QKD systems, and attracted widespread concerns. Although measurement-device-independent QKD can eliminate all the imperfections of the detector, it still faces experimental challenges.

A practical approach would be to install countermeasures against specific loopholes. There are several countermeasures focusing on specific detector control attacks in specific SPDs, but not all types of detector-control attacks. Taking the method of monitoring the photocurrent of the avalanche photodiode as the example, it is effective in finding the detector-control attack with blinding light, but would fail to detect the recent avalanche-transition region attack.

In this study, HAN Zhengfu’s group from CAS Key Laboratory of Quantum Information proposed a robust countermeasure against the detector control attacks just by introducing a variable attenuator (VA) in front of the detector. With the random change of attenuation of the VA and the analysis of the corresponding detection events and quantum bit error rates, the countermeasure criteria are proven effective against the detector-control attack.

Besides theoretical proof, the countermeasure is also supported by an experimental demonstration. This countermeasure is not only effective against the attacks with blinding light, but also robust against the attacks without blinding light which are more concealed and threatening. It is general in sense that it is independent of implementation of the SPD, which is different from previous technical methods. Thus it is suitable for different kinds of SPDs, such as photomultiplier tubes, superconducting SPDs and semiconductor SPDs.

The countermeasure can be easily applied to the existing QKD systems, just by adding a VA in front of SPD. This study provides a perfect balance between security claims and practicality claims. Since it is independent of the technical details of the detector, it can offer practical and effective guidance in the security evaluation and standards of QKD systems.