Akihisa Tomita

Since the first QKD protocol was proposed by Bennett and Brassard in 1984, practical QKD systems have been successfully demonstrated. Despite these developments, QKD systems have not been widely deployed due to imperfections in real-world devices. Many eavesdropping methods have been proposed to exploit the imperfections. One of those is the time-shift attack that exploits mismatches in temporal responses of avalanche photodiodes (APDs) to control the receiver’s detection. Zhao’s group demonstrated the time-shift attack successfully in 2008. The time-shift attack can be realized with current technology, and it will threaten the secure key distribution. We propose a simple and effective countermeasure to the time-shift attack by connecting additional APDs to detection ports through beamsplitters to average temporal responses of APDs. This method can reduce the photon detection mismatches between port 0 and port 1. Our simulation shows that averaging the temporal responses of APDs efficiently neutralizes the attack.