Robust countermeasure against detector control attack in a practical quantum key distribution system: comment

The physical imperfections of quantum key distribution systems compromise their information-theoretic security. By exploiting the imperfections on the detection unit, an eavesdropper can launch various detector-control attacks to steal the secret key. Recently, in Optica6, 1178 (2019)OPTIC82334-253610.1364/OPTICA.6.001178 entitled “Robust countermeasure against detector control attack in a practical quantum key distribution system,” Qian et al. proposed a countermeasure using variable attenuators in the detection unit that was claimed to be effective against detector-control attacks with or without blinding light. We comment on this paper, disputing this countermeasure by showing that their assumptions for proving this effectiveness are unrealistic.