Single photon detectors (SPDs) based on passively quenchedavalanche photodiodes can be temporarily blinded by relatively bright light, ofintensity less than 1nW. A bright-light regime suitable for attacking a quantumkey distribution system containing such detectors is described in this paper. Inthis regime, all SPDs in the receiver Bob are uniformly blinded by continuousillumination coming from the eavesdropper Eve. When Eve needs a certaindetector in Bob to produce a click, she modifies the polarization (or otherparameters used to encode quantum states) of the light she sends to Bob suchthat the target detector stops receiving light, while the other detector(s) continueto be illuminated. The target detector regains single photon sensitivity and, whenEve modifies the polarization again, produces a single click. Thus, Eve hasfull control of Bob and can perform a successful intercept–resend attack. Tocheck the feasibility of the attack, three different models of passively quencheddetectors have been tested. In the experiment, I have simulated the intensitydiagrams the detectors would receive in a real QKD system under attack. Controlparameters and side effects are considered. It appears that the attack could bepractically possible.
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