Exploiting wavelength-dependent beam splitter to attack the calibration of practical quantum key distribution systems

We propose a modified attack on the calibration stage of polarized-based passive-basis-choice BB84 quantum key distribution (QKD) systems, where the activation windows of the detectors are scanned independently, by exploiting the wavelength-dependent defect of practical beam splitters. The eavesdropper may induce large basis dependent detector efficiency mismatch (BEM) in every attempt without failure. Moreover, the coupling radios of practical beam splitters at different wavelengths do not need to be close to 0 or 1, which makes our attack more applicative in practical QKD systems. Theoretical analysis and numerical simulation of BEM-based faked states attack on decoy-state QKD are also given out, which indicates that the eavesdropper can steal information about the final key if legitimate users are not aware of the induced BEM and re-emphasizes the inappropriate design of independently scanned activation windows of gated single photon detectors should be treated strictly. Countermeasures are also discussed.