After-gate attack on a quantum cryptosystem

We present a method to control the detection events in quantum key distribution systems that use gated single-photon detectors. We employ bright pulses as faked states, timed to arrive at the avalanche photodiodes outside the activation time. The attack can remain unnoticed, since the faked states do not increase the error rate per se. This allows for an intercept–resend attack, where an eavesdropper transfers her detection events to the legitimate receiver without causing any errors.