Patent ID: 8890049

Claim:
A receiver for a quantum communication system, the receiver being configured to receive light pulses encoded using a basis selected from at least two bases, the receiver comprising: a decoder configured to perform a measurement in a basis selected from possible bases used to encode the pulses and a photon detection system configured to receive an output of the decoder, wherein the photon detection system comprises: an avalanche photodiode; a biasing circuit configured to reverse bias said avalanche photodiode, the biasing circuit comprising a DC path and an AC path, the DC path comprising a DC voltage source configured to supply a DC voltage and the AC path comprising an AC voltage source configured to provide a modulating AC bias voltage which is superimposed on the DC voltage such that said avalanche photodiode is periodically biased above its breakdown voltage for photon detection; a first measuring circuit configured to measure avalanche events occurring in said avalanche photodiode as a transient current due to photon absorption; and a second measuring circuit configured to measure the bias current flowing through the DC path, the second measuring circuit being configured to indicate to a controller that the receiver is experiencing a blinding attack when said measured bias current exceeds a threshold value, wherein the threshold value is less than a maximum photocurrent for the avalanche photodiode in a single photon regime.