Receiver for a quantum communication system

1. 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.

2. A receiver according to claim 1 , wherein the second measuring circuit is configured to measure the bias current with an integration time of 1 microsecond or longer.

3. A receiver according to claim 1 , wherein the second measuring circuit is provided in series with the DC voltage source.

4. A receiver according to claim 3 , wherein the bias circuit comprises a quenching resistor provided in series with said DC voltage source, said quenching resistor having a resistance of 20 kΩ or more.

5. A receiver according to claim 1 , wherein the second measuring circuit further indicates that the receiver is experiencing the blinding attack by issuing a predetermined erroneous signal to indicate that what is detected by the APD should be ignored.

6. A receiver according to claim 5 , wherein the predetermined erroneous signal is a fixed electrical pulse stream.

7. A receiver according to claim 1 , wherein the second measuring circuit further indicates that the receiver is experiencing the blinding attack by disabling a detector output.

8. A receiver according to claim 1 , wherein the second measuring circuit is configured to indicate if the bias current exceeds the value corresponding to 10% of the of the maximum photocurrent of the avalanche photodiode in the single photon regime.

9. A receiver according to claim 1 , wherein said modulating AC bias voltage is in the form of a pulse, square wave or sinusoidal wave.

10. A receiver according to claim 1 , wherein the modulating AC bias voltage is periodic.

11. A receiver according to claim 1 , wherein the first measuring circuit further comprises a unit for compensating for the capacitive response of the avalanche photodiode.

12. A quantum communication system, comprising a sending unit configured to send light pulses encoded using a basis selected from at least two bases and a receiver according to claim 1 .

13. A method of detecting a photon in 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 including 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 includes an avalanche photodiode, the method comprising:

periodically reverse biasing said avalanche photodiode above its breakdown voltage for photon detection by supplying a DC voltage from a DC voltage source and a modulating AC bias voltage superimposed on the DC voltage from an AC voltage source;

measuring avalanche events occurring in said avalanche photodiode as a transient current due to photon absorption;

measuring the bias current flowing through a DC path comprising the DC voltage source; and

indicating 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.