Single-photon watch dog detector for folded quantum key distribution system
View Patent ↗A single-photon “watch dog” detector for a two-way quantum key distribution (QKD) system. The detector can detect weak probe signals associated with a Trojan horse attack, or weak substitute signals associated with a “man in the middle” attacks. The detector provides for a significant increase in security for a two-way QKD system over the prior art that employs a conventional detector such as a photodiode. By counting the number of weak pulses entering and/or leaving the reflecting QKD station (Alice), an eavesdropper that attempts to add weak pulses to the quantum channel in order to gain phase information from the phase modulator at Alice can be detected.
1. A key encoding station for a two-way quantum key distribution (QKD) system, comprising:
a phase modulator;
a Faraday mirror arranged to reflect pulses of radiation arriving from a transmitting/receiving key encoding station through the phase modulator;
a controller, coupled to the phase modulator, that provides a first gating signal to the phase modulator to activate the phase modulator to modulate one of the pulses of radiation; and
a single-photon detector coupled to the controller and gated with a second gating signal from the controller to detect radiation pulses entering and/or leaving the reflecting key encoding station in a given time interval, said detector and controller being operable to compare the number of radiation pulses entering and leaving the key encoding station in a given time interval to determine if any of the incoming radiation pulses were not sent by the transmitting/receiving station.
2. A key encoding station according to claim 1 , further including a beamsplitter arranged to direct a portion of the radiation pulses to the single-photon detector.
3. A key encoding station according to claim 1 , further including a photon-emitting device arranged to provide a calibration radiation beam to the single-photon detector.
4. A key encoding station according to claim 1 , including an optical coupler arranged to direct at least a portion of the radiation pulses to the single-photon detector.
5. A key encoding station according to claim 1 , including an optical switch arranged to selectively reflect radiation pulses to the single-photon detector.
6. A key encoding station according to claim 1 , further including a photon-emitting device optically coupled to the single-photon detector and adapted to emit single-photon pulses in order to calibrate the single-photon detector.
7. A method of improving the security of a two-way quantum key distribution (QKD) system, comprising:
providing a key encoding station having a single-photon detector; and
monitoring radiation pulses incoming to and outgoing from the key encoding station in a given time interval using the single-photon detector;
determining an average number of incoming radiation pulses and an average number of outgoing radiation pulses in the given time interval; and
comparing the average number of incoming radiation pulses to the average number of outgoing radiation pulses to detect the presence of an eavesdropper.
8. A method according to claim 7 , further including calibrating the single-photon detector with a photon-emitting device located within the key encoding station.
9. A method according to claim 7 , including gating the single-photon detector so that the single-photon detector detects radiation pulses corresponding to a time period during which a phase-modulator in the encoding station is activated.
10. A method according to claim 9 , including sending a gating pulse from a controller to the single-photon detector to gate the single-photon detector.
11. A method according to claim 7 , including:
randomly activating an optical switch to randomly direct some of the incoming radiation pulses to the single-photon detector; and
gating the single-photon detector to detect the randomly directed incoming radiation pulses.
12. A method according to claim 11 , including:
randomly activating an optical switch to direct some of the outgoing radiation pulses to the single photon detector; and
gating the single-photon detector to detect the randomly directed outgoing radiation pulses.
13. A method according to claim 7 , including
randomly activating an optical switch to direct some of both the incoming and outgoing radiation pulses to the single-photon detector; and
gating the single-photon detector to detect the randomly directed incoming and outgoing radiation pulses.