Optical switching for quantum key distribution
A system and method for operating a quantum key distribution system including a receiver to receive an optical signal from a transmitter, the receiver including a first signal detection unit (1SDU) and a second signal detection unit (2SDU) for detecting events associated with single photons; an optical switch to alternate between a first switch position to direct a received optical signal to the 1SDU and a second switch position to direct a received optical signal to the 2SDU, such that the 1SDU and the 2SDU correspond to the first and second switch positions respectively; a detection time tagger to tag the events at the 1SDU and the 2SDU according to time, switch position, and signal detection unit, to introduce a correlation between a time in which an event is registered, the switch position, and the signal detection unit; and a timing recovery module connected to the detection time tagger.
1 . A receiver configured to receive an optical signal from a transmitter in an optical communication system, the receiver comprising:
at least a first and a second signal detection unit for detecting detection events associated with single photons;
at least one optical switch configured to alternate between engaging at least a first switch position and a second switch position, the first switch position being arranged to direct a received optical signal to the first signal detection unit and the second switch position being arranged to direct a received optical signal to the second signal detection unit, such that the first and second signal detection units correspond to the first and second switch positions respectively;
a detection time tagger configured to tag the detection events at the first and second signal detection units according to time, switch position, and signal detection unit, thereby introducing a correlation between a time period in which a detection event is registered, the switch position, and the signal detection unit; and
a timing recovery module connected to the detection time tagger, wherein the timing recovery module is configured to:
use the correlation to verify whether or not a detection event has been tagged in the correct time period, and, if a detection event is found to be tagged in the incorrect time period,
re-assign the detection event to the correct time period.
2 . The receiver of claim 1 , wherein the timing recovery module is configured to re-assign the detection event to the correct time period when:
the switch position tagged for the detection event is the first switch position, and the signal detection unit for the detection event is the second signal detection unit; or
when the switch position tagged for the detection event is the second switch position, and the signal detection unit tagged for the detection event is the first signal detection unit.
3 . The receiver according to claim 1 , wherein the time period corresponds to a time-bin, wherein the timing recovery module is configured to re-assign the detection event, found to be tagged in the incorrect time period, by reassigning the detection event to a previous time bin.
4 . The receiver according to claim 1 , wherein the detection time tagger is further configured to tag each detection event of the detection events with the encoding of photons in associated with each respective detection event.
5 . The receiver according to claim 1 , further comprising at least a first and second optical decoding system, wherein the at least one optical switch is configured to:
direct, when the optical switch is engaged at the first switch position, the received optical signal through the first optical decoding system to produce a first decoded optical signal; and
direct the first decoded optical signal to the first signal detection unit for detecting detection events; and
when the optical switch is engaged at the second switch position, direct the received optical signal through a second optical decoding system to produce a second decoded optical signal; and
direct the second decoded optical signal to the second signal detection unit for detecting detection events.
6 . The receiver of claim 1 , wherein the at least one optical switch is configured to alternate between engaging at least the first switch position and the second switch position at a rate equal to a repetition rate of the received optical, such that the alternating of the at least one optical switch between the first and second switch positions is synchronised with the repetition rate of the received optical signal.
7 . The receiver of claim 6 , wherein the repetition rate of the received optical signal is calculated based on a repetition rate of a corresponding transmitted optical signal, transmitted from a transmitter to the receiver, adjusted according to a Doppler shift correction factor.
8 . The receiver according to claim 1 , wherein the at least one optical switch is configured to alternate independent of the wavelength and encoding of photons in the received optical signal.
9 . The receiver according to claim 1 , wherein the first and second signal detection units each comprise a plurality of single photon detectors.
10 . The receiver according to claim 9 , wherein the first and second signal detection units each comprise four single photon detectors.
11 . The receiver according to claim 9 , wherein the single photon detectors are SPADs.
12 . The receiver according to claim 1 , wherein the receiver is a ground-based receiver.
13 . The receiver of claim 1 , comprising a plurality of optical switches, wherein each of the plurality of optical switches is configured to alternate between engaging a first respective switch position and a second respective switch position, and wherein each of the plurality of optical switches is associated with a first and second respective signal detection unit that are independent for each optical switch.
14 . The receiver of claim 1 , wherein the at least one optical switch is configured to alternate between a plurality of switch positions, each of the plurality of switch positions being associated with a respective signal detection unit of a plurality of signal detection units.
15 . A method of operating an optical communication system, the method comprising, at a receiver:
receiving an optical signal;
alternating at least one optical switch between engaging at least a first switch position and a second switch position,
directing, when the optical switch is engaged at the first switch position, the received optical signal to a first signal detection unit;
directing, when the optical switch is engaged at the second switch position, the received optical signal to a second signal detection unit, such that the first and second signal detection units correspond to the first and second switch positions respectively;
detecting detection events at the first and second signal detection units;
tagging the detection events at the first and second signal detection units according to time, switch position, and signal detection unit, thereby introducing a correlation between a time period in which a detection event is registered, the switch position, and the signal detection units; and
using the correlation to verify whether or not a detection event has been tagged in the correct time period, and if a detection event is found to be tagged in the incorrect time period, re-assigning the detection event to the correct time period.
16 . The method of claim 15 , wherein the re-assigning the detection event to the correct time period occurs when:
the switch position tagged for the detection event is the first switch position, and the signal detection unit for the detection event is the second signal detection unit; or when
the switch position tagged for the detection event is the second switch position, and the signal detection unit tagged for the detection event is the first signal detection unit.
17 . The method of claim 15 , wherein the time period corresponds to a time-bin, and reassigning the detection event, found to be tagged in the incorrect time period, includes reassigning the detection event to a previous time bin.
18 . The method of claim 15 , wherein tagging detection events further comprises tagging detection events with encodings of photons associated with the detection events.
19 . The method of claim 15 , wherein the directing, when the optical switch is engaged at the first switch position, the received optical signal to a first signal detection unit further comprises:
directing the received optical signal through a first optical decoding system to produce a first decoded optical signal; and
directing the first decoded optical signal to the first signal detection unit for detecting detection events;
and wherein the directing, when the optical switch is engaged at the second switch position, the received optical signal to a second signal detection unit further comprises:
directing the received optical signal through a second optical decoding system to produce a second decoded optical signal; and
directing the second decoded optical signal to the second signal detection unit for detecting detection events.
20 . The method of claim 15 , wherein alternating the at least one optical switch between engaging the at least first switch position and the second switch position occurs at a rate equal to a repetition rate of the received optical, such that the alternating of the at least one optical switch between the first and second switch positions is synchronised with the repetition rate of the received optical signal.
21 . The method of claim 20 , further comprising:
calculating the repetition rate of the received optical signal based on a repetition rate of a corresponding transmitted optical signal, transmitted from a transmitter to the receiver, adjusted according to a Doppler shift correction factor.
22 . An optical communication system comprising:
a transmitter; and
a receiver configured to receive an optical signal from the transmitter, the receiver comprising:
at least a first and a second signal detection unit for detecting detection events associated with single photons;
at least one optical switch configured to alternate between engaging at least a first switch position and a second switch position, the first switch position being arranged to direct a received optical signal to the first signal detection unit and the second switch position being arranged to direct a received optical signal to the second signal detection unit, such that the first and second signal detection units correspond to the first and second switch positions respectively;
a detection time tagger configured to tag the detection events at the first and second signal detection units according to time, switch position, and signal detection unit, thereby introducing a correlation between a time period in which a detection event is registered, the switch position, and the signal detection unit; and
a timing recovery module connected to the detection time tagger, wherein the timing recovery module is configured to:
use the correlation to verify whether or not a detection event has been tagged in the correct time period, and, if a detection event is found to be tagged in the incorrect time period,
re-assign the detection event to the correct time period.
23 . The optical communication system of claim 22 , wherein at least one of the transmitter and the receiver is ground-based or wherein at least one of the transmitter and the receiver is a satellite.
24 . A non-transitory computer-readable storage medium comprising code or computer instructions stored thereon, which when executed by a processor, causes the processor to, upon a receiver receiving an optical signal:
alternate at least one optical switch between engaging at least a first switch position and a second switch position,
direct, when the optical switch is engaged at the first switch position, the received optical signal to a first signal detection unit;
direct, when the optical switch is engaged at the second switch position, the received optical signal to a second signal detection unit, such that the first and second signal detection units correspond to the first and second switch positions respectively;
detect detection events at the first and second signal detection units;
tag the detection events at the first and second signal detection units according to time, switch position, and signal detection unit, thereby introducing a correlation between a time period in which a detection event is registered, the switch position, and the signal detection units; and
use the correlation to verify whether or not a detection event has been tagged in the correct time period, and if a detection event is found to be tagged in the incorrect time period, re-assigning the detection event to the correct time period.