Optical switching for quantum key distribution

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.

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.

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.