Method and system for ongoing spectral feedback and control during quantum key distribution using data analysis

A quantum communication system receiver comprising:

• an interference device with multiple inputs configured to receive multiple series of temporally spaced apart qubit signals from multiple transmitters, and multiple outputs configured to output an output signal based on the interference of the received qubit signals at the interference device;
• multiple detectors configured to detect the output signal at multiple outputs of the interference device;
• and
• a receiver controller configured to:
• transmit information concerning the detected output signals to the transmitters;
• monitor a coincidence rate of simultaneously arriving signals at multiple different detectors;
• and,
• provide spectral feedback based on the outputs and based on the determined coincidence rate, wherein the feedback is derived from information determined through an analysis of a plurality of the detected output signals and pertains to spectral similarity of the qubit signals.

A method of establishing at least a portion of a cryptographic key, the method comprising:

• transmitting a controlled series of temporally spaced apart qubit signals from each of a plurality of transmitters, each qubit signal being within a respective time window, and corresponding to a basis within a plurality of bases, and a mode within a plurality of modes of the basis;
• receiving the qubit signals at a receiver;
• interfering multiple received qubit signals from the multiple transmitters to produce an output signal based on an interference of the received qubit signals at the interference device;
• monitoring a coincidence rate of simultaneously arriving signals at multiple different detectors;
• detecting the output signals;
• and
• transmitting information relating to the detected output signals to the transmitters, automatically providing spectral feedback based on information determined from an analysis of a plurality of the detected output signals and based on the determined coincidence rate, wherein the feedback corresponds to spectral characteristics of the qubit signals, the characteristics comprising:
• spectral similarity of the qubit signals;
• concurrently adjusting subsequently transmitted qubit signals based on the received feedback and adjusting the central frequency of the emitted qubit signals by an amount determined from a deviation of the coincidence rate from a predetermined value;
• and
• establishing at least part of a cryptographic key between the transmitters based on the transmitted information relating to the detected output signals.