Detector autocalibration in QKD systems

1. A method of autocalibrating a single-photon detector arranged to detect weak photon pulses in a quantum key distribution (QKD) system, comprising:

a) performing a detector gate scan by sending a detector gate pulse to the single-photon detector and varying an arrival time T of the detector gate pulse over a first select timing range R 1 to determine an optimal arrival time T MAX that corresponds to a maximum number of photon counts N MAX from the single-photon detector by detecting the weak photon pulses; and

b) performing detector gate dithering of the detector gate pulse by varying the arrival time T over a second select timing range R 2 surrounding optimal arrival time T MAX to maintain the photon count at the maximum number of photon counts.

2. The method of claim 1 , including:

terminating the detector gate dithering and performing another detector gate scan.

3. The method of claim 1 , further including performing the detector gate scan and detector gate dithering multiple times during operation of the QKD system.

4. The method of claim 1 , wherein performing the detector gate scan includes varying a detector gate pulse width W over a first range of pulse widths RW 1 to establish an optimal detector gate pulse width W MAX .

5. The method of claim 4 , wherein performing detector gate dithering includes varying the detector gate pulse width W over a second range of pulse widths RW 2 <RW 1 to maintain an optimal pulse width.

6. A computer-readable medium having instructions embodied therein to direct a computer in a quantum key distribution (QKD) system to execute instructions to perform the following method of performing autocalibration of a single-photon detector arranged to detect weak photon pulses in the QKD system:

a) sending a detector gate pulse to the single-photon detector and varying an arrival time T of the detector gate pulse over a first timing range R 1 to determine an optimal arrival time T MAX that corresponds to a maximum number of photon counts N MAX from the single-photon detector; and

b) dithering the detector gate pulse by varying the arrival time T over a second timing range R 2 surrounding optimal arrival time T MAX to maintain the number of photon counts at either said maximum number of photon counts N MAX or a new maximum number of photon counts N′ MAX .

7. The computer-readable medium of claim 6 , wherein the method further includes performing acts a) and b) multiple times during operation of the QKD system.

8. The computer-readable medium of claim 6 , wherein the method further includes varying a detector gate pulse width W over a range of pulse widths to establish an optimal detector gate pulse width W MAX for the single-photon detector.

9. A method of operating a quantum key distribution (QKD) system having a single-photon detector operably coupled to a controller, comprising:

sending weak photon pulses between encoding stations in the QKD system;

performing a first detector gate scan by sending, from the controller to the single-photon detector, a detector gate pulse having an arrival time, including varying the arrival time over a first range of detector gate pulse arrival times to establish a first optimal arrival time T MAX corresponding to a first maximum number of photon counts N MAX determined by detecting said weak photon pulses by the single-photon detector;

terminating the first detector gate scan when the first optimal arrival time T MAX is established; and

performing a first detector gate dither by varying the arrival time over a second range of detector gate pulse arrival times about the first optimal arrival time T MAX to maintain either the first maximum number of photon counts N MAX or a second maximum number of photon counts N′ MAX , wherein the second range of detector gate pulse arrival times is within the first range of detector gate arrival times.

10. The method of claim 9 , wherein performing the first detector gate dither results in a second optimal arrival time T′ MAX different from the first optimal arrival time T MAX .

11. The method of claim 9 , further including:

terminating the performing of the first detector gate dither;

performing a second detector gate scan;

terminating the second detector gate scan; and

performing a second detector gate dither.

12. The method of claim 9 , further including terminating and repeating the first detector gate dither periodically so as to perform a series of detector gate dithers.

13. The method of claim 9 , further including varying a detector gate pulse width W over a range of pulse widths to establish an optimal detector gate pulse width W MAX for the single-photon detector.

14. A computer-readable medium having instructions embodied therein to direct a computer in a quantum key distribution (QKD) system to execute instructions to perform the following method of autocalibrating a single-photon detector arranged to detect photons in the QKD system:

sending weak photon pulses between encoding stations in the QKD system; performing a first detector gate scan by sending, from a controller to the detector, a detector gate pulse having an arrival time, including varying the arrival time over a first range of detector gate pulse arrival times to establish a first optimal arrival time T MAX corresponding to a first maximum number of photon counts N MAX determined by detecting said weak photon pulses by the single-photon detector;

terminating the first detector gate scan when the first optimal photon count N MAX is established; and

performing a first detector gate dither by varying the arrival time over a second range of detector gate pulse arrival times about the first optimal arrival time T MAX to maintain either the first maximum number of photon counts N MAX or a second maximum number of photon counts N′ MAX , wherein the second range of detector gate pulse arrival times is within the first range of detector gate arrival times.

15. A method of autocalibrating a single-photon detector in a quantum key distribution (QKD) system having a controller, comprising:

sending weak photon pulses between encoding stations in the QKD system;

performing a first detector gate scan to determine an optimum arrival time of a detector gate pulse sent from a controller to the single-photon detector;

terminating the first detector gate scan; and

periodically performing a first detector gate dither to maintain a maximum number of photon counts from the single-photon detector.

16. the method of claim 15 , further including:

terminating the first detector gate dither; and

performing a second detector gate scan.

17. The method of claim 15 , further including varying a detector gate pulse width W over a range of pulse widths to establish an optimal detector gate pulse width W MAX for the single-photon detector.

18. A method of performing photon detector autocalibration in quantum key distribution (QKD) system having a single-photon detector coupled to a controller adapted to generate a detector gate pulse having a corresponding arrival time at the single-photon detector, the method comprising:

performing a detector gate scan to establish an optimum arrival time of the detector gate pulse that corresponds with a first maximum number of photon counts of weak photon pulses detected by the single-photon detector;

terminating the detector gate scan; and

performing a detector gate dither process by varying the detector gate pulse arrival time around the optimal arrival time to adjust the arrival time to ensure that the single-photon detector produces either the maximum number of photon counts or a second maximum number of photon counts when detecting the weak photon pulses.

19. The method of claim 18 , further including varying a detector gate pulse width W over a range of pulse widths to establish an optimal detector gate pulse width W MAX for the single-photon detector.