Phase locking in a multi-channel quantum communication system
View Patent ↗A communication system adapted to use wavelength (frequency) division multiplexing for quantum-key distribution (QKD) and having a transmitter coupled to a receiver via a transmission link. In one embodiment, the receiver is adapted to (i) phase-shift a local oscillator (LO) signal generated at the receiver, (ii) combine the LO signal with a quantum-information (QI) signal received via the transmission link from the transmitter to produce interference signals, (iii) measure an intensity difference for these interference signals, and (iv) phase-lock the LO signal to the QI signal based on the measurement result. In one configuration, the QI signal has a plurality of pilot frequency components, each carrying a training signal, and a plurality of QKD frequency components, each carrying quantum key data. Advantageously, the system can maintain a phase lock for the QKD frequency components of the QI and LO signals, while the QKD frequency components of the QI signal continuously carry quantum key data.
1. A method of receiving quantum information at a receiver of a communication system having a transmitter coupled to the receiver via a transmission link, the method comprising:
(A) receiving via the transmission link a quantum-information (QI) signal generated by the transmitter using a first optical source;
(B) phase-locking to the QI signal a local oscillator (LO) signal generated using a second optical source;
(C) combining the LO signal and the QI signal to produce first and second interference signals;
(D) measuring an intensity difference between the first and second interference signals; and
(E) phase-shifting the LO signal based on the measurement result to achieve the phase lock.
2. The invention of claim 1 , wherein:
for step (A), the QI signal comprises a training signal; and
step (B) comprises:
determining a reference phase shift for the LO signal based on the training signal; and
phase-shifting the LO signal using the reference phase shift to achieve the phase lock.
3. The invention of claim 2 , wherein step (B) comprises:
generating a probability distribution function corresponding to the training signal; and
determining the reference phase shift based on the probability distribution function.
4. The invention of claim 3 , wherein step (B) comprises:
generating a plurality of probability distribution functions corresponding to the training signal using sliding-window processing; and
adaptively adjusting the reference phase shift based on said plurality of probability distribution functions.
5. The invention of claim 3 , wherein step (B) comprises:
determining a distribution average for the probability distribution function; and
determining the reference phase shift based on the distribution average.
6. The invention of claim 5 , comprising adaptively tracking a null of the distribution average.
7. The invention of claim 2 , wherein the training signal comprises a known sequence of bits encoded using a known basis-set selection.
8. The invention of claim 7 , wherein:
the known sequence of bits corresponds to a first basis set and a single bit value; and
step (B) comprises selecting a second basis set, different from the first basis set, for the determination of the reference phase shift.
9. The invention of claim 1 , comprising compiling a quantum key based on the QI signal, wherein the QI signal comprises a signal having quantum key data.
10. The invention of claim 1 , wherein each of the QI and LO signals has a plurality of frequency components, wherein at least one frequency component of the LO signal is phase-locked to a corresponding frequency component of the QI signal.
11. The invention of claim 10 , wherein each frequency component of the QI signal alternates between carrying a training signal and a signal having quantum key data.
12. The invention of claim 11 , wherein, for at least a first period of time, all frequency components of the QI signal carry training signals and, for at least a second period of time, all frequency components of the QI signal carry signals having quantum key data.
13. The invention of claim 1 , wherein:
for step (A), the QI signal has one or more pilot frequency components, each characterized by a corresponding optical frequency, wherein each pilot frequency component carries a training signal;
for step (B), the LO signal has one or more pilot frequency components having the one or more optical frequencies;
for each pilot frequency component of the LO signal, step (B) comprises:
determining a reference phase shift based on the training signal; and
phase-shifting the pilot frequency component of the LO signal using the reference phase shift.
14. The invention of claim 13 , wherein, for at least one frequency component of the LO signal having a signal frequency other than one of the optical frequencies of the one or more pilot frequency components, step (B) comprises determining a reference phase shift based on the one or more reference phase shifts determined for the one or more pilot frequency components of the LO signal.
15. The invention of claim 13 , wherein, in addition to the one or more pilot frequency components, each of the QI and LO signals has one or more quantum-key-distribution (QKD) frequency components, wherein the method comprises receiving quantum key data, in parallel with the one or more training signals, using said QKD frequency components.
16. The invention of claim 15 , comprising changing over time allocation of a frequency component as a pilot or QKD frequency component.
17. The invention of claim 16 , comprising changing the allocation based on a condition of the transmission link.
18. A communication system for transmission of quantum information, comprising a transmitter coupled to a receiver via a transmission link, wherein:
the receiver is adapted to:
receive via the transmission link a quantum-information (QI) signal generated by the transmitter using a first optical source; and
phase-lock to the QI signal a local oscillator (LO) signal generated using a second optical source; and
the receiver comprises:
an optical modulator adapted to phase-shift the LO signal;
a detector adapted to (i) combine the LO signal with the QI signal to produce first and second interference signals and (ii) measure an intensity difference between the first and second interference signals; and
a processor adapted to process the measurement result to generate a control signal, which configures the optical modulator to phase-shift the LO signal to achieve the phase lock.
19. The invention of claim 18 , wherein each of the first and second optical sources is referenced to a frequency standard.
20. The invention of claim 18 , wherein:
the QI signal comprises a training signal; and
the processor is adapted to (i) determine a reference phase shift for the LO signal based on a measurement result corresponding to the training signal and (ii) configure the optical modulator to phase-shift the LO signal using the reference phase shift.
21. The invention of claim 20 , wherein the processor is configured to:
generate a probability distribution function corresponding to the training signal; and
determine the reference phase shift based on the probability distribution function.
22. The invention of claim 21 , wherein the processor is configured to:
generate a plurality of probability distribution functions corresponding to the training signal using sliding-window processing; and
adjust the reference phase shift in real time based on said plurality of probability distribution functions.
23. The invention of claim 21 , wherein the processor is configured to:
determine a distribution average for the probability distribution function; and
determine the reference phase shift based on the distribution average.
24. The invention of claim 23 , the processor is configured to adaptively track a null of the distribution average.
25. The invention of claim 20 , wherein, the training signal comprises a known sequence of bits encoded using a known basis-set selection.
26. The invention of claim 18 , wherein:
the QI signal has one or more pilot frequency components, each characterized by a corresponding optical frequency, wherein each pilot frequency component carries a training signal;
the LO signal has one or more pilot frequency components having the one or more optical frequencies;
for each pilot frequency component of the LO signal, the processor is adapted to:
determine a reference phase shift based on a measurement result corresponding to the training signal; and
configure the OM to phase shift the pilot frequency component of the LO signal using the reference phase shift.
27. The invention of claim 26 , wherein, for at least one frequency component of the LO signal having a signal frequency other than one of the optical frequencies of the one or more pilot frequency components, the processor is adapted to determine a reference phase shift based on the one or more reference phase shifts determined for the one or more pilot frequency components of the LO signal.
28. The invention of claim 26 , wherein, in addition to the one or more pilot frequency components, each of the QI and LO signals has one or more quantum-key-distribution (QKD) frequency components, wherein the receiver is adapted to receive quantum key data, in parallel with the one or more training signals, using the QKD frequency components.
29. The invention of claim 28 , wherein the system is adapted to change over time allocation of a frequency component as a pilot or QKD frequency component.
30. The invention of claim 29 , wherein the allocation is changed based on a condition of the transmission link.
31. The invention of claim 18 , wherein the receiver is adapted to compile a quantum key based on the QI signal, wherein the QI signal comprises a signal having quantum key data.
32. The invention of claim 18 , wherein each of the QI and LO signals has a plurality of frequency components, wherein at least one frequency component of the LO signal is phase-locked to a corresponding frequency component of the QI signal.
33. The invention of claim 32 , wherein each frequency component of the QI signal alternates between carrying a training signal and a signal having quantum key data.
34. The invention of claim 33 , wherein, for at least a first period of time, all frequency components of the QI signal carry training signals and, for at least a second period of time, all frequency components of the QI signal carry signals having quantum key data.
35. A receiver for a communication system adapted for transmission of quantum information and having a transmitter optically coupled to the receiver, wherein the receiver is adapted to:
receive a quantum-information (QI) signal generated by the transmitter using a first optical source; and
phase-lock to the QI signal a local oscillator (LO) signal generated using a second optical source, wherein:
the QI signal is a frequency-multiplexed QI signal having a first plurality of independently modulated frequency components;
the LO signal has a second plurality of frequency components; and
the receiver is adapted to phase-lock at least one frequency component of the second plurality to a corresponding frequency component of the first plurality.
36. A method of receiving quantum information at a receiver of a communication system having a transmitter optically coupled to the receiver, the method comprising:
(A) receiving a quantum-information (QI) signal generated by the transmitter using a first optical source; and
(B) phase-locking to the QI signal a local oscillator (LO) signal generated using a second optical source, wherein:
for step (A), the QI signal comprises a training signal; and
step (B) comprises:
determining a reference phase shift for the LO signal based on the training signal; and
phase-shifting the LO signal using the reference phase shift to achieve the phase lock.
37. The invention of claim 36 , wherein step (B) comprises:
generating a probability distribution function corresponding to the training signal; and
determining the reference phase shift based on the probability distribution function.
38. The invention of claim 37 , wherein step (B) comprises:
generating a plurality of probability distribution functions corresponding to the training signal using sliding-window processing; and
adaptively adjusting the reference phase shift based on said plurality of probability distribution functions.
39. The invention of claim 37 , wherein step (B) comprises:
determining a distribution average for the probability distribution function; and
determining the reference phase shift based on the distribution average.
40. The invention of claim 39 , comprising adaptively tracking a null of the distribution average.
41. The invention of claim 36 , wherein the training signal comprises a known sequence of bits encoded using a known basis-set selection.
42. The invention of claim 41 , wherein:
the known sequence of bits corresponds to a first basis set and a single bit value; and
step (B) comprises selecting a second basis set, different from the first basis set, for the determination of the reference phase shift.
43. A method of receiving quantum information at a receiver of a communication system having a transmitter optically coupled to the receiver, the method comprising:
(A) receiving a quantum-information (QI) signal generated by the transmitter using a first optical source; and
(B) phase-locking to the QI signal a local oscillator (LO) signal generated using a second optical source, wherein:
for step (A), the QI signal has one or more pilot frequency components, each characterized by a corresponding optical frequency, wherein each pilot frequency component carries a training signal;
for step (B), the LO signal has one or more pilot frequency components having the one or more optical frequencies;
for each pilot frequency component of the LO signal, step (B) comprises:
determining a reference phase shift based on the training signal; and
phase-shifting the pilot frequency component of the LO signal using the reference phase shift.
44. The invention of claim 43 , wherein, for at least one frequency component of the LO signal having a signal frequency other than one of the optical frequencies of the one or more pilot frequency components, step (B) comprises determining a reference phase shift based on the one or more reference phase shifts determined for the one or more pilot frequency components of the LO signal.
45. The invention of claim 43 , wherein, in addition to the one or more pilot frequency components, each of the QI and LO signals has one or more quantum-key-distribution (QKD) frequency components, wherein the method comprises receiving quantum key data, in parallel with the one or more training signals, using said QKD frequency components.
46. The invention of claim 45 , comprising changing over time allocation of a frequency component as a pilot or QKD frequency component.
47. The invention of claim 46 , comprising changing the allocation based on a condition of a transmission link via which the QI signal is transmitted.
48. A method of receiving quantum information at a receiver of a communication system having a transmitter optically coupled to the receiver, the method comprising:
(A) receiving a quantum-information (QI) signal generated by the transmitter using a first optical source; and
(B) phase-locking to the QI signal a local oscillator (LO) signal generated using a second optical source, wherein:
the QI signal is a frequency-multiplexed QI signal having a first plurality of independently modulated frequency components; and
the LO signal has a second plurality of frequency components, wherein at least one frequency component of the second plurality is phase-locked to a corresponding frequency component of the first plurality.
49. The invention of claim 48 , wherein each frequency component of the first plurality alternates between carrying a respective training signal and a respective signal having quantum key data.
50. The invention of claim 49 , wherein, for at least a first period of time, all frequency components of the first plurality carry training signals and, for at least a second period of time, all frequency components of the first plurality carry respective signals having quantum key data.
51. A communication system for transmission of quantum information, comprising a transmitter optically coupled to a receiver, wherein:
the receiver is adapted to:
receive a quantum-information (QI) signal generated by the transmitter using a first optical source; and
phase-lock to the QI signal a local oscillator (LO) signal generated using a second optical source;
the transmitter is adapted to independently modulate frequency components of a first plurality of frequency components to make the QI signal a frequency-multiplexed QI signal;
the LO signal has a second plurality of frequency components; and
the receiver is further adapted to phase-lock at least one frequency component of the second plurality to a corresponding frequency component of the first plurality.
52. The invention of claim 51 , wherein the transmitter is further adapted to alternate each frequency component of the first plurality between carrying a respective training signal and a respective signal having quantum key data.
53. The invention of claim 52 , wherein the transmitter is further adapted to modulate the frequency components so that, for at least a first period of time, all frequency components of the first plurality carry training signals and, for at least a second period of time, all frequency components of the first plurality carry signals having quantum key data.