Quantum cryptography device and method

• US 6,438,234 B1
• Filed: 03/05/1999
• Issued: 08/20/2002
• Est. Priority Date: 09/05/1996
• Status: Expired due to Term

• Current Assignee: Swisscom AG (Government of Switzerland)
• Original Assignee: Swisscom AG (Government of Switzerland)
• Inventors: Perny, Beat, Gisin, Nicolas, Muller, Antoine, Huttner, Bruno, Zbinden, Hugo
• Primary Examiner(s): Hayes, Gail
• Assistant Examiner(s): Seal, James
• Application Number:
  US09/262,748
• Time in Patent Office: 1,264 Days
• Field of Search: 380/44, 380/278, 380/283, 380/260, 380/256, 359/112
• US Class Current: 380/256
• CPC Class Codes: H04L 9/0852 Quantum cryptography transmission systems employing electromagnetic waves other than radio waves, e.g. light, infrared H04B10/00; wavelength-division multiplex systems H04J14/02; WDM arrangements H04J14/03

Abstract

A system and method for communicating a key between two stations using an interferometric system for quantum cryptography. The method includes sending at least two light pulses over a quantum channel and detecting the interference created by the light pulses. The interfering pulses traverse the same arms of an interferometer but in a different sequence such that the pulses are delayed when traversing a quantum channel. The pulses are reflected by Faraday mirrors at the ends of the quantum channel so as to cancel any polarization effects. Because the interfering pulses traverse the same arms of an interferometer, there is no need to align or balance between multiple arms of an interferometer.

Claims

1. Method of communicating between two stations using an interferometric system for quantum cryptography, comprising:

sending at least two light pulses over a quantum channel coupled to the two stations; and

detecting interference created by said pulses in one station;

wherein said pulses traverse the same branches of the interferometric system, each of said pulses traversing the same branches in a different sequence so that said pulses are delayed when traversing said quantum channel.

2. Method according to claim 1, wherein said pulses are reflected by at least one Faraday mirror on at least one end of said quantum channel.

3. Method according to claim 1 or claim 2, wherein the average number of photons in said pulses is less than 1.

4. Method according to claim 1, wherein said pulses include a first pulse and a second pulse;

and

wherein said pulses are sent by a source in a sending/receiving station which delays the second pulse with a delay line, and received by at least one key encoding station which phase modulates the second pulse and reflects both pulses toward said sending/receiving station which delays and phase modulates said first pulse.

5. Method according to claim 4, wherein said second pulse is attenuated in said key encoding station so that an average number of photons in said second pulse reflected back to said sending/receiving station is less than 1.

6. Method according to one of claims 4 or 5, herein both stations choose at random phase shifts applied to said first and second pulses.

7. Method according to claim 6, wherein said phase shifts are chosen as either the value 0 or the value π

, and

wherein the interference between said first pulse and said second pulse is constructive when both stations have applied the same phase shift, and totally destructive when they apply different phase shifts.

8. Method according to claim 1, wherein said pulses include at least two orthogonal polarization components;

and herein said components traverse the same branches of the interferometric system, each of said components traversing the same branches in a different sequence.

9. Method according to claim 8, wherein one of said stations chooses at random the phase of one of the above polarization components with respect to the phase of another polarization component, thus creating a random output polarization.

10. Interferometric system configured for the distribution of a key over a quantum channel using quantum cryptography, comprising:

at least one sending/receiving station and at least one key encoding station, both coupled to said quantum channel,

means in at least one of said stations for sending at least two light pulses over said quantum channel to at least one other said station,

detectors in at least one of said stations for detecting interference created by said pulses in said stations,

wherein said light pulses traverse the same branches of the interferometric system, each of said pulses traversing the same branches in a different sequence so that said pulses are delayed when traversing said quantum channel.

11. System according to claim 10, wherein at least one of said stations comprises at least one Faraday mirror on at least one end of said quantum channel.

12. System according to one of claims 10 or 11, wherein at least one of said stations comprises means for attenuating the intensity of said light pulses so that the average number of photons in said pulses is less than 1.

13. System according to claim 10, wherein

said at least two light pulses include a first pulse and a second pulse;

said sending/receiving station includes a delay line for delaying said first pulse before it is sent over said quantum channel and said second pulse received over said quantum channel, and at least one single photon detector for detecting interference between said first and second pulses; and

said key encoding station includes mirrors for reflecting said first and second pulses and at least one phase modulator for modulating the phase of at least one of said pulses.

14. System according to claim 13, wherein said key encoding station comprises means for attenuating the intensity of at least one of said pulses so that the average number of photons in said second pulse reflected back to said sending/receiving station is less than 1.

15. System according to claim 13, wherein both stations choose at random phase shifts applied to said first and second pulses.

16. System according to claim 15, wherein both stations choose said phase shifts as either the value 0 or the value π

, and wherein the interference between said first pulse and said second pulse is constructive when both stations have applied the same phase shift, and totally destructive when the stations have applied different phase shifts.

17. System according to claim 10, wherein the light pulses sent by said sending/receiving station comprise at least two orthogonal polarization components, and wherein said components traverse the same branches of said interferometric system, each of said components traversing the same branches in a different sequence.

18. System according to claim 17, wherein one of said stations chooses at random the phase of one of said polarization components with respect to another polarization component, thus creating a random output polarization.

19. Key encoding station for communicating a key to at least one sending/receiving station through a quantum channel comprising:

reflecting means for reflecting a first pulse sent by a receiving station back to said receiving station;

reflecting means for reflecting a second pulse, sent by said receiving station shortly after said first pulse, back to said receiving station; and

modulating means for modulating the phase of said second pulse with respect to said first pulse.

20. Key encoding station according to claim 19, further comprising detecting means for detecting said first pulse.

21. Key encoding station according to claim 20, wherein said first pulse and said second pulse both run through said modulating means and are both reflected by the same reflecting means;

and

wherein said detecting means adjust the phase shift applied by said modulating means immediately after having received said first pulse, so that only said second pulse is phase modulated by said phase modulating means.

22. Key encoding station according to claim 21, wherein the key encoding station chooses at random the phase shift applied to said second pulse.

23. Key encoding station according to claim 21, wherein the phase shift applied by said modulating means is chosen at random among as either the value 0 or the value π

.

24. Key encoding station according to claim 19, further comprising an attenuating means for attenuating the light intensity of said second pulse so that the average number of photons in said second pulse reflected back is less than 1.

25. Key encoding station according to claim 24, wherein said attenuating means comprises a coupler sending most of the received light to said detecting means.

26. Key encoding station according to claim 24, wherein said attenuating means comprise an attenuator controlled by said detecting means.

27. Key encoding station according to claim 19, wherein said reflecting means are composed of a Faraday mirror.

28. Key encoding station according to claim 20, wherein said detecting means are not single-photon detectors.

29. Key encoding station according to claim 19, wherein said modulating means are made of a Lithium Niobate (LiNbO₃) modulator.

43. Device for the distribution of a key over a quantum channel using quantum cryptography, comprising a sending/receiving station according to claim 30 and a key encoding station according to claim 20.

44. Multi-station system for the distribution of a key over a quantum channel using quantum cryptography between at least one sending/receiving station and at least one key encoding station, comprising at least one sending/receiving station according to claim 30 and at least one key encoding station according to claim 20.

30. Sending/receiving station for receiving a key sent from one key encoding station through a quantum channel, comprising:

a pulsed laser source;

a delay line;

detecting means; and

a first coupler connected in such a way that the pulses emitted by said pulsed laser source are split in two pulses;

wherein said delay line is configured such that a first split pulse is directly sent to said quantum channel and a second split pulse is delayed by said delay line before being sent to said quantum channel; and

wherein the pulses received from said quantum channel are split in two pulses, the first pulse being directly sent to said detecting means and the second pulse being delayed by said delay line before being sent to said detecting means.

31. Sending/receiving station according to claim 30, further comprising modulating means for modulating the phase of the received pulses delayed by said delay line.

32. Sending/receiving station according to claim 31, wherein said modulating means choose at random phase shifts applied to said delayed pulses.

33. Sending/receiving station according to claim 32, wherein said modulating means choose said phase shifts at random as either the value 0 or the value π

.

34. Sending/receiving station according to one of the claims 31 to 33, wherein said modulating means are made of a Lithium Niobate (LiNbO₃) modulator.

35. Sending/receiving station according to claim 30, further comprising two detectors, wherein said first coupler is a 3×

3 coupler connected to said detectors, and wherein a pulse will be sent either to the first or to the second of said detectors depending on the interference created in said coupler.

36. Sending/receiving station according to claim 30, wherein said laser source sends light pulses with a circular polarization, and wherein said first coupler is a polarization coupler that separates the vertical and horizontal polarizations of the pulses.

37. Sending/receiving station according to claim 30, wherein said delay line comprises two Faraday mirrors reflecting the delayed pulses.

38. Sending/receiving station according to claim 30, wherein said detecting means are single-photon detectors.

39. Sending/receiving station according to claim 38, wherein said single-photon detectors are avalanche photo diodes biased beyond reverse breakdown and operating in the Geiger mode.

40. Sending/receiving station according to claim 38, wherein the single-photon detectors are activated only each time a photon is expected.

41. Sending/receiving station according to claim 30, wherein said pulsed laser source is a DFB laser.

42. Sending/receiving station according to claim 30, further comprising error correcting means.