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Title:
QKD SYSTEM WITH COMMON-MODE DITHERING
Document Type and Number:
WIPO Patent Application WO/2009/014601
Kind Code:
A1
Abstract:
A QKD system (10) having two QKD stations (Alice and Bob) optically coupled by an optical fiber link (FL), wherein Bob includes a vaπable timing delay arranged between Bob's controller (CB) and modulator (MB) or detector unit (40) A set-up and calibration procedure is performed wherein delay DL2 is adjusted until the timings for the modulator and detector unit (TS8 and TS42, respectively) are established Delay (DL2) is then fixed so that the detector unit and modulator operate in a common timing mode that is not changed if the synchronization signal is changed The timing (TSS) of the synchronization (sync) signals (SS) sent from Alice to Bob is adjusted to arrive at optimum system performance

Inventors:
VIG HARRY (US)
Application Number:
PCT/US2008/008407
Publication Date:
January 29, 2009
Filing Date:
July 09, 2008
Export Citation:
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Assignee:
MAGIQ TECHNOLOGIES INC (US)
VIG HARRY (US)
International Classes:
H04K1/00
Foreign References:
US20060239463A12006-10-26
US20070110241A12007-05-17
US20070133799A12007-06-14
US20070165862A12007-07-19
Attorney, Agent or Firm:
GORTYCH, Joseph, E. (PLLC7791 Alister Mackenzie D, Sarasota FL, US)
Download PDF:
Claims:

What is claimed is:

1. A method of operating a QKD system that includes first and second QKD stations (Alice and Bob) optically coupled to one another, with Bob comprising a controller, a detector unit and a phase modulator, the method comprising: setting a timing T S s of synchronization (sync) signals SS that travel between Alice and Bob; sending optical signals from Alice to Bob to establish a timing TSB of a phase modulator activation signal SB and a timing T S4 o of a detector gating signal S42 based on said sync signals SS, wherein established timings TSB and Ts4o correspond to maximum number of optical signal counts NMAX; and fixing the relative timing δTFI between the signals SB and S42 so that a change in sync signal timing Tss does not change δTFI .

2. The method of claim 1 , further including: after fixing the relative timing δT F i, operating the system using quantum signals; and dithering the timing Tss of the synchronization signals SS to maintain an optimum sync signal timing Tss without changing the relative timing δT F i between signals SB and S42.

3. The method of claim 1 , wherein the optical signals used to establish timings Ts4o and TSB are strong (non-quantum) optical signals.

4. The method of claim 1 , further including setting a width δTSB to phase modulator activation signal SB and a width δT S42 to detector gating signal S42.

5. The method of claim 1 , wherein Alice includes a modulator MA, and including fixing a modulation value for modulator MA when establish timing TSB and timing Ts4o-

6. The method of claim 1 , including providing no modulation for modulator MA.

7. The method of claim 1 , including fixing a modulation value for modulator MB when establish the timing TSB and timing Ts 4 o-

8. The method of claim 2, wherein said dithering is performed at different times during the operation of the QKD system.

9. The method of claim 8, wherein said dithering is performed periodically during the operation of the QKD system.

10. The method of claim 8, wherein the optimum performance corresponds to a maximum number NMAX of detector counts N, and performing said dithering when N drops from a maximum number of detector counts N M AX by a threshold number of counts NTH-

11. A QKD station (Bob) operably couplable to another QKD station (Alice) via an optical fiber link and a synchronization channel that supports synchronization (sync) signals having a timing Tss, the QKD station comprising: a modulator adapted to receive and selectively randomly modulate quantum signals sent by Alice to Bob over the optical fiber link; a detector unit optically coupled to the modulator; a controller operably coupled to the detector unit and the modulator; a variable delay arranged between the controller and either the detector unit or the modulator, wherein the variable delay is set to define a fixed timing interval δT F i between a timing TSB for a modulator activation signal SB and a timing T S42 for a detector gating signal S42 that corresponds to an optimum detector count NMAX from the detector unit; and wherein a change in the sync signal timing Tss does not cause a change in the fixed timing interval δT F i.

12. The QKD station of claim 11 , wherein the change in the sync signal timing includes dithering the sync signal timing.

13. The QKD station of claim 12, further including another variable delay adapted to perform the dithering of the sync signal timing.

14. The QKD station of claim 11 , wherein the detector unit includes first and second SPDs.

15. A QKD system, including: the QKD station Bob of claim 11 ; the second QKD station Alice; and the optical fiber link and synchronization channel that operably couple Alice and Bob.

16. The QKD system of claim 15, wherein the optical fiber link carries the synchronization channel.

17. A QKD system, including: the QKD station Bob of claim 12; the second QKD station Alice; the optical fiber link and synchronization channel that operably couple Alice and Bob; and wherein Alice includes another variable delay operably coupled to Alice's controller and adapted to perform the dithering of the sync signal timing.

Description:

QKD SYSTEM WITH COMMON-MODE DITHERING

Claim of Priority

This patent application claims priority from U.S. Patent Application Serial No. 11/880,340, filed on July 20, 2007.

Field of the Invention

The present invention relates to quantum cryptography, and in particular relates to quantum key distribution (QKD) systems, and systems for and methods of maintaining the performance of same during operation.

Background Art

Quantum key distribution (QKD) involves establishing a key between a sender ("Alice") and a receiver ("Bob") by using weak (i.e., 1 photon or less, on average, and typically 0.1 photon on average) optical signals or "qubits" transmitted over a "quantum channel." Rather than relying on computational impracticality, the security of the key distribution is based on the quantum mechanical principle that any measurement of a quantum system in an unknown state will modify its state. Thus, an eavesdropper ("Eve") that attempts to intercept or otherwise measure the exchanged qubits will introduce errors that reveal her presence.

The general principles of quantum cryptography were first set forth by Bennett and Brassard in their article "Quantum Cryptography: Public key distribution and coin tossing," Proceedings of the International Conference on Computers, Systems and Signal Processing, Bangalore, India, 1984, pp. 175- 179 (IEEE, New York, 1984). Specific QKD systems are described in U.S. Patent No. 5,307,410 to Bennett, and in the article by CH. Bennett entitled "Quantum Cryptography Using Any Two Non-Orthogonal States", Phys. Rev. Lett. 68 3121 (1992). The general process for performing QKD is described in the book by Bouwmeester et al., "The Physics of Quantum Information," Springer-Verlag 2001, in Section 2.3, pages 27-33.

The above mentioned references by Bennett each describe a QKD system wherein Alice randomly encodes the polarization or phase of single photons at one end of the system, and Bob randomly measures the polarization or phase of the photons at the other end of the system. The oneway system described in the Bennett 1992 paper is based on two optical fiber Mach-Zehnder interferometers. Respective parts of the interferometric system are accessible by Alice and Bob so that each can control the phase of the interferometer. The interferometers need to be actively stabilized to within a portion of quantum signal wavelength during transmission to compensate for thermal drifts.

U.S. Patent No. 6,438,234 to Gisin (the '234 patent) discloses a so- called "two-way" QKD system that employs an autocompensating interferometer of the type invented by Dr. Joachim Meier of Germany and published in 1995 (in German) as "Stabile lnterferometrie des nichtlinearen Brechzahl-Koeffizienten von Quarzglasfasern der optischen Nachrichtentechnik," Joachim Meier. - AIs Ms. gedr.. - Dϋsseldorf : VDI-Verl., Nr. 443, 1995 (ISBN 3-18-344308-2). Because the Meier interferometer is autocompensated, the two-way QKD system based thereon is generally less susceptible to environmental effects than a one-way system.

In a typical QKD system, Alice generates a quantum signal and randomly modulates this signal based on a select number of possible basis modulations. This process is referred to herein as "selective random modulation." The once-modulated quantum signal is then sent to Bob, who receives this signal and selectively randomly modulates it to form a twice- modulated quantum signal. The twice-modulated quantum signal is then detected at Bob at one of two single-photon detectors (SPDs). Bob is arranged so that an overall modulation of one value (e.g., an overall phase modulation of 0) is detected at one of SPD, while an overall modulation of another value (e.g., an overall phase modulation of ττ/2) is detected at the other SPD. This quantum signal exchange process is repeated for a large number of photons (e.g., 10 4 photons), and known QKD protocols and procedures (e.g., sifting, error correction, privacy amplification, etc., as

described in the above-cited reference by Bouwmeester et al.) are then followed to establish a secure "quantum key" between Alice and Bob.

The operation of the QKD system relies on the synchronized operation of its key active elements — namely, the light source, the modulators and the SPDs. The activation of the key elements is based on the expected arrival times of the quantum signals (photons). While it may be fairly straightforward to operate a QKD system in a laboratory environment with rudimentary synchronization of the aforementioned active elements, a commercially viable QKD system needs to have a simple and robust synchronization system that can be adjusted to maintain the stability of the QKD system over time, as well as provide for ongoing efficient (e.g., optimized) system performance. This is true even for the so-called autocompensated system, because the "autocompensation" applies to the quantum signals and not to the synchronization signals used to coordinate system operation.

Summary of the Invention

An aspect of the present invention is a method of operating a QKD system that includes first and second QKD stations (Alice and Bob) optically coupled to one another, with Bob comprising a controller, a detector unit and a phase modulator. The method includes setting a timing T S s of synchronization (sync) signals SS that travel between Alice and Bob. The method further includes sending optical signals from Alice to Bob to establish a timing T S B of a phase modulator activation signal SB and a timing Ts4o of a detector gating signal S42 based on the sync signals SS 1 wherein established timings TSB and Ts4o correspond to maximum number of optical signal counts NMAX- The method also includes fixing the relative timing δT F i between the signals SB and S42 so that a change in sync signal timing T S s does not change δT F i.

Another aspect of the invention is a QKD station (Bob) operably couplable to another QKD station (Alice) via an optical fiber link and a synchronization channel that supports synchronization (sync) signals having a timing T S s- The QKD station includes a modulator adapted to receive and selectively randomly modulate quantum signals sent by Alice to Bob over the optical fiber link and a detector unit optically coupled to the modulator. The

QKD station also includes a controller operably coupled to the detector unit and the modulator. A variable delay is arranged between the controller and either the detector unit or the modulator. The variable delay is set to define a fixed timing interval ATp 1 between a timing TSB for a modulator activation signal SB and a timing T S 4 2 for a detector gating signal S42 that corresponds to an optimum detector count NMAX from the detector unit. A change in the sync signal timing T S s does not cause a change in the fixed timing interval δTFI , which allows for Bob's modulator and detector unit to be dithered in a "common mode" simply by dithering the sync signal timing.

Brief Description of the Drawings

FIG. 1 is a schematic diagram of an example QKD system according to the present invention, illustrating the key elements of the QKD system that allow for the system to perform common-mode dithering; and

FIG. 2 is a timing diagram of the sync signal (SS) along with the modulator signal (SB) and the detector gating signal (S42), illustrating the common mode dithering of the modulator (MB) and single-photon detectors (42A 1 42B) by dithering the sync signal.

The various elements depicted in the drawings are merely representational and are not necessarily drawn to scale. Certain sections thereof may be exaggerated, while others may be minimized. The drawings are intended to illustrate various embodiments of the invention that can be understood and appropriately carried out by those of ordinary skill in the art.

In the Figures, like elements are identified by like reference numbers.

Detailed Description of the Invention

FIG. 1 is a schematic diagram of an example QKD system 10 according to the present invention, illustrating the key elements of the QKD system that allow the system to operate using the common-mode dithering method of the present invention. QKD system 10 includes a first QKD station Alice optically coupled to a second QKD station Bob via an optical fiber link FL.

Alice

Alice includes a light source 12, a variable optical attenuator 14 arranged downstream of and optically coupled to the light source, and a phase modulator MA arranged downstream of and optically coupled to the VOA. Phase modulator MA is optically coupled to optical fiber link FL and is operably coupled to a modulator driver 2OA.

Alice further includes a controller CA operably coupled to light source 12, VOA 14 and modulator driver 2OA. Controller CA controls the operation of light source 12 and modulator driver 2OA via respective timed control signals S12 and S20A. VOA 14 is controlled so as to have a select attenuation by a control signal S14 from controller CA. In an example embodiment, controller CA includes a field-programmable gate array (FPGA) 22A programmed to control the operation of Alice, as well as to communicate with Bob's controller (discussed below) in coordinating the overall operation of QKD system 10. Controller CA (or Bob's controller CB, as described below) also includes a variable delay DL1 operably arranged in a synchronization ("sync") channel SC that connects to Alice to Bob and carries sync signals SS used to coordinate the operation of key active elements of QKD system 10. In an example embodiment, delay DL1 is or includes an electronic circuit adapted to impart a given time delay to an electronic signal passing therethrough, and in particular is adapted to dither the electronic signal. Delay DL1 can be arranged anywhere between Alice's controller CA and Bob's controller CB.

Bob

With continuing reference to FIG. 1, Bob includes a phase modulator MB optically coupled to optical fiber link FL and also operably coupled to a modulator driver 2OB. Bob also includes a detector unit 40 optically coupled to phase modulator MB. In an example embodiment, detector unit 40 includes two single-photon detectors (SPDs) 42A and 42B. A discriminator 50 is operably coupled to SPDs 42A and 42B.

Bob also includes a controller CB operably coupled to modulator driver 2OB and detector unit 40, as well as to Alice's controller CA via synchronization channel SC. In an example embodiment, Bob's controller CB

includes a FPGA 22B programmed to control the operation of Bob, as well as communicate with Alice over synchronization channel SC via synchronization ("sync") signals SS. Bob rather than Alice may include variable delay DL1 and control the operation of this variable delay as described below.

Though synchronization channel SC and optical fiber link FL are shown as separate connections for the sake of illustration, in an example embodiment the synchronization channel is carried multiplexed over the optical fiber link (i.e., the optical fiber link carries both the quantum channel and the synchronization channel).

Bob also includes an adjustable electronic delay element ("delay") DL. In an example embodiment, delay DL2 is or includes an electronic circuit adapted to impart a given time delay to an electronic signal passing therethrough. Delay DL2 can be arranged between controller CB and phase modulator MB or between controller CB and SPD unit 40.

QKD system general method of operation

The operation of QKD system 10 is now described, assuming for the moment that delay DL2 is set to zero and that the system is ready to operate (i.e., is set up and calibrated). Controller CA sends signal S12 to light source 12 to initiate the generation of an optical pulse PO. Controller CA records the timing Tsi 2 at which signal S12 was generated. Optical pulse PO travels through VOA 14 and is attenuated thereby to form a quantum signal (pulse) QS having a mean photon number μ of 1 photon or less on average, and preferably on the order of 0.1 photon on average.

Quantum signal QS passes through phase modulator MA, which is activated based on the expected arrival time of the quantum signal. Specifically, controller CA generates control signal S20A, which activates modulator driver 2OA to generate a voltage signal SA representative of a phase randomly selected from a set of basis phase modulations (voltages). The selectively randomly modulated quantum signal QS' then enters optical fiber link FL and travels over to Bob.

At Bob, quantum signal QS 1 is modulated in essentially the same way via controller CB, which sends a modulator control signal S20B timed to the

expected arrival of quantum signal QS at modulator MA. Modulator control signal activate modulator driver 2OB, which generates a voltage signal SB (hereinafter, the "modulator activation signal") representative of a phase randomly selected from a set of basis phase modulations (voltages), thereby selectively randomly modulating quantum signal QS 1 to form a twice- modulated quantum signal QS". Modulator activation signal SB has an associated timing TSB and a (gating) width δTSB, wherein timing TSB coincides with the arrival time of quantum signal QS' at modulator MB. Gating width δTSB is sized to account for uncertainty in the arrival time of the quantum signal.

Twice-modulated quantum signal QS" then proceeds to detector unit 40. Depending on the overall phase imparted to quantum signal QS", a click will register in one of SPDs 42A and 42B. SPDs 42A and 42B generate corresponding detector signals S42A and S42B that travel to discriminator 50. Discriminator 50 is used to ensure that two detector signals are not generated at the same time, e.g., due to dark count errors. If only one detector signal S42A or S42B is detected at discriminator 50, the signal is passed to controller CB, which receives and processes the signal e.g., records in FPGA 22B the modulation state of modulator MB, which of the SPDs clicked, and which quantum signal QS" in the stream of quantum signals was detected.

SPDs 42A and 42B in detector unit 40 are gated via a detector gating signal S42 from controller CB. Gating signal S42 has an associated timing Ts 42 and a gating width δTs4 2 - Detector gating signal S42 is timed to the expected arrival time of twice-modulated quantum signal QS". The detector gating width δTs 42 is sized to accommodate uncertainty in the arrival time of quantum signal QS". In an example embodiment, the detector gating signal timing Ts 42 is established by sync signal SS sent from Bob to Alice. The timing T S s of sync signal SS, in turn, is based on the timing Tsi 2 of control signal S12 sent to light source 12 to generate initial optical pulse PO.

QKD system calibration

The above description of the operation of QKD system 10 assumed that delays DL1 and DL2 were zero, and that the QKD system was already set up

to operate in its normal operating condition. However, in the present invention, delays DL1 and DL2 are used to set up and calibrate the QKD system and then maintain QKD system performance in a desired state, which is usually the optimum operating state as reflected by a maximum number NMAX of SPD counts from detector unit 40 for a given operating time interval (e.g., minutes). This is accomplished by using variable delay DL1 at Alice or Bob to dither the timing of sync signal SS.

The discussion below makes reference to FIG. 1 and also to FIG. 2, the latter being a timing diagram of the sync signal SS, the modulator activation signal SB and the detector gating signal S42.

Set-up and calibration of QKD system 10 involves performing a first step that includes setting an initial sync signal timing T S s and setting gating widths δTSB and δTs42 to be relatively wide to account for the relatively large timing uncertainty in this initial set-up stage. Variable delay DL1 is initially set to have zero delay (i.e., no dithering)

The system is then operated while varying the variable delay DL2 at Bob until the activation of phase modulator MB via modulation signal SB and the gating of detectors 42A and 42B via gating signal S42 is properly coordinated, e.g., until a maximum photon count NMAX is obtained. In an example embodiment, relatively strong (e.g., non-quantum) optical signals SOS are sent by Alice. In an example embodiment, strong optical signals SOS are generated by reducing the amount of attenuation applied by VOA 14 via control signal S14 so that initial optical pulses PO retain a larger portion of their power.

Further in an example embodiment, Alice's modulator MA is set to a fixed value so that only Bob's modulator MB is activated. In addition, only a single modulation value is preferably used for modulator MB so that the maximum interference is obtained at detector unit 40.

This establishes the coarse timing T S B and coarse modulator gating width δT S B for Bob's modulator MB and the timing Ts4 2 and coarse detector gating width δT S42 for SPDs 42.

Also in an example embodiment, the final modulator gating width δTSB of modulator activation signal SB and the final detector gating width δTs 42 of

detector gating signal S42 are established by starting with relatively large (coarse) gating widths and then repeating the above process with narrower gating widths until the minimum acceptable gating width for each is established.

Once the modulator and detector gating widths are established and the appropriate timing interval δT F i between signals SB and S42 is established (FIG. 2), variable delay DL2 is then fixed so that the timing interval δTFI is also fixed. At this point, controller CB is configured so that triggering modulator MB via modulator activation signal SB is tied directly to the gating of SPDs 42A and 42B rather than on sync signal SS being received by controller CB. Thus, when controller CB generates detector gating signal S42 in response to sync signal SS, it simultaneously generates signal S20B, which in turn generates modulator activation signal SB. In an example embodiment, modulator control signal S20B is preferably just a replicated version of gating signal S42.

Because the timing interval δT F i between the activation of modulator MB and the gating of SPDs 42A and 42B is fixed, the modulator and SPDs operate in a "common mode," as opposed to their timing being varied independently and relying independently on the timing Tss of sync signal SS.

In an example embodiment of the second step of the set-up and calibration procedure, Bob's modulator MB is set to a fixed value, and relatively strong (i.e., non-quantum) optical signals are exchanged between Alice and Bob to ascertain the proper timing TMA for Alice's modulator MA.

The varying of the sync signal timing Tss within a coarse timing interval δTi (e.g., using variable delay DL1) is carried out until optimum performance is obtained, e.g., as reflected by a maximum photon count NMAX- In an example embodiment, timing interval δTi is then reduced repeatedly (to δT'i, δT"i, etc . ) and the process repeated as many times as necessary to hone in on a precise sync signal timing Tss- Once the sync signal timing TSS is established in this iterative fashion, then the system timing is set and the QKD system is ready for operation.

QKD operation with common-mode dithering

Once QKD system 10 is set up and calibrated as described above, in an example embodiment the system is then operated with set delay DL2 for an initial operation time To, say a few minutes. After time To, either Bob's controller or Alice's controller, depending on the location of variable delay DL1 , causes this variable delay to dither the sync signal timing T S s back and forth in small time increments δt over a dither interval δT D . The QKD system is operated for the different values of T S s ± nδt (where n is an integer) to establish whether or not it is necessary to set Tss to a new value that optimizes QKD system performance, e.g., one that leads to an increased photon (detector) count N. Note that the timing interval δT F i between the activation of modulator MB and the gating of SPDs 42A and 42B remains fixed while only the sync signal timing is dithered.

If necessary, the sync-signal dithering process is repeated during QKD system operation. The dithering process may be repeated periodically or aperiodically, such as when the detector count N drops by a threshold number of counts NTH from a maximum number of detector counts NMAX over a given time interval.

The sync-signal dithering process provides for common-mode dithering of the modulator timing and the SPD timing, since the timing interval δTFI between modulator MB and SPDs 42A and 42B is fixed. This allows for a single dithering operation performed on the sync signal SS at Alice or Bob to maintain optimum performance of the QKD system without the need to independently vary the timing of the other elements, particularly Bob's modulator and SPD unit. This is possible because the timing of modulator MB and SPDs 42A and 42B generally remains unchanged, with most of the timing variation in the QKD system being due to variations in the transmission of sync signal Tss timing over a long fiber that experiences regular temperature change. However, the relative timing of modulator MB and SPDs 42A and 42B initially needs to be adjustable using variable delay DL2 to establish proper timing set-up and calibration of the QKD system as a whole, and to make any necessary adjustments in modulator and/or detector timing at Bob should these elements experience a timing drift.

While the present invention has been described in connection with preferred embodiments, it will be understood that it is not so limited. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined in the appended claims.