WO2023093867A1 - 一种tf-qkd系统及方法 - Google Patents

一种tf-qkd系统及方法 Download PDF

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WO2023093867A1
WO2023093867A1 PCT/CN2022/134500 CN2022134500W WO2023093867A1 WO 2023093867 A1 WO2023093867 A1 WO 2023093867A1 CN 2022134500 W CN2022134500 W CN 2022134500W WO 2023093867 A1 WO2023093867 A1 WO 2023093867A1
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polarization
frequency
reference light
locked
phase
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PCT/CN2022/134500
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English (en)
French (fr)
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汤艳琳
唐世彪
许穆岚
赵梅生
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科大国盾量子技术股份有限公司
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L9/00—Cryptographic mechanisms or cryptographic arrangements for secret or secure communications; Network security protocols
    • H04L9/08—Key distribution or management, e.g. generation, sharing or updating, of cryptographic keys or passwords
    • H04L9/0816—Key establishment, i.e. cryptographic processes or cryptographic protocols whereby a shared secret becomes available to two or more parties, for subsequent use
    • H04L9/0852—Quantum cryptography
    • H04L9/0858—Details about key distillation or coding, e.g. reconciliation, error correction, privacy amplification, polarisation coding or phase coding
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/50—Transmitters
    • H04B10/516—Details of coding or modulation
    • H04B10/548—Phase or frequency modulation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/60—Receivers
    • H04B10/61—Coherent receivers
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/60—Receivers
    • H04B10/61—Coherent receivers
    • H04B10/614—Coherent receivers comprising one or more polarization beam splitters, e.g. polarization multiplexed [PolMux] X-PSK coherent receivers, polarization diversity heterodyne coherent receivers
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/70—Photonic quantum communication
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L9/00—Cryptographic mechanisms or cryptographic arrangements for secret or secure communications; Network security protocols
    • H04L9/08—Key distribution or management, e.g. generation, sharing or updating, of cryptographic keys or passwords
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L9/00—Cryptographic mechanisms or cryptographic arrangements for secret or secure communications; Network security protocols
    • H04L9/08—Key distribution or management, e.g. generation, sharing or updating, of cryptographic keys or passwords
    • H04L9/0816—Key establishment, i.e. cryptographic processes or cryptographic protocols whereby a shared secret becomes available to two or more parties, for subsequent use
    • H04L9/0819—Key transport or distribution, i.e. key establishment techniques where one party creates or otherwise obtains a secret value, and securely transfers it to the other(s)
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L9/00—Cryptographic mechanisms or cryptographic arrangements for secret or secure communications; Network security protocols
    • H04L9/08—Key distribution or management, e.g. generation, sharing or updating, of cryptographic keys or passwords
    • H04L9/0816—Key establishment, i.e. cryptographic processes or cryptographic protocols whereby a shared secret becomes available to two or more parties, for subsequent use
    • H04L9/0852—Quantum cryptography
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02D—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN INFORMATION AND COMMUNICATION TECHNOLOGIES [ICT], I.E. INFORMATION AND COMMUNICATION TECHNOLOGIES AIMING AT THE REDUCTION OF THEIR OWN ENERGY USE
    • Y02D30/00—Reducing energy consumption in communication networks
    • Y02D30/70—Reducing energy consumption in communication networks in wireless communication networks

Definitions

  • the invention relates to the field of quantum secure communication, in particular to a TF-QKD system and method, which allow stable polarization consistent coupling.
  • the security of the ideal BB84QKD protocol is based on some basic assumptions. If the actual equipment cannot perfectly satisfy these assumptions, security holes will occur.
  • the ideal BB84QKD protocol uses a single-photon source, using the principle of single-photon non-cloning to ensure communication security.
  • the real single-photon source cannot be realized in reality.
  • the decoy state quantum key distribution (QKD) protocol proposed in 2005 uses a weakly coherent light source instead of a single photon source, which can solve the problem of imperfect light sources in real devices.
  • the detection system is the most vulnerable part of the QKD implementation. Many attacks are carried out against the security vulnerabilities of the detector-side equipment, including typical strong light blinding attacks and time-shifting attacks.
  • MDI-QKD measurement device-independent class QKD
  • H-K Lo group proposed a measurement device-independent quantum key distribution protocol (original MDI-QKD protocol) based on entanglement exchange technology and time-reversal EPR scheme, which can eliminate all detector side channels, and further in 2018
  • TF-QKD protocol two-field quantum key distribution protocol
  • the TF-QKD protocol is different from the original MDI-QKD protocol.
  • the main technical difficulty lies in how to realize the high-contrast interference of two independent laser sources.
  • NPP -QKD need further fast phase feedback compensation.
  • the global phase difference corresponds to the control of the optical path difference at the sub-wavelength level, and the contribution of the phase difference includes the phase disturbance introduced in the quantum optical transmission channel in addition to the difference caused by the difference in frequency and wavelength of the light sources of Alice and Bob.
  • the disturbance is on the order of 10rad/ms, and the change is very drastic, which needs to be monitored quickly.
  • the prior art there are usually two types of methods for quickly monitoring channel phase disturbance: one is to perform fast time-division multiplexing on the signal light, and a part of the light is used as a phase reference light for monitoring and estimating the relative fast drift of the optical fiber; the other is to Use different wavelengths of reference light for wavelength division multiplexing, and monitor the phase disturbance of the reference light to reflect the phase disturbance of the signal light.
  • the reference light power can be increased to support higher phase disturbance monitoring accuracy and long-distance degree .
  • Figure 1 shows an existing solution for quickly monitoring channel phase disturbances using wavelength division multiplexing technology, in which the frequency-locked light L1 (wavelength ⁇ 1) and phase reference light L2 (wavelength ⁇ 2) are wavelength-division multiplexed at the Charlie end Use to share the same fiber channel to send to Alice (and send another to Bob through a splitter).
  • L1 and L2 demultiplex at Alice (Bob), and use L1 to frequency-lock and phase-lock Alice (Bob) local L1A (L1B) signal light, so the signal light L1A (L1B) will be in phase with L1 form a relationship.
  • L2 and L1A are then wavelength-division multiplexed to share the same optical fiber for sending to the Charlie end.
  • the Charlie end interferes both the signal light and the reference light.
  • the interference signal of the reference light is sent to the D2 detector through demultiplexing, and the interference signal of the signal light is sent to the D0 and D1 detectors.
  • 2a-2b show an optical path structure for realizing polarization consistent coupling and polarization compensation in the prior art.
  • the polarization of L1A (wavelength ⁇ 1) at the sending end is adjusted through the first EPC, so that the polarization is consistent after coupling with L2 (wavelength ⁇ 2), achieving polarization consistent coupling; after that, the second EPC at the sending end EPC implements polarization pre-compensation.
  • a polarization beam splitter PBS is placed on each of the two arms (respectively corresponding to the light from Alice and Bob) at the Charlie end, and the reflection ends of the PBS are coupled together to enter the Dp detector.
  • the counts will be alternately sent to Alice and Bob through the classical network for alternate polarization feedback to control the second EPC of Alice and Bob to complete polarization compensation.
  • the polarization consistent coupling and polarization compensation optical path at the transmitting end are complicated, the cost is high, the implementation is difficult, and the stability is poor, requiring manual calibration.
  • the transmitter has two EPCs, and needs to be equipped with two EPC high-voltage control circuits, which increases the complexity of electronics and the cost of the transmitter; and when the transmitter is affected by environmental changes such as temperature and vibration, its internal L1A Or the polarization state of L2 will change, resulting in that the polarization of L1A and L2 coupled together are not completely consistent, and eventually the polarization states of the two optical signals at the Charlie end will also be inconsistent, so the polarization feedback scheme based on the Dp detector cannot simultaneously The polarization states of the two wavelengths are simultaneously fed back to a consistent state (such as the state of the maximum polarization extinction ratio). Therefore, when affected by environmental disturbances, the scheme needs to be combined with manual calibration, which is not conducive to industrial production
  • L1 and L2 are wavelength-division multiplexed at the Charlie end, and then share the same optical fiber channel to send to Alice, but no polarization compensation is performed after reaching Alice, which will make the system performance unstable.
  • the polarization disturbance of the fiber channel will cause the polarization of L2 to change, resulting in a result similar to the above: the polarizations of L1 and L2 coupled together are not completely consistent, and eventually the polarization states of the two optical signals at the Charlie end will also be inconsistent. ;
  • the polarization of L1 also changes, causing the frequency locking effect of the subsequent frequency locking (OPLL) to change. The worst case is that the OPLL may lose lock completely when the polarization changes too much.
  • the optical path of the polarization consistent coupling scheme is complicated, the cost is high, the implementation is difficult, and the stability is poor, which requires manual calibration, or the polarization state of the frequency-locked light is unstable, resulting in poor frequency-locking effect. Stablize.
  • the present invention discloses a TF-QKD system and a stable TF-QKD implementation method, wherein, by performing polarization-maintaining wavelength division coupling of the frequency-locked reference light and the phase reference light at the Charlie end into a reference light signal, allowing Alice to In the end and the Bob end, combined with the frequency locking technology, two polarization-maintaining wavelength division multiplexers are used to cooperate to realize the wavelength division coupling of the quantum optical signal and the phase reference light in the same polarization without EPC or manual intervention, so that a simple optical path Guarantees the stability of polarization consistent coupling.
  • the first aspect of the present invention relates to a kind of TF-QKD system, and it comprises Alice end, Bob end and Charlie end;
  • the Charlie end includes a measurement module, a first wavelength division multiplexer and a first beam splitter module;
  • the first wavelength division multiplexer is used to perform wavelength division coupling of the frequency-locked reference light and the phase reference light in a polarization-maintaining manner to form a reference combined beam, and the frequency-locked reference light and the phase reference light have different wavelengths;
  • the first beam splitting module is used to split the reference beam combining light to form two reference beam combining light components
  • the Alice end is configured to receive one of the two reference beam combining light components, and send the first signal beam combining light to the Charlie end, and includes a first frequency-locked light source, a first quantum state preparation module, and a first polarization consistent coupling module;
  • the first polarization consistent coupling module is configured to demultiplex the frequency-locked reference light and phase reference light in the reference combined beam component, and output the frequency-locked reference light to the a first frequency-locked light source, and performing wavelength division coupling of the first quantum optical signal and the phase reference light in a polarization-maintaining manner to form the first signal combined beam;
  • the first frequency-locked light source is used to generate a first optical signal in a frequency-locked manner by using the frequency-locked reference light;
  • the first quantum state preparation module is used to prepare a quantum state on the first optical signal to generate the first quantum optical signal
  • the Bob end is configured to receive the other of the two reference beam combining light components, and send the second signal beam combining light to the Charlie end, and includes a second frequency-locked light source, a second quantum state preparation module and a second polarization consistent coupling module;
  • the second polarization consistent coupling module is configured to demultiplex the frequency-locked reference light and phase reference light in the reference combined beam component, and output the frequency-locked reference light to the a second frequency-locked light source, and performing wavelength division coupling of the second quantum optical signal and the phase reference light in a polarization-maintaining manner to form the second signal combined beam;
  • the second frequency-locked light source is used to generate a second optical signal in a frequency-locked manner by using the frequency-locked reference light;
  • the second quantum state preparation module is used to prepare a quantum state on the second optical signal to generate the second quantum optical signal
  • the measurement module is used for measuring the interference signal of the first and second quantum optical signals.
  • the polarization consistent coupling module includes two polarization-maintaining wavelength division multiplexers
  • One of the two polarization-maintaining wavelength division multiplexers is configured to demultiplex the frequency-locked reference light and phase reference light in the reference combined beam component, and respectively demultiplex the frequency-locked reference light and phase reference light
  • the phase reference light is output to the frequency-locked light source and the other of the two polarization-maintaining wavelength division multiplexers in a polarization-maintaining manner;
  • the other of the two polarization-maintaining wavelength division multiplexers is configured to perform wavelength-division coupling on the phase reference light and the quantum optical signal in a polarization-maintaining manner to generate the combined signal beam.
  • At least one of the Alice end and the Bob end also includes a polarization monitoring compensation module, which is arranged before the polarization consistency coupling module, and is used to perform polarization state compensation on the reference combined beam, so that Has a preset state of polarization.
  • the polarization monitoring compensation module may include a polarization controller, a polarization beam splitter and a photodetector.
  • the TF-QKD system of the present invention may further include two polarization monitoring compensation modules, which are respectively used to perform polarization state compensation on the first and second combined signal beams.
  • the first of the two polarization monitoring compensation modules includes a polarization controller, a polarization beam splitter and a photodetector, wherein the polarization controller is arranged at the Alice end or the Charlie end, and the A polarizing beam splitter and a photodetector are arranged at the Charlie end;
  • the second of the two polarization monitoring compensation modules includes a polarization controller, a polarization beam splitter and a photodetector, wherein the polarization controller is arranged at the Bob end or the Charlie end, and the polarization beam splitter and a photodetector disposed at the Charlie end.
  • the two polarization monitoring compensation modules multiplex photodetectors through a beam splitter.
  • the Charlie end further includes a frequency-locking reference light source for generating the frequency-locking reference light, and a phase reference light source for generating the phase reference light; and/or,
  • the Charlie end also includes a slow phase compensation module and/or a fast phase compensation module, wherein the slow phase compensation module is configured to compensate for the gap between the quantum optical signal and the phase reference light in the combined signal beam. phase disturbance, the fast phase compensation module is configured to compensate the phase disturbance between the first and second signal combined beams.
  • the frequency-locked light source uses the frequency-locked reference light to generate the optical signal based on injection locking, optical phase-locked loop or time-frequency transmission.
  • the total number of the Alice end and the Bob end is N, and N is a positive integer greater than 1; and the Charlie end is also provided with an optical switch supporting 2:N ports for allowing Alice to selectively access end and/or Bob end.
  • a second aspect of the present invention relates to a method for stably realizing TF-QKD, comprising the following steps:
  • the frequency-locked reference light and the phase reference light are subjected to wavelength-division coupling to form a reference combined beam;
  • the frequency-locked reference light and the phase reference light in the reference combined beam are demultiplexed by means of a polarization-maintaining wavelength division multiplexer, and the frequency-locked reference light is polarization-maintaining Output to the frequency-locked light source;
  • the frequency-locked light source is used to generate an optical signal in a frequency-locked manner using the frequency-locked reference light, and the optical signal is used to prepare a quantum optical signal and output it in a polarization-maintaining manner;
  • the phase reference light and the quantum optical signal are subjected to wavelength division coupling by means of another polarization-maintaining wavelength division multiplexer to form signal beam combining light;
  • the quantum optical signals from the Alice end and the Bob end are interfered to generate an interference signal, and the interference signal is measured.
  • the method of the present invention may also include the following steps:
  • the polarization compensation is performed on the reference beam combining light before it is demultiplexed, and the polarization is maintained for output; and/or,
  • the polarization compensation is performed on the quantum optical signal before it is interfered, and the polarization is maintained for output.
  • the method of the present invention may further include the step of performing slow phase compensation and/or fast phase compensation on the combined signal beam at the Charlie end.
  • the method of the present invention can be realized by means of the above-mentioned TF-QKD system.
  • Fig. 1 shows a kind of TF-QKD system in the prior art
  • Figures 2a-2b show an example of a polarization consistent coupling and polarization compensation structure used in a TF-QKD system in the prior art
  • Fig. 3 shows an example according to the TF-QKD system of the present invention
  • Fig. 4 shows an example of a polarization monitoring compensation module according to the present invention
  • Fig. 5 shows an example of a polarization consistent coupling module according to the present invention.
  • Fig. 3 shows an example of the TF-QKD system according to the present invention, which includes an Alice end, a Bob end and a Charlie end.
  • the Charlie end may include a frequency-locked reference light source, a phase reference light source, a polarization-maintaining first wavelength division multiplexer (WDM), a first beam splitting module, and a measurement module.
  • WDM wavelength division multiplexer
  • the frequency-locked reference light source has a first wavelength and is used to generate frequency-locked reference light.
  • the phase reference light source has a second wavelength for generating phase reference light, wherein the first wavelength is different from the second wavelength.
  • the first wavelength division multiplexer has a common terminal, a first wavelength terminal and a second wavelength terminal.
  • the first wavelength end is connected to the frequency-locking reference light source
  • the second wavelength end is connected to the phase reference light source, so the frequency-locking reference light and the phase reference light are allowed to be input into the first wavelength division multiplexer for wavelength division coupling in a polarization-maintaining manner Form the reference combined beam and output it through the common port.
  • the common end of the first wavelength division multiplexer can be connected to the first beam splitting module, for example, through a single-mode or polarization-maintaining optical fiber. Therefore, the reference beam combining light can be split into two reference beam combining light components through the first beam splitting module.
  • an output end of the first beam splitting module is connected to the Alice end through the first single-mode or polarization-maintaining fiber channel, to allow a reference beam combining light component to be sent to the Alice end; the other end of the first beam splitting module One output end is connected to the Bob end through another first single-mode or polarization-maintaining optical fiber channel, so as to allow another reference combined beam component to be sent to the Bob end.
  • the first beam splitting module may include a beam splitter.
  • a polarization monitoring compensation module can be set in the Alice end and the Bob end to perform polarization monitoring feedback compensation for the reference beam component (whose polarization state is unstable) arriving at the Alice end and the Bob end, so that it has a preset polarization state, and output the polarization-stabilized reference beam component in a polarization-maintaining manner.
  • Fig. 4 shows an example of a polarization monitoring compensation module according to the present invention, which may include a polarization controller, a polarization beam splitter and a photodetector.
  • the polarization controller is used to provide polarization compensation for the reference beam components.
  • the polarization controller may be an electrodynamic polarization controller (EPC).
  • the polarization beam splitter and the photodetector constitute the polarization monitoring part, wherein the polarization beam splitter is used to split the reference beam components output by the polarization controller, so one of the split beams can be detected by the photodetector to obtain the reference
  • the polarization state of the beam-combined light components that is, obtains the polarization deviation information about the preset polarization state, thereby allowing a feedback control signal to be generated according to the polarization deviation information, and provided to the polarization controller to finally complete the polarization closed-loop feedback control.
  • the Alice end may also include a first frequency-locked light source, a first quantum state preparation module, and a first polarization consistent coupling module
  • Bob's end may include a second frequency-locked light source, a second quantum state preparation module, and a second Polarization Consistent Coupling Module.
  • the polarization monitoring compensation module outputs the polarization-maintaining reference beam component with a stable polarization state to the first polarization consistent coupling module, which is configured to provide frequency-locked reference light to the first frequency-locked light source, and
  • the phase reference light and the first quantum optical signal generated at the Alice terminal are wavelength-division-coupled in the same polarization state to form the first signal combined beam.
  • the polarization monitoring compensation module outputs the polarization-maintaining reference beam component with a stable polarization state to the second polarization consistent coupling module, which is configured to provide a frequency-locking reference for the second frequency-locking light source
  • the light, as well as the phase reference light and the second quantum optical signal generated at the Bob terminal, are wavelength-division-coupled in the same polarization state to form the second signal combined beam.
  • Fig. 5 shows an example of a polarization consistent coupling module according to the present invention.
  • the first (second) polarization-consistent coupling module may include two polarization-maintaining wavelength division multiplexers, such as a second wavelength division multiplexer and a third wavelength division multiplexer, both of which have A common end, a first wavelength end and a second wavelength end.
  • two polarization-maintaining wavelength division multiplexers such as a second wavelength division multiplexer and a third wavelength division multiplexer, both of which have A common end, a first wavelength end and a second wavelength end.
  • the common end is used as the dual-wavelength input end 1 of the polarization consistent coupling module for receiving (for example, the polarization maintaining output by the polarization monitoring compensation module) reference beam component, therefore, allowing the The frequency-locked reference light and phase reference light in the reference combined beam component are demultiplexed, and the first wavelength end (which serves as the single-wavelength output end 2 of the polarization consistent coupling module) maintains the polarization and outputs the frequency-locked reference light.
  • the frequency-locked reference light After the frequency-locked reference light is output from the first wavelength end, it will enter the first (second) frequency-locked light source.
  • the frequency-locked light source uses (but not limited to) injection locking technology, optical phase-locked loop OPLL technology, time-frequency transmission technology, etc. to lock its frequency on the first wavelength of the frequency-locked reference light, so that its output frequency is stable at the first wavelength.
  • a first (second) optical signal at a wavelength uses (but not limited to) injection locking technology, optical phase-locked loop OPLL technology, time-frequency transmission technology, etc.
  • the reference beam that reaches the second wavelength division multiplexer (that is, the polarization consistent coupling module) can be made
  • the light component has a stable polarization state, therefore, the frequency-locked reference light and the phase reference light demultiplexed and output by the second wavelength division multiplexer can have stable light intensity. Since the frequency-locking reference light used in the frequency-locking process of the frequency-locked light source has a stable light intensity, its frequency-locking effect is also stabilized, thereby allowing the first (second) frequency-locked light source to output the first (second) frequency-locked light source stably. ) optical signal, which will be prepared by the first (second) quantum state preparation module to generate the first (second) quantum optical signal, and the quantum optical signal has the same wavelength as the frequency-locked reference light.
  • the first wavelength end (as the single-wavelength input end 3 of the polarization consistent coupling module) is connected to the first (second) quantum state preparation module with polarization maintaining, to receive the first (second) ) quantum optical signal, the second wavelength end is polarization-maintaining and connected to the second wavelength end of the second wavelength division multiplexer, therefore, allowing the first (second) quantum optical signal and phase reference light to be wavelength-divided in a polarization-maintaining manner coupled, and output the first (second) signal combined beam through the common port (which serves as the dual-wavelength output port 4 of the polarization consistent coupling module).
  • the demultiplexed phase reference light will also have a stable light intensity, which is conducive to the output of stable signal combining light.
  • the phase reference light and the frequency-locking reference light are wavelength-division-coupled in a polarization-consistent manner to form a reference combined beam.
  • Setting a polarization-consistent coupling module composed of two polarization-maintaining wavelength division multiplexers can allow the realization of The polarization consistency coupling of the first (second) quantum optical signal and the phase reference light, the realization of this polarization consistency will not be affected by the temperature and vibration of Alice and Bob, and the implementation is more stable without complex control , can effectively reduce system cost, improve system integration and stability.
  • the existing scheme of using EPC to achieve polarization consistent coupling can only realize the polarization stabilization of the quantum optical signal path, but cannot feed back the stable phase reference light path, and its polarization feedback control effect is limited.
  • it can simultaneously The polarization state stabilization effect of the phase reference light is realized, thereby effectively ensuring the polarization stability of the entire Alice and Bob end optical path.
  • the first (second) quantum state preparation module may include a phase modulation unit, an amplitude modulation unit and a light attenuation unit.
  • the first (second) signal combined beam output by the first (second) polarization-consistent coupling module can be transmitted to the Charlie end through the second single-mode or polarization-maintaining optical fiber channel.
  • two polarization monitoring and compensating modules can also be set up, which are respectively used to perform polarization feedback compensation on the first and second combined signal beams (which have unstable polarization states), so that they have a preset polarization state, ensuring that The combined first and second signal beams of the measurement module have a stable polarization state.
  • the polarization monitoring and compensating module used for signal beam combining may adopt the same structure as that of the above-mentioned polarization monitoring and compensating module at Alice's end or Bob's end, so details will not be repeated here.
  • the polarization controller used for polarization compensation can be set in the Alice end/Bob end, or in the Charlie end (as shown in Figure 3 shown), a polarization beam splitter and a photodetector for polarization monitoring can be placed at the Charlie end.
  • the two polarization monitoring and compensating modules used for combining signal beams may share one photodetector by means of a beam splitter, so as to improve system integration.
  • the beam splitter can be arranged before the photodetector to couple the split light from the first and second combined signal beams into one path, so as to allow the detection of the two combined signal beams by means of one photodetector.
  • the Charlie end may also include a slow phase compensation module for compensating the slow phase disturbance between the quantum optical signal and the phase reference light in the same signal combining light, and a slow phase compensation module for compensating the first and second signal combining light A fast phase compensation module for fast phase perturbations between them.
  • a fast phase compensation module may include a phase modulator.
  • the slow phase compensation module may include a polarization maintaining fifth wavelength division multiplexer and a polarization maintaining sixth wavelength division multiplexer, both of which also have a common terminal, a first wavelength terminal and a second wavelength terminal.
  • the first wavelength ends of the fifth and sixth wavelength division multiplexers are connected through a first arm, and the second wavelength ends of the fifth and sixth wavelength division multiplexers are connected through a second arm.
  • the common end of the fifth wavelength division multiplexer is set to receive the signal combined beam, so that the signal combined beam can be subjected to wave division multiplexing, and the quantum optical signal and the phase reference light can be respectively output through the first and second wavelength ends , so that they are respectively transmitted along the first and second arms towards the sixth wavelength division multiplexer.
  • a phase shifter may be provided on at least one of the first and second arms for phase compensation between the quantum light signal and the phase reference light.
  • the quantum optical signal and the phase reference light respectively enter the sixth wavelength division multiplexer from the first and second wavelength ends, and undergo wavelength division coupling in a polarization-maintaining manner to form signal combined beams again.
  • the phase shifter With the help of the phase shifter, the slow phase disturbance between the quantum optical signal and the phase reference light in the combined signal beam can be compensated.
  • the combined beam light of the two signals after polarization compensation, slow phase compensation or fast phase compensation will finally enter the measurement module.
  • the first and second signal combined beams will interfere to generate and detect the interference signal.
  • the measurement module may include a polarization maintaining second beam splitter having first and second inputs, and first and second outputs.
  • the first and second input ends are respectively used to receive the first and second combined signal beams, thus allowing the two paths of signal combined beams to interfere to generate an interference signal.
  • the interference signal will be output through the first and second output terminals respectively, and enter the first detection optical path and the second detection optical path.
  • the interference signal at this time includes the interference signal of the quantum optical signal and the interference signal of the phase reference light.
  • a fourth wavelength division multiplexer may be provided on at least one of the first and second detection optical paths, for separating the interference signal about the quantum optical signal and the interference signal about the phase reference light among the interference signals, so as to allow the The interference signal of the quantum optical signal and the interference signal of the phase reference light are detected.
  • the fourth wavelength division multiplexer may also have a common terminal, a first wavelength terminal and a second wavelength terminal.
  • the common end is used to receive the interference signal
  • the first wavelength end is connected to the first photodetector to allow detection of the interference signal of the quantum optical signal
  • the second wavelength end is connected to the second photodetector to allow the phase reference light The interference signal is detected.
  • the detection result of the interference signal about the phase reference light can be used for the feedback control of the fast phase compensation module; a part of the detection result about the interference signal of the quantum optical signal can be used for the feedback control of the slow phase compensation module, and the other part can be used for the feedback control of the slow phase compensation module.
  • a polarization-maintaining wavelength division multiplexer is set at the Charlie end to couple the frequency-locked reference light and the phase reference light wavelength division together, and at the same time, two polarization-maintaining wavelength division multiplexers are set at the Alice and Bob ends.
  • the polarization consistent coupling module formed by the multiplexer can allow the polarization consistent coupling of the quantum optical signal and the phase reference light to be realized by relying on the characteristics of the optical device itself without EPC and manual intervention, so that the system optical path and control The process is simplified, and the stability of the system is enhanced, and the integrated design is easy.
  • the polarization monitoring compensation module before the polarization consistent coupling module, the light intensity of the frequency-locking reference light used for the frequency-locked light source and the phase reference light used for polarization-consistent coupling can be guaranteed to be stable, which is conducive to realizing stable The frequency-locking effect, and the implementation of fast phase compensation in Charlie and the generation of quantum keys.
  • the TF-QKD system may include N user terminals serving as Alice's terminal/Bob's terminal, and an optical switch supporting 2:N ports is provided on Charlie's terminal.
  • two user terminals among the N user terminals can be switched as the Alice terminal and the Bob terminal to form a connection with the Charlie terminal.
  • Another aspect of the present invention relates to a method for stably achieving TF-QKD.
  • the Charlie end sends the reference combined beam to the Alice end and the Bob end respectively.
  • the reference combined beam may be divided into two components to be sent to Alice and Bob respectively.
  • the polarization monitoring compensation module In the Alice end and the Bob end, it is preferable to use the polarization monitoring compensation module to perform polarization compensation on the reference combined beam light so that it has a preset polarization state, which is conducive to the stable progress of the frequency locking process and the stable generation of
  • the optical signal and the phase reference light are polarization-maintainingly coupled to form a combined signal beam.
  • the polarization-compensated reference beam combination light is demultiplexed in a polarization-maintaining manner to separate the frequency-locked reference light and the phase reference light.
  • the frequency-locked reference light is output to the frequency-locked light source in a polarization-maintaining manner, for allowing the frequency-locked light source to generate an optical signal in a frequency-locked manner, and its wavelength is locked to the wavelength of the frequency-locked reference light.
  • a quantum state is prepared on the optical signal to generate a quantum optical signal.
  • the quantum optical signal and the phase reference light can be polarization-maintainingly coupled to form a combined signal beam.
  • the quantum optical signal and the phase reference light in the combined signal beam will have the same polarization state, so as to realize the polarization consistent coupling of the two.
  • the realization of this polarization consistent coupling does not require EPC or manual intervention, and only relies on the characteristics of the optical device itself, which is not easily affected by the external environment, thereby ensuring high stability.
  • the Alice end and the Bob end respectively send the generated signal combined beams to the Charlie end, so that the interference occurs at the Charlie end, and the TF-QKD process can be realized with the help of the interference signal of the quantum optical signal.
  • polarization monitoring and compensation may be performed on the combined signal beams sent from Alice and Bob, so that they have a stable preset polarization state.
  • the Charlie end can also perform slow phase compensation and fast phase compensation on the combined signal beams before interferometry to eliminate slow phase disturbances and fast phase disturbances introduced in various optical signals during system operation. Thereby ensuring the stability and efficiency of the intervention process.
  • the TF-QKD method can be realized by means of the TF-QKD system of the present invention.

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Abstract

本发明公开了一种TF-QKD系统及稳定的TF-QKD实现方法，其中，通过在Charlie端将锁频参考光和相位参考光进行保偏地波分耦合成参考光信号，使得允许在Alice端和Bob端中，结合锁频技术，利用两个保偏的波分复用器配合实现量子光信号和相位参考光偏振一致地波分耦合，而无需EPC或人工干预，从而以简单的光路保证偏振一致性耦合的稳定性。此外，还可以通过在Alice端和Bob端中，首先对参考光信号进行偏振补偿，使得还允许借助本发明的偏振一致性耦合模块保证锁频参考光和相位参考光的稳定性，从而确保锁频效果的稳定性。

Description

一种TF-QKD系统及方法
本申请要求于2021年11月29日提交中国国家知识产权局、申请号为202111455471.8、申请名称为“一种TF-QKD系统及方法”的中国专利申请的优先权，其全部内容通过引用结合在本申请中。
技术领域
本发明涉及量子保密通信领域，尤其涉及一种TF-QKD系统及方法，其允许稳定地实现偏振一致性耦合。
背景技术
理想BB84QKD协议的安全性基于一些基本假设，若现实设备不能够完美的满足这些假设就会产生安全漏洞。例如，理想的BB84QKD协议使用单光子源，利用单光子不可克隆原理保证通信安全，然而，真正的单光子源现实中还无法实现。2005年提出的诱骗态量子密钥分发(QKD)协议使用弱相干光源代替单光子源，可解决现实设备光源端不完美问题。探测系统是QKD实现中最脆弱的部分，众多攻击都是针对探测器端设备的安全漏洞实施的攻击，包括典型的强光致盲攻击、时移攻击等。为抵抗探测器端攻击，除了使用安全补丁方法、将设备参数模型化后包含在安全性证明中，还有一种方法是采用测量设备无关类QKD(MDI-QKD)。最早于2012年由H-K Lo小组提出基于纠缠交换技术和时间反演EPR方案的测量设备无关量子密钥分发协议(原始MDI-QKD协议)，可消除所有的探测器端侧信道，进一步地在2018年，东芝欧洲研究中心A.J.Shields研究团队提出双场量子密钥分发协议(TF-QKD协议)，该协议继承了MDI-QKD的测量设备无关的安全性并具有更远的安全距离，引起了国内外广泛关注。
TF-QKD协议不同于原始的MDI-QKD协议，主要技术难点在于如何实现两个独立的激光源的高对比度干涉，同时，还需要对全局相位差进行快速监测，无相位后选择的QKD(NPP-QKD)还需要进一步进行快速相位反馈补偿。这里的全局相位差对应光程差的控制要求在亚波长量级，而相位差的贡献除了Alice、Bob的光源本身频率和波长差异导致的差异，还包括量子光传输信道中引入的相位扰动，该扰动在10rad/ms量级，变化非常剧烈，需要快速监测。
现有技术通常存在两类快速监测信道相位扰动的方法：一是对信号光进行 快速的时分复用，将其中一部分光作为相位参考光，用于监测和估计光纤的相对相位快速漂移；二是采用不同波长的参考光进行波分复用，通过监测参考光的相位扰动来体现信号光的相位扰动，此时，可以通过提高参考光功率来支持更高的相位扰动监测的精度和远距离程度。
图1示出了现有采用波分复用技术快速监测信道相位扰动的一种方案，其中，在Charlie端将锁频光L1(波长λ1)和相位参考光L2(波长λ2)进行波分复用，以共用同一光纤信道发给Alice(通过分束器另一束发给Bob)。L1和L2在Alice(Bob)端解波分复用，并利用L1对Alice(Bob)本地的L1A(L1B)信号光进行锁频锁相，因此信号光L1A(L1B)将在相位上与L1形成关联。在发送端，再将L2和L1A(L1B)进行波分复用，以共用同一光纤发给Charlie端。Charlie端对信号光和参考光都进行干涉，干涉后通过解波分复用分出参考光的干涉信号给D2探测器，分出信号光的干涉信号给D0、D1探测器。
然后，这种方案需要实现波长不同的信号光和参考光的偏振一致性耦合及偏振补偿。图2a-2b示出了现有技术中的一种用于实现偏振一致性耦合及偏振补偿的光路结构。如图2a所示，发送端的L1A(波长λ1)通过第一个EPC调节偏振，以便和L2(波长λ2)耦合在一起后偏振一致，实现偏振一致性耦合；之后，再经过发送端的第二个EPC实现偏振预补偿。此时，如图2b所示，Charlie端则在两臂(分别对应Alice和Bob过来的光)各放置一个偏振分束器PBS，PBS的反射端耦合在一起进入Dp探测器，该探测器的计数会通过经典网络交替发给Alice和Bob进行交替的偏振反馈以控制Alice和Bob的第二个EPC，完成偏振补偿。
在例如图2a-2b所示的这种实现结构中，发送端的偏振一致性耦合和偏振补偿光路复杂，成本较高，实现困难，且稳定性较差需要配合人工标校。具体地，发送端有两个EPC，且需要配备2个EPC的高压控制电路，增加了电子学复杂度和发送端成本；且发送端在受到温度、振动等环境变化影响时，其内部的L1A或L2的偏振状态还会发生变化，导致L1A和L2耦合在一起的偏振并不完全一致，最终会使得Charlie端的两个光信号的偏振状态也不一致，则基于Dp探测器的偏振反馈方案不能同时将两种波长的偏振态同时反馈到一致状态(如都为最大偏振消光比的状态)。因此，在受环境扰动影响时该方案还 需结合人工标校，不利于进行工业化生产和设备自动化运行。
除此之外，在现有技术中，L1和L2在Charlie端进行波分复用，然后共用同一光纤信道发给Alice，但到Alice之后未进行偏振补偿，会使得系统性能不稳定。一方面，该光纤信道的偏振扰动会导致L2的偏振发生变化，导致类似上述的结果：L1和L2耦合在一起的偏振并不完全一致，最终会使得Charlie端的两个光信号的偏振状态也不一致；另一方面，L1的偏振也发生变化，导致后续锁频(OPLL)的锁频效果发生变化，最差情况是当偏振变化太大时OPLL可能完全失锁。
发明内容
针对现有技术存在上述问题，例如偏振一致性耦合方案光路复杂，成本较高，实现困难，且稳定性较差需要配合人工标校，或者锁频光的偏振态不稳定，导致锁频效果不稳定。本发明公开了一种TF-QKD系统及稳定的TF-QKD实现方法，其中，通过在Charlie端将锁频参考光和相位参考光进行保偏地波分耦合成参考光信号，使得允许在Alice端和Bob端中，结合锁频技术，利用两个保偏的波分复用器配合实现量子光信号和相位参考光偏振一致地波分耦合，而无需EPC或人工干预，从而以简单的光路保证偏振一致性耦合的稳定性。此外，还可以通过在Alice端和Bob端中，首先对参考光信号进行偏振补偿，使得还允许借助本发明的偏振一致性耦合模块保证锁频参考光和相位参考光的稳定性，从而确保锁频效果的稳定性。
本发明的第一方面涉及一种TF-QKD系统，其包括Alice端、Bob端和Charlie端；
所述Charlie端包括测量模块、第一波分复用器和第一分束模块；
所述第一波分复用器用于以保偏的方式将锁频参考光和相位参考光进行波分耦合，形成参考合束光，所述锁频参考光和相位参考光具有不同的波长；
所述第一分束模块用于将所述参考合束光分光，形成两个参考合束光分量；
所述Alice端被设置成接收所述两个参考合束光分量中的一个，以及向所述Charlie端发送第一信号合束光，且包括第一锁频光源、第一量子态制备模块和第一偏振一致性耦合模块；
所述第一偏振一致性耦合模块被设置用于将所述参考合束光分量中的锁频参考光和相位参考光解复用，并将所述锁频参考光保偏地输出给所述第一锁频光源，以及以保偏的方式将第一量子光信号和所述相位参考光进行波分耦合，形成所述第一信号合束光；
所述第一锁频光源用于利用所述锁频参考光，以锁频的方式生成第一光信号；
所述第一量子态制备模块用于在所述第一光信号上制备量子态，生成所述第一量子光信号；
所述Bob端被设置成接收所述两个参考合束光分量中的另一个，以及向所述Charlie端发送第二信号合束光，且包括第二锁频光源、第二量子态制备模块和第二偏振一致性耦合模块；
所述第二偏振一致性耦合模块被设置用于将所述参考合束光分量中的锁频参考光和相位参考光解复用,并将所述锁频参考光保偏地输出给所述第二锁频光源，以及以保偏的方式将第二量子光信号和所述相位参考光进行波分耦合，形成所述第二信号合束光；
所述第二锁频光源用于利用所述锁频参考光，以锁频的方式生成第二光信号；
所述第二量子态制备模块用于在所述第二光信号上制备量子态，生成所述第二量子光信号；
所述测量模块用于测量所述第一和第二量子光信号的干涉信号。
进一步地，所述偏振一致性耦合模块包括两个保偏的波分复用器；
所述两个保偏的波分复用器中的一个被设置成将所述参考合束光分量中的锁频参考光和相位参考光解复用，并分别将所述锁频参考光和相位参考光保偏地输出给所述锁频光源和所述两个保偏的波分复用器中的另一个；
所述两个保偏的波分复用器中的另一个被设置成以保偏的方式将所述相位参考光和量子光信号进行波分耦合，生成所述信号合束光。
进一步地，所述Alice端和Bob端中的至少一个还包括偏振监测补偿模块，其设在所述偏振一致性耦合模块之前，用于对所述参考合束光进行偏振态补偿，以使其具有预设的偏振态。其中，所述偏振监测补偿模块可以包括偏振控 制器、偏振分束器和光电探测器。
进一步地，本发明的TF-QKD系统还可以包括两个偏振监测补偿模块，分别用于对所述第一和第二信号合束光进行偏振态补偿。
更进一步地，所述两个偏振监测补偿模块中的第一个包括偏振控制器、偏振分束器和光电探测器，其中，所述偏振控制器设置在所述Alice端或Charlie端，所述偏振分束器和光电探测器设置在所述Charlie端；
所述两个偏振监测补偿模块中的第二个包括偏振控制器、偏振分束器和光电探测器，其中，所述偏振控制器设置在所述Bob端或Charlie端，所述偏振分束器和光电探测器设置在所述Charlie端。
可选地，所述两个偏振监测补偿模块通过分束器复用光电探测器。
进一步地，所述Charlie端还包括用于生成所述锁频参考光的锁频参考光源，以及用于生成所述相位参考光的相位参考光源；以及/或者，
所述Charlie端还包括慢速相位补偿模块和/或快速相位补偿模块，其中，所述慢速相位补偿模块被设置用于补偿所述信号合束光中的量子光信号和相位参考光之间的相位扰动，所述快速相位补偿模块被设置用于补偿所述第一和第二信号合束光之间的相位扰动。
可选地，所述锁频光源基于注入锁定、光学锁相环或时频传输，利用所述锁频参考光生成所述光信号。
优选地，所述Alice端和Bob端的总数量为N个,N为大于1的正整数；并且，所述Charlie端还设置有支持2:N端口的光开关，用于允许选择接入的Alice端和/或Bob端。
本发明的第二方面涉及一种用于稳定实现TF-QKD的方法，其包括以下步骤：
在Charlie端，借助保偏的波分复用器，将锁频参考光和相位参考光进行波分耦合，形成参考合束光；
在所述Alice端和Bob端中，借助一个保偏的波分复用器将所述参考合束光中的锁频参考光和相位参考光解复用，并将锁频参考光保偏地输出给锁频光源；
在所述Alice端和Bob端中，使所述锁频光源利用所述锁频参考光以锁频 的方式生成光信号，利用所述光信号制备量子光信号并将其保偏地输出；
在所述Alice端和Bob端中，借助另一保偏的波分复用器将所述相位参考光和量子光信号进行波分耦合，形成信号合束光；
在所述Charlie端中，使分别来自所述Alice端和Bob端的量子光信号发生干涉以生成干涉信号，并对干涉信号进行测量。
进一步地，本发明的方法还可以包括以下步骤：
在所述Alice端和Bob端中，将所述参考合束光解复用之前对其进行偏振补偿并保偏输出；以及/或者，
在使所述量子光信号发生干涉之前对其进行偏振补偿并保偏输出。
进一步地，本发明的方法还可以包括在所述Charlie端中，对所述信号合束光进行慢速相位补偿和/或快速相位补偿的步骤。
优选地，本发明的方法可以借助上述TF-QKD系统来实现。
附图说明
下面结合附图对本发明的具体实施方式作进一步详细的说明。
为了更清楚地说明本发明实施例或现有技术中的技术方案，下面将对实施例或现有技术描述中所需使用的附图作简单地介绍，显而易见，下面描述中的附图仅仅是本发明的一些实施例，对于本领域普通技术人员来讲，在不付出创造性劳动的前提下，还可以根据这些附图来获得其他的附图。
图1示出了现有技术中的一种TF-QKD系统；
图2a-2b示出了现有技术中用于TF-QKD系统的偏振一致性耦合及偏振补偿结构的一种示例；
图3示出了根据本发明的TF-QKD系统的一种示例；
图4示出了根据本发明的偏振监测补偿模块的一种示例；
图5示出了根据本发明的偏振一致性耦合模块的一种示例。
具体实施方式
在下文中，本发明的示例性实施例将参照附图来详细描述。下面的实施例以举例的方式提供，以便充分传达本发明的精神给本发明所属领域的技术人员。因此，本发明不限于本文公开的实施例。
图3示出了根据本发明的TF-QKD系统的一种示例，其包括Alice端、Bob 端和Charlie端。
Charlie端可以包括锁频参考光源、相位参考光源、保偏的第一波分复用器(WDM)、第一分束模块及测量模块。
锁频参考光源具有第一波长，用于生成锁频参考光。
相位参考光源具有第二波长，用于生成相位参考光，其中，第一波长不同于第二波长。
第一波分复用器具有公共端、第一波长端和第二波长端。其中，第一波长端连接锁频参考光源，第二波长端连接相位参考光源，因此允许锁频参考光和相位参考光输入第一波分复用器中，以保偏的方式进行波分耦合形成参考合束光，并通过公共端输出。
第一波分复用器的公共端例如可以通过单模或保偏光纤连接第一分束模块，因此，可以通过第一分束模块将参考合束光分成两个参考合束光分量。
如图3所示，第一分束模块的一个输出端通过第一单模或保偏光纤信道连接Alice端，以允许将一个参考合束光分量发送给Alice端；第一分束模块的另一个输出端通过另一第一单模或保偏光纤信道连接Bob端，以允许将另一个参考合束光分量发送给Bob端。作为示例，第一分束模块可以包括分束器。
如前所述，当参考合束光分量经例如第一单模光纤信道到达Alice端或Bob端时，其偏振态可能会发生变化。因此，可以在Alice端和Bob端中设置偏振监测补偿模块，用于对到达Alice端和Bob端的参考合束光分量(其偏振态不稳定)进行偏振监测反馈补偿，使其具有预设的偏振态，并以保偏的方式输出偏振态稳定的参考合束光分量。
图4示出了根据本发明的偏振监测补偿模块的一种示例，其可以包括偏振控制器、偏振分束器和光电探测器。
偏振控制器用于对参考合束光分量提供偏振补偿。例如，偏振控制器可以为电动偏振控制器(EPC)。
偏振分束器和光电探测器构成偏振监测部分，其中，偏振分束器用于对偏振控制器输出的参考合束光分量进行分光，因此，可以借助光电探测器对其中一路分光进行探测，获取参考合束光分量的偏振态，即获取关于预设偏振态的偏振偏差信息，从而允许根据该偏振偏差信息生成反馈控制信号，提供给偏振 控制器，最终完成偏振闭环反馈控制。
继续参见图3，Alice端还可以包括第一锁频光源、第一量子态制备模块和第一偏振一致性耦合模块，Bob端可以包括第二锁频光源、第二量子态制备模块和第二偏振一致性耦合模块。
在Alice端中，偏振监测补偿模块将具有稳定偏振态的参考合束光分量保偏输出给第一偏振一致性耦合模块，其被设置用于向第一锁频光源提供锁频参考光，以及将相位参考光和Alice端中生成的第一量子光信号，在相同的偏振态下波分耦合形成第一信号合束光。
类似地，在Bob端中，偏振监测补偿模块将具有稳定偏振态的参考合束光分量保偏输出给第二偏振一致性耦合模块，其被设置用于向第二锁频光源提供锁频参考光，以及将相位参考光和Bob端中生成的第二量子光信号，在相同的偏振态下波分耦合形成第二信号合束光。
图5示出了根据本发明的偏振一致性耦合模块的一种示例。
如图5所示，第一(第二)偏振一致性耦合模块可以包括两个保偏的波分复用器，例如第二波分复用器和第三波分复用器，其均具有公共端、第一波长端和第二波长端。
在第二波分复用器中，公共端作为偏振一致性耦合模块的双波长输入端1，用于接收(例如由偏振监测补偿模块保偏输出的)参考合束光分量，因此，允许将参考合束光分量中的锁频参考光和相位参考光解复用，并由第一波长端(其作为偏振一致性耦合模块的单波长输出端2)保偏输出锁频参考光，由第二波长端保偏输出相位参考光。
锁频参考光经第一波长端输出后，将进入第一(第二)锁频光源。锁频光源利用例如(但不限于)注入锁定技术、光学锁相环OPLL技术、时频传输技术等，将其频率锁定在锁频参考光的第一波长上，从而输出其频率稳定在第一波长上的第一(第二)光信号。至此，本领域技术人员能够理解，通过事先利用偏振监测补偿模块对参考合束光分量进行偏振态补偿，可以使得到达第二波分复用器(即，偏振一致性耦合模块)的参考合束光分量具有稳定的偏振态，因此，经第二波分复用器解复用输出的锁频参考光和相位参考光可以具有稳定的光强。由于用于锁频光源的锁频过程的锁频参考光具有稳定光强，因此其锁 频效果也得以稳定，由此可以允许第一(第二)锁频光源稳定地输出第一(第二)光信号，其将由第一(第二)量子态制备模块制备生成第一(第二)量子光信号，该量子光信号具有与锁频参考光一致的波长。
在第三波分复用器中，第一波长端(其作为偏振一致性耦合模块的单波长输入端3)保偏连接第一(第二)量子态制备模块，以接收第一(第二)量子光信号，第二波长端保偏连接第二波分复用器的第二波长端，因此，允许以保偏的方式将第一(第二)量子光信号和相位参考光进行波分耦合，并经公共端(其作为偏振一致性耦合模块的双波长输出端4)向外输出第一(第二)信号合束光。如前所述，由于偏振监测补偿模块的设置，解复用的相位参考光也将具有稳定的光强，这有利于输出稳定的信号合束光。
综上可知，通过在Charlie端中设置保偏的第一波分复用器将相位参考光和锁频参考光以偏振一致的方式进行波分耦合形成参考合束光，同时在Alice和Bob端设置由两个保偏的波分复用器构成的偏振一致性耦合模块，可以允许在无需EPC参与和人工干预的情况下，仅借助光学器件本身固有的光学特性，在Alice(Bob)端实现第一(第二)量子光信号和相位参考光的偏振一致性耦合，这种偏振一致性的实现不会受到Alice和Bob端温度、振动等环境影响，实现上更为稳定，且无需复杂控制，能够有效减低系统成本、提高系统集成度和稳定性。此外，采用EPC实现偏振一致性耦合的现有方案只能实现量子光信号这一路上的偏振稳定，不能反馈稳定相位参考光这一路，其偏振反馈控制效果有限，而在本发明中，可以同时实现相位参考光的偏振态稳定效果，由此有效保证整个Alice和Bob端光路的偏振稳定性。
作为示例，第一(第二)量子态制备模块可以包括相位调制单元、幅度调制单元和光衰减单元。
继续参见图3，由第一(第二)偏振一致性耦合模块输出的第一(第二)信号合束光可以借助第二单模或保偏光纤信道，传输至Charlie端。
类似地，第一(第二)信号合束光经例如第二单模光纤信道到达Charlie端时，其偏振态同样可能会发生变化。因此，还可以设置两个偏振监测补偿模块，分别用于对第一和第二信号合束光(其具有不稳定的偏振态)进行偏振反馈补偿，使其具有预设的偏振态，确保用于测量模块的第一和第二信号合束光 具有稳定的偏振态。
用于信号合束光的偏振监测补偿模块可以采用与上述Alice端或Bob端中的偏振监测补偿模块相同的结构，因此在此不再赘述。需要说明的是，在用于信号合束光的偏振监测补偿模块中，用于偏振补偿的偏振控制器既可以设置在Alice端/Bob端中，也可以设置在Charlie端中(如图3所示)，用于偏振监测的偏振分束器和光电探测器可以设置在Charlie端。
在一种可选的示例中，用于信号合束光的两个偏振监测补偿模块可以借助分束器共用一个光电探测器，以提高系统的集成度。其中，分束器可以被设置在光电探测器之前，将分别来自第一和第二信号合束光的分光耦合成一路，以允许借助一个光电探测器实现对两路信号合束光的探测。
进一步地，Charlie端还可以包括用于补偿同一信号合束光中量子光信号与相位参考光之间慢速相位扰动的慢速相位补偿模块，以及用于补偿第一和第二信号合束光之间快速相位扰动的快速相位补偿模块。
作为示例，快速相位补偿模块可以包括相位调制器。
作为示例，慢速相位补偿模块可以包括保偏的第五波分复用器和保偏的第六波分复用器，两者同样均具有公共端、第一波长端和第二波长端。
第五和第六波分复用器的第一波长端之间通过第一臂连接，第五和第六波分复用器的第二波长端之间通过第二臂连接。
第五波分复用器的公共端被设置成接收信号合束光，因此允许将信号合束光进行波分解复用，并分别通过第一和第二波长端输出量子光信号和相位参考光，以使它们分别沿第一和第二臂朝向第六波分复用器传输。
可以在第一和第二臂中的至少一个上设置相移器，以在量子光信号和相位参考光之间进行相位补偿。
量子光信号和相位参考光分别从第一和第二波长端进入第六波分复用器，并以保偏的方式发生波分耦合，再次形成信号合束光。此时，借助相移器的作用，信号合束光中量子光信号和相位参考光之间的慢速相位扰动可以得到补偿。
继续参见图3，经偏振补偿、慢速相位补偿或快速相位补偿作用后的两路信号合束光最终将进入测量模块。
在测量模块中，第一和第二信号合束光将发生干涉作用，生成干涉信号，并对干涉信号进行检测。
作为示例，测量模块可以包括保偏的第二分束器，其具有第一和第二输入端，以及第一和第二输出端。
第一和第二输入端分别用于接收第一和第二信号合束光，因此允许这两路信号合束光发生干涉以生成干涉信号。干涉信号将分别经由第一和第二输出端输出，进入第一检测光路和第二检测光路。本领域技术人员能够理解，此时的干涉信号包括量子光信号的干涉信号，以及相位参考光的干涉信号。
可以在第一和第二检测光路中的至少一个上设置第四波分复用器，用于将干涉信号中关于量子光信号的干涉信号和关于相位参考光的干涉信号分开，以允许分别对量子光信号的干涉信号和相位参考光的干涉信号进行检测。
第四波分复用器同样可以具有公共端、第一波长端和第二波长端。其中，公共端用于接收干涉信号，第一波长端连接第一光电探测器，以允许对量子光信号的干涉信号进行检测，第二波长端连接第二光电探测器，以允许对相位参考光的干涉信号进行检测。
因此，可以利用关于相位参考光的干涉信号的检测结果，进行快速相位补偿模块的反馈控制；利用关于量子光信号的干涉信号的检测结果的一部分进行慢速相位补偿模块的反馈控制，利用另一部分进行量子密钥的生成。
综上可知，在本发明中，通过Charlie端设置保偏的波分复用器将锁频参考光和相位参考光波分耦合在一起，同时在Alice和Bob端设置由两个保偏的波分复用器形成的偏振一致性耦合模块，可以允许在不借助EPC及人工干预的情况下，依靠光学器件本身的特性实现量子光信号和相位参考光的偏振一致性耦合，从而使系统光路及控制过程均得到简化，同时增强系统的稳定性，易于集成化设计。此外，还通过在偏振一致性耦合模块之前设置偏振监测补偿模块，可以保证用于锁频光源的锁频参考光和用于偏振一致性耦合的相位参考光的光强稳定，有利于实现稳定的锁频效果，以及Charlie端中快速相位补偿的实现及量子密钥的生成。
进一步地，根据本发明的TF-QKD系统可以包括N个用作Alice端/Bob端的用户端，且在Charlie端设置有支持2：N端口的光开关。
因此，可以通过控制支持2：N端口的光开关，在N个用户端中切换2个用户端作为Alice端和Bob端与Charlie端形成连接。
本发明的另一方面涉及一种用于稳定实现TF-QKD的方法。
如前所述，为在TF-QKD过程中稳定实现量子光信号和相位参考光的偏振一致性耦合，首先需要在Charlie端，利用保偏的第一波分复用器，以保偏的方式将锁频参考光和相位参考光进行波分耦合，形成参考合束光。
Charlie端分别将该参考合束光发送给Alice端和Bob端。例如，可以将该参考合束光分成两个分量，以分别发送给Alice端和Bob端。
在Alice端和Bob端中，优选先利用偏振监测补偿模块对参考合束光进行偏振补偿，以使其具有预设的偏振态，这有利于锁频过程的稳定进行，以及稳定地生成由量子光信号和相位参考光保偏地波分耦合形成的信号合束光。
随后，在Alice端和Bob端中，以保偏的方式将经偏振补偿的参考合束光解复用，以将锁频参考光和相位参考光分开。其中，锁频参考光被保偏地输出给锁频光源，用于允许锁频光源以锁频的方式生成光信号，其波长锁定在锁频参考光的波长上。在光信号上制备量子态，以生成量子光信号。
因此，在Alice端和Bob端中，可以将量子光信号和相位参考光保偏地进行波分耦合，形成信号合束光。此时，信号合束光中的量子光信号和相位参考光将具有一致的偏振态，从而实现两者的偏振一致性耦合。并且，这种偏振一致性耦合的实现，无需EPC或者人工干预，仅仅依靠光学器件本身的特性，其不容易受到外界环境的影响，从而可以保证高的稳定性。
Alice端和Bob端分别将其中生成的信号合束光发送给Charlie端，以便在Charlie端发生干涉，并且借助其中量子光信号的干涉信号可以实现TF-QKD过程。
优选地，为保证干涉测量过程的稳定性和效率，还可以在干涉测量之前，对Alice端和Bob端发送过来的信号合束光进行偏振监测补偿，以使其具有稳定的预设偏振态。更进一步地，Charlie端还可以在干涉测量之前，对信号合束光进行慢速相位补偿和快速相位补偿，以消除系统运行过程在各种光信号中引入的慢速相位扰动和快速相位扰动，从而保证干涉过程的稳定性和效率。
作为一种优选方式，该TF-QKD方法可以借助本发明的TF-QKD系统来 实现。
尽管前面结合附图通过具体实施例对本发明进行了说明，但是，本领域技术人员容易认识到，上述实施例仅仅是示例性的，用于说明本发明的原理，其并不会对本发明的范围造成限制，本领域技术人员可以对上述实施例进行各种组合、修改和等同替换，而不脱离本发明的精神和范围。

Claims (14)

  1. 一种TF-QKD系统，其包括Alice端、Bob端和Charlie端；
    所述Charlie端包括测量模块、第一波分复用器和第一分束模块；
    所述第一波分复用器用于以保偏的方式将锁频参考光和相位参考光进行波分耦合，形成参考合束光，所述锁频参考光和相位参考光具有不同的波长；
    所述第一分束模块用于将所述参考合束光分光，形成两个参考合束光分量；
    所述Alice端被设置成接收所述两个参考合束光分量中的一个，以及向所述Charlie端发送第一信号合束光，且包括第一锁频光源、第一量子态制备模块和第一偏振一致性耦合模块；
    所述第一偏振一致性耦合模块被设置用于将所述参考合束光分量中的锁频参考光和相位参考光解复用，并将所述锁频参考光保偏地输出给所述第一锁频光源，以及以保偏的方式将第一量子光信号和所述相位参考光进行波分耦合，形成所述第一信号合束光；
    所述第一锁频光源用于利用所述锁频参考光，以锁频的方式生成第一光信号；
    所述第一量子态制备模块用于在所述第一光信号上制备量子态，生成所述第一量子光信号；
    所述Bob端被设置成接收所述两个参考合束光分量中的另一个，以及向所述Charlie端发送第二信号合束光，且包括第二锁频光源、第二量子态制备模块和第二偏振一致性耦合模块；
    所述第二偏振一致性耦合模块被设置用于将所述参考合束光分量中的锁频参考光和相位参考光解复用,并将所述锁频参考光保偏地输出给所述第二锁频光源，以及以保偏的方式将第二量子光信号和所述相位参考光进行波分耦合，形成所述第二信号合束光；
    所述第二锁频光源用于利用所述锁频参考光，以锁频的方式生成第二光信号；
    所述第二量子态制备模块用于在所述第二光信号上制备量子态，生成所述第二量子光信号；
    所述测量模块用于测量所述第一和第二量子光信号的干涉信号。
  2. 如权利要求1所述的TF-QKD系统，其中，所述偏振一致性耦合模块包括两个保偏的波分复用器；
    所述两个保偏的波分复用器中的一个被设置成将所述参考合束光分量中的锁频参考光和相位参考光解复用，并分别将所述锁频参考光和相位参考光保偏地输出给所述锁频光源和所述两个保偏的波分复用器中的另一个；
    所述两个保偏的波分复用器中的另一个被设置成以保偏的方式将所述相位参考光和量子光信号进行波分耦合，生成所述信号合束光。
  3. 如权利要求1所述的TF-QKD系统，其中，所述Alice端和Bob端中的至少一个还包括偏振监测补偿模块，其设在所述偏振一致性耦合模块之前，用于对所述参考合束光进行偏振态补偿，以使其具有预设的偏振态。
  4. 如权利要求3所述的TF-QKD系统，其中，所述偏振监测补偿模块包括偏振控制器、偏振分束器和光电探测器。
  5. 如权利要求1所述的TF-QKD系统，其还包括两个偏振监测补偿模块，分别用于对所述第一和第二信号合束光进行偏振态补偿。
  6. 如权利要求5所述的TF-QKD系统，其中：
    所述两个偏振监测补偿模块中的第一个包括偏振控制器、偏振分束器和光电探测器，其中，所述偏振控制器设置在所述Alice端或Charlie端，所述偏振分束器和光电探测器设置在所述Charlie端；
    所述两个偏振监测补偿模块中的第二个包括偏振控制器、偏振分束器和光电探测器，其中，所述偏振控制器设置在所述Bob端或Charlie端，所述偏振分束器和光电探测器设置在所述Charlie端。
  7. 如权利要求6所述的TF-QKD系统，其中，所述两个偏振监测补偿模块通过分束器复用光电探测器。
  8. 如权利要求1所述的TF-QKD系统，其中，所述Charlie端还包括用于生成所述锁频参考光的锁频参考光源，以及用于生成所述相位参考光的相位参考光源；以及/或者，
    所述Charlie端还包括慢速相位补偿模块和/或快速相位补偿模块，其中，所述慢速相位补偿模块被设置用于补偿所述信号合束光中的量子光信号和相位参考光之间的相位扰动，所述快速相位补偿模块被设置用于补偿所述第一和 第二信号合束光之间的相位扰动。
  9. 如权利要求1所述的TF-QKD系统，其中，所述锁频光源基于注入锁定、光学锁相环或时频传输，利用所述锁频参考光生成所述光信号。
  10. 如权利要求1所述的TF-QKD系统，其中，所述Alice端和Bob端的总数量为N个,N为大于1的正整数；并且，
    所述Charlie端还设置有支持2:N端口的光开关，用于允许选择接入的Alice端和/或Bob端。
  11. 一种用于稳定实现TF-QKD的方法，其包括以下步骤：
    在Charlie端，借助保偏的波分复用器，将锁频参考光和相位参考光进行波分耦合，形成参考合束光；
    在所述Alice端和Bob端中，借助一个保偏的波分复用器将所述参考合束光中的锁频参考光和相位参考光解复用，并将锁频参考光保偏地输出给锁频光源；
    在所述Alice端和Bob端中，使所述锁频光源利用所述锁频参考光以锁频的方式生成光信号，利用所述光信号制备量子光信号并将其保偏地输出；
    在所述Alice端和Bob端中，借助另一保偏的波分复用器将所述相位参考光和量子光信号进行波分耦合，形成信号合束光；
    在所述Charlie端中，使分别来自所述Alice端和Bob端的量子光信号发生干涉以生成干涉信号，并对干涉信号进行测量。
  12. 如权利要求11所述的方法，其还包括以下步骤：
    在所述Alice端和Bob端中，将所述参考合束光解复用之前对其进行偏振补偿并保偏输出；以及/或者，
    在使所述量子光信号发生干涉之前对其进行偏振补偿并保偏输出。
  13. 如权利要求11所述的方法，其还包括在所述Charlie端中，对所述信号合束光进行慢速相位补偿和/或快速相位补偿的步骤。
  14. 如权利要求11所述的方法，其借助如权利要求1-10中任一项所述的TF-QKD系统来实现。
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Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN117375833A (zh) * 2023-12-07 2024-01-09 济南量子技术研究院 一种基于芯片光梳的发送端全片上tf-qkd系统
CN117459152A (zh) * 2023-12-20 2024-01-26 济南量子技术研究院 基于光学频率梳的射频与光频参考复用的tf-qkd实施方法
CN117914411A (zh) * 2024-03-19 2024-04-19 济南量子技术研究院 一种用于tf-qkd系统的相位反馈方法
CN117914411B (zh) * 2024-03-19 2024-06-04 济南量子技术研究院 一种用于tf-qkd系统的相位反馈方法

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN116633541B (zh) * 2023-07-19 2023-09-29 济南量子技术研究院 一种基于气室频率参考的双场光源锁频方法及系统

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN109379188A (zh) * 2018-12-10 2019-02-22 山西大学 一种测量设备无关相位匹配量子密钥分发装置
CN110896328A (zh) * 2018-09-12 2020-03-20 中国科学技术大学 基于单个参考光脉冲单零差探测的连续变量量子密钥分发系统
CN112929161A (zh) * 2021-01-22 2021-06-08 西安电子科技大学 即插即用型参考系无关的双场量子密钥分发协议实现方法
CN216531333U (zh) * 2021-11-29 2022-05-13 科大国盾量子技术股份有限公司 一种稳定的双场量子密钥分发系统

Family Cites Families (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN112039666B (zh) * 2020-09-03 2023-09-19 北京量子信息科学研究院 基于量子密钥分发的频率锁定与相位稳定方法及系统

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN110896328A (zh) * 2018-09-12 2020-03-20 中国科学技术大学 基于单个参考光脉冲单零差探测的连续变量量子密钥分发系统
CN109379188A (zh) * 2018-12-10 2019-02-22 山西大学 一种测量设备无关相位匹配量子密钥分发装置
CN112929161A (zh) * 2021-01-22 2021-06-08 西安电子科技大学 即插即用型参考系无关的双场量子密钥分发协议实现方法
CN216531333U (zh) * 2021-11-29 2022-05-13 科大国盾量子技术股份有限公司 一种稳定的双场量子密钥分发系统

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
WEI YADONG, TANG ZHI-LIE, LIU XIAO-BAO, LIAO CHANG-JUN, LIU SONG-HAO: "Study on Sending after Verify Scheme in Quantum Channel for Quantum Key Distribution System Based on Polarization Coding", ACTA PHOTONICA SINICA, SCIENCE PRESS, BEIJING., CN, vol. 38, no. 7, 15 July 2009 (2009-07-15), CN , pages 1852 - 1857, XP093068029, ISSN: 1004-4213 *

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN117375833A (zh) * 2023-12-07 2024-01-09 济南量子技术研究院 一种基于芯片光梳的发送端全片上tf-qkd系统
CN117375833B (zh) * 2023-12-07 2024-03-08 济南量子技术研究院 一种基于芯片光梳的发送端全片上tf-qkd系统
CN117459152A (zh) * 2023-12-20 2024-01-26 济南量子技术研究院 基于光学频率梳的射频与光频参考复用的tf-qkd实施方法
CN117459152B (zh) * 2023-12-20 2024-04-30 济南量子技术研究院 基于光学频率梳的射频与光频参考复用的tf-qkd实施方法
CN117914411A (zh) * 2024-03-19 2024-04-19 济南量子技术研究院 一种用于tf-qkd系统的相位反馈方法
CN117914411B (zh) * 2024-03-19 2024-06-04 济南量子技术研究院 一种用于tf-qkd系统的相位反馈方法

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