Phase-stable TF-QKD method, system and its phase perturbation monitoring method

Abstract

The present invention proposes a method for monitoring phase perturbation in a TF-QKD system, as well as a TF-QKD method and system for achieving phase stability thereby, which allows the detection function in the TF-QKD system to be realized by means of an avalanche detector, and at the same time can meet the high signal-to-noise ratio of the quantum optical signal and the high count rate of the phase reference optical signal.

Description

Technical Field

[0001] The present invention relates to the field of quantum secure communication, and in particular, to a method for monitoring phase perturbation for a TF-QKD system, and a TF-QKD method and system for achieving phase stability thereby. Background Art

[0002] The security of the ideal BB84 QKD protocol is based on some basic assumptions. If real devices cannot perfectly meet these assumptions, security vulnerabilities will occur. For example, the ideal BB84 QKD protocol uses a single-photon source and utilizes the principle of non-clonability of single photons to ensure communication security. However, a true single-photon source cannot be realized in reality. The decoy-state quantum key distribution (QKD) protocol proposed in 2005 uses a weak coherent light source instead of a single-photon source to solve the problem of imperfect light sources in real devices. The detection system is the most vulnerable part in the implementation of QKD, and many attacks are carried out against the security vulnerabilities of the detector-side devices, including typical strong-light blinding attacks, time-shift attacks, etc. To resist detector-side attacks, in addition to using security patch methods and modeling device parameters and including them in the security proof, there is also a method of using measurement-device-independent QKD (MDI-QKD). The measurement-device-independent quantum key distribution protocol (original MDI-QKD protocol) based on entanglement swapping technology and time-reversal EPR scheme was first proposed by the H-K Lo group in 2012, which can eliminate all detector-side channels. Further, in 2018, the research team of A.J. Shields from Toshiba Research Europe proposed the twin-field quantum key distribution protocol (TF-QKD protocol). This protocol inherits the measurement-device-independent security of MDI-QKD and has a longer secure distance, which has attracted extensive attention at home and abroad.

[0003] The TF-QKD protocol is different from the original MDI-QKD protocol. The main technical difficulty lies in how to achieve high-contrast interference of two independent laser sources. At the same time, it is also necessary to quickly monitor the global phase difference. For non-post-selected QKD (NPP-QKD), rapid phase feedback compensation is further required. The global phase difference here corresponds to the control requirement of the optical path difference at the sub-wavelength level. In addition to the differences caused by the frequency and wavelength differences of the light sources of Alice and Bob themselves, the contribution of the phase difference also includes the phase perturbation introduced in the quantum optical transmission channel. This perturbation is on the order of 10 rad / ms and changes very violently, requiring rapid monitoring.

[0004] To achieve fast monitoring of channel phase perturbation, there are currently two types of solutions. One is to perform fast time-division multiplexing on the signal light, using a part of the light as the phase reference light to monitor and estimate the relative phase fast drift of the optical fiber. The other is to perform wavelength-division multiplexing using reference lights of different wavelengths, and reflect the phase perturbation of the signal light by monitoring the phase perturbation of the reference light. Among them, higher phase perturbation monitoring accuracy and longer distance can be supported by increasing the reference light power. However, this solution still has quantum optical residual phase perturbation that needs to be monitored and also needs to be combined with the first time-division multiplexing solution.

[0005] In the prior art, the phase reference light separated from the signal light in time-division multiplexing needs to accumulate a large counting rate in a short time to reduce the calculation result deviation caused by the counting rate fluctuation. At the same time, the quantum optical component in the signal light needs to maintain a high signal-to-noise ratio, and the influence of the phase reference light of the same wavelength on the signal-to-noise ratio of the quantum light needs to be minimized. This poses relatively high requirements for the detector, namely both high counting rate and high signal-to-noise ratio.

[0006] Due to the limitations of the performance of the avalanche detector itself, for example, there is a mutual restraint among its high signal-to-noise ratio, low after-pulse, and high counting rate performances, making it difficult to be directly applied to the above-mentioned phase perturbation monitoring scheme. Therefore, in the existing TF-QKD phase perturbation monitoring scheme, a superconducting detector is usually adopted, which leads to a significant increase in the complexity and size of the existing system. Summary of the Invention

[0007] Aiming at the above problems in the prior art, the present invention proposes a method for monitoring phase perturbation in a TF-QKD system, as well as a TF-QKD method and system for achieving phase stability thereby, which allows the detection function in the TF-QKD system to be realized by means of an avalanche detector, and at the same time can meet the high signal-to-noise ratio of the quantum optical signal and the high counting rate of the phase reference optical signal.

[0008] Specifically, the first aspect of the present invention relates to a method for monitoring phase perturbation in a TF-QKD system, which includes the following steps:

[0009] Sending a second output light from a sending end to a receiving end, wherein by providing different modulations to the optical signal at different times, the second output light contains a quantum optical signal and a first phase reference optical signal in a time-division multiplexing manner;

[0010] In the receiving end, interfering the second output lights from different sending ends to generate an interference signal, wherein the interference signal contains a first interference signal generated by interfering the quantum optical signals and a second interference signal generated by interfering the first phase reference optical signals in a time-division multiplexing manner; and

[0011] In the receiving end, an avalanche detector is used to detect the interference signal, so as to monitor the phase perturbation in the TF-QKD system according to the second interference signal. Among them, the avalanche detector detects the first interference signal at a first dead time and detects the second interference signal at a second dead time, and the first dead time is greater than the second dead time.

[0012] Further, the phase perturbation monitoring method of the present invention may further include the following steps:

[0013] In the receiving end, the frequency-locked reference light with a first wavelength and the second phase reference light signal with a second wavelength are wavelength-division coupled in a polarization-maintaining manner to form a first output light, where the first wavelength is different from the second wavelength;

[0014] In the sending end, the frequency-locked reference light and the second phase reference light signal in the first output light are demultiplexed in a polarization-maintaining manner, the optical signal is generated in a frequency-locked manner based on the frequency-locked reference light, and the quantum optical signal, the first phase reference light signal and the second phase reference light signal are wavelength-division coupled in a polarization-maintaining manner to form the second output light;

[0015] In the receiving end, the first wavelength component and the second wavelength component in the interference signal are demultiplexed, and an avalanche detector is used to detect a third interference signal generated by the interference of the second phase reference light signal, so as to monitor the phase perturbation in the TF-QKD system according to the third interference signal.

[0016] The second aspect of the present invention relates to a phase-stable TF-QKD system, which includes a receiving end and a plurality of sending ends;

[0017] The receiving end includes a measurement module, a frequency-locked reference light source and a first beam splitter;

[0018] The frequency-locked reference light source is configured to generate a frequency-locked reference light;

[0019] The first beam splitter is configured to split the first output light of the receiving end, where the first output light includes the frequency-locked reference light;

[0020] The sending end includes a frequency-locked light source, a reference light modulation module and a quantum state preparation module;

[0021] The frequency-locked light source is configured to generate an optical signal in a frequency-locked manner based on the frequency-locked reference light in the first output light;

[0022] The quantum state preparation module is configured to prepare a quantum state on the optical signal;

[0023] The reference optical signal modulation module is configured to modulate the optical signal so that the second output optical signal of the transmitting end includes a quantum optical signal and a first phase reference optical signal in a time-division multiplexing manner;

[0024] The measurement module includes a polarization-maintaining second beam splitter, an avalanche photodetector, and a control unit;

[0025] The second beam splitter is configured to allow the second output optical signals from different transmitting ends to interfere to generate an interference signal, which includes a first interference signal generated by the interference of the quantum optical signal and a second interference signal generated by the interference of the first phase reference optical signal in a time-division multiplexing manner;

[0026] The avalanche photodetector is configured to detect the interference signal;

[0027] The control unit is configured to make the avalanche photodetector have a first dead time when detecting the first interference signal and a second dead time when detecting the second interference signal, and the first dead time is greater than the second dead time.

[0028] Further, the control unit is configured to: control the dead time by controlling the avalanche gating signal for the avalanche photodetector; or control the dead time by setting the valid time interval of the detection data of the avalanche photodetector.

[0029] Further, the transmitting end further includes a continuous optical chopping unit; and / or, the quantum state preparation module includes a decoy state modulation unit, a phase modulation unit, and an optical attenuation unit.

[0030] Further, the transmitting end further includes a polarization compensation module for compensating the polarization state of the first output optical signal to make it have a preset polarization state; and / or, the receiving end further includes a polarization compensation module for compensating the polarization state of the second output optical signal to make it have a preset polarization state.

[0031] Further, the receiving end further includes a fast phase compensation module for compensating the phase perturbation between the quantum optical signals in the second output optical signals from different transmitting ends.

[0032] Further, the frequency-locked light source generates the optical signal by using injection locking, an optical phase-locked loop, or time-frequency transmission with the frequency-locked reference optical signal.

[0033] Further, the number of the transmitting ends is N, where N is a positive integer greater than 1, and the receiving end is further provided with an optical switch supporting 2:N ports for allowing selection of the accessing transmitting end.

[0034] Further, the receiving end further includes a phase reference light source and a first wavelength division multiplexer;

[0035] The phase reference light source is used to generate a second phase reference optical signal with a second wavelength, and the frequency-locked reference light has a first wavelength, and the first wavelength is different from the second wavelength;

[0036] The first wavelength division multiplexer is arranged to perform wavelength division coupling on the frequency-locked reference light and the second phase reference optical signal in a polarization-maintaining manner, so that the first output light includes the combined beam of the frequency-locked reference light and the second phase reference optical signal;

[0037] The sending end further includes a polarization consistency coupling module, which is arranged to demultiplex the frequency-locked reference light and the second phase reference optical signal in the first output light, and output the frequency-locked reference light to the frequency-locked light source in a polarization-maintaining manner, and perform wavelength division coupling on the quantum optical signal, the first phase reference optical signal and the second phase reference optical signal in a polarization-maintaining manner, so that the second output light includes the combined beam of the quantum optical signal, the first phase reference optical signal and the second phase reference optical signal;

[0038] The measurement module further includes a wavelength division multiplexer, which is arranged before the avalanche detector and is used to demultiplex the first wavelength component and the second wavelength component in the interference signal, so as to be detected by different avalanche detectors respectively.

[0039] Furthermore, the polarization consistency coupling module includes two polarization-maintaining wavelength division multiplexers;

[0040] One of the two polarization-maintaining wavelength division multiplexers is arranged to demultiplex the frequency-locked reference light and the second phase reference optical signal in the first output light, and output the frequency-locked reference light and the second phase reference optical signal to the frequency-locked light source and the other of the two polarization-maintaining wavelength division multiplexers in a polarization-maintaining manner respectively;

[0041] The other of the two polarization-maintaining wavelength division multiplexers is arranged to perform wavelength division coupling on the quantum optical signal, the first phase reference optical signal and the second phase reference optical signal in a polarization-maintaining manner.

[0042] Furthermore, the receiving end further includes a slow phase compensation module for compensating the phase perturbation between the quantum optical signal and the second phase reference optical signal in the second output light.

[0043] The third aspect of the present invention relates to a TF-QKD method, which includes the following steps:

[0044] The receiving end sends the first output light to the sending end, and the first output light includes the frequency-locked reference light;

[0045] At the sending end, an optical signal is generated in a frequency-locking manner based on the frequency-locking reference light, and modulation is performed on the optical signal to generate a second output light, where the second output light includes a quantum optical signal and a first phase reference optical signal in a time-division multiplexing manner;

[0046] At the receiving end, the second output lights from different sending ends are made to interfere to generate an interference signal, where the interference signal includes a first interference signal generated by the interference of the quantum optical signal and a second interference signal generated by the interference of the first phase reference optical signal in a time-division multiplexing manner;

[0047] At the receiving end, an avalanche detector is used to detect the interference signal, where the avalanche detector detects the first interference signal at a first dead time and detects the second interference signal at a second dead time, and the first dead time is greater than the second dead time;

[0048] At the receiving end, the phase perturbation in the TF-QKD system is calculated according to the detection data of the second interference signal, and phase compensation is provided according to the phase perturbation; and the detection data of the first interference signal is sent to the sending end.

[0049] Further, the TF-QKD method of the present invention may further include the following steps:

[0050] At the receiving end, the frequency-locking reference light and a second phase reference optical signal having a second wavelength are wavelength-division coupled in a polarization-maintaining manner to form the first output light, where the frequency-locking reference light has a first wavelength, and the first wavelength is different from the second wavelength;

[0051] At the sending end, the frequency-locking reference light and the second phase reference optical signal in the first output light are demultiplexed in a polarization-maintaining manner, and the quantum optical signal, the first phase reference optical signal and the second phase reference optical signal are wavelength-division coupled in a polarization-maintaining manner to form the second output light; and

[0052] At the receiving end, the first wavelength component and the second wavelength component in the interference signal are demultiplexed to be detected by different avalanche detectors respectively.

[0053] Further, the TF-QKD method of the present invention may further include the following steps:

[0054] Polarization compensation is performed on the first output light at the sending end and polarization-maintaining output is performed; and / or

[0055] Polarization compensation is performed on the second output light at the receiving end and polarization-maintaining output is performed.

[0056] Furthermore, the TF-QKD method of the present invention further includes the following steps:

[0057] In the receiving end, quickly compensate for the phase perturbation between the quantum optical signals in the second output light from different sending ends; and / or,

[0058] In the receiving end, slowly compensate for the phase perturbation between the quantum optical signal and the second phase reference optical signal in the second output light.

[0059] Preferably, this method is implemented by means of the above TF-QKD system. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] The following further elaborates on the specific embodiments of the present invention in conjunction with the drawings.

[0061] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following briefly introduces the drawings required for use in the description of the embodiments or the prior art. Obviously, the following-described drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0062] Figure 1 Shows an embodiment of the TF-QKD system according to the present invention;

[0063] Figure 2 Shows a further embodiment of the TF-QKD system according to the present invention;

[0064] Figure 3 Shows an example of the polarization consistency coupling module according to the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0065] Hereinafter, the exemplary embodiments of the present invention will be described in detail with reference to the drawings. The following embodiments are provided by way of example to fully convey the spirit of the present invention to those skilled in the art to which the present invention pertains. Therefore, the present invention is not limited to the embodiments disclosed herein.

[0066] Figure 1 Shows an embodiment of the TF-QKD system according to the present invention, which includes a sending end 1, a sending end 2, and a receiving end.

[0067] The receiving end may include a frequency-locked reference light source, a first beam splitter, and a measurement module.

[0068] The frequency-locked reference light source has a first wavelength and is used to generate a frequency-locked reference light.

[0069] The frequency-locked reference light serves as the first output light and is split by a first beam splitter at the receiving end to be output to transmitter 1 and transmitter 2 respectively.

[0070] Since the transmitters in the TF-QKD system can have the same structure, for the sake of simplicity, the structure of the transmitter will be described below taking transmitter 1 as an example.

[0071] In the present invention, the first output light of the receiving end can be transmitted to transmitter 1 through a first single-mode or polarization-maintaining optical fiber channel.

[0072] Since the polarization state of the optical signal may change during transmission, a polarization compensation module can be provided in transmitter 1 to perform polarization compensation on the first output light arriving at transmitter 1, so that it has a preset polarization state and is output in a polarization-maintaining manner.

[0073] As an example, the polarization compensation module can achieve polarization compensation by means of a polarization controller. For example, the polarization controller can be an electric polarization controller (EPC).

[0074] Those skilled in the art can easily understand that the polarization state of an optical signal such as the first output light can be monitored by means of a polarization beam splitter and a photodetector, so as to perform polarization compensation on it to achieve a preset polarization state on the optical signal.

[0075] Continue to refer to Figure 1 , transmitter 1 may further include a frequency-locked light source, a reference light modulation module, and a quantum state preparation module.

[0076] In transmitter 1, the frequency-locked reference light in the first output light continues to be transmitted into the frequency-locked light source. The frequency-locked light source uses, for example (but not limited to), injection locking technology, optical phase-locked loop (OPLL) technology, time-frequency transmission technology, etc., to lock its frequency to the first wavelength of the frequency-locked reference light, thereby outputting an optical signal with its frequency stabilized at the first wavelength.

[0077] The quantum state preparation module is used to prepare a quantum state on the optical signal.

[0078] As an example, the quantum state preparation module may include a phase modulation unit, a decoy state modulation unit, and an optical attenuation unit.

[0079] Furthermore, the transmitter may further include a continuous light chopping unit for shaping the continuous optical signal output by the frequency-locked light source into a pulsed optical signal.

[0080] The reference optical modulation module can provide different modulations on the optical signal to form a quantum optical signal and a first phase reference optical signal. For example, the reference optical modulation module can modulate the optical signal in a preset reference optical time interval to form a first phase reference optical signal, and modulate the optical signal in a preset quantum optical time interval to form a quantum optical signal.

[0081] Therefore, in the second output optical signal transmitted from the transmitting end 1 to the receiving end through the second single-mode / polarization-maintaining optical fiber channel, the quantum optical signal and the first phase reference optical signal exist in a time-division multiplexing manner.

[0082] Similarly, when the second output optical signal reaches the receiving end through, for example, the second single-mode / polarization-maintaining optical fiber channel, its polarization state may also change. Therefore, a polarization compensation module can also be set up to perform polarization compensation on the second output optical signal so that it has a preset polarization state.

[0083] It should be noted that when performing polarization compensation on the second output optical signal, the polarization state monitoring of the second output optical signal needs to be carried out at the receiving end. In other words, a polarization beam splitter and a photodetector can be set in the receiving end to monitor the polarization state of the second output optical signal.

[0084] Those skilled in the art can easily understand that since it is necessary to perform polarization compensation on the second output optical signals from two different transmitting ends at the receiving end, a beam splitter can be additionally set in the receiving end to allow sharing a single photodetector to realize the polarization state monitoring of different second output optical signals, so as to improve the integration of the system. Among them, the additionally set beam splitter can be set before the photodetector to split and combine the second output optical signals from different transmitting ends into one path, so as to allow the detection of the split second output optical signals by means of a single photodetector.

[0085] Continue to refer to Figure 1 , the measurement module can include a polarization-maintaining second beam splitter (PMBS), avalanche detectors 1, 2, and a control unit.

[0086] The second beam splitter has two input ends and two output ends. Among them, the two second output optical signals from the transmitting end 1 and the transmitting end 2 are respectively input through the two input ends and interfere, and finally the interference signals are respectively output through the two output ends.

[0087] Those skilled in the art can understand that since the second output optical signal contains the quantum optical signal and the first phase reference optical signal in a time-division multiplexing manner, the interference signal also correspondingly contains, in a time-division multiplexing manner, a first interference signal generated by the interference of the quantum optical signal and a second interference signal generated by the interference of the first phase reference optical signal.

[0088] Two avalanche detectors are respectively arranged on the two output ends of the second beam splitter to detect the interference signal.

[0089] When the first interference signal arrives at the avalanche detector, the control unit makes the avalanche detector have a relatively long first dead time, so that high-signal-to-noise quantum light detection with a low afterpulse probability can be ensured; when the second interference signal arrives at the avalanche detector, the control unit makes the avalanche detector have a relatively short second dead time, so that a relatively high afterpulse probability can be allowed to achieve a high counting rate for the phase reference light. Thus, when using the avalanche detector to realize the detection function in the TF-QKD system, a high counting rate of the phase reference light and a high signal-to-noise ratio of the quantum signal light can be achieved simultaneously, that is, high-precision phase reference light detection and quantum signal light detection can be allowed to be achieved simultaneously.

[0090] As an example, to control the dead time of the avalanche detector, the control unit can be achieved by controlling the avalanche gating signal for the avalanche detector. For example, the avalanche gating signal can be stopped to extend the dead time of the avalanche detector.

[0091] As another example, the control unit can also control the dead time by setting the valid time interval of the detection data of the avalanche detector. For example, the control unit can discard the detection data of the avalanche detector for a period of time to extend the dead time of the avalanche detector.

[0092] In an example of the present invention, the reference light modulation module can realize the switching modulation between the quantum light signal and the first phase reference light signal according to a preset switching period. For example, the switching period can be 1 - 100 microseconds, or 1 - 100 milliseconds.

[0093] By setting an appropriate switching period, the monitoring of fast phase perturbation or slow phase perturbation in the TF-QKD system can be realized by means of the second interference signal.

[0094] As Figure 1 shown, a fast phase compensation module can also be arranged in the receiving end to allow the phase perturbation between the quantum light signals in the second output light of different sending ends (such as sending end 1 and sending end 2) to be compensated according to the monitoring result of the fast phase perturbation.

[0095] As an example, the fast phase compensation module can include a phase modulator (PM).

[0096] Figure 2 Shows a further embodiment of the TF-QKD system according to the present invention.

[0097] As Figure 2 shown, a phase reference light source and a first wavelength division multiplexer (WDM) can also be arranged in the receiving end.

[0098] The phase reference light source has a second wavelength and is used to generate a second phase reference optical signal, where the first wavelength is different from the second wavelength.

[0099] The first wavelength division multiplexer has a common end, a first wavelength end, and a second wavelength end. Among them, the first wavelength end is connected to the frequency-locked reference light source, and the second wavelength end is connected to the phase reference light source. Therefore, the frequency-locked reference light and the second phase reference optical signal are allowed to be input into the first wavelength division multiplexer, and wavelength division coupling is performed in a polarization-maintaining manner to form a combined beam, and the combined beam is output through the common end. Therefore, the first output light at the receiving end can include the combined beam of the frequency-locked reference light and the second phase reference optical signal.

[0100] The common end of the first wavelength division multiplexer is connected to the first beam splitter through, for example, a single-mode or polarization-maintaining optical fiber, so as to allow the first output light to be sent to two transmitting ends simultaneously, such as transmitting end 1 and transmitting end 2.

[0101] Continue to refer to Figure 2 , the transmitting end may further include a polarization-consistent coupling module, which is used to demultiplex the frequency-locked reference light and the second phase reference optical signal in the first output light, and output the frequency-locked reference light to the frequency-locked light source in a polarization-maintaining manner, and perform wavelength division coupling on the quantum optical signal, the first phase reference optical signal, and the second phase reference optical signal in a polarization-maintaining manner, so that the second output light includes the combined beam of the quantum optical signal, the first phase reference optical signal, and the second phase reference optical signal.

[0102] Figure 3 An example of the polarization-consistent coupling module according to the present invention is shown.

[0103] As Figure 3 shown, the 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.

[0104] In the second wavelength division multiplexer, the common end serves as the dual-wavelength input end 1 of the polarization-consistent coupling module and is used to receive the first output light (for example, polarization-maintaining output by the polarization compensation module). Therefore, the frequency-locked reference light and the second phase reference optical signal in the first output light are allowed to be demultiplexed, and the frequency-locked reference light is output in a polarization-maintaining manner by the first wavelength end (which serves as the single-wavelength output end 2 of the polarization-consistent coupling module), and the second phase reference optical signal is output in a polarization-maintaining manner by the second wavelength end.

[0105] After the frequency-locked reference light is output from the first wavelength end, it will enter the frequency-locked light source. Those skilled in the art can understand that by using the polarization compensation module to compensate the polarization state of the first output light in advance, the first output light reaching the second wavelength division multiplexer (i.e., the polarization-consistent coupling module) can have a stable polarization state. Therefore, the frequency-locked reference light and the second phase reference light signal demultiplexed and output by the second wavelength division multiplexer can have a stable optical intensity. Since the frequency-locked reference light used in the frequency-locking process of the frequency-locked light source has a stable optical intensity, its frequency-locking effect is also stabilized, thereby allowing the frequency-locked light source to stably output optical signals.

[0106] In the third wavelength division multiplexer, the first wavelength end (which serves as the single-wavelength input end 3 of the polarization-consistent coupling module) receives the quantum optical signal and the first phase reference light signal in a polarization-maintaining manner, and the second wavelength end is polarization-maintainingly connected to the second wavelength end of the second wavelength division multiplexer. Therefore, it is allowed to perform wavelength division coupling of the quantum optical signal, the first phase reference light signal, and the second phase reference light signal in a polarization-maintaining manner, and output the second output light through the common end (which serves as the dual-wavelength output end 4 of the polarization-consistent coupling module). At this time, the second output light includes the combined beam of the quantum optical signal, the first phase reference light signal, and the second phase reference light signal.

[0107] By setting a polarization-maintaining first wavelength division multiplexer in the receiving end to perform wavelength division coupling of the second phase reference light signal and the frequency-locked reference light in a polarization-consistent manner to form the first output light, and at the same time setting a polarization-consistent coupling module composed of two polarization-maintaining wavelength division multiplexers at the sending end, it is allowed to achieve polarization-consistent coupling of the quantum optical signal, the first phase reference light signal, and the second phase reference light signal at the sending end without the participation of EPC and manual intervention, only by relying on the inherent optical characteristics of the optical devices themselves. The realization of this polarization consistency is not affected by environmental factors such as temperature and vibration at the sending end, is more stable in implementation, and does not require complex control, which can effectively reduce the system cost, improve the system integration and stability.

[0108] Continue to refer to Figure 2 , in the measurement module at the receiving end, fourth wavelength division multiplexers can also be respectively set at the two output ends of the second beam splitter for separating the first wavelength components (the first interference signal and the second interference signal) from the second wavelength components (the third interference signal formed by the interference of the second phase reference light signal) in the interference signal.

[0109] The fourth wavelength division multiplexer can also have a common end, a first wavelength end, and a second wavelength end. Among them, the common end is connected to the output end of the second beam splitter to receive the interference signal, and the first wavelength end and the second wavelength end are respectively connected with avalanche detectors.

[0110] Therefore, as mentioned above, the avalanche detector at the first wavelength end is used to detect the first interference signal and the second interference signal in a time division multiplexing manner. The avalanche detector at the second wavelength end is used to detect the third interference signal to provide phase disturbance monitoring in the system.

[0111] exist Figure 2 In the illustrated embodiment, by detecting the third interference signal, the fast phase disturbance monitoring in the TF-QKD system can be well realized. Therefore, by setting a suitable switching period for the reference light modulation module, the slow phase disturbance monitoring in the TF-QKD system can be realized with the help of the second interference signal.

[0112] Therefore, a slow phase compensation module may be correspondingly provided at the receiving end to compensate for the phase disturbance between the quantum light signal in the second output light and the second phase reference light signal.

[0113] As an example, 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 end, a first wavelength end, and a second wavelength end.

[0114] The first wavelength ends of the fifth and sixth wavelength division multiplexers are connected via a first arm, and the second wavelength ends of the fifth and sixth wavelength division multiplexers are connected via a second arm.

[0115] The common end of the fifth wavelength division multiplexer is configured to receive the second output light, thereby allowing the second output light to be wavelength division multiplexed, and outputting the quantum optical signal, the first phase reference optical signal and the second phase reference optical signal through the first and second wavelength ends respectively, so that they are transmitted along the first and second arms respectively toward the sixth wavelength division multiplexer.

[0116] A phase shifter (PS) may be provided on at least one of the first and second arms to perform phase compensation between the quantum optical signal and the second phase reference optical signal.

[0117] The first phase reference optical signal, the quantum optical signal and the second phase reference optical signal enter the sixth wavelength division multiplexer from the first and second wavelength ends respectively, and are wavelength-division coupled in a polarization-maintaining manner to form the second output light again. At this time, with the help of the phase shifter, the slow phase disturbance between the quantum optical signal and the second phase reference optical signal in the second output light can be compensated.

[0118] Further, the TF-QKD system according to the present invention may include N transmitting ends, and an optical switch supporting 2:N ports is provided at the receiving end, wherein N is a positive integer greater than 1.

[0119] Therefore, by controlling the optical switch supporting 2:N ports, 2 transmitting ends among N transmitting ends can be switched to form a connection with the receiving end.

[0120] In addition, the present invention also proposes a method for monitoring phase perturbation in a TF-QKD system.

[0121] According to the present invention, a first output light can be output from the receiving end to the transmitting end, wherein the first output light can include a frequency-locked reference light.

[0122] The transmitting end then uses the frequency-locked reference light to generate an optical signal in a frequency-locked manner, generates a quantum optical signal or a first phase reference optical signal by providing different modulations to the optical signal at different times, and sends it to the receiving end in the form of a second output light. Therefore, the second output light contains a quantum optical signal and a first phase reference optical signal in a time-division multiplexing manner.

[0123] In this process, preferably, polarization compensation can also be performed on the received first output light to make it have a preset polarization state.

[0124] After the receiving end receives two paths of second output lights from different transmitting ends, the two paths of second output lights can be made to interfere to generate an interference signal. At this time, the interference signal correspondingly contains a first interference signal generated by the interference of the quantum optical signal and a second interference signal generated by the interference of the first phase reference optical signal in a time-division multiplexing manner.

[0125] Therefore, an avalanche detector can be used to detect the interference signal to monitor the phase perturbation in the TF-QKD system according to the second interference signal.

[0126] In the detection process, the avalanche detector can be made to detect the first interference signal at a first dead time and the second interference signal at a second dead time with the help of a control unit, and the first dead time is greater than the second dead time. The control process of the dead time can be referred to above and will not be elaborated here.

[0127] In a further embodiment of the phase perturbation monitoring method of the present invention, a second phase reference optical signal can also be generated at the receiving end by using a phase reference light source, wherein the second phase reference optical signal has a second wavelength different from the first wavelength of the frequency-locked reference light.

[0128] Therefore, the frequency-locked reference light and the second phase reference optical signal are also wavelength-division coupled in a polarization-maintaining manner at the receiving end to form a first output light.

[0129] Correspondingly, the frequency-locked reference light and the second phase reference optical signal in the first output light are also demultiplexed in a polarization-maintaining manner at the transmitting end, and the quantum optical signal, the first phase reference optical signal and the second phase reference optical signal are wavelength-division coupled in a polarization-maintaining manner to form a second output light.

[0130] When two second output lights from different transmitters reach the receiver and interfere, the generated interference signal will contain a first wavelength component and a second wavelength component. Among them, the first wavelength component includes a first interference signal and a second interference signal, and the second wavelength component will include a third interference signal formed by the interference of the second phase reference optical signal. Therefore, it is necessary to demultiplex the first wavelength component and the second wavelength component in the interference signal at the receiver, and use an avalanche detector to detect the third interference signal to monitor the phase perturbation in the TF-QKD system according to the third interference signal. At this time, since the third interference signal can reflect the rapid phase perturbation existing in the system, the switching period between the quantum optical signal and the first phase reference optical signal in the second output light can be reasonably set to allow the slow phase perturbation existing in the system to be carried by the second interference signal.

[0131] In addition, the present invention also discloses a TF-QKD method, which allows the use of an avalanche detector to accurately and real-time monitor the phase perturbation in the system, thereby allowing the realization of the phase stability of the system.

[0132] In the TF-QKD method of the present invention, a first output light is sent from the receiver to the transmitter, and the first output light includes a frequency-locked reference light.

[0133] The transmitter can then use the frequency-locked reference light to generate an optical signal in a frequency-locked manner, generate a quantum optical signal or a first phase reference optical signal by providing different modulations to the optical signal at different times, and send it to the receiver in the form of a second output light. Therefore, the second output light contains a quantum optical signal and a first phase reference optical signal in a time-division multiplexing manner.

[0134] The transmitter then uses the frequency-locked reference light to generate an optical signal in a frequency-locked manner, generates a quantum optical signal or a first phase reference optical signal by providing different modulations to the optical signal at different times, and sends it to the receiver in the form of a second output light. Therefore, the second output light contains a quantum optical signal and a first phase reference optical signal in a time-division multiplexing manner.

[0135] In this process, it is preferably also possible to perform polarization compensation on the received first output light to make it have a preset polarization state.

[0136] After the receiver receives two second output lights from different transmitters, the two second output lights can be made to interfere to generate an interference signal. At this time, the interference signal correspondingly contains, in a time-division multiplexing manner, a first interference signal generated by the interference of the quantum optical signal, and a second interference signal generated by the interference of the first phase reference optical signal.

[0137] Therefore, an avalanche detector can be used to detect the interference signal, so as to monitor the phase perturbation in the TF-QKD system according to the second interference signal.

[0138] During the detection process, the avalanche detector can be made to detect the first interference signal at a first dead time and the second interference signal at a second dead time with the help of a control unit, where the first dead time is greater than the second dead time. The control process of the dead time can be referred to above and will not be elaborated here.

[0139] After obtaining the detection data of the interference signal, the receiving end can send the detection data of the first interference signal to the sending end, so that the sending end can perform data processing such as basis vector comparison, error correction, and privacy amplification in combination with the phase perturbation information. At the same time, corresponding phase compensation can also be provided according to the phase perturbation calculated from the second interference signal.

[0140] In a further embodiment of the TF-QKD method of the present invention, a second phase reference optical signal can also be generated at the receiving end using a phase reference light source, where the second phase reference optical signal has a second wavelength different from the first wavelength of the frequency-locked reference light.

[0141] Therefore, the frequency-locked reference light and the second phase reference optical signal are also wavelength-division coupled in a polarization-maintaining manner at the receiving end to form a first output light.

[0142] Correspondingly, the frequency-locked reference light and the second phase reference optical signal in the first output light are also demultiplexed in a polarization-maintaining manner at the sending end, and the quantum optical signal, the first phase reference optical signal, and the second phase reference optical signal are wavelength-division coupled in a polarization-maintaining manner to form a second output light.

[0143] When two second output lights from different sending ends reach the receiving end and interfere, the generated interference signal will contain a first wavelength component and a second wavelength component. Among them, the first wavelength component includes the first interference signal and the second interference signal, and the second wavelength component will include a third interference signal formed by the interference of the second phase reference optical signal. Therefore, it is necessary to demultiplex the first wavelength component and the second wavelength component in the interference signal at the receiving end, and use an avalanche detector to detect the third interference signal, so as to monitor the phase perturbation in the TF-QKD system according to the third interference signal. At this time, since the third interference signal can reflect the rapid phase perturbation existing in the system, the switching period between the quantum optical signal and the first phase reference optical signal in the second output light can be reasonably set to allow the slow phase perturbation existing in the system to be carried by the second interference signal.

[0144] Thus, it is possible to quickly compensate for the phase perturbations between the quantum optical signals in the second output light from different transmitters, and to slowly compensate for the phase perturbations between the quantum optical signals in the second output light and the second phase reference optical signal.

[0145] As a preferred method, the TF-QKD method can be implemented by means of the TF-QKD system of the present invention.

[0146] With the aid of the present invention, it is possible to use an avalanche detector in TF-QKD, and at the same time to meet the requirements of a high count rate of the phase reference optical signal and a high signal-to-noise ratio of the quantum optical signal. Compared with the TF-QKD using a superconducting detector in the prior art, it has better maturity, stability, ease of use, and the system is easy to miniaturize.

[0147] Although the present invention has been described above with reference to specific embodiments in conjunction with the accompanying drawings, it is easy for those skilled in the art to recognize that the above embodiments are merely exemplary and are used to illustrate the principle of the present invention, which will not limit the scope of the present invention. Those skilled in the art can make various combinations, modifications, and equivalent replacements of the above embodiments without departing from the spirit and scope of the present invention.

Claims

1. A method for monitoring phase perturbation in a TF-QKD system, comprising the following steps: Transmitting a second output light from a transmitting end to a receiving end, wherein by providing different modulations to an optical signal at different times, the second output light contains a quantum optical signal and a first phase reference optical signal in a time-division multiplexing manner; In the receiving end, causing the second output lights from different transmitting ends to interfere to generate an interference signal, wherein the interference signal contains a first interference signal generated by the interference of the quantum optical signal and a second interference signal generated by the interference of the first phase reference optical signal in a time-division multiplexing manner; and In the receiving end, using an avalanche detector to detect the interference signal to monitor the phase perturbation in the TF-QKD system according to the second interference signal, wherein the avalanche detector detects the first interference signal at a first dead time and detects the second interference signal at a second dead time, and the first dead time is greater than the second dead time.

2. The phase perturbation monitoring method according to claim 1, further comprising the following steps: In the receiving end, wavelength-division coupling a frequency-locked reference light with a first wavelength and a second phase reference optical signal with a second wavelength in a polarization-maintaining manner to form a first output light, wherein the first wavelength is different from the second wavelength; In the transmitting end, demultiplexing the frequency-locked reference light and the second phase reference optical signal in the first output light in a polarization-maintaining manner, generating the optical signal in a frequency-locked manner based on the frequency-locked reference light, and wavelength-division coupling the quantum optical signal, the first phase reference optical signal and the second phase reference optical signal in a polarization-maintaining manner to form the second output light; In the receiving end, demultiplexing the first wavelength component and the second wavelength component in the interference signal, and using an avalanche detector to detect a third interference signal generated by the interference of the second phase reference optical signal to monitor the phase perturbation in the TF-QKD system according to the third interference signal.

3. A phase-stable TF-QKD system, comprising a receiving end and a plurality of transmitting ends; The receiving end includes a measurement module, a frequency-locked reference light source and a first beam splitter; The frequency-locked reference light source is configured to generate a frequency-locked reference light; The first beam splitter is configured to split the first output light of the receiving end, wherein The first output light includes the frequency-locked reference light; The transmitting end includes a frequency-locked light source, a reference light modulation module and a quantum state preparation module; The frequency-locked light source is configured to generate an optical signal in a frequency-locked manner based on the frequency-locked reference light in the first output light; The quantum state preparation module is configured to prepare a quantum state on the optical signal; The reference light modulation module is configured to modulate the optical signal so that the second output light of the transmitting end contains a quantum optical signal and a first phase reference optical signal in a time-division multiplexing manner; The measurement module includes a polarization-maintaining second beam splitter, an avalanche detector and a control unit; The second beam splitter is configured to allow the second output lights from different transmitters to interfere to generate an interference signal, which includes, in a time-division multiplexing manner, a first interference signal generated by the interference of the quantum optical signal and a second interference signal generated by the interference of the first phase reference optical signal; The avalanche detector is configured to detect the interference signal; The control unit is configured to make the avalanche detector have a first dead time when detecting the first interference signal and a second dead time when detecting the second interference signal, and the first dead time is greater than the second dead time.

4. The TF-QKD system according to claim 3, wherein, The control unit is configured to: control the dead time by controlling the avalanche gating signal for the avalanche detector; or control the dead time by setting the valid time interval of the detection data of the avalanche detector.

5. The TF-QKD system according to claim 3, wherein, The transmitter further includes a continuous light chopping unit; and / or, the quantum state preparation module includes a decoy state modulation unit, a phase modulation unit, and an optical attenuation unit.

6. The TF-QKD system according to claim 3, wherein, The transmitter further includes a polarization compensation module for compensating the polarization state of the first output light so that it has a preset polarization state; and / or, the receiver further includes a polarization compensation module for compensating the polarization state of the second output light so that it has a preset polarization state.

7. The TF-QKD system according to claim 3, wherein, The receiver further includes a fast phase compensation module for compensating the phase perturbation between the quantum optical signals in the second output lights from different transmitters.

8. The TF-QKD system according to claim 3, wherein, The frequency-locked light source generates the optical signal by using the frequency-locked reference light based on injection locking, an optical phase-locked loop, or time-frequency transmission.

9. The TF-QKD system according to claim 3, wherein, The number of the transmitters is N, where N is a positive integer greater than 1, and the receiver is further provided with an optical switch supporting 2:N ports for allowing the selection of the accessed transmitters.

10. The TF-QKD system according to any one of claims 3-9, wherein: The receiver further includes a phase reference light source and a first wavelength division multiplexer; The phase reference light source is configured to generate a second phase reference optical signal having a second wavelength, and the frequency-locked reference light has a first wavelength, and the first wavelength is different from the second wavelength; The first wavelength division multiplexer is configured to perform wavelength division coupling of the frequency-locked reference light and the second phase reference optical signal in a polarization-maintaining manner so that the first output light includes the combined beam of the frequency-locked reference light and the second phase reference optical signal; The transmitter further includes a polarization consistency coupling module, which is configured to demultiplex the frequency-locked reference light and the second phase reference optical signal in the first output light, output the frequency-locked reference light to the frequency-locked light source in a polarization-maintaining manner, and perform wavelength division coupling of the quantum optical signal, the first phase reference optical signal, and the second phase reference optical signal in a polarization-maintaining manner so that the second output light includes the combined beam of the quantum optical signal, the first phase reference optical signal, and the second phase reference optical signal; The measurement module further includes a wavelength division multiplexer, which is disposed before the avalanche detector and is configured to demultiplex the first wavelength component and the second wavelength component in the interference signal so as to be detected by different avalanche detectors respectively.

11. The TF-QKD system according to claim 10, wherein, 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 the second phase reference light signal in the first output light, and output the frequency-locked reference light and the second phase reference light signal to the frequency-locked light source and the other of the two polarization-maintaining wavelength division multiplexers in a polarization-maintaining manner respectively; The other of the two polarization-maintaining wavelength division multiplexers is configured to perform wavelength division coupling on the quantum optical signal, the first phase reference light signal and the second phase reference light signal in a polarization-maintaining manner.

12. The TF-QKD system according to claim 10, wherein, The receiving end further includes a slow phase compensation module for compensating the phase perturbation between the quantum optical signal and the second phase reference light signal in the second output light.

13. A TF-QKD method, which includes the following steps: The receiving end sends a first output light to the sending end, and the first output light includes a frequency-locked reference light; In the sending end, an optical signal is generated in a frequency-locked manner based on the frequency-locked reference light, and modulation is performed on the optical signal to generate a second output light, and the second output light includes a quantum optical signal and a first phase reference light signal in a time-division multiplexing manner; In the receiving end, the second output lights from different sending ends are made to interfere to generate an interference signal, wherein the interference signal includes a first interference signal generated by the interference of the quantum optical signal and a second interference signal generated by the interference of the first phase reference light signal in a time-division multiplexing manner; In the receiving end, an avalanche detector is used to detect the interference signal, wherein the avalanche detector detects the first interference signal at a first dead time and detects the second interference signal at a second dead time, and the first dead time is greater than the second dead time; In the receiving end, the phase perturbation in the TF-QKD system is calculated according to the detection data of the second interference signal, and phase compensation is provided according to the phase perturbation; and the detection data of the first interference signal is sent to the sending end.

14. The TF-QKD method according to claim 13, which further includes the following steps: In the receiving end, the frequency-locked reference light and a second phase reference light signal with a second wavelength are wavelength division coupled in a polarization-maintaining manner to form the first output light, wherein the frequency-locked reference light has a first wavelength, and the first wavelength is different from the second wavelength; In the sending end, the frequency-locked reference light and the second phase reference light signal in the first output light are demultiplexed in a polarization-maintaining manner, and the quantum optical signal, the first phase reference light signal and the second phase reference light signal are wavelength division coupled in a polarization-maintaining manner to form the second output light; and In the receiving end, the first wavelength component and the second wavelength component in the interference signal are demultiplexed to be detected by different avalanche detectors respectively.

15. The TF-QKD method according to claim 13, which further includes the following steps: Polarization compensation is performed on the first output light at the sending end and the output is polarization-maintained; And / or At the receiving end, perform polarization compensation on the second output light and output it while maintaining polarization.

16. The TF-QKD method according to claim 14, further comprising the following steps: At the receiving end, quickly compensate for the phase perturbation between the quantum optical signals in the second output light from different sending ends; and / or, At the receiving end, slowly compensate for the phase perturbation between the quantum optical signal in the second output light and the second phase reference optical signal.

17. The TF-QKD method according to any one of claims 13-16 is implemented by means of the TF-QKD system according to any one of claims 3-12.

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