A Method for Implementing a TF-QKD System and Frequency Synchronization

Abstract

The present invention proposes a method for realizing a TF-QKD system and frequency synchronization. A first beam splitter splits the optical signal emitted by a first signal light source and transmits the first split optical signal to the transmitting end of the synchronization system. The transmitting end of the synchronization system generates a correction optical signal and transmits it to the receiving end of the synchronization system. A second beam splitter transmits the third split optical signal corresponding to a second signal light source to the receiving end of the synchronization system. The receiving end of the synchronization system generates a control signal according to the correction optical signal and the third split optical signal, and transmits the control signal to the second signal light source to control its frequency adjustment so that the second signal light source is frequency synchronized with the first signal light source. Without the need for an externally provided frequency-locked reference light, directly using the frequency of the optical signal emitted by the first signal light source in the first terminal as a reference to control the second signal light source, enabling its frequency to be synchronized with the signal light source at the transmitting end, saving costs, simplifying the process, and improving the efficiency of realizing frequency synchronization.

Description

Technical Field

[0001] The present invention relates to the field of quantum, and more particularly, to a method for implementing a TF-QKD system and frequency synchronization. Background Art

[0002] Quantum key distribution (QKD) is based on the fundamental principles of quantum mechanics and provides a secure key sharing mechanism at the information-theoretic level for both communicating parties. Combined with the one-time pad information encryption method, unconditional secure communication can be achieved. In terms of signal transmission, both QKD and classical optical fiber communication face the problem that the signal strength decays exponentially with increasing distance. Classical optical communication can deploy optical amplifiers to enhance the transmitted signal and thus build a long-distance optical fiber communication network. However, for QKD, due to the limitation of the quantum no-cloning theorem, it is impossible to extend the transmission distance of quantum signals through simple optical amplification, otherwise additional bit error rates will be generated, directly threatening the legitimacy and security of the communication services of authorized users. Therefore, channel loss has become the core factor restricting long-distance quantum key distribution.

[0003] Compared with the classical BB84 protocol, Twin-Field Quantum Key Distribution (TF-QKD) can improve the linear relationship between the key rate and the channel transmittance to a more efficient square root correlation, thus greatly increasing the secure transmission distance of QKD without relays. With the gradual maturity of TF-QKD technology, applying the TF-QKD protocol in the existing network can also significantly reduce the number of trusted relay nodes required in the long-distance quantum communication network, improve the overall security of long-haul trunk QKD, and effectively reduce its security cost. However, the implementation conditions of TF-QKD technology are quite strict. It relies on the interference of single photons from independent lasers of both communicating parties. Tiny deviations in the light source frequency and fluctuations in the fiber optic link will be amplified during long-term and long-distance transmission, and the accumulated phase noise will greatly reduce the quality of single-photon interference and affect the secure coding rate. Therefore, the frequency synchronization module plays a crucial role in ensuring the precise synchronization of the frequencies of the lasers of both communicating parties and is an important component of the implementation of TF-QKD technology. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for implementing a TF-QKD system and frequency synchronization to improve the above problems.

[0005] To achieve the above purpose, the technical solutions adopted in the embodiments of the present invention are as follows:

[0006] In a first aspect, an embodiment of the present invention provides a TF-QKD system, and the TF-QKD system includes a first terminal and a second terminal;

[0007] The first terminal includes a first signal light source, a first beam splitter, and a transmitting end of a synchronization system;

[0008] The second terminal includes a second signal light source, a second beam splitter, and a receiving end of the synchronization system;

[0009] The first beam splitter is used to split the optical signal emitted by the first signal light source and transmit the first split optical signal to the transmitting end of the synchronization system;

[0010] The transmitting end of the synchronization system is used to generate a correction optical signal according to the first split optical signal and transmit the correction optical signal to the receiving end of the synchronization system;

[0011] The second beam splitter is used to split the optical signal emitted by the second signal light source and transmit the third split optical signal to the receiving end of the synchronization system;

[0012] The receiving end of the synchronization system is used to generate a control signal according to the correction optical signal and the third split optical signal, and transmit the control signal to the second signal light source to control its frequency adjustment so that the second signal light source is frequency synchronized with the first signal light source.

[0013] Optionally, the TF-QKD system further includes a third terminal; the first terminal further includes a first quantum state modulation module; the second terminal further includes a second quantum state modulation module;

[0014] When the second signal light source is frequency synchronized with the first signal light source, the first beam splitter is further used to transmit the second split optical signal to the first quantum state modulation module;

[0015] The first quantum state modulation module is used to modulate the second split optical signal and send the obtained first modulation signal to the third terminal;

[0016] When the second signal light source is frequency synchronized with the first signal light source, the second beam splitter is further used to transmit the fourth split optical signal to the second quantum state modulation module;

[0017] The second quantum state modulation module is used to modulate the fourth split optical signal and send the obtained second modulation signal to the third terminal;

[0018] The third terminal is used to determine the interference result between the first signal light source and the second signal light source according to the first modulation signal and the second modulation signal.

[0019] Optionally, the transmitting end of the synchronization system includes: a radio frequency signal generator, a frequency divider, a first intensity modulator, a first auxiliary light source, a downconverter, a first intensity detector, a second intensity modulator, and a first wavelength division multiplexer;

[0020] The output end of the radio frequency signal generator is respectively connected to the input end of the frequency divider and the first input end of the downconverter;

[0021] The output end of the frequency divider is connected to the loading end of the first intensity modulator, and the input end of the first intensity modulator is connected to the first beam splitter;

[0022] The second input end of the downconverter is connected to the output end of the first intensity detector; the output end of the downconverter is connected to the loading end of the second intensity modulator, and the input end of the second intensity modulator is connected to the first auxiliary light source;

[0023] The output end of the first intensity modulator, the input end of the first intensity detector, and the output end of the second intensity modulator are all connected to the first wavelength division multiplexer.

[0024] Optionally, the radio frequency signal generator is used to generate a radio frequency signal , and transmit the radio frequency signal to the frequency divider and the downconverter;

[0025] The first intensity detector is used to detect the synchronization signal light collected by the first wavelength division multiplexer, and transmit the detected radio frequency signal to the downconverter, where the synchronization signal light comes from the second terminal;

[0026] The downconverter is used to perform downconversion processing on the radio frequency signal using the radio frequency signal , and transmit the obtained radio frequency signal to the second intensity modulator;

[0027] The second intensity modulator is used to modulate the radio frequency signal onto the first auxiliary optical signal emitted by the first auxiliary light source, and transmit the modulated first auxiliary optical signal to the first wavelength division multiplexer, so as to transmit it to the second terminal;

[0028] The frequency divider is used to perform frequency division by two on the radio frequency signal , and transmit the obtained radio frequency signal to the first intensity modulator;

[0029] The first intensity modulator is used to modulate the radio frequency signal Modulate on the first split optical signal, and transmit the corrected optical signal obtained by modulation to the first wavelength division multiplexer, so as to transmit it to the second terminal.

[0030] Optionally, the down-converter includes a first band-pass filter, a mixer, and a second band-pass filter;

[0031] The first input end of the mixer is connected to the output end of the radio frequency signal generator, the second input end of the mixer is connected to the output end of the first band-pass filter, and the input end of the first band-pass filter is connected to the output end of the first intensity detector;

[0032] The output end of the mixer is connected to the input end of the second band-pass filter, and the output end of the second band-pass filter is connected to the loading end of the second intensity modulator.

[0033] Optionally, the receiving end of the synchronization system includes a second wavelength division multiplexer, a second intensity detector, a third band-pass filter, a third intensity modulator, a fourth intensity modulator, a second auxiliary light source, a coupler, and a feedback module;

[0034] The second wavelength division multiplexer is respectively connected to the input end of the second intensity detector, the output end of the third intensity modulator, and the first input end of the coupler;

[0035] The input end of the third intensity modulator is connected to the output end of the second auxiliary light source;

[0036] The output end of the second intensity detector is connected to the input end of the third band-pass filter, and the output end of the third band-pass filter is respectively connected to the loading end of the third intensity modulator and the loading end of the fourth intensity modulator;

[0037] The input end of the fourth intensity modulator is connected to the second beam splitter, the output end of the fourth intensity modulator is connected to the second input end of the coupler, the output end of the coupler is connected to the feedback module, and the output end of the feedback module is connected to the second signal light source.

[0038] Optionally, the second intensity detector is used to detect the radio frequency signal with a wavelength of collected by the second wavelength division multiplexer, and transmit the detected radio frequency signal to the third band-pass filter, where the wavelength of the first auxiliary optical signal is ;

[0039] The third band-pass filter is used to filter the radio frequency signal and transmit the filtered radio frequency signal Transmitted to the third intensity modulator and the fourth intensity modulator;

[0040] The third intensity modulator is used to modulate the radio frequency signal onto the second auxiliary optical signal emitted by the second auxiliary light source, and transmit the modulated synchronous signal light to the second wavelength division multiplexer, so as to transmit it to the first terminal;

[0041] The fourth intensity modulator is used to modulate the radio frequency signal onto the third split optical signal, and transmit the modulated third split optical signal to the coupler;

[0042] The coupler is used to couple the corrected optical signal collected by the second wavelength division multiplexer and the modulated third split optical signal, and transmit the obtained coupled optical signal to the feedback module;

[0043] The feedback module is used to generate a control signal according to the coupled optical signal, and transmit the control signal to the second signal light source.

[0044] Optionally, the feedback module includes a large-bandwidth intensity detector, a power amplifier, a fourth low-pass filter, and a processor connected in sequence;

[0045] The input end of the large-bandwidth intensity detector is connected to the output end of the coupler, and the output end of the processor is connected to the control end of the second signal light source;

[0046] The large-bandwidth intensity detector is used to detect the coupled optical signal, and transmit the obtained stray radio frequency signal to the power amplifier;

[0047] The power amplifier is used to amplify the received radio frequency signal, and transmit the amplified radio frequency signal to the fourth low-pass filter;

[0048] The fourth low-pass filter is used to perform low-pass filtering on the received radio frequency signal, and transmit the obtained low-frequency stray signal to the processor;

[0049] The processor is used to generate a control signal according to the low-frequency stray signal, and transmit the control signal to the second signal light source.

[0050] Optionally, both the first quantum state modulation module and the second quantum state modulation module include a phase modulator, an amplitude modulator, and an optical attenuator connected in sequence.

[0051] In a second aspect, an embodiment of the present invention provides a frequency synchronization implementation method, which is applied to the above TF-QKD system. The method includes:

[0052] The first beam splitter splits the optical signal emitted by the first signal light source and transmits the first split optical signal to the transmitting end of the synchronization system;

[0053] The transmitting end of the synchronization system generates a correction optical signal according to the first split optical signal and transmits the correction optical signal to the receiving end of the synchronization system;

[0054] The second beam splitter splits the optical signal emitted by the second signal light source and transmits the third split optical signal to the receiving end of the synchronization system;

[0055] The receiving end of the synchronization system generates a control signal according to the correction optical signal and the third split optical signal, and transmits the control signal to the second signal light source to control its frequency adjustment so that the second signal light source is frequency synchronized with the first signal light source.

[0056] Compared with the prior art, in a TF-QKD system and a frequency synchronization implementation method provided by an embodiment of the present invention, the first beam splitter splits the optical signal emitted by the first signal light source and transmits the first split optical signal to the transmitting end of the synchronization system. The transmitting end of the synchronization system generates a correction optical signal and transmits it to the receiving end of the synchronization system. The second beam splitter transmits the third split optical signal corresponding to the second signal light source to the receiving end of the synchronization system. The receiving end of the synchronization system generates a control signal according to the correction optical signal and the third split optical signal, and transmits the control signal to the second signal light source to control its frequency adjustment so that the second signal light source is frequency synchronized with the first signal light source. There is no need for an externally provided frequency-locked reference light. Directly using the frequency of the optical signal emitted by the first signal light source in the first terminal as a reference to control the second signal light source to keep its frequency synchronized with the signal light source at the transmitting end saves costs, simplifies the process, and improves the implementation efficiency of frequency synchronization.

[0057] In order to make the above objects, features, and advantages of the present invention more obvious and understandable, the following specifically enumerates preferred embodiments and, in conjunction with the accompanying drawings, makes the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for use in the embodiments. It should be understood that the following drawings only show some embodiments of the present invention, and therefore should not be regarded as limiting the scope. For those of ordinary skill in the art, without creative efforts, other related drawings can also be obtained based on these drawings.

[0059] Figure 1 It is a schematic structural diagram of the TF-QKD system provided by the embodiment of the present invention.

[0060] Figure 2Schematic diagram of the transmitting end of the synchronization system provided by the embodiment of the present invention.

[0061] Figure 3 Schematic diagram of the receiving end of the synchronization system provided by the embodiment of the present invention. Detailed implementation manners

[0062] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some but not all of the embodiments of the present invention. Components of the embodiments of the present invention generally described and illustrated in the accompanying drawings here can be arranged and designed in a variety of different configurations.

[0063] Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed present invention, but merely represents selected embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

[0064] It should be noted that like reference numerals and letters denote like items in the following drawings. Therefore, once an item is defined in one drawing, it does not require further definition and explanation in subsequent drawings. At the same time, in the description of the present invention, terms such as "first", "second", etc. are only used for differential description and cannot be construed as indicating or implying relative importance.

[0065] It should be noted that, in this document, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or sequence between these entities or operations. Moreover, the term "comprising", "including" or any other variation thereof is intended to cover a non-exclusive inclusion, such that a process, method, article or device comprising a series of elements includes not only those elements, but also other elements not expressly listed, or elements inherent to such process, method, article or device. Without further limitation, an element defined by the phrase "comprising a..." does not exclude the presence of additional identical elements in the process, method, article or device comprising the element.

[0066] In the description of the present invention, it should be noted that the orientation or positional relationship indicated by the terms "upper", "lower", "inner", "outer", etc. is based on the orientation or positional relationship shown in the drawings, or the orientation or positional relationship in which the inventive product is customarily placed during use. It is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on the present invention.

[0067] In the description of the present invention, it should also be noted that unless otherwise clearly specified and defined, the terms "arrangement" and "connection" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, and it can be the communication inside two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0068] The following will describe in detail some embodiments of the present invention with reference to the drawings. Without conflict, the following embodiments and the features in the embodiments can be combined with each other.

[0069] To solve the defects such as complexity and high cost existing in the frequency synchronization in the TF-QKD system, the embodiments of the present invention provide a method for realizing a TF-QKD system and frequency synchronization, which ensures to reduce the system cost under the condition of realizing the frequency synchronization of the TF-QKD communication transceiver with high precision, and provides a technical basis for the large-scale application of TF-QKD in the future industry.

[0070] Please refer to Figure 1 , Figure 1 , which is a schematic structural diagram of the TF-QKD system provided by the embodiments of the present invention. The TF-QKD system includes a first terminal and a second terminal, and the first terminal and the second terminal are fiber-optically connected.

[0071] The first terminal includes a first signal light source, a first beam splitter, and a transmitting end of the synchronization system. Among them, the output end of the first signal light source is connected to the input end of the first beam splitter, and the first output end of the first beam splitter is connected to the input end of the transmitting end of the synchronization system.

[0072] The second terminal includes a second signal light source, a second beam splitter, and a receiving end of the synchronization system. Among them, the receiving end of the synchronization system is connected to the transmitting end of the synchronization system through an optical fiber, the output end of the receiving end of the synchronization system is connected to the control end of the second signal light source, the output end of the second signal light source is connected to the input end of the second beam splitter, and the first output end of the second beam splitter is connected to the input end of the receiving end of the synchronization system.

[0073] The first beam splitter is used to split the optical signal emitted by the first signal light source and transmit the first split optical signal to the transmitting end of the synchronization system.

[0074] The transmitting end of the synchronization system is used to generate a correction optical signal according to the first split optical signal and transmit the correction optical signal to the receiving end of the synchronization system.

[0075] The second beam splitter is used to split the optical signal emitted by the second signal light source and transmit the third split optical signal to the receiving end of the synchronization system.

[0076] The receiving end of the synchronization system is used to generate a control signal according to the correction optical signal and the third split optical signal, and transmit the control signal to the second signal light source to control its frequency adjustment so that the second signal light source is frequency synchronized with the first signal light source.

[0077] In the TF-QKD system of the embodiment of the present invention, there is no need for an externally provided frequency-locked reference light. The frequency of the optical signal emitted by the first signal light source in the first terminal is directly used as a reference, and it is transmitted to the second terminal through the transmitting end of the synchronization system; in the second terminal, it is received by the receiving end of the synchronization system, and a feedback control signal is generated through the internal operation of the second terminal and transmitted to the second signal light source in the second terminal for negative feedback control to keep its frequency synchronized with the signal light source at the transmitting end. Innovatively, the frequency-locked reference light generation device is omitted, and the synchronization system is directly integrated between the first terminal and the second terminal, saving costs, simplifying the process, and improving the implementation efficiency of frequency synchronization.

[0078] Please continue to refer to Figure 1 , in an optional embodiment, the TF-QKD system further includes a third terminal, and the first terminal, the second terminal, and the third terminal are respectively fiber-optically communicatively connected to each other.

[0079] The first terminal further includes a first quantum state modulation module. The second output end of the first beam splitter is connected to the input end of the first quantum state modulation module, and the output end of the first quantum state modulation module is connected to the third terminal.

[0080] The second terminal further includes a second quantum state modulation module. The second output end of the second beam splitter is connected to the input end of the second quantum state modulation module, and the output end of the second quantum state modulation module is connected to the third terminal.

[0081] When the second signal light source is frequency synchronized with the first signal light source, the first beam splitter is further used to transmit the second split optical signal (corresponding to the first split optical signal) to the first quantum state modulation module.

[0082] The first quantum state modulation module is used to modulate the second split optical signal and send the obtained first modulation signal to the third terminal.

[0083] When the second signal light source and the first signal light source achieve frequency synchronization, the second beam splitter is further configured to split the optical signal emitted by the second signal light source, and transmit the fourth split optical signal (corresponding to the third split optical signal) to the second quantum state modulation module.

[0084] The second quantum state modulation module is configured to modulate the fourth split optical signal, and send the obtained second modulation signal to the third terminal;

[0085] The third terminal is configured to determine the interference result between the first signal light source and the second signal light source according to the first modulation signal and the second modulation signal (when the interference result is correct, it indicates that the two have completed quantum entanglement and quantum key negotiation can be performed).

[0086] When both the frequency and the phase are synchronized, if the interference result is correct, the first terminal and the second terminal can perform quantum key negotiation.

[0087] Certainly, when the second signal light source and the first signal light source do not achieve frequency synchronization, the first beam splitter is further configured to transmit the second split optical signal to the first quantum state modulation module, and the second beam splitter can also transmit the fourth split optical signal to the second quantum state modulation module. However, at this time, the third terminal will not obtain the interference result.

[0088] In the transmitting end of the synchronization system and the receiving end of the synchronization system in the embodiments of the present invention, an optical carrier radio frequency structure is adopted instead of a traditional optical comb and a frequency locking reference system, effectively reducing the system complexity and cost, and providing technical support for the large-scale industrial deployment of TF-QKD devices. On the Figure 1 basis, regarding the specific structure of the transmitting end of the synchronization system, the embodiments of the present invention further provide an optional implementation manner. Please refer to Figure 2 , Figure 2 which is the schematic structural diagram of the transmitting end of the synchronization system provided by the embodiments of the present invention.

[0089] The transmitting end of the synchronization system includes: a radio frequency signal generator, a frequency divider, a first intensity modulator, a first auxiliary light source, a downconverter, a first intensity detector, a second intensity modulator, and a first wavelength division multiplexer, where the wavelength division multiplexer is also called WDM.

[0090] The output end of the radio frequency signal generator is respectively connected to the input end of the frequency divider and the first input end of the downconverter.

[0091] The output end of the frequency divider is connected to the loading end of the first intensity modulator, and the input end of the first intensity modulator is connected to the first output end of the first beam splitter (not shown in the figure).

[0092] The second input terminal of the downconverter is connected to the output terminal of the first intensity detector; the output terminal of the downconverter is connected to the loading terminal of the second intensity modulator, and the input terminal of the second intensity modulator is connected to the first auxiliary light source.

[0093] The output terminal of the first intensity modulator, the input terminal of the first intensity detector, and the output terminal of the second intensity modulator are all connected to the first wavelength division multiplexer.

[0094] The radio frequency signal generator is used to generate a radio frequency signal , and transmit the radio frequency signal to the frequency divider and the downconverter.

[0095] Optionally, the frequency and phase information of the radio frequency signal is expressed as follows:

[0096]

[0097] wherein, represents the frequency of the radio frequency signal , represents the phase of the radio frequency signal .

[0098] The first intensity detector is used to detect the synchronous signal light (wavelength is ) collected by the first wavelength division multiplexer, and transmit the detected radio frequency signal to the downconverter, wherein the synchronous signal light comes from the second terminal.

[0099] Optionally, the frequency and phase information of the radio frequency signal is expressed as follows:

[0100]

[0101] wherein, represents the frequency of the radio frequency signal , represents the phase of the radio frequency signal .

[0102] The downconverter is used to perform downconversion processing on the radio frequency signal using the radio frequency signal , and transmit the obtained radio frequency signal to the second intensity modulator.

[0103] Optionally, the radio frequency signal and the radio frequency signal undergo mixing and band-pass filtering to achieve downconversion, and the obtained radio frequency signal . Preset , so the obtained radio frequency signal Expressed as:

[0104]

[0105] Wherein, represents the frequency of the radio frequency signal and represents the phase of the radio frequency signal and represents the frequency of the radio frequency signal and represents the phase of the radio frequency signal and

[0106] The second intensity modulator is used to (by means of intensity modulation) modulate the radio frequency signal onto the first auxiliary optical signal (continuous light with a wavelength of ) emitted by the first auxiliary light source, and transmit the modulated first auxiliary optical signal to the first wavelength division multiplexer, so as to transmit it to the second terminal (the receiving end of the medium synchronization system).

[0107] The frequency divider is used to perform frequency division by two on the radio frequency signal and transmit the obtained radio frequency signal to the first intensity modulator.

[0108] Optionally, the radio frequency signal obtained after frequency division by two has an expression of:

[0109]

[0110] Wherein, represents the frequency of the radio frequency signal and represents the phase of the radio frequency signal and

[0111] The first intensity modulator is used to (by means of intensity modulation) modulate the radio frequency signal onto the first split optical signal (with a wavelength of ) and transmit the corrected optical signal obtained by modulation to the first wavelength division multiplexer, so as to transmit it to the second terminal (the receiving end of the medium synchronization system).

[0112] Please continue to refer to Figure 2 , in an optional implementation manner, the down-converter includes a first band-pass filter, a mixer, and a second band-pass filter.

[0113] The first input end of the mixer is connected to the output end of the radio frequency signal generator, the second input end of the mixer is connected to the output end of the first band-pass filter, and the input end of the first band-pass filter is connected to the output end of the first intensity detector.

[0114] The output end of the mixer is connected to the input end of the second band-pass filter, and the output end of the second band-pass filter is connected to the loading end of the second intensity modulator.

[0115] The first band-pass filter is used to filter out the noise of the radio frequency signal and transmit the filtered radio frequency signal to the mixer.

[0116] The mixer is used to mix the radio frequency signal and the radio frequency signal, and transmit the mixed signal to the second band-pass filter.

[0117] The second band-pass filter is used to perform lower sideband filtering on the mixed signal (filter out the upper sideband signal therein) to obtain and output the radio frequency signal .

[0118] Based on Figure 1 , regarding the specific structure of the receiving end of the synchronization system, the embodiment of the present invention further provides an optional implementation manner. Please refer to Figure 3 , Figure 3 , which is the schematic structural diagram of the receiving end of the synchronization system provided by the embodiment of the present invention.

[0119] The receiving end of the synchronization system includes a second wavelength division multiplexer, a second intensity detector, a third band-pass filter, a third intensity modulator, a fourth intensity modulator, a second auxiliary light source, a coupler, and a feedback module.

[0120] The second wavelength division multiplexer is respectively connected to the input end of the second intensity detector, the output end of the third intensity modulator, and the first input end of the coupler.

[0121] The input end of the third intensity modulator is connected to the output end of the second auxiliary light source.

[0122] The output end of the second intensity detector is connected to the input end of the third band-pass filter, and the output end of the third band-pass filter is respectively connected to the loading end of the third intensity modulator and the loading end of the fourth intensity modulator.

[0123] The input end of the fourth intensity modulator is connected to the first output end of the second beam splitter (not shown in the figure), the output end of the fourth intensity modulator is connected to the second input end of the coupler, the output end of the coupler is connected to the feedback module, and the output end of the feedback module is connected to the control end of the second signal light source (not shown in the figure).

[0124] The second intensity detector is used to detect the radio frequency signal with a wavelength of collected by the second wavelength division multiplexer, and the detected radio frequency signal Transmitted to the third band-pass filter, where the wavelength of the first auxiliary optical signal is .

[0125] Optionally, the radio frequency signal has the expression:

[0126]

[0127] where represents the frequency of the radio frequency signal , represents the phase of the radio frequency signal , represents the frequency of the radio frequency signal , represents the phase of the radio frequency signal , is the phase error accumulated during the transmission of the radio frequency signal from the first terminal to the second terminal.

[0128] The third band-pass filter is used to filter the radio frequency signal and transmit the filtered radio frequency signal to the third intensity modulator and the fourth intensity modulator.

[0129] The third intensity modulator is used to (by means of intensity modulation) modulate the radio frequency signal onto the second auxiliary optical signal (continuous light with a wavelength of ) emitted by the second auxiliary light source, and transmit the modulated synchronous signal light (with a wavelength of ) to the second wavelength division multiplexer, thereby transmitting it to the first terminal (the transmitting end of the synchronization system).

[0130] At the transmitting end of the synchronization system, the synchronous signal light passes through the first intensity detector to obtain the radio frequency signal . At this time, the signal will be additionally added with the phase error accumulated during the transmission from the second terminal to the first terminal. Therefore, the expression form of the signal is:

[0131]

[0132]

[0133] and are different expression forms of the same signal. Therefore, the two are equivalent and have the following relationship:

[0134]

[0135] The above relationship can be expressed as:

[0136]

[0137] Substitute the above relationship into the expression of the radio frequency signal for simplification, and the simplification result is as follows:

[0138]

[0139]

[0140] After simplification, the expression of the radio frequency signal is the same as the expression of the radio frequency signal , indicating that the radio frequency signal is in the same frequency and in phase with the radio frequency signal .

[0141] The fourth intensity modulator is used to modulate the radio frequency signal onto the third split optical signal (by means of intensity modulation), and transmit the modulated third split optical signal to the coupler.

[0142] The coupler is used to couple the corrected optical signal (with a wavelength of ) collected by the second wavelength division multiplexer and the modulated third split optical signal, and transmit the obtained coupled optical signal to the feedback module.

[0143] The feedback module is used to generate a control signal according to the coupled optical signal and transmit the control signal to the second signal light source.

[0144] Optionally, the feedback module includes a large bandwidth intensity detector, a power amplifier, a fourth low-pass filter, and a processor connected in sequence.

[0145] Among them, the large bandwidth intensity detector is an intensity detector supporting GHz signal detection. The input end of the large bandwidth intensity detector is connected to the output end of the coupler, and the output end of the processor is connected to the control end of the second signal light source.

[0146] The large bandwidth intensity detector is used to detect the coupled optical signal and transmit the obtained spurious radio frequency signal to the power amplifier. Among them, the spurious radio frequency signal originates from light sources with different frequencies.

[0147] The power amplifier is used to amplify the received radio frequency signal and transmit the amplified radio frequency signal to the fourth low-pass filter.

[0148] The fourth low-pass filter is used to perform low-pass filtering on the received radio frequency signal and transmit the obtained low-frequency spurious signal to the processor.

[0149] The processor is used to generate a control signal according to the low-frequency spurious signal (in combination with the negative feedback regulation algorithm). And transmit the control signal to the second signal light source for feedback control, so that the second signal light source is frequency-synchronized with the first signal light source.

[0150] In the embodiment of the present invention, the frequency synchronization control of the second signal light source and the first signal light source is obtained through the comprehensive operation of the radio frequency signal and the spurious signal. Specifically, the frequency of the first signal light source is denoted as and the frequency of the second signal light source is denoted as At the receiving end of the synchronization system, after large-bandwidth intensity detection, not only can the radio frequency signal with a frequency of be obtained, but also the radio frequency spurious signal caused by the non-synchronization of the light sources can be obtained, and its main spectral components include , , . As the frequency difference between the second signal light source and the first signal light source gradually decreases, the first item in the spurious signal gradually decreases to 0, and the latter two items gradually approach the synchronized radio frequency signal. Therefore, by collecting the power value of the signal and the power value of the spurious signal, and using their ratio as the feedback amount of the feedback control, the frequency synchronization of the second signal light source and the first signal light source can be achieved, ensuring that the frequencies of the two light sources are locked and precise frequency synchronization is achieved.

[0151] Please continue to refer to Figure 1 In an optional implementation manner, both the first quantum state modulation module and the second quantum state modulation module include a phase modulator, an amplitude modulator, and an optical attenuator connected in sequence.

[0152] Optionally, the processor may be an integrated circuit chip with signal processing capabilities. The above-mentioned processor may be a general-purpose processor, including a central processing unit (CPU for short), a network processor (NP for short), etc.; it may also be a digital signal processor (DSP for short), an application specific integrated circuit (ASIC for short), a field-programmable gate array (FPGA for short), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components.

[0153] An embodiment of the present invention also provides a method for realizing frequency synchronization, which can be but is not limited to being applied to the above-mentioned TF-QKD system. The method for realizing frequency synchronization includes: S101, S102, S103, and S104, which are specifically described as follows.

[0154] S101, a first beam splitter splits the optical signal emitted by the first signal light source and transmits the first split optical signal to the transmitting end of the synchronization system.

[0155] S102, the transmitting end of the synchronization system generates a corrected optical signal according to the first split optical signal and transmits the corrected optical signal to the receiving end of the synchronization system.

[0156] S103, a second beam splitter splits the optical signal emitted by the second signal light source and transmits the third split optical signal to the receiving end of the synchronization system.

[0157] S104, the receiving end of the synchronization system generates a control signal according to the corrected optical signal and the third split optical signal, and transmits the control signal to the second signal light source to control its frequency adjustment so that the second signal light source is frequency synchronized with the first signal light source.

[0158] It should be noted that the method for realizing frequency synchronization provided in this embodiment can perform the functions and uses shown in the above system embodiment to achieve the corresponding technical effects. For the sake of brief description, for the parts not mentioned in this embodiment, reference can be made to the corresponding content in the above embodiment.

[0159] In summary, for a TF-QKD system and a method for realizing frequency synchronization provided by an embodiment of the present invention, a first beam splitter splits the optical signal emitted by the first signal light source and transmits the first split optical signal to the transmitting end of the synchronization system. The transmitting end of the synchronization system generates a corrected optical signal and transmits it to the receiving end of the synchronization system. The second beam splitter transmits the third split optical signal corresponding to the second signal light source to the receiving end of the synchronization system. The receiving end of the synchronization system generates a control signal according to the corrected optical signal and the third split optical signal, and transmits the control signal to the second signal light source to control its frequency adjustment so that the second signal light source is frequency synchronized with the first signal light source. It does not require a frequency-locked reference light provided externally, directly uses the frequency of the optical signal emitted by the first signal light source in the first terminal as a reference to control the second signal light source, so that its frequency is synchronized with the signal light source at the transmitting end, saving costs, simplifying the process, and improving the efficiency of realizing frequency synchronization.

[0160] The above are only the preferred embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

[0161] It is obvious to those skilled in the art that the present invention is not limited to the details of the above-described exemplary embodiments, and that the present invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the present invention. Therefore, from any point of view, the embodiments should be regarded as exemplary and non-restrictive. The scope of the present invention is defined by the appended claims rather than the above description. Therefore, all changes falling within the meaning and scope of the equivalent elements of the claims are intended to be embraced within the present invention. Any reference signs in the claims should not be construed as limiting the claims involved.

Claims

1. A TF-QKD system, characterized in that, The TF-QKD system includes a first terminal and a second terminal; The first terminal includes a first signal light source, a first beam splitter, and a transmitting end of the synchronization system; The second terminal includes a second signal light source, a second beam splitter, and a receiving end of the synchronization system; The first beam splitter is used to split the optical signal emitted by the first signal light source and transmit the first split optical signal to the transmitting end of the synchronization system; The transmitting end of the synchronization system is used to generate a correction optical signal based on the first split optical signal and transmit the correction optical signal to the receiving end of the synchronization system; The second beam splitter is used to split the optical signal emitted by the second signal light source and transmit the third split optical signal to the receiving end of the synchronization system; The receiving end of the synchronization system is used to generate a control signal based on the correction optical signal and the third split optical signal, and transmit the control signal to the second signal light source to control its frequency adjustment, so that the second signal light source is frequency synchronized with the first signal light source; The transmitting end of the synchronization system includes: a radio frequency signal generator, a frequency divider, a first intensity modulator, a first auxiliary light source, a downconverter, a first intensity detector, a second intensity modulator, and a first wavelength division multiplexer; The output end of the radio frequency signal generator is respectively connected to the input end of the frequency divider and the first input end of the downconverter; The output end of the frequency divider is connected to the loading end of the first intensity modulator, and the input end of the first intensity modulator is connected to the first beam splitter; The second input end of the downconverter is connected to the output end of the first intensity detector; the output end of the downconverter is connected to the loading end of the second intensity modulator, and the input end of the second intensity modulator is connected to the first auxiliary light source; The output end of the first intensity modulator, the input end of the first intensity detector, and the output end of the second intensity modulator are all connected to the first wavelength division multiplexer; The radio frequency signal generator is used to generate a radio frequency signal and transmit the radio frequency signal to the frequency divider and the downconverter; The first intensity detector is used to detect the synchronous signal light collected by the first wavelength division multiplexer and transmit the detected radio frequency signal to the down-converter, where the synchronous signal light comes from the second terminal; The downconverter is used to utilize a radio frequency signal for the radio frequency signal to perform downconversion processing, and transmit the obtained radio frequency signal to the second intensity modulator; The second intensity modulator is used to modulate a radio frequency signal onto the first auxiliary optical signal emitted by the first auxiliary light source, and transmit the modulated first auxiliary optical signal to the first wavelength division multiplexer, so as to transmit it to the second terminal; The frequency divider is used to divide the radio frequency signal by two and transmit the obtained radio frequency signal to the first intensity modulator; The first intensity modulator is used to modulate a radio frequency signal onto the first split optical signal, and transmit the corrected optical signal obtained by modulation to the first wavelength division multiplexer, so as to transmit it to the second terminal.

2. The TF-QKD system according to claim 1, wherein The TF-QKD system further includes a third terminal; the first terminal further includes a first quantum state modulation module; the second terminal further includes a second quantum state modulation module; When the second signal light source is frequency synchronized with the first signal light source, the first beam splitter is further used to transmit the second split optical signal to the first quantum state modulation module; The first quantum state modulation module is used to modulate the second split optical signal and send the obtained first modulation signal to the third terminal; When the second signal light source is frequency synchronized with the first signal light source, the second beam splitter is further used to transmit the fourth split optical signal to the second quantum state modulation module; The second quantum state modulation module is used to modulate the fourth split optical signal and send the obtained second modulation signal to the third terminal; The third terminal is used to determine the interference result between the first signal light source and the second signal light source based on the first modulation signal and the second modulation signal.

3. The TF-QKD system according to claim 1, wherein The downconverter includes a first bandpass filter, a mixer, and a second bandpass filter; The first input terminal of the mixer is connected to the output terminal of the radio frequency signal generator, the second input terminal of the mixer is connected to the output terminal of the first band-pass filter, and the input terminal of the first band-pass filter is connected to the output terminal of the first intensity detector; The output terminal of the mixer is connected to the input terminal of the second band-pass filter, and the output terminal of the second band-pass filter is connected to the loading terminal of the second intensity modulator.

4. The TF-QKD system according to claim 1, wherein The receiving end of the synchronization system includes a second wavelength division multiplexer, a second intensity detector, a third band-pass filter, a third intensity modulator, a fourth intensity modulator, a second auxiliary light source, a coupler, and a feedback module; The second wavelength division multiplexer is respectively connected to the input terminal of the second intensity detector, the output terminal of the third intensity modulator, and the first input terminal of the coupler; The input terminal of the third intensity modulator is connected to the output terminal of the second auxiliary light source; The output terminal of the second intensity detector is connected to the input terminal of the third band-pass filter, and the output terminal of the third band-pass filter is respectively connected to the loading terminal of the third intensity modulator and the loading terminal of the fourth intensity modulator; The input terminal of the fourth intensity modulator is connected to a second beam splitter, the output terminal of the fourth intensity modulator is connected to the second input terminal of the coupler, the output terminal of the coupler is connected to the feedback module, and the output terminal of the feedback module is connected to the second signal light source.

5. The TF-QKD system according to claim 4, wherein The second intensity detector is used to detect the radio frequency signal with a wavelength of collected by the second wavelength division multiplexer, and transmit the detected radio frequency signal to the third band-pass filter, wherein the wavelength of the first auxiliary optical signal is ; The third band-pass filter is used to filter the radio frequency signal and transmit the filtered radio frequency signal to the third intensity modulator and the fourth intensity modulator; The third intensity modulator is used to modulate a radio frequency signal onto the second auxiliary optical signal emitted by the second auxiliary light source, and transmit the modulated synchronous signal light to the second wavelength division multiplexer, so as to transmit it to the first terminal; The fourth intensity modulator is used to modulate a radio frequency signal onto the third split optical signal and transmit the modulated third split optical signal to the coupler; The coupler is used to couple the corrected optical signal collected by the second wavelength division multiplexer and the modulated third split optical signal, and transmit the obtained coupled optical signal to the feedback module; The feedback module is used to generate a control signal according to the coupled optical signal and transmit the control signal to the second signal light source.

6. The TF-QKD system according to claim 5, characterized in that, The feedback module includes a large bandwidth intensity detector, a power amplifier, a fourth low-pass filter, and a processor connected in sequence; The input terminal of the large bandwidth intensity detector is connected to the output terminal of the coupler, and the output terminal of the processor is connected to the control terminal of the second signal light source; The large-bandwidth intensity detector is used to detect the coupled optical signal and transmit the obtained stray radio frequency signal to the power amplifier; The power amplifier is used to amplify the received radio frequency signal and transmit the amplified radio frequency signal to the fourth low-pass filter; The fourth low-pass filter is used to perform low-pass filtering on the received radio frequency signal and transmit the obtained low-frequency stray signal to the processor; The processor is used to generate a control signal according to the low-frequency stray signal and transmit the control signal to the second signal light source.

7. The TF-QKD system according to claim 2, wherein Both the first quantum state modulation module and the second quantum state modulation module include a phase modulator, an amplitude modulator, and an optical attenuator connected in sequence.

8. A method for realizing frequency synchronization, characterized in that, Applied to the TF-QKD system according to any one of claims 1-7, the method includes: The first beam splitter splits the optical signal emitted by the first signal light source and transmits the first split optical signal to the transmitting end of the synchronization system; The transmitting end of the synchronization system generates a corrected optical signal according to the first split optical signal and transmits the corrected optical signal to the receiving end of the synchronization system; The second beam splitter splits the optical signal emitted by the second signal light source and transmits the third split optical signal to the receiving end of the synchronization system; The receiving end of the synchronization system generates a control signal based on the corrected optical signal and the third split optical signal, and transmits the control signal to the second signal light source to control its frequency adjustment so that the second signal light source is frequency synchronized with the first signal light source.

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