TF-QKD system and frequency synchronization realization method

Abstract

The invention proposes a TF-QKD system and frequency synchronization realization method, and the method comprises the steps: carrying out the beam splitting of an optical signal emitted by a first signal light source through a first beam splitter, transmitting a first beam-split optical signal to a synchronization system transmitting end, generating a correction optical signal through the synchronization system transmitting end, and transmitting the correction optical signal to a synchronization system receiving end; the second beam splitter transmits a third beam splitting optical signal corresponding to the second signal light source to the synchronous system receiving end; and the synchronization system receiving end generates a control signal according to the correction light signal and the third beam splitting light signal, transmits the control signal to the second signal light source, and controls the second signal light source to perform frequency adjustment, so that the second signal light source and the first signal light source realize frequency synchronization. The second signal light source is controlled by directly taking the frequency of the optical signal emitted by the first signal light source in the first terminal as the reference without externally provided frequency locking reference light, so that the frequency of the second signal light source is kept synchronous with the emitting end signal light source, the cost is saved, the process is simplified, and the implementation efficiency of frequency synchronization is improved.

Description

Technical Field

[0001] The present invention relates to the field of quantum, and more particularly, to a TF-QKD system and a frequency synchronization implementation method. 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 theory level for both communicating parties. Combining with the one-time pad information encryption method can achieve unconditional secure communication. In terms of signal transmission, both QKD and classical optical fiber communication face the problem that the signal intensity decays exponentially with the increase of distance. Classical optical communication can deploy optical amplifiers to enhance the transmitted signal and then 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 improving 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 QKD, and effectively reduce its security cost. However, the implementation conditions of TF-QKD technology are quite stringent, and 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 optical fiber link will be amplified during long-time 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 laser frequencies of both communicating parties and is an important component for the implementation of TF-QKD technology. Summary of the Invention

[0004] The purpose of the present invention is to provide a TF-QKD system and a frequency synchronization implementation method 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: 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; 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 configured 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 configured 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; The second beam splitter is configured 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 configured 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.

[0006] 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; When the second signal light source is frequency synchronized with the first signal light source, the first beam splitter is further configured to transmit the second split optical signal to the first quantum state modulation module; The first quantum state modulation module is configured 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 configured to transmit the fourth split optical signal to the second quantum state modulation module; 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; 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.

[0007] 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; 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 down converter is connected to the output end of the first intensity detector; the output end of the down converter 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.

[0008] 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 down converter; 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 down converter is used to perform down-conversion processing on the radio frequency signal using the radio frequency signal , and transmit the obtained radio frequency signal to the second intensity modulator; 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; 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; The first intensity modulator is used to modulate the radio frequency signal onto the first split optical signal, and transmit the modulated corrected optical signal to the first wavelength division multiplexer, so as to transmit it to the second terminal.

[0009] Optionally, the down converter includes a first band-pass filter, a mixer, and a second band-pass filter; 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; 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.

[0010] 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; 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; The input end of the third intensity modulator is connected to the output end of the second auxiliary light source; 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; The input end of the fourth intensity modulator is connected to a 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.

[0011] 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 ; 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 the radio frequency signal onto the second auxiliary optical signal emitted by the second auxiliary light source, and transmit the modulated synchronization 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 the radio frequency signal onto the third beam-split optical signal, and transmit the modulated third beam-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 beam-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.

[0012] Optionally, 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 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; 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.

[0013] 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.

[0014] 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: 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 according to the first split optical signal and transmits the correction 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 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.

[0015] Compared with the prior art, for a TF-QKD system and a frequency synchronization implementation method provided by an embodiment of the present invention, 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 a 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. There is no need for an externally provided frequency-locked reference light, and 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, so that its frequency is synchronized with the signal light source at the transmitting end, saving costs, simplifying the process, and improving the implementation efficiency of frequency synchronization.

[0016] To make the above objects, features, and advantages of the present invention more obvious and understandable, the following specifically gives preferred embodiments and, in conjunction with the accompanying drawings, makes a detailed description as follows. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] To more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required to be used 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.

[0018] Figure 1 It is a schematic structural diagram of the TF-QKD system provided by an embodiment of the present invention.

[0019] Figure 2 It is a schematic structural diagram of the transmitting end of the synchronization system provided by an embodiment of the present invention.

[0020] Figure 3 It is a schematic structural diagram of the receiving end of the synchronization system provided by an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0021] To make the objects, technical solutions, and advantages of the embodiments of the present invention clearer, the following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. Usually, the components of the embodiments of the present invention described and shown in the drawings here can be arranged and designed in various different configurations.

[0022] Accordingly, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed 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 shall fall within the scope of protection of the present invention.

[0023] 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, the terms "first", "second", etc. are only used for descriptive distinction and cannot be construed as indicating or implying relative importance.

[0024] 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 order between these entities or operations. Moreover, the terms "comprise", "include" or any other variation thereof are intended to cover 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 statement "comprising a..." does not exclude the presence of additional identical elements in the process, method, article or device comprising the element.

[0025] 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, and does not indicate or imply 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 of the present invention.

[0026] In the description of the present invention, it should also be noted that, unless otherwise clearly specified and limited, the terms "set", "connect" 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.

[0027] The following will describe in detail some embodiments of the present invention in conjunction with the accompanying drawings. Without conflict, the following embodiments and the features in the embodiments can be combined with each other.

[0028] To solve the defects such as complexity and high cost existing in the frequency synchronization of the TF-QKD system, the embodiments of the present invention provide a method for realizing a TF-QKD system and frequency synchronization, which can ensure the reduction of the system cost under the condition of realizing the frequency synchronization of the TF-QKD communication transceiver with high precision, and provide a technical basis for the large-scale application of TF-QKD in the future industry.

[0029] 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 connected by optical fiber communication.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] In the TF-QKD system of the embodiment of the present invention, there is no need for an externally provided frequency-locked reference light. Instead, 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. At the second terminal, it is received through the receiving end of the synchronization system, and a feedback control signal is generated through internal operation of the second terminal and transmitted to the second signal light source in the second terminal to perform negative feedback control on it, so that its frequency is 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.

[0037] Please continue to refer to Figure 1 , in an alternative 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.

[0038] 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.

[0039] 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.

[0040] When the second signal light source is frequency-synchronized with the first signal light source, the first beam splitter is further configured to transmit the second split optical signal (corresponding to the first split optical signal) to the first quantum state modulation module.

[0041] The first quantum state modulation module is configured to modulate the second split optical signal and send the obtained first modulation signal to the third terminal.

[0042] When the second signal light source is frequency-synchronized with the first signal light source, 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.

[0043] 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; 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 are quantum-entangled and quantum key negotiation can be performed).

[0044] 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.

[0045] Of course, when the second signal light source and the first signal light source are not frequency-synchronized, 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 may 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.

[0046] In the transmitting end and the receiving end of the synchronization system in the embodiments of the present invention, an optical-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 Figure 1 the basis of this, 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 a schematic structural diagram of the transmitting end of the synchronization system provided by the embodiments of the present invention.

[0047] 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 down converter, a first intensity detector, a second intensity modulator, and a first wavelength division multiplexer. Among them, the wavelength division multiplexer is also called WDM.

[0048] 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 down converter.

[0049] 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).

[0050] The second input end of the down converter is connected to the output end of the first intensity detector; the output end of the down converter 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.

[0051] 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.

[0052] The radio frequency signal generator is configured to generate a radio frequency signal , and transmit the radio frequency signal

[0053] Optionally, the frequency and phase information of the radio frequency signal is expressed as follows:

[0054] Among them, represents the frequency of the radio frequency signal , represents the radio frequency signal phase.

[0055] 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, where the synchronous signal light comes from the second terminal.

[0056] Optionally, the frequency and phase information of the radio frequency signal are represented as follows:

[0057] Among them, represents the frequency of the radio frequency signal , represents the radio frequency signal phase.

[0058] 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.

[0059] Optionally, the radio frequency signal and the radio frequency signal are downconverted after mixing and band-pass filtering to obtain the radio frequency signal . Preset , so the obtained radio frequency signal is represented as:

[0060] Among them, represents the frequency of the radio frequency signal , represents the radio frequency signal phase, represents the frequency of the radio frequency signal , represents the radio frequency signal phase.

[0061] 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 synchronization system in the middle).

[0062] 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.

[0063] Optionally, the radio frequency signal obtained after frequency division by two has the following expression:

[0064] where represents the frequency of the radio frequency signal , represents the radio frequency signal phase.

[0065] The first intensity modulator is used to (by means of intensity modulation) modulate the radio frequency signal onto the first split optical signal (wavelength ), 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).

[0066] Please continue to refer to Figure 2 , in an alternative embodiment, the downconverter includes a first band-pass filter, a mixer, and a second band-pass filter.

[0067] 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.

[0068] 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.

[0069] 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.

[0070] 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.

[0071] 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 .

[0072] In Figure 1Based on the above, for the specific structure of the receiving end of the synchronization system, an optional implementation manner is further provided in the embodiments of the present invention. Please refer to Figure 3 , Figure 3 which is a schematic structural diagram of the receiving end of the synchronization system provided by the embodiments of the present invention.

[0073] 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.

[0074] 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.

[0075] The input end of the third intensity modulator is connected to the output end of the second auxiliary light source.

[0076] 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.

[0077] The input end of the fourth intensity modulator is connected to the first output end of a 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).

[0078] 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 .

[0079] Optionally, the expression of the radio frequency signal is:

[0080] 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.

[0081] The third band-pass filter is used for the radio frequency signal to perform filtering processing, and transmit the filtered radio frequency signal to the third intensity modulator and the fourth intensity modulator.

[0082] 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 obtained synchronous signal light (with a wavelength of ) to the second wavelength division multiplexer, so as to transmit it to the first terminal (the transmitting end of the synchronization system).

[0083] At the transmitting end of the synchronization system, the synchronous signal light obtains a radio frequency signal through the first intensity detector. At this time, the signal will be appended with the phase error accumulated during the transmission process from the second terminal to the first terminal. Therefore, the expression form of the signal is:

[0084]

[0085] and are different expression forms of the same signal. Therefore, the two are equivalent and have the following relationship:

[0086] The above relationship can be expressed as:

[0087] Substitute the above relationship into the expression of the radio frequency signal for simplification. The simplification result is as follows:

[0088]

[0089] 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 phase as the radio frequency signal .

[0090] The fourth intensity modulator is used to (by means of intensity modulation) modulate the radio frequency signal onto the third split optical signal, and transmit the modulated third split optical signal to the coupler.

[0091] The coupler is used to correct the optical signal (with a wavelength of )( ) and the modulated third split optical signal are coupled, and the obtained coupled optical signal is transmitted to the feedback module.

[0092] 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.

[0093] Optionally, the feedback module includes a large-bandwidth intensity detector, a power amplifier, a fourth low-pass filter, and a processor connected in sequence.

[0094] Among them, the large-bandwidth intensity detector is an intensity detector that supports 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.

[0095] 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.

[0096] 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.

[0097] 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.

[0098] The processor is used to generate a control signal according to the low-frequency spurious signal (combining the negative feedback regulation algorithm) and transmit the control signal to the second signal light source to perform feedback control on it, so that the second signal light source and the first signal light source achieve frequency synchronization.

[0099] 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 , 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 asynchronization of the light sources can be obtained. 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 signal The power value of the [signal] and the power value of the spurious signal are used, and their ratio is used as the feedback quantity for feedback control, so that the frequency synchronization between the second signal light source and the first signal light source can be realized, the frequencies of the two light sources are ensured to be locked, and accurate frequency synchronization is achieved.

[0100] 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.

[0101] 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.

[0102] The 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.

[0103] S101, 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.

[0104] S102, 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.

[0105] S103, 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.

[0106] S104, 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.

[0107] It should be noted that the frequency synchronization implementation method provided in this embodiment can perform the functions and uses shown in the above system embodiment to achieve 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.

[0108] In summary, for a TF-QKD system and a frequency synchronization implementation method provided in an embodiment of the present invention, 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 a 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. There is no need for an externally provided frequency-locked reference light. Directly taking the frequency of the optical signal emitted by the first signal light source in the first terminal as a reference, the second signal light source is controlled to keep its frequency synchronized with the signal light source at the transmitting end, saving costs, simplifying the process, and improving the implementation efficiency of frequency synchronization.

[0109] 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.

[0110] For those skilled in the art, it is obvious that the present invention is not limited to the details of the above exemplary embodiments, and can be implemented in other specific forms without departing from the spirit or basic characteristics of the present invention. Therefore, from any point of view, the embodiments should be regarded as exemplary and non-limiting. The scope of the present invention is defined by the appended claims rather than the above description. Therefore, it is intended to embrace all changes falling within the meaning and scope of the equivalent elements of the claims in the present invention. Any reference signs in the claims should not be regarded as limiting the claimed rights.

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 synchronization system transmitting end; The second terminal includes a second signal light source, a second beam splitter and a synchronization system receiving end; 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 synchronization system transmitting end; The synchronization system transmitting end is used to generate a corrected optical signal according to the first split optical signal, and transmit the corrected optical signal to the synchronization system receiving end; 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 synchronization system receiving end; The receiving end of the synchronization system is used to generate a control signal according to the corrected optical signal and the third split optical signal, and transmit the control signal to the second signal light source to control it to perform frequency adjustment so that the second signal light source can achieve frequency synchronization with the first signal light source.

2. The TF-QKD system according to claim 1, characterized in that 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; In the case where the second signal light source achieves frequency synchronization with the first signal light source, the first beam splitter is further used to transmit the second split light signal to the first quantum state modulation module; The first quantum state modulation module is used to modulate the second split light signal and send the obtained first modulated signal to the third terminal; When the second signal light source achieves frequency synchronization with the first signal light source, the second beam splitter is further used to transmit the fourth split light 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 modulated signal to the third terminal; The third terminal is used to determine an 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.

3. The TF-QKD system according to claim 1 or 2, characterized in that: The synchronous system transmitting end comprises: a radio frequency signal generator, a two-frequency divider, a first intensity modulator, a first auxiliary light source, a down converter, a first intensity detector, a second intensity modulator and a first wavelength division multiplexer; The output end of the radio frequency signal generator is connected to the input end of the binary frequency divider and the first input end of the down converter respectively; The output end of the binary 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 down converter is connected to the output end of the first intensity detector; the output end of the down converter 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.

4. The TF-QKD system according to claim 3, characterized in that The radio frequency signal generator is used to generate a radio frequency signal , and the RF signal Transmitted to the binary frequency divider and the down converter; The first intensity detector is used to detect the synchronization signal light collected by the first wavelength division multiplexer, and to detect the radio frequency signal Transmitted to the down converter, wherein the synchronization signal light comes from the second terminal; The down converter is used to utilize the radio frequency signal For RF signals Perform down-conversion processing and obtain the RF signal transmitting to the second intensity modulator; The second intensity modulator is used to convert the radio frequency signal modulating the first auxiliary optical signal emitted by the first auxiliary light source, and passing the modulated first auxiliary optical signal to the first wavelength division multiplexer, thereby transmitting it to the second terminal; The two-frequency divider is used to Perform two-way frequency division and obtain the RF signal transmitting to the first intensity modulator; The first intensity modulator is used to convert the radio frequency signal The first split optical signal is modulated, and the modulated corrected optical signal is passed to the first wavelength division multiplexer, thereby transmitting it to the second terminal.

5. The TF-QKD system according to claim 4, characterized in that The down converter includes a first bandpass filter, a mixer and a second bandpass filter; The first input end of the mixer is connected to the output end of the RF 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; 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.

6. The TF-QKD system of claim 4, wherein: The synchronous system receiving end includes a second wavelength division multiplexer, a second intensity detector, a third bandpass 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 end of the second intensity detector, the output end of the third intensity modulator and the first input end of the coupler; The input end of the third intensity modulator is connected to the output end of the second auxiliary light source; 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 connected to the loading end of the third intensity modulator and the loading end of the fourth intensity modulator respectively; 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.

7. The TF-QKD system of claim 6, wherein: The second intensity detector is used to detect the wavelength collected by the second wavelength division multiplexer. The radio frequency signal detected is transmitted to the third bandpass filter, wherein the wavelength of the first auxiliary optical signal is ; The third bandpass filter is used to Perform filtering and convert the filtered RF signal transmitting to the third intensity modulator and the fourth intensity modulator; The third intensity modulator is used to convert the radio frequency signal modulating the second auxiliary optical signal emitted by the second auxiliary light source, and passing the modulated synchronization signal light to the second wavelength division multiplexer, thereby transmitting it to the first terminal; The fourth intensity modulator is used to convert the radio frequency signal modulating the third split optical signal and transmitting 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.

8. The TF-QKD system according to claim 7, 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 end of the wide 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; The wide bandwidth intensity detector is used to detect the coupled optical signal and obtain the stray radio frequency signal transmitting 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 spurious 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.

9. The TF-QKD system of claim 2, wherein: The first quantum state modulation module and the second quantum state modulation module both include a phase modulator, an amplitude modulator and an optical attenuator which are connected in sequence.

10. A method for implementing frequency synchronization, characterized in that: The TF-QKD system applied to any one of claims 1 to 9, the method comprising: The first beam splitter splits the optical signal emitted by the first signal light source, and transmits the first split optical signal to the synchronization system transmitting end; The synchronous system transmitting end generates a corrected optical signal according to the first split optical signal, and transmits the corrected optical signal to the synchronous system receiving end; The second beam splitter splits the optical signal emitted by the second signal light source, and transmits the third split optical signal to the synchronization system receiving end; 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 it to adjust the frequency so that the second signal light source can achieve frequency synchronization with the first signal light source.

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