A synchronization correction method for quantum key distribution system

Abstract

This invention discloses a synchronization correction method particularly suitable for time-division multiplexing-based quantum key distribution systems. It proposes a specific scheme for determining the relative time position of the first signal light pulse at the receiving end with respect to the synchronization light pulse by using detection counting. This allows for easy determination of the relative time position drift, thereby achieving time synchronization correction for the quantum key distribution system. This method is applicable to various encoding schemes, such as polarization encoding, phase encoding, and time phase encoding, and can be easily implemented using controllers such as FPGAs and CPUs.

Description

Technical Field

[0001] The present invention relates to the field of quantum secure communication technology, and in particular to a method for realizing time synchronization correction in a quantum key distribution system. The method is particularly suitable for a quantum key distribution system based on time division multiplexing, and can be used in a quantum key distribution system using polarization coding, phase coding and time phase coding. Background Art

[0002] To ensure the smooth implementation of the quantum key distribution process, the quantum key distribution system usually requires time synchronization correction between the transmitter and the receiver to ensure the correct completion of the basis vector comparison. The prior art has disclosed some time synchronization correction schemes for quantum key distribution systems. For example, in a Chinese patent document entitled "A Synchronization Correction Method and Controller for a Quantum Key Distribution System", a synchronization correction method and controller are disclosed that can establish a correct one-to-one correspondence between the transmitter and the receiver when the time difference is greater than one synchronization light cycle. Specifically, after the initial synchronization sequence number is agreed upon, the transmitter sends a signal light every preset number of synchronization lights, and does not send signal light for other synchronization sequence numbers. The receiver measures the sampled values ​​of the synchronization correction values ​​corresponding to multiple different arrival times, and calculates the final result of the synchronization correction value based on the fine synchronization correction value (a signal light is sent for each synchronization light cycle). Another Chinese patent document entitled "A time synchronization method and system in quantum secure communication" also discloses a method for time synchronization of signal light between a transmitter and a receiver, wherein the transmitter labels each signal light according to the time interval between the signal light and the synchronization light of the frame to which it belongs, and the receiver measures the time interval between the signal light and the synchronization light of the frame to which it belongs, and determines the label of the received signal light according to the time interval, and matches the received signal light cursor number with the sent signal light cursor number one by one, and then corrects the time position of the signal light through a correction module.

[0003] However, these existing solutions usually describe the time synchronization correction method in principle, without involving the specific implementation process, making it difficult to directly implement the time synchronization of quantum key distribution systems in engineering applications, especially the time synchronization correction in quantum key distribution systems based on time division multiplexing. Summary of the Invention

[0004] To address the aforementioned issues in the prior art, the present invention discloses a synchronization correction method particularly suitable for quantum key distribution systems based on time-division multiplexing. This method proposes a specific scheme for determining the relative time position of the first signal light pulse at the receiving end with respect to the synchronization light pulse by using detection counting. This allows for easy determination of the relative time position drift, thereby achieving time synchronization correction for the quantum key distribution system. This method is applicable to various encoding schemes, such as polarization encoding, phase encoding, and temporal phase encoding, and can be easily implemented using controllers such as FPGAs and CPUs.

[0005] Specifically, the present invention relates to a synchronization correction method for a quantum key distribution system, which includes a data acquisition step, a main peak search step, a first peak search step, and a correction step;

[0006] In the data acquisition step, detection data within a first time range is acquired, and the first time range is [T a -T th1 , T a +T th2 ], T a is the relative time position of the signal light pulse at the transmitting end with respect to the synchronization light pulse, T th1 、T th2 are respectively the first and second time thresholds which are preset, and the detection data includes the detection count N i The relative time position T of the synchronous light pulse and the detection count Ni , i=1,2,...;

[0007] In the main peak search step, the relative time position T of the main pulse at the receiving end relative to the synchronous optical pulse is determined based on the detection data. Peak The receiving end main pulse has a maximum detection count Max(N i );

[0008] In the step of searching for the first peak, at the relative time position T Peak In the second time range from the beginning to the end, the relative time position T of the first pulse of the receiving end with respect to the synchronous optical pulse is determined according to the detection data. F , wherein the relative time position T F The detection count on the i )*K, K is the preset ratio;

[0009] In the correction step, according to the relative time position T F Perform synchronization corrections.

[0010] Furthermore, in the data acquisition step, when the acquired detection count reaches a preset threshold, a statistical sub-step is executed, which is used to count the relative time position T from the detection data.Ni The detection count N on i .

[0011] Furthermore, in the statistical sub-step, according to the relative time position T Ni Set the RAM address and write the relative time position T at the RAM address Ni The detection count N on i .

[0012] Furthermore, the maximum drift of the relative time position of the signal light pulse with respect to the synchronization light pulse in the quantum key distribution system is estimated, and the first and / or second time threshold T is set according to the maximum drift. th1 、T th2 ; and / or, the first and / or second time threshold T th1 、T th2 is set to 1μs.

[0013] Furthermore, the maximum theoretical time interval T between multiple signal light pulses generated by the receiving end in a time division multiplexing manner based on a single signal light pulse is determined. interval , and according to the maximum theoretical time interval T interval Set the size of the second time range.

[0014] Furthermore, for a quantum key distribution system that forms the plurality of signal light pulses by means of an unequal arm structure, the maximum theoretical time interval T is determined according to the arm length difference of the unequal arm structure. interval ; and / or, setting the size of the second time range to be not less than the maximum theoretical time interval T interval .

[0015] Furthermore, the preset ratio K is determined according to the dark count. Preferably, the preset ratio K is set to 30% or greater.

[0016] Furthermore, in the first peak search step, the first peak whose detection count is equal to or greater than Max(N) is searched from the time starting point of the second time range. i )*K, the detection count relative to the time position T of the synchronization light pulse F1 ; At the relative time position T F1 In the third time range from t = t ... Fmax ; and, the relative time position T Fmax Let the relative time position T of the first pulse of the receiving end with respect to the synchronous optical pulse be F .

[0017] Furthermore, the size of the third time range is set to be greater than or equal to the pulse width of the signal light pulse and less than the time interval between the first two signal light pulses formed by the receiving end in a time division multiplexing manner based on a single signal light pulse. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0019] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0020] Figure 1 An example of a quantum key distribution system based on time division multiplexing is shown;

[0021] Figure 2 It shows that during the time synchronization correction process, Figure 1 An example of a synchronous optical pulse and a single signal optical pulse generated by a transmitter A in FIG;

[0022] Figure 3 It shows that during the time synchronization correction process, Figure 1 An example of a plurality of signal light pulses formed by a receiving end B based on a single signal light pulse;

[0023] Figure 4 Schematically shows a flow chart of a synchronization correction method for a quantum key distribution system according to the present invention;

[0024] Figure 5 Shown with Figure 3 An example of corresponding detection count statistics at the receiving end within a synchronous optical cycle;

[0025] Figure 6 The flowchart of the main peak searching step and the first peak searching step according to the present invention is schematically shown. DETAILED DESCRIPTION

[0026] Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments are provided by way of example so as to fully convey the spirit of the present invention to those skilled in the art to which the present invention belongs. Therefore, the present invention is not limited to the embodiments disclosed herein.

[0027] During quantum key distribution, transmitter A and receiver B need to perform a basis vector comparison. This involves comparing the basis vectors used by transmitter A to send a photon at a specific time and the basis vectors used by receiver B to detect the photon at that time. To ensure that transmitter A and receiver B perform basis vector comparisons at the same time, time synchronization corrections are required between transmitter A and receiver B during quantum key distribution to eliminate the offset between the transmission and detection times.

[0028] The quantum key distribution system of polarization decoding, phase decoding or time phase decoding requires the use of four detectors to measure the original code. However, in order to eliminate the inconsistent detection efficiency under the same basis vector, it is preferred to use two detectors in a time-division multiplexing manner to achieve the detection requirements, while also saving the number of detectors and reducing costs.

[0029] Figure 1 An example of a quantum key distribution system based on time division multiplexing is shown. In this example, the quantum key distribution system adopts a time phase encoding scheme.

[0030] Figure 2 It shows that during the time synchronization correction process, Figure 1 An example of a synchronous optical pulse and a single signal optical pulse generated by the transmitter A in FIG.

[0031] Figure 3 It shows that during the time synchronization correction process, Figure 1 An example of a receiving end B in which multiple signal light pulses are formed in a time division multiplexing manner based on a single signal light pulse.

[0032] like Figure 1-3 As shown, in this quantum key distribution system, in order to perform time synchronization correction between the transmitter A and the receiver B, the transmitter A usually sends a synchronization light pulse and a single signal light pulse (for example, a signal light pulse under the time basis vector), wherein the relative time position Ta of the transmitter's signal light pulse with respect to the synchronization light pulse is known.

[0033] When time division multiplexing is used, an unequal-arm interferometer (arm length difference Δt2) for coherent decoding and a time division multiplexing unit with a delay difference Δt3 can be set at the receiving end B. Therefore, when a single signal light pulse arrives at the receiving end B, it will be divided into four signal light pulses in the receiving end B, namely, signal light pulse 1, signal light pulse 2, signal light pulse 3 and signal light pulse 4, as shown in FIG. Figure 3As shown in the figure, those skilled in the art will readily understand that the time intervals between signal light pulses 1 and 2, and between signal light pulses 3 and 4, are determined by the arm length difference of the unequal-arm interferometer, both Δt2; while the time interval between signal light pulses 1 and 3 is determined by the delay difference provided by the time division multiplexing unit, Δt3. Therefore, when transmitter A transmits a single signal light pulse, the DX detector at receiver B receives and detects multiple signal light pulses thanks to the dual effects of the unequal-arm interferometer and the time division multiplexing unit.

[0034] Due to time differences in the transmission paths and detection processes of the signal and synchronization light pulses between transmitter A and receiver B, as well as differences in the transmission speeds of light of different wavelengths in the optical fiber channel, the timing of signal light pulses received by different detectors varies relative to the timing of the synchronization light pulses. For example, the DX detector at receiver B will receive the first signal light pulse at a relative time position Tb relative to the synchronization light pulse. This means that the relative time position of the signal light pulse relative to the synchronization light pulse has drifted by ΔT = Tb - Ta in the quantum key distribution system. This drift needs to be detected during the time synchronization correction process so that time synchronization correction can be implemented accordingly.

[0035] As previously mentioned, the prior art does not specifically disclose how to determine this relative temporal position drift using detection data. In particular, in a quantum key distribution system employing time-division multiplexing (TDM), the detection counts of multiple signal light pulses generated at receiving end B, for example by the dual action of an unequal-arm interferometer and a TDM unit, are randomly distributed. Determining the relative temporal position drift from these detection counts is one of the issues addressed by the present invention.

[0036] Figure 4 A flow chart of the synchronization correction method of the quantum key distribution system according to the present invention is schematically shown.

[0037] like Figure 4 As shown, when the quantum key distribution system starts the synchronization correction process, for example, through the controller, according to the present invention, a data acquisition step will be first performed, which is used to obtain a sufficient amount of detection data within a preset first time range.

[0038] In the present invention, in order to reduce the amount of data that needs to be processed, the maximum drift of the relative time position of the signal light pulse with respect to the synchronization light pulse in the quantum key distribution system can be estimated, and the first and second time thresholds T can be set based on the maximum drift. th1 、T th2 To ensure that all (one or more) signal light pulses formed by the receiving end based on a single signal light pulse fall within the first time range [Ta -T th1 , T a +T th2 ]Inside, T a is the relative time position of the signal light pulse at the transmitting end with respect to the synchronization light pulse.

[0039] Therefore, by setting the first time range, the relative time position T of the detection count with respect to the synchronization light pulse can be determined in the data acquisition step. Ni Whether it falls within the first time range, it is determined whether it is valid data, and the valid data is written into the cache for subsequent use.

[0040] For example, the transmitter is at the relative time position T with respect to the synchronization optical pulse. a =5μs, a signal light pulse is sent. When the first and second time thresholds T are set according to the estimation result of the maximum drift amount, th1 、T th2 When the time interval is 1 μs, the relative time position of the detection data falling within [4 μs, 6 μs] with respect to the synchronization light pulse can be written into the cache as valid data at the receiving end for subsequent time synchronization correction.

[0041] When the collected valid data reaches the preset threshold, a statistical sub-step can be performed, which is used to count the relative time positions T of the synchronous optical pulse. Ni (i=1, 2, ...), the detection count N obtained i That is, the detection data may include the detection count N i and the detection count relative to the time position T of the synchronization light pulse Ni .

[0042] As an example, the statistical sub-step can be implemented with the help of RAM. For example, the RAM can be initialized first (i.e., the initial data values ​​in all addresses are zero), and then the data in the cache can be read (i.e., the relative time position). The detection count can be calculated with respect to the relative time position T of the synchronization light pulse. Ni As the RAM read address, add 1 when reading the data in the RAM address, and write it back to the RAM according to the original address, thereby realizing the statistical sub-step. At this time, the data in each RAM address is the relative time position T corresponding to the RAM address. Ni The number of probes that occur on .

[0043] Figure 5 Shown with Figure 3 Correspondingly, an example of the statistical results of the detection count at the receiving end within a synchronous optical cycle. Figure 5As shown, in this example, the time interval between the first signal light pulse 1 and the second signal light pulse 2 is t1, the time interval between the second signal light pulse 2 and the third signal light pulse 3 is t2, and the time interval between the third signal light pulse 3 and the fourth signal light pulse 4 is t3.

[0044] According to the present invention, in order to find the relative time position of the first signal light pulse, it is necessary to first perform a main peak search step, which is used to find the main peak with the maximum detection count in the collected detection data and determine the relative time position T of the main pulse at the receiving end with respect to the synchronization light pulse. Peak Then the first peak search step is performed, which is used to find the peak according to the relative time position T Peak , determine the relative time position T of the first signal light pulse with respect to the synchronization light pulse F .

[0045] After determining the relative time position T through the peak finding step F After that, the peak search step can be ended and the correction step can be started to calculate the peak value according to the relative time position T. F Realize time synchronization correction of quantum key distribution system.

[0046] Figure 6 The flowchart of the main peak searching step and the first peak searching step according to the present invention is schematically shown.

[0047] According to the present invention, in the step of finding the main peak, the detection count N at each address in the RAM can be i Perform pairwise comparison to obtain the maximum detection count Max(N i ), and then determine the address max_cnt_addr corresponding to the maximum detection count Max(Ni), that is, the relative time position T of the main pulse of the receiving end with respect to the synchronous optical pulse Peak .

[0048] After the main peak search step is completed, the first peak search step can be started, based on the relative time position T of the main pulse of the receiving end to the synchronous optical pulse. Peak , find the relative time position T of the first signal light pulse with respect to the synchronization light pulse F That is, the first pulse peak search starts.

[0049] In the first peak search step, it is first necessary to determine the relative time position range of the first signal light pulse, ie, the first pulse peak search address interval (hereinafter referred to as the "second time range").

[0050] In a quantum key distribution system based on time division multiplexing and / or using an unequal-arm interferometer structure, multiple signal light pulses are usually generated at the receiving end based on a single signal light pulse. However, by analyzing the optical path used to generate the multiple signal light pulses, the time intervals between the multiple signal light pulses and the maximum time interval T can always be determined theoretically. interval For example, in Figure 1 In the quantum key distribution system shown in the figure, the receiving end uses the dual functions of the unequal-arm interferometer and the time division multiplexing unit to generate four signal light pulses based on a single signal light pulse. Theoretically, the maximum time interval between these four signal light pulses is T interval It is determined by the arm length difference Δt2 of the unequal-arm interferometer and the delay Δt3 provided by the time division multiplexing unit. Therefore, the maximum theoretical time interval T can be easily determined. interval (ie, t1+t2+t3), which is, for example, 5.6 ns.

[0051] At this time, the size of the second time range is set to be no less than the maximum theoretical time interval T interval Therefore, the relative time position T of the main pulse with respect to the synchronization light pulse at the receiving end can be guaranteed. Peak In the second time range going forward, the first signal light pulse must exist, no matter which of the multiple signal light pulses the main pulse determined in the main peak searching step is.

[0052] Taking into account the actual situation such as fiber drift, the size of the second time range can be set to be larger than the maximum theoretical time interval T interval For example, when the maximum theoretical time interval T interval =5.6ns, the size of the second time range can be set to 10ns.

[0053] After determining the second time range (i.e., the first pulse peak-seeking address interval), the signal light pulse can be searched starting from the start time of the second time range (the start address statistics_ram_addr0 of its first pulse peak-seeking address interval). At this time, the first signal light pulse found is the first of the multiple signal light pulses formed by the receiving end based on the single signal light pulse.

[0054] like Figure 6 As shown, to ensure that the detection counts sought correspond to the signal light pulses, a ratio value K can be preset, for example, based on the dark counts of the detection part of the quantum key distribution system, such as K=30%. By setting the ratio value K reasonably, for example, it significantly exceeds the ratio that may be formed due to factors such as dark counts, it is ensured that any value thereof is greater than or equal to Max(N iThe detection counts of )*K are all counts corresponding to the signal light pulses, rather than interference counts caused by noise factors such as dark counts.

[0055] On this basis, we can start from the start time of the second time range to find the first time whose value is greater than or equal to Max(N i )*K detection count, thereby knowing its relative time position T with respect to the synchronization light pulse F1 .

[0056] Those skilled in the art will appreciate that the relative time position T F1 Therefore, in order to accurately determine the relative time position of the first signal light pulse with respect to the synchronization light pulse, the present invention further provides a third time range as the next search address interval.

[0057] According to the present invention, the size of the third time range can be set to be greater than or equal to the pulse width of a single signal light pulse, but less than the time interval between the first two signal light pulses formed by the receiving end in a time division multiplexing manner based on the single signal light pulse, thereby ensuring that the data of the second signal light pulse is not included in the search range, thereby causing an error.

[0058] Therefore, it is possible to F1 In the third time range after the start of the synchronization light pulse, the detection count with the maximum value is found, and its relative time position T with respect to the synchronization light pulse is determined. Fmax (i.e. its RAM address). The relative time position T Fmax That is, the relative time position of the highest peak of the first signal light pulse, which can be used as the relative time position T of the first pulse of the receiving end with respect to the synchronization light pulse. F .

[0059] At this time, in the correction step, the relative time position T of the first pulse of the receiving end with respect to the synchronous optical pulse can be used. F , determine the relative time position drift ΔT = T F -Ta, and make synchronous corrections accordingly.

[0060] In summary, the present invention proposes a time synchronization correction method for quantum key distribution systems. This method is particularly suitable for quantum key distribution systems based on time division multiplexing, but can also be used in the case of a single pulse at the receiving end. Those skilled in the art will appreciate that the method of the present invention is applicable to various encoding schemes, such as polarization encoding, phase encoding, and time phase encoding, and can be easily implemented using controllers such as FPGAs and CPUs.

[0061] Although the present invention has been described above through specific embodiments in conjunction with the accompanying drawings, it is easy for those skilled in the art to recognize that the above embodiments are merely exemplary and are used to illustrate the principles of the present invention. They do not limit the scope of the present invention. Those skilled in the art can make various combinations, modifications and equivalent substitutions to the above embodiments without departing from the spirit and scope of the present invention.

Claims

1. A synchronization correction method for a quantum key distribution system, comprising a data acquisition step, a main peak search step, a first peak search step, and a correction step; In the data acquisition step, detection data within a first time range is acquired, and the first time range is [T a -T th1 ,T a +T th2 ], T a is the relative time position of the signal light pulse at the transmitting end with respect to the synchronization light pulse, T th1 、T th2 are respectively the first and second time thresholds which are preset, and the detection data includes the detection count N i The relative time position T of the synchronous light pulse and the detection count Ni , i=1,2,…; In the main peak search step, the relative time position T of the main pulse at the receiving end relative to the synchronous optical pulse is determined based on the detection data. Peak The receiving end main pulse has a maximum detection count Max(N i ); In the step of searching for the first peak, at the relative time position T Peak In the second time range from the beginning to the end, the relative time position T of the first pulse of the receiving end with respect to the synchronous optical pulse is determined according to the detection data. F ,in, The relative time position T F The detection count on the i )*K, K is a preset ratio, and the size of the second time range is based on the maximum theoretical time interval T between multiple signal light pulses formed by the receiving end in a time division multiplexing manner based on a single signal light pulse. interval Set up; In the correction step, according to the relative time position T F Perform synchronization corrections.

2. The synchronization correction method for a quantum key distribution system according to claim 1, wherein: In the data acquisition step, when the acquired detection count reaches a preset threshold, a statistical sub-step is executed, which is used to count the relative time position T from the detection data. Ni The detection count N on i .

3. The synchronization correction method for a quantum key distribution system according to claim 2, wherein: In the statistical sub-step, according to the relative time position T Ni Set the RAM address and write the relative time position T at the RAM address Ni The detection count N on i .

4. The synchronization correction method for a quantum key distribution system according to claim 1, wherein: Estimate the maximum drift of the relative time position of the signal light pulse with respect to the synchronization light pulse in the quantum key distribution system, and set the first and / or second time threshold T according to the maximum drift th1 、T th2 ; and / or, the first and / or second time threshold T th1 、T th2 is set to 1μs.

5. The synchronization correction method for a quantum key distribution system according to claim 1, wherein: For a quantum key distribution system that forms the plurality of signal light pulses by means of an unequal arm structure, the maximum theoretical time interval T is determined according to the arm length difference of the unequal arm structure. interval ; and / or, setting the size of the second time range to be not less than the maximum theoretical time interval T interval .

6. The synchronization correction method for a quantum key distribution system according to claim 1, wherein: The preset ratio K is determined according to the dark count.

7. The synchronization correction method for a quantum key distribution system according to claim 6, wherein: The preset ratio K is set to 30% or greater.

8. The synchronization correction method for a quantum key distribution system according to any one of claims 1 to 7, wherein: In the first peak search step, starting from the time starting point of the second time range, the first peak whose detection count is equal to or greater than Max(N i )*K, the detection count relative to the time position T of the synchronization light pulse F1 ; At the relative time position T F1 In the third time range from t = t ... Fmax ;and , the relative time position T Fmax Let the relative time position T of the first pulse of the receiving end with respect to the synchronous optical pulse be F .

9. The synchronization correction method for a quantum key distribution system according to claim 8, wherein: The size of the third time range is set to be greater than or equal to the pulse width of the signal light pulse and less than the time interval between the first two signal light pulses formed by the receiving end in a time division multiplexing manner based on a single signal light pulse.

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