Pulse synchronization method and system of quantum key distribution system

Abstract

The invention discloses a pulse synchronization method and system of a quantum key distribution system. A sending end sends a frame signal through a classic common channel and sends signal light through a quantum channel; the signal light S comprises M frames of frame signals, M frames of frame inversion signals and key signals; the frame signal comprises N random binary bits, and the frame inversion signal is obtained by inverting the frame signal; the receiving end calculates a signal period according to the arrival time of the signal light and obtains a decoding signal, calculates a first bit error rate between a frame signal of a classic channel and the decoding signal bit by bit, and performs bit positioning; and then calculating a second bit error rate between the frame signal of the classic channel and the decoded signal frame by frame, and performing pulse synchronization. According to the method, part of quantum bit signal light is used for transmitting synchronization information, the method does not depend on auxiliary equipment, the signal cycle is accurately calculated to achieve clock synchronization, on the basis of in-place positioning, the signal pulse positions of the sending end and the receiving end are further accurately aligned, and the accuracy and success rate of pulse synchronization are ensured.

Description

Technical Field

[0001] The present invention relates to the field of quantum communication technology, in particular to a pulse synchronization method and system of a quantum key distribution system. Background Art

[0002] Quantum key distribution (QKD) is a method of distributing symmetric keys using the basic laws of quantum physics, which can ensure the unconditional security of encryption keys. Since the first QKD protocol was proposed in 1984, QKD has made significant progress in fiber optic links, free space links and network structures. Since the QKD system relies on an extremely short time window to transmit quantum states at the single photon level, a slight deviation in the clock will cause signal misalignment, which in turn affects the system's bit error rate, so clock synchronization between the sender and receiver is crucial. In addition, the precise alignment of pulses between sending and receiving can ensure the accuracy of transmitted information, which places high demands on the pulse synchronization between the sender and receiver.

[0003] In existing QKD systems, locking the reference clock and sending the synchronization signal are the most common methods to ensure the clock synchronization between the transmitter and receiver. In the former, the reference clock is provided by the rubidium reference clock or the global navigation satellite system, which is highly dependent on auxiliary equipment and has a high cost. In the latter, the synchronization laser is used together with the optical clock recovery instrument, which not only requires the transmission of a strong synchronization optical signal and occupies a certain amount of channel resources, but also the receiver needs to provide additional equipment to recover the clock information from the synchronization light.

[0004] Common pulse synchronization methods include strong light synchronization and wavelength division multiplexing (WDM). The traditional strong light synchronization scheme inserts a pre-set synchronization frame, that is, adds a designed strong light synchronization signal before the quantum signal, so that the receiving end can obtain the synchronization signal with a higher signal-to-noise ratio. Adding a strong light synchronization signal not only takes up resources, but may also be used to implement certain types of side channel attacks, such as detector attacks. However, the isolation of actual wavelength division multiplexing devices is limited, and some stray light may enter the signal light channel, resulting in an increase in the dark count rate and bit error rate. In addition, wavelength division multiplexing is highly dependent on and complex to the equipment. Summary of the invention

[0005] Purpose of the invention: The purpose of the present invention is to provide a method for realizing clock synchronization and pulse synchronization through partial single photons, which does not require auxiliary equipment, does not occupy additional channels, and uses weak light to realize precise synchronization between the sender and the receiver in the QKD system.

[0006] Technical solution: The pulse synchronization method of the quantum key distribution system described in the present invention comprises the following steps:

[0007] The transmitting end sends a frame signal through a classical public channel and sends a signal light through a quantum channel; the signal light S includes a frame signal of M frames, a frame inversion signal of M frames, and a key signal; the frame signal includes N random binary bits, and the frame inversion signal is obtained by inverting the frame signal;

[0008] The receiving end calculates the signal period according to the arrival time of the signal light to achieve clock synchronization, and decodes the signal light to obtain a decoded signal;

[0009] The receiving end calculates the first bit error rate between the frame signal received in the classic public channel and the 2M frame signal before the decoded signal bit by bit, and realizes bit positioning when the first bit error rate is within a preset range;

[0010] After achieving position positioning, the receiving end calculates the second bit error rate between the frame signal received in the classic common channel and the 2M frame signal before the decoded signal frame by frame, and achieves pulse synchronization when the second bit error rate is within a preset range.

[0011] The present invention adds a frame signal and a frame inversion signal to the signal light, and pulse synchronization can be completed using only part of the signal light, so that the QKD system no longer relies on auxiliary equipment.

[0012] Furthermore, the receiving end collects the arrival time of the signal light, estimates the signal frequency through fast Fourier transform, calculates the corresponding signal period; calculates the position where the signal light arrives in the signal period, and obtains the decoded signal.

[0013] Furthermore, the receiving end in the signal period T signal_FFT The arrival time t of the signal light within the nearby δ range i Perform a modulo operation to obtain the remainder result M i , M i =Mod(t i ,T signal_FFT ±δ); according to different signal cycles M i The histogram distribution of the signal is analyzed, and the histogram with the most concentrated signal distribution is found. The corresponding period is the optimal period. The position where the signal light reaches in the optimal period is calculated to obtain the decoded signal.

[0014] Furthermore, when the next signal light is received, the optimal period of the signal light is searched within a range of δ around the optimal period of the previous signal light.

[0015] Furthermore, the receiving end sets a sliding window for extracting a signal from the decoded signal S', and the width of the sliding window is N; each time the sliding window is shifted right by one bit, the number of inconsistencies between the signal in the sliding window and the corresponding bit of the frame signal received in the classic public channel is counted, and the first bit error rate is calculated, and bit positioning is achieved when the first bit error rate is within a preset range.

[0016] Furthermore, after achieving bit positioning, the receiving end calculates the second positive bit error rate between the frame signal received on the classic common channel and the frame signal in the decoded signal frame by frame, and calculates the second reverse bit error rate between the frame signal received on the classic common channel and the frame inversion signal in the decoded signal; pulse synchronization is achieved when the second positive bit error rate and the second reverse bit error rate are both within their preset ranges, otherwise this section of the decoded signal is discarded.

[0017] Further, the preset range of the second positive bit error rate is [0%, 5%], and the preset range of the second negative bit error rate is [95%, 100%].

[0018] Due to the addition of the frame inversion signal, the bit error rate suddenly changes from a lower bit error rate to a higher bit error rate in pulse synchronization, which means that the signal position can be accurately aligned and the pulse position can be clearly known.

[0019] The pulse synchronization system of the quantum key distribution system of the present invention comprises:

[0020] A signal transceiver unit, used to send a frame signal through a classical public channel at the transmitting end, and send a signal light through a quantum channel; the signal light S includes a frame signal of M frames, a frame inversion signal of M frames, and a key signal; the frame signal includes N random binary bits, and the frame inversion signal is obtained by inverting the frame signal; and is also used to calculate a signal period according to the arrival time of the signal light at the receiving end and decode the signal light to obtain a decoded signal;

[0021] A bit positioning unit is used to calculate the first bit error rate between the frame signal received in the classic common channel and the 2M frame signal before the decoded signal bit by bit at the receiving end, and to achieve bit positioning when the first bit error rate is within a preset range;

[0022] The pulse synchronization unit is used to calculate the second bit error rate between the frame signal received in the classic public channel and the 2M frame signal before the decoded signal frame by frame at the receiving end after achieving bit positioning, and to achieve pulse synchronization when the second bit error rate is within a preset range.

[0023] The electronic device described in the present invention includes a memory, a processor, and a computer program stored in the memory and executable on the processor. When the computer program is loaded into the processor, the pulse synchronization method of the quantum key distribution system is implemented.

[0024] The computer-readable storage medium described in the present invention stores a computer program, and is characterized in that when the computer program is executed by a processor, the pulse synchronization method of the quantum key distribution system is implemented.

[0025] Beneficial effects: The present invention uses part of the quantum bit signal light to transmit synchronization information, calculates the precise clock cycle frequency through the modulus algorithm, and accurately restores the signal offset at both ends of the transmitter and receiver through bit operation bit positioning and frame synchronization frame positioning algorithm. Compared with the traditional QKD system, the advantages of the present invention are:

[0026] 1. Low resource consumption: Traditional QKD requires additional channels or additional equipment to ensure that the clocks of the sender and receiver are consistent, and the synchronization signal light is additionally generated and is strong light; the present invention no longer relies on auxiliary equipment and does not require additional channels; by utilizing part of the transmitted quantum bits to achieve clock synchronization and pulse synchronization, the clock synchronization and pulse synchronization of the sender and receiver are achieved, reducing the complexity of the system; no strong light signal is required for synchronization, only part of the single photons (weak light) are used to achieve synchronization, and the quantum bits consumed by the frame positioning algorithm are less than 1% of the total quantum bits.

[0027] 2. Excellent synchronization effect: The higher the cycle calculation accuracy, the better the clock synchronization effect. The present invention calculates the precise cycle through the modulo algorithm, with the highest accuracy up to 10 -5 ps, ensuring a very high clock synchronization effect; based on the bit positioning, the frame positioning further accurately aligns the signal pulse positions of the transmitter and receiver through the bit error rate calculation of multiple consecutive frames, ensuring the accuracy and success rate of pulse synchronization.

[0028] 3. Enhanced system robustness: In the high attenuation environment of the quantum channel, the present invention can clearly determine the pulse position and achieve fast and accurate signal alignment by introducing a frame inversion signal and utilizing the sudden change of the bit error rate from low to high. Therefore, the present invention still has excellent robustness and synchronization capabilities under large attenuation conditions.

[0029] 4. Wide applicability: The present invention has been experimentally verified in the BB84-QKD system, and the results show that it performs well in long-distance optical fiber links and has high practicality and reliability. The present invention is applicable to a variety of quantum communication application scenarios and can effectively meet the needs of quantum key distribution in different environments. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a flow chart of the pulse synchronization method of the present invention;

[0031] Figure 2 It is a schematic diagram of the composition of signals in the quantum channel and the classical channel of the present invention;

[0032] Figure 3 This is a schematic diagram of the time error changing with time in an embodiment of the present invention;

[0033] Figure 4 A schematic diagram of bit error rates in the bit positioning and frame positioning steps in an embodiment of the present invention;

[0034] Figure 5 It is a comparison chart of the theoretical simulation and experimental results of the BB84 protocol in an embodiment of the present invention. DETAILED DESCRIPTION

[0035] The technical solution of the present invention is further described below in conjunction with the accompanying drawings.

[0036] like Figure 1 As shown in the figure, the pulse synchronization method of the present invention realizes clock synchronization and pulse synchronization through partial single photons, so that the synchronization of the QKD system no longer depends on auxiliary equipment and does not occupy additional channels, and is experimentally verified in BB84-QKD. The main steps include signal transmission, rough estimation of period by FFT algorithm, calculation of precise period by modulus algorithm, signal recovery, bit positioning and frame positioning.

[0037] Step 1, such as Figure 2 As shown, in the signal sending step, the frame signal S sync Composed of N random 0 and 1 bits, frame inversion signal By the frame signal S sync To ensure that the frame signal can still be detected under large attenuation conditions, the value of N should be appropriately changed according to the attenuation degree. The signal light S is composed of five repeated frame signals S sync , five-frame repeated frame inversion signal and key signal S bit The frame signal and frame inversion signal are used for pulse synchronization, and the key signal is used to transmit quantum information in the QKD system. The transmitter sends the frame signal S through the classical public channel. sync , signal light S is sent through the quantum channel.

[0038] In this embodiment, to ensure that the frame signal can still be detected under large attenuation conditions, N increases with increasing attenuation. In actual QKD experiments, the values ​​of the frame signal length N at 50 km and 100 km are 5000 and 20000, respectively.

[0039] Step 2: The receiving end collects the arrival time t of the signal light S i Finally, the fast Fourier transform (FFT) algorithm is used to roughly estimate the frequency of some signal lights, and the arrival time t of an appropriate number of signal lights is taken out according to the attenuation degree of the signal light and the repetition frequency of the signal source. i , set the corresponding sampling frequency Sample as the input parameter of the fast Fourier transform, and calculate the rough clock frequency f through the fast Fourier transform signal_FFT , and the corresponding signal period T signal_FFT .

[0040] Step 3, in the step of calculating the exact period by the modulo algorithm, the rough period of the signal is roughly estimated by the fast Fourier transform. It can be known that the actual period of the signal should be within a certain range. The exact period is calculated by the modulo operation within this period range, that is, in T signal_FFT Within ±δ, the arrival time t of the signal light i Perform a modulo operation to obtain the remainder result M i , M i =Mod(t i ,T signal_FFT ±δ). Through different cycles M i The histogram distribution of the signal is found, and the corresponding period is the optimal period T. signal_Mod During the calculation process, when δ = 0.001ps, the single calculation step size is 2 / 10 -6 ps, the cycle accuracy can reach 10 -5 p.s. The next frequency calculation does not need to be roughly estimated using the FFT algorithm first, but rather it can be found near the frequency obtained by the previous modulo algorithm.

[0041] Figure 3 The figure is a schematic diagram of the time error variation over time when the signal light passes through 100km (18.4dB) optical fiber in nearly 200s. The time error is the difference between the expected arrival time of the signal light and the measured arrival time. The smaller the time error, the more accurate the period calculation. The time error in the figure is basically less than 1ns, which shows the accuracy of the period calculation, indicating that the method of calculating the signal period using the fast Fourier transform (FFT) combined with the modulus algorithm is very practical.

[0042] Step 4: In the signal recovery step, according to the optimal period T signal_Mod and the signal light arrival time t i , calculate the position where each signal light arrives in the cycle and obtain the decoded signal S'.

[0043] Step 5, in the bit positioning step, set the frame signal S sync A sliding window with the same length and a window width of N is used to extract the signal from the decoded signal S'. Every time the window is shifted right, the signal in the window and the frame signal S are counted. sync The number of inconsistencies in the corresponding bits is the number of bit errors, and the bit error rate is calculated. When the bit error rate is within the preset range, the frame signal S sync Preliminary alignment with the decoded signal S' to achieve bit positioning.

[0044] Step 6: After completing the position positioning step, the frame positioning step begins, by calculating the frame signal S frame by frame. syncThe bit error rate of the decoded signal S' is compared. When the bit error rates of the five-frame signal are within [0%, 5%], and the bit error rates of the five-frame frame inversion signal are within [95%, 100%], it indicates that the frame signal S sync The pulse position of the decoded signal S' is completely aligned, achieving pulse synchronization. Otherwise, the decoded signal is discarded.

[0045] In pulse synchronization, since the signal light will be greatly attenuated after passing through the quantum channel, if only the frame signal is used without the frame inversion signal, the pulse position may be misjudged, making it impossible to align the transmitter signal with the receiver signal. Therefore, the frame inversion signal is added, and the bit error rate changes from a lower bit error rate to a higher bit error rate. This process just shows that the signal position can be accurately aligned. Therefore, in the environment of large attenuation such as the quantum channel, the addition of the frame inversion signal can clearly know the pulse position, thereby better aligning the transmitter and receiver signals.

[0046] Figure 4 The figure shows the change of the bit error rate at 50km during the pulse synchronization process. The dots on the left represent the change of the bit error rate during the position positioning process, and the triangles on the right represent the change of the bit error rate during the frame positioning process. First, align the frame signal and the decoded signal by bit, and keep the decoded signal unchanged. During the position positioning process, shift the frame signal right by one bit, and calculate the bit error rate of the frame signal and the decoded signal by bit. When the position positioning is unsuccessful, the bit error rate fluctuates around 50%. After the position positioning is successful, the bit error rate will drop below 5%. At this time, frame positioning begins, shift the frame signal right by N bits, that is, one frame, and calculate the bit error rate of the frame signal and the decoded signal by bit. The bit error rate of the frame signal in the first five frames is within [0%, 5%], and the bit error rate of the frame inversion signal in the last five frames is within [95%, 100%].

[0047] Figure 5 This is a comparison image of the experimental results and theoretical simulation when the present invention implements the BB84-QKD protocol. The number of pulses N in the simulation is 10 10 , all parameters of simulation and experiment are consistent. The values ​​of other parameters are shown in Table I:

[0048] Table I List of simulation system parameters

[0049]

[0050] Where f(x) is the error correction efficiency, Y 0 is the dark rate of the detector, and are the background bit errors when selecting Z basis and X basis respectively, η D is the detector efficiency, and α is the loss coefficient of the optical fiber.

[0051] Figure 5The horizontal axis is the transmission distance (km), and the vertical axis is the security key rate (bit / s). The solid line in the figure represents the simulation result of the security key rate, the circle represents the experimental measurement value using an attenuator to simulate the optical fiber, and the triangle represents the experimental measurement value using the optical fiber. Under a 50 km (100 km) optical fiber link, the theoretical simulation key rate is 36936.77746bps (3556.26786bps), and the actual measured key rate is 34158.48062bps (3333.89468bps). The total number of quantum signals received at 50km (100km) is 55112374 (5588572), the key amount is 52062341 (5561097), and the synchronization consumption is 213047 (27475). The synchronization consumption accounts for less than 1% of the total number of quantum signals received, and the key signal accounts for more than 99% of the total number of quantum signals received. The experimental results are very close to the theoretical simulation. These experimental data verify the feasibility and efficiency of the present invention to achieve clock synchronization and pulse synchronization through very few single photons.

[0052] It can be seen that the present invention proposes a method for realizing clock synchronization and pulse synchronization through partial single photons, so that the QKD system no longer relies on auxiliary equipment. Clock synchronization is realized by roughly estimating the period through Fourier transform (FFT) and calculating the precise period through modulus algorithm, and the calculation accuracy can reach 10 -5 ps; The signal offset at both ends of the transmitter and receiver is restored through bit positioning and frame positioning to achieve pulse synchronization. The quantum bits consumed by the frame positioning algorithm are less than 1% of the total quantum bits, and the key transmission efficiency is as high as over 99%. The present invention has been experimentally verified in the BB84-QKD system, and the actual key rate measured under 50 km and 100 km optical fiber is close to the theoretical value, which shows that the present invention has high practicality and high reliability.

[0053] The pulse synchronization system of the quantum key distribution system of the present invention comprises:

[0054] A signal transceiver unit, used to send a frame signal through a classical public channel at the transmitting end, and send a signal light through a quantum channel; the signal light S includes a frame signal of M frames, a frame inversion signal of M frames, and a key signal; the frame signal includes N random binary bits, and the frame inversion signal is obtained by inverting the frame signal; and is also used to calculate a signal period according to the arrival time of the signal light at the receiving end and decode the signal light to obtain a decoded signal;

[0055] A bit positioning unit is used to calculate the first bit error rate between the frame signal received in the classic common channel and the 2M frame signal before the decoded signal bit by bit at the receiving end, and to achieve bit positioning when the first bit error rate is within a preset range;

[0056] The pulse synchronization unit is used to calculate the second bit error rate between the frame signal received in the classic public channel and the 2M frame signal before the decoded signal frame by frame at the receiving end after achieving bit positioning, and to achieve pulse synchronization when the second bit error rate is within a preset range.

[0057] The electronic device described in the present invention includes a memory, a processor, and a computer program stored in the memory and executable on the processor. When the computer program is loaded into the processor, the pulse synchronization method of the quantum key distribution system is implemented.

[0058] The computer-readable storage medium described in the present invention stores a computer program, and is characterized in that when the computer program is executed by a processor, the pulse synchronization method of the quantum key distribution system is implemented.

[0059] The computer-readable storage media may include RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, flash memory, or any other medium that can be used to store the desired program code in the form of instructions or data structures and that can be accessed by a computer.

[0060] The processor is used to execute the computer program stored in the memory to implement each step of the method involved in the above embodiment.

Claims

1. A pulse synchronization method for a quantum key distribution system, characterized in that: The steps include: The transmitting end sends a frame signal through a classical public channel and sends a signal light through a quantum channel; the signal light S includes a frame signal of M frames, a frame inversion signal of M frames, and a key signal; the frame signal includes N random binary bits, and the frame inversion signal is obtained by inverting the frame signal; The receiving end calculates the signal period according to the arrival time of the signal light to achieve clock synchronization, and decodes the signal light to obtain a decoded signal; The receiving end calculates the first bit error rate between the frame signal received in the classic public channel and the 2M frame signal before the decoded signal bit by bit, and realizes bit positioning when the first bit error rate is within a preset range; After achieving position positioning, the receiving end calculates the second bit error rate between the frame signal received in the classic common channel and the 2M frame signal before the decoded signal frame by frame, and achieves pulse synchronization when the second bit error rate is within a preset range.

2. The pulse synchronization method of the quantum key distribution system according to claim 1, characterized in that: The receiving end collects the arrival time of the signal light, estimates the signal frequency through fast Fourier transform, and calculates the corresponding signal period; The position where the signal light arrives in the signal cycle is calculated to obtain the decoded signal.

3. The pulse synchronization method of the quantum key distribution system according to claim 2, characterized in that: The receiving end is in the signal period T sugnal_FFT The arrival time t of the signal light within the nearby δ range i Perform a modulo operation to obtain the remainder result M i , M i =Mod(t i ,T signal_FFT ±δ); According to different signal cycles, M i The histogram distribution of the signal is analyzed, and the histogram with the most concentrated signal distribution is found, and the corresponding period is the optimal period; The position where the signal light arrives in the optimal period is calculated to obtain the decoded signal.

4. The pulse synchronization method of the quantum key distribution system according to claim 3, characterized in that: When the next signal light is received, the optimal period of the signal light is searched within a range of δ near the optimal period of the previous signal light.

5. The pulse synchronization method of the quantum key distribution system according to claim 1, characterized in that: The receiving end sets a sliding window for extracting a signal from the decoded signal S', and the width of the sliding window is N; each time the sliding window is shifted right by one bit, the number of inconsistencies between the signal in the sliding window and the corresponding bit of the frame signal received in the classic public channel is counted, and the first bit error rate is calculated. When the first bit error rate is within a preset range, bit positioning is achieved.

6. The pulse synchronization method of the quantum key distribution system according to claim 1, characterized in that: After the bit positioning is achieved, the receiving end calculates the second positive bit error rate between the frame signal received on the classic common channel and the frame signal in the decoded signal frame by frame, and calculates the second inverse bit error rate between the frame signal received on the classic common channel and the frame inversion signal in the decoded signal; Pulse synchronization is achieved when the second positive bit error rate and the second negative bit error rate are both within their preset ranges.

7. The pulse synchronization method of the quantum key distribution system according to claim 6, characterized in that: The preset range of the second positive bit error rate is [0%, 5%], and the preset range of the second negative bit error rate is [95%, 100%].

8. A pulse synchronization system for a quantum key distribution system, characterized in that: include: A signal transceiver unit, used to send a frame signal through a classical public channel at the transmitting end and send a signal light through a quantum channel; the signal light S includes a frame signal of M frames, a frame inversion signal of M frames, and a key signal; the frame signal includes N random binary bits, and the frame inversion signal is obtained by inverting the frame signal; It is also used to calculate the signal period according to the arrival time of the signal light at the receiving end and decode the signal light to obtain a decoded signal; A bit positioning unit is used to calculate the first bit error rate between the frame signal received in the classic common channel and the 2M frame signal before the decoded signal bit by bit at the receiving end, and to achieve bit positioning when the first bit error rate is within a preset range; The pulse synchronization unit is used to calculate the second bit error rate between the frame signal received in the classic public channel and the 2M frame signal before the decoded signal frame by frame at the receiving end after achieving bit positioning, and to achieve pulse synchronization when the second bit error rate is within a preset range.

9. An electronic device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that: When the computer program is loaded into a processor, the pulse synchronization method of the quantum key distribution system according to any one of claims 1 to 7 is implemented.

10. A computer-readable storage medium storing a computer program, characterized in that: When the computer program is executed by a processor, the pulse synchronization method of the quantum key distribution system according to any one of claims 1 to 7 is implemented.

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