A pulse synchronization method and system for a quantum key distribution system

Abstract

This invention discloses a pulse synchronization method and system for a quantum key distribution system. The transmitting end sends frame signals through a classical common channel and signal light through a quantum channel. The signal light S includes M frame signals, M frame inversion signals, 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 based on the arrival time of the signal light and obtains the decoded signal. It then calculates the first bit error rate between the classical channel frame signal and the decoded signal bit by bit for bit positioning. Finally, it calculates the second bit error rate between the classical channel frame signal and the decoded signal frame by frame for pulse synchronization. This invention utilizes a portion of the quantum bit signal light to transmit synchronization information, without relying on auxiliary equipment. It accurately calculates the signal period to achieve clock synchronization. Based on bit positioning, it further precisely aligns the signal pulse positions of the transmitting and receiving ends, ensuring the accuracy and success rate of pulse synchronization.

Description

Technical Field

[0001] This invention relates to the field of quantum communication technology, and in particular to a pulse synchronization method and system for quantum key distribution systems. Background Technology

[0002] Quantum key distribution (QKD) is a method for distributing symmetric keys using the fundamental laws of quantum physics, guaranteeing unconditional security of the 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 architectures. Because QKD systems rely on extremely short time windows to transmit single-photon-level quantum states, even minute clock deviations can cause signal misalignment, thus affecting the system's bit error rate. Therefore, clock synchronization between the sender and receiver is crucial. Furthermore, precise alignment of pulses between the transmitter and receiver is essential to ensure the accuracy of transmitted information, placing high demands on pulse synchronization between the sender and receiver.

[0003] In existing QKD systems, locking the reference clock and transmitting a synchronization signal are the most common methods to ensure clock synchronization between the transmitting and receiving ends. In the former, the reference clock is provided by a rubidium reference clock or a global navigation satellite system, which is highly dependent on auxiliary equipment and is costly. In the latter, a synchrotron laser is used in conjunction with an optical clock recovery instrument, requiring not only the transmission of a strong synchronization optical signal, occupying channel resources, but also additional equipment at the receiving end to recover clock information from the synchronization light.

[0004] Common pulse synchronization methods include strong optical synchronization and wavelength division multiplexing (WDM). Traditional strong optical synchronization schemes insert a pre-set synchronization frame, adding a designed strong optical synchronization signal before the quantum signal, allowing the receiver to acquire the synchronization signal with a high signal-to-noise ratio. However, adding a strong optical synchronization signal not only consumes resources but can also be used to launch certain types of side-channel attacks, such as detector attacks. Furthermore, practical WDM devices have limited isolation, allowing some stray light to enter the signal optical channel, leading to increased dark count rate and bit error rate. WDM also has high equipment dependence and complexity. Summary of the Invention

[0005] Purpose of the invention: The purpose of this invention is to provide a method for achieving clock synchronization and pulse synchronization using a portion of single photons, without requiring auxiliary equipment or occupying additional channels, and utilizing weak light to achieve precise synchronization between the transmitter and receiver in a QKD system.

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

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

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

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

[0010] After bit positioning is achieved, the receiving end calculates the second bit error rate between the frame signal received on the classic common channel and the first 2M frames of the decoded signal frame by frame. When the second bit error rate is within a preset range, pulse synchronization is achieved.

[0011] This invention incorporates frame signals and frame inversion signals into the signal light, enabling pulse synchronization to be completed using only a portion of the signal light, thus freeing the QKD system from dependence on auxiliary equipment.

[0012] Furthermore, the receiving end acquires the arrival time of the signal light, estimates the signal frequency through fast Fourier transform, calculates the corresponding signal period, and calculates the position of the signal light in the signal period to obtain the decoded signal.

[0013] Furthermore, the receiving end during the signal period T signal_FFT Within a radius of 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); M under different signal periods i Find the histogram distribution of the signal, identify the histogram with the most concentrated signal distribution, and determine the optimal period corresponding to that period. Calculate the position of the signal light within the optimal period to obtain the decoded signal.

[0014] Furthermore, upon receiving the next signal light, the optimal period of the signal light is searched within a range δ near the optimal period of the previous signal light.

[0015] Furthermore, the receiving end sets a sliding window to extract the signal from the decoded signal S', and the width of the sliding window is N; every time the sliding window moves one bit to the right, the number of bits in the signal in the sliding window that are inconsistent with the corresponding bits of the frame signal received in the classic common 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.

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

[0017] Furthermore, 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] Because of the addition of the frame inversion signal, the bit error rate suddenly changes from a low bit error rate to a high bit error rate during pulse synchronization, indicating that the signal position can be accurately aligned and the pulse position can be clearly determined.

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

[0020] The signal transceiver unit is used to transmit frame signals through a classical common channel and signal light through a quantum channel at the transmitting end; the signal light S includes M frame signals, M frame inversion signals, 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 based on the arrival time of the signal light at the receiving end and decode the signal light to obtain a decoded signal;

[0021] The bit positioning unit is used to calculate the first bit error rate between the frame signal received in the classic common channel and the first 2M frames of the decoded signal bit by bit at the receiving end, and to realize 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 on the classic common channel and the first 2M frames of the decoded signal after bit positioning is achieved. Pulse synchronization is achieved when the second bit error rate is within a preset range.

[0023] The electronic device of 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 onto the processor, it implements the pulse synchronization method of the quantum key distribution system.

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

[0025] Beneficial effects: This invention utilizes partial quantum bit signal optical transmission synchronization information, calculates the precise clock cycle frequency through a modulo algorithm, and accurately recovers the signal offset between the transmitting and receiving ends through bitwise operation bit positioning and frame synchronization frame positioning algorithms. Compared with traditional QKD systems, the advantages of this invention are:

[0026] 1. Low resource consumption: Traditional QKD requires additional channels or equipment to ensure clock synchronization between the transmitter and receiver, and the synchronization signal light is generated separately and is strong light. This invention no longer relies on auxiliary equipment and does not require additional channels. By using a portion of the transmitted qubits to achieve clock synchronization and pulse synchronization, the clock and pulse synchronization between the transmitter and receiver are realized, reducing the complexity of the system. No strong light signal is required for synchronization; synchronization is achieved using only a portion of single photons (weak light), and the qubits consumed by the frame positioning algorithm are less than 1% of the total number of qubits.

[0027] 2. Excellent synchronization effect: The higher the accuracy of the period calculation, the better the clock synchronization effect. This invention calculates the precise period through a modulo algorithm, with a maximum accuracy of 10. -5 PS ensures extremely high clock synchronization. Based on bit positioning, frame positioning further aligns the signal pulse positions of the transmitting and receiving ends by calculating the bit error rate of multiple consecutive frames, ensuring the accuracy and success rate of pulse synchronization.

[0028] 3. Enhanced System Robustness: In the high-attenuation environment of quantum channels, this invention introduces a frame inversion signal and utilizes the abrupt change in bit error rate from low to high to clearly determine the pulse position, achieving rapid and accurate signal alignment. Therefore, this invention still possesses excellent robustness and synchronization capabilities under high attenuation conditions.

[0029] 4. Wide Applicability: This invention has been experimentally verified in the BB84-QKD system, demonstrating excellent performance in long-distance fiber optic links and possessing high practicality and reliability. This invention is applicable to various quantum communication application scenarios and can effectively meet the quantum key distribution requirements in different environments. Attached Figure Description

[0030] Figure 1 This is a flowchart of the pulse synchronization method of the present invention;

[0031] Figure 2 This is a schematic diagram illustrating the signal composition in the quantum channel and classical channel of this invention;

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

[0033] Figure 4 This is a schematic diagram illustrating the bit error rate of the bit positioning and frame positioning steps in an embodiment of the present invention;

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

[0035] The technical solution of the present invention will be further described below with reference to the accompanying drawings.

[0036] like Figure 1 As shown, the pulse synchronization method of this invention achieves clock synchronization and pulse synchronization through partial single-photon synchronization, making the synchronization of the QKD system independent of auxiliary equipment and without occupying additional channels. Experimental verification was conducted in BB84-QKD. The main steps include signal transmission, FFT algorithm for coarse period estimation, modulo algorithm for precise period calculation, signal recovery, bit positioning, and frame positioning.

[0037] Step 1, as follows Figure 2 As shown, in the signal transmission step, the frame signal S sync The frame inversion signal consists of N random 0 and 1 bits. From frame signal S sync The value of N is obtained by inverting the signal. To ensure that the frame signal can still be detected under conditions of high attenuation, the value of N should be appropriately changed according to the degree of attenuation. The signal light S is composed of five repeated frame signals Si. sync Five-frame repeating frame inversion signal and key signal S bit The system consists of a frame signal and a frame inversion signal used for pulse synchronization, and a key signal used to transmit quantum information in the QKD system. The transmitting end sends the frame signal S through a classical common channel. sync The signal light S is transmitted through a quantum channel.

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

[0039] Step 2: The receiving end collects the arrival time t of the signal light S. i Then, the Fast Fourier Transform (FFT) algorithm is used to roughly estimate the frequencies of some signal light. Based on the signal light attenuation and the repetition frequency of the signal source, an appropriate number of signal light arrival times t are selected. i Set the corresponding sampling frequency (Sample) as the input parameter for the Fast Fourier Transform (FFT), and calculate the approximate clock frequency (f) using the FFT. signal_FFT At the same time, the signal period T is obtained accordingly. signal_FFT .

[0040] Step 3, in the modulo algorithm for calculating the precise period, the coarse period of the signal is roughly estimated by the Fast Fourier Transform. It can be seen that the actual period of the signal should be within a certain range. The precise period is then calculated using modulo operations on this range, i.e., within T... signal_FFT Within the range of ±δ, the arrival time t of the signal light i Perform modulo operation to obtain the remainder result M. i M i =Mod(t) i ,T signal_FFT ±δ). Through M under different periods i By analyzing the histogram distribution, we can find the histogram with the most concentrated signal distribution; the corresponding period is the optimal period T. signal_Mod During the calculation, when δ = 0.001 ps, the step size for a single calculation is taken as 2 / 10. -6 ps, at this point the period accuracy can reach 10. -5 P.S. The next frequency calculation does not need to use the FFT algorithm for a rough estimate first, but instead searches for the frequency in the vicinity of the frequency obtained by the modulo algorithm in the previous calculation.

[0041] Figure 3 This diagram illustrates the change in time error over approximately 200 seconds as the signal light travels through 100km (18.4dB) of optical fiber. The time error is the difference between the expected arrival time and the measured arrival time of the signal light. A smaller time error results in more accurate period calculation. The time error in the diagram is generally less than 1ns, demonstrating the accuracy of the period calculation. This illustrates the practicality of the method used in this invention, which combines Fast Fourier Transform (FFT) with analog-to-digital algorithm to calculate the signal period.

[0042] Step 4, in the signal recovery step, according to the optimal period T signal_Mod and signal light arrival time t i The position of each signal light in the period is calculated to obtain the decoded signal S'.

[0043] Step 5, in the bit positioning step, set the frame signal S sync A sliding window of uniform length and width N is used to extract the signal from the decoded signal S'. Each time the window moves one position to the right, the signal within the window is compared with the frame signal S. sync The number of inconsistent bits, i.e., the number of bit errors, is used to calculate the bit error rate. When the bit error rate is within a preset range, the frame signal S... sync The signal is initially aligned with the decoded signal S' to achieve bit positioning.

[0044] Step 6: After completing the bit 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 with the bit error rate of the other two frames. When the bit error rates of all five frames are within [0%, 5%] and the bit error rates of all five frame inversion signals are within [95%, 100%], it indicates that the frame signal S' is at a good condition. sync The pulse position is now perfectly aligned with the decoded signal S', achieving pulse synchronization. Otherwise, the decoded signal is discarded.

[0045] In pulse synchronization, the signal light will be significantly 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 transmitting and receiving signals. Therefore, the frame inversion signal is added. Through the change of bit error rate, the bit error rate suddenly changes from a low bit error rate to a high bit error rate. This process precisely shows that the signal position can be accurately aligned. Therefore, in the environment of high attenuation in the quantum channel, the addition of the frame inversion signal can clearly determine the pulse position, thereby better aligning the transmitting and receiving signals.

[0046] Figure 4 The diagram illustrates the change in bit error rate (BER) at 50km during pulse synchronization. The left dot represents the BER change during bit positioning, and the right triangle represents the BER change during frame positioning. First, the frame signal and decoded signal are aligned bit-by-bit, with the decoded signal remaining stationary. During bit positioning, the frame signal is shifted right by one bit, and the BER of both the frame and decoded signals is calculated bit-by-bit. If bit positioning fails, the BER fluctuates around 50%. After successful bit positioning, the BER drops sharply to below 5%. Frame positioning then begins. The frame signal is shifted right by N bits (one frame), and the BER of both the frame and decoded signals is calculated bit-by-bit. The BER of the first five frames is within [0%, 5%], and the BER of the last five frames (frame reversal signal) is within [95%, 100%].

[0047] Figure 5 This is a comparison image showing the experimental results and theoretical simulations when the BB84-QKD protocol is executed in this invention. In the simulation, the number of pulses N is 10. 10 All parameters in the simulation and experiment are consistent. Other parameter values ​​are shown in Table I:

[0048] Table I. List of Simulation System Parameters

[0049]

[0050] Where f(x) is the error correction efficiency, and Y0 is the dark mark rate of the detector. and The background error values ​​η represent the base values ​​for Z-based and X-based selections, respectively. D Here, α represents the detector efficiency, and α represents the fiber loss coefficient.

[0051] Figure 5The horizontal axis represents transmission distance (km), and the vertical axis represents the security key rate (bit / s). Solid lines in the graph represent simulated security key rates, circles represent experimental measurements using attenuators to simulate fiber optic connections, and triangles represent experimental measurements using actual fiber optic connections. Under a 50km (100km) fiber optic link, the theoretical simulated key rate is 36936.77746bps (3556.26786bps), while the actual measured key rate is 34158.48062bps (3333.89468bps). The total number of quantum signals received at 50km (100km) was 55,112,374 (5,588,572), the key quantity was 52,062,341 (5,561,097), and the synchronization consumption was 213,047 (27,475). The synchronization consumption accounted for less than 1% of the total number of received quantum signals, and the key signal accounted for more than 99% of the total number of received quantum signals. The experimental results are very close to the theoretical simulation. These experimental data verify the feasibility and efficiency of the present invention in achieving clock synchronization and pulse synchronization with very few single photons.

[0052] As can be seen, this invention proposes a method for achieving clock and pulse synchronization using a subset of single photons, eliminating the reliance on auxiliary equipment in QKD systems. Clock synchronization is achieved by coarsely estimating the period using Fourier Transform (FFT) and calculating the precise period using a modulo algorithm, achieving a computational accuracy of up to 10. -5 PS: By recovering the signal offset between the transmitting and receiving ends through bit positioning and frame positioning, pulse synchronization is achieved. The frame positioning algorithm consumes less than 1% of the total number of qubits, and the key transmission efficiency is as high as 99% or more. This invention has been experimentally verified in the BB84-QKD system. The actual key rates measured under 50 km and 100 km optical fiber are close to the theoretical values, which shows that this invention has high practicality and high reliability.

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

[0054] The signal transceiver unit is used to transmit frame signals through a classical common channel and signal light through a quantum channel at the transmitting end; the signal light S includes M frame signals, M frame inversion signals, 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 based on the arrival time of the signal light at the receiving end and decode the signal light to obtain a decoded signal;

[0055] The bit positioning unit is used to calculate the first bit error rate between the frame signal received in the classic common channel and the first 2M frames of the decoded signal bit by bit at the receiving end, and to realize 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 on the classic common channel and the first 2M frames of the decoded signal after bit positioning is achieved. Pulse synchronization is achieved when the second bit error rate is within a preset range.

[0057] The electronic device of 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 onto the processor, it implements the pulse synchronization method of the quantum key distribution system.

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

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

[0060] The processor is used to execute a computer program stored in memory to implement the various steps in the methods described in the above embodiments.

Claims

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

2. The pulse synchronization method for a 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 of the signal light during the signal period is calculated to obtain the decoded signal.

3. The pulse synchronization method for a quantum key distribution system according to claim 2, characterized in that, The receiver during the signal period T sugnal_FFT Within a radius of 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); According to M under different signal periods i Find the histogram distribution of the signal and identify the histogram with the most concentrated signal distribution. The corresponding period is the optimal period. The position where the signal light arrives during the optimal period is calculated to obtain the decoded signal.

4. The pulse synchronization method for 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 for a quantum key distribution system according to claim 1, characterized in that, The receiving end sets up a sliding window to extract the signal from the decoded signal S'. The width of the sliding window is N. Every time the sliding window moves one bit to the right, the number of bits in the signal in the sliding window that are inconsistent with the corresponding bits of the frame signal received in the classic common 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 for a quantum key distribution system according to claim 1, characterized in that, After bit positioning is achieved, the receiving end calculates the second positive bit error rate between the frame signal received in the classical common channel and the frame signal in the decoded signal frame by frame, and calculates the second negative bit error rate between the frame signal received in the classical common channel and the frame inversion signal in the decoded signal. Pulse synchronization is achieved when both the second positive bit error rate and the second negative bit error rate are within their preset ranges.

7. The pulse synchronization method for a 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: The signal transceiver unit is used to transmit frame signals through a classical common channel and signal light through a quantum channel at the transmitting end; the signal light S includes frame signals of M frames, frame inversion signals of M frames, and key signals; the frame signals include N random binary bits, and the frame inversion signals are obtained by inverting the frame signals; It is also used to calculate the signal period at the receiving end based on the arrival time of the signal light and to decode the signal light to obtain a decoded signal; The bit positioning unit is used to calculate the first bit error rate between the frame signal received in the classic common channel and the first 2M frames of the decoded signal bit by bit at the receiving end, and to realize 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 on the classic common channel and the first 2M frames of the decoded signal after bit positioning is achieved. Pulse synchronization is achieved 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 the processor, it implements the pulse synchronization method of the quantum key distribution system according to any one of claims 1-7.

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

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