Method for reducing light source phase noise in double-field quantum key distribution system

Abstract

The invention provides a method for reducing light source phase noise in a double-field quantum key distribution system, which combines an optical phase-locked loop (OPLL) and a proportional integral (PI) control technology, and comprises the following steps of: firstly, performing beat frequency on laser signals of light source ends Alice and Bob, and realizing frequency locking and phase synchronization of a laser through the OPLL; on the basis, the system introduces frequency deviation through an acousto-optic modulator (AOM) to ensure the stability of a beat frequency signal. Further, the system couples the optical signals after frequency beating through an optical beam splitter, and detects the power of the coupled optical signals. The detected power value is transmitted to the FPGA control module to serve as an input parameter of PI control, a proportionality coefficient and an integral coefficient are adjusted in real time, a feedback control signal is generated and used for dynamically adjusting a front phase modulator of a receiving end Bob, and therefore the phase noise of a light source end is effectively reduced.

Description

Technical Field

[0001] The present invention relates to the field of quantum communication, and in particular to a method for reducing the phase noise of a light source in a dual-field quantum key distribution TF-QKD system. Background Art

[0002] Quantum key distribution (QKD) is one of the important applications of quantum communication technology. It ensures the secure transmission of keys based on the basic principles of quantum mechanics and can effectively resist the attack threats of traditional computing and quantum computing. However, in actual QKD systems, signal transmission and processing will be interfered by various noises, among which the phase noise of the light source is one of the key issues affecting system performance. Especially in the dual-field quantum key distribution TF-QKD system, the phase consistency of the light source at the transmitter and receiver is crucial. If the accumulation of light source phase noise is not controlled, it will inevitably lead to an increase in the bit error rate of key distribution, seriously affecting the key generation rate and security of the system.

[0003] Most traditional QKD systems rely on fixed-frequency light sources for coherent transmission, but in long-distance quantum communications, lasers will inevitably be affected by external factors, such as temperature fluctuations, mechanical jitter, and device aging, which will cause frequency and initial phase deviations. These deviations will introduce phase noise, especially in long-distance TF-QKD systems. The longer the fiber transmission distance, the greater the impact of phase noise. To this end, existing technologies usually suppress noise by means of fiber link length stabilization and temperature control, but these methods can only be based on the physical environment. It is difficult to fundamentally solve the problem of phase stability of the light source, especially in dual-field quantum key distribution systems that require high phase consistency. The effect of these methods is limited.

[0004] At present, the common solution to control phase noise is to use optical phase-locked loop (OPLL) technology to synchronize the frequency and phase of the laser at the light source end by detecting the beat signal. The optical phase-locked loop (OPLL) uses an optical converter to obtain the frequency difference of the laser signal and feedback to adjust the frequency and phase of the laser from the laser Bob end. However, when faced with dynamically changing high-frequency phase noise with small and fast fluctuations, the feedback adjustment speed and phase-locking accuracy of the optical phase-locked loop (OPLL) are limited, making it difficult to cope with the rapid changes in the global phase of the optical fiber link.

[0005] On the other hand, proportional-integral PI control technology has been applied in phase noise suppression, but when used alone, its adjustment accuracy and real-time responsiveness to noise are still insufficient. Proportional-integral PI control can only maintain stability under slower environmental changes and cannot make timely and accurate adjustments to fast phase drift. Summary of the invention

[0006] In view of the shortcomings of the prior art, the present invention proposes a method for reducing phase noise in a dual-field quantum key distribution TF-QKD system. The continuous light generated by the laser at Bob's end passes through the phase modulator PM, and then is split by a 50:50 optical beam splitter BS3 into a third light beam and a fourth light beam. One of the light beams generates a beat frequency signal with the light beam at Alice's end after passing through the beam splitter BS4.

[0007] The beat frequency signal is converted into an electrical signal by an optical detector PD and input into an optical phase-locked loop OPLL for frequency and phase locking, generating a feedback control signal which is fed back to the Bob-end laser to adjust the frequency and phase of the Bob-end laser to achieve synchronization.

[0008] Furthermore, the optical phase-locked loop OPLL includes a local oscillator LO, a mixer and a loop filter Loopfilter; the beat frequency signal converted into an electrical signal is mixed with a reference frequency signal generated by the local oscillator LO in the mixer to generate an error signal, which is processed by the loop filter to generate the feedback control signal for adjusting the frequency and phase of Bob's laser, so that the frequency of Bob's laser is synchronized with Alice's laser to achieve frequency and phase locking.

[0009] Furthermore, the light beam at the Alice end that generates the beat signal after passing through the beam splitter BS4 is the light beam modulated by the acousto-optic modulator AOM; a specific frequency offset is introduced into the optical signal at the receiving end Bob through the acousto-optic modulator AOM to ensure the stability of the beat signal, and the frequency offset is 300.7 MHz, ensuring that the generated beat signal can be detected and used for subsequent processing.

[0010] As a slave laser, Bob locks its own optical frequency to the optical frequency of the master laser at Alice's end, and regulates the laser at Bob's end to make the optical frequency difference between Alice's and Bob's lasers;

[0011] The frequency of the third light beam or the fourth light beam is shifted by using an acousto-optic modulator AOM, and the frequency-shifted light beam and the light beam at the Alice end generate a beat frequency signal after passing through the beam splitter BS4;

[0012] The signal of the local oscillator LO in the optical phase-locked loop OPLL is amplified and used to drive the acousto-optic modulator AOM.

[0013] Further, the continuous light at Alice's end is split by a 50:50 optical beam splitter BS1 into a first light beam and a second light beam;

[0014] One of the first light beam and the second light beam is used to beat the frequency with the frequency-shifted third light beam or the fourth light beam;

[0015] After the other of the first light beam and the second light beam passes through the beam splitter BS2, an optical signal S1 for power detection and another signal for quantum key encoding are obtained;

[0016] After the light beam in the third light beam or the fourth light beam that has not been frequency shifted passes through the beam splitter BS5, an optical signal S2 for power detection and another signal for quantum key encoding are obtained;

[0017] The optical signal S1 and the optical signal S2 are coupled via a 50:50 optical beam splitter BS6, and finally form an interference signal;

[0018] An optical power meter is used to measure the power value w of any output optical signal in the optical path after coupling to obtain a power signal w(t); this signal is used to reflect the phase relationship and interference intensity of the two optical signals, and has the characteristics of small and fast high-frequency phase noise;

[0019] The power signal is input into the proportional-integral PI feedback module to generate a pre-PM modulation signal p(t) loaded to the Bob end, and this signal is used to dynamically adjust the pre-phase modulator PM at the Bob end; the control signal p(t) is transmitted to the phase modulator PM of Bob to dynamically adjust the initial phase of the optical signal at the Bob end, ensuring that the output signal of the phase modulator PM effectively reduces the initial phase noise at the Bob end, thereby effectively reducing the phase noise at the light source end;

[0020] Furthermore, the proportional-integral PI feedback module includes a low-pass filter LPF and a PI module; the power signal w(t) is input into the proportional-integral PI feedback module, mixed with the low-frequency sinusoidal signal generated by the FPGA to generate a periodic signal with high-frequency noise, and then filtered through the low-pass filter LPF in the FPGA to generate an input signal for the PI module, and the PI module dynamically adjusts the proportional coefficient gp and the integral coefficient fI according to the real-time changes of the signal.

[0021] Furthermore, the continuous light at Alice's end is split by a 50:50 beam splitter BS1, specifically:

[0022] The continuous light at the Alice end first passes through a polarization controller PC and then is split by a 50:50 beam splitter BS1.

[0023] A structure for reducing the phase noise of a light source in a dual-field quantum key distribution system of the present invention comprises a beat frequency signal generation module and an optical phase-locked loop OPLL;

[0024] The laser signals of Alice and Bob generate beat frequency signals through the beat frequency signal generation module, which are converted into electrical signals through the optical detector PD and input into the optical phase-locked loop OPLL to generate the feedback control signal, which is fed back to the laser at the Bob end to adjust the frequency and phase of the laser at the Bob end.

[0025] Further, the beat frequency signal generating module includes a beam splitter BS1, a beam splitter BS3, a beam splitter BS4 and an AOM;

[0026] The continuous light at the Alice end is split by the 50:50 beam splitter BS1 into a first light beam and a second light beam;

[0027] The continuous light generated by the Bob-end laser passes through the phase modulator PM and then is split by the 50:50 optical beam splitter BS3 into the third light beam and the fourth light beam;

[0028] The third light beam or the fourth light beam is frequency-shifted by the AOM to obtain a frequency-shifted light beam;

[0029] One of the first light beam and the second light beam and the frequency-shifted light beam generate a beat signal through the beam splitter BS4.

[0030] Furthermore, the optical phase-locked loop OPLL includes a local oscillator LO, a mixer and a loop filter Loopfilter; the beat frequency signal is converted into an electrical signal by an optical detector PD and input into the optical phase-locked loop OPLL, mixed with a reference frequency signal generated by the local oscillator LO in the mixer to generate an error signal, and after being processed by the loop filter, the feedback control signal is generated.

[0031] Furthermore, a structure for reducing the phase noise of a light source in a dual-field quantum key distribution system of the present invention further includes a beam splitter BS2, a beam splitter BS5, a beam splitter BS6 and a proportional-integral PI feedback module

[0032] The proportional-integral PI feedback module includes a low-pass filter LPF and a PI module;

[0033] After the other of the first light beam and the second light beam passes through the beam splitter BS2, an optical signal S1 and another signal for quantum key encoding are obtained;

[0034] After the light beam in the third light beam or the fourth light beam that has not been frequency shifted passes through the beam splitter BS5, an optical signal S2 and another signal for quantum key encoding are obtained;

[0035] The optical signal S1 and the optical signal S2 are coupled through a 50:50 optical beam splitter BS6, and an optical power meter is used to measure the power value w of the output optical signal in any optical path after coupling to obtain a power signal, and the power signal is input into a proportional-integral PI feedback module;

[0036] The power signal is input into the proportional-integral PI feedback module, mixed with the low-frequency sinusoidal signal generated by the FPGA, and then filtered through a low-pass filter LPF to generate an input signal for the PI module. The PI module dynamically adjusts the proportional coefficient gp and the integral coefficient fI according to the real-time changes of the signal.

[0037] Beneficial effects of the present invention:

[0038] The present invention provides a method for reducing phase noise in a dual-field quantum key distribution TF-QKD system. By combining the innovative solution of an optical phase-locked loop OPLL and a proportional-integral PI control module, the shortcomings of OPLL in the face of dynamic high-frequency phase noise are overcome. After OPLL achieves frequency locking and phase synchronization, the phase noise of the signal is still significant, and it needs to be adjusted by real-time feedback through the PI module. The PI module can quickly respond to environmental changes and signal fluctuations, and dynamically adjust system parameters, thereby effectively reducing phase noise. This combined method not only improves the phase stability of the light source end, but also significantly enhances the anti-interference ability of the TF-QKD system. Specifically:

[0039] 1. Significant reduction in phase noise: By using the optical phase-locked loop (OPLL) for frequency and phase synchronization and combining it with the PI feedback control system, the phase modulator of the receiving end Bob can be effectively adjusted in real time, which significantly reduces the phase noise of the light source, thereby improving the stability and phase consistency of the optical signal in the TF-QKD system.

[0040] 2. Improved real-time performance and precision: Compared with the single frequency-locked phase-locked technology, the present invention dynamically adjusts the error through the PI module, can respond to environmental changes in real time, achieve high-precision control of the optical signal power, and ensure the phase stability of the optical signal during system operation. At the same time, by dynamically adjusting the PI parameters (gp, fI), the present invention can adapt to complex transmission environments and effectively improve the system's ability to resist noise interference.

[0041] 3. Enhanced system robustness: Due to the combination of frequency offset technology (implemented by AOM), the beat frequency signal is more stable, reducing the loss of lock caused by frequency and phase fluctuations during transmission. Even under complex transmission conditions, the phase noise control technology proposed in this invention can still maintain the synchronization and stable operation of the system.

[0042] 4. Wide applicability: The technical solution of the present invention is not only applicable to the light source phase noise control of the TF-QKD system, but can also be applied to other quantum communication systems with high requirements on phase stability, especially in long-distance communication and high-speed key distribution scenarios, and has broad application prospects.

[0043] 5. Improved system security: By reducing the phase noise of the light source, the problems of information leakage and system performance degradation caused by phase instability during the key distribution process are effectively alleviated, thereby enhancing the security of the quantum key distribution system and ensuring the reliable transmission of the key. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 This is the block diagram of the frequency-locked and phase-locked structure at the independent light source end;

[0045] Figure 2 is the Allan variance plot of the beat frequency signal;

[0046] Figure 3 is the spectrum of the beat frequency signal and its Lorentz fitting diagram; DETAILED DESCRIPTION

[0047] The present invention discloses a method for reducing the phase noise of a light source in a dual-field quantum key distribution system. After the light signals of Alice and Bob at the light source end are frequency locked and phase synchronized through a frequency-locked phase-locked OPLL module, the phase noise in the system is reduced in real time through a proportional-integral PI feedback module. In the proportional-integral PI module, the signal p(t) loaded by the FPGA to the phase modulator PM is adjusted by real-time monitoring of the signal power of the Alice and Bob ends. Therefore, the phase noise in the TF-QKD system can be greatly reduced on the basis of the original frequency-locked phase-locked, thereby reducing the impact of the phase noise on the entire system. It is particularly suitable for application scenarios with high requirements for phase stability in long-distance quantum communication and high-speed quantum key distribution systems. The main process includes frequency locking, phase locking, beam splitting, coupling, photoelectric conversion, mixing, filtering, and PM phase modulation through proportional-integral PI module feedback. The specific steps are as follows: Step 1, Bob, as a slave laser, locks its own optical frequency to the optical frequency of the master laser at Alice's end. Both Alice and Bob's lasers use narrow-linewidth semiconductor continuous lasers (Rio PLANEX), with a linewidth of about 2KHz, an optical frequency stability of less than 5MHz / 6h, an output wavelength of 1549.32nm±0.5nm, and an output power of less than 23mW. Before the optical phase-locked loop OPLL is running, the semiconductor refrigeration plate (Thermo Electric Cooler, TEC) at the bottom of the laser at Bob's end is adjusted to make the optical frequency difference between Alice and Bob's lasers around 300.7MHz;

[0048] In order to make the optical frequencies of Alice and Bob as close to zero frequency difference as possible, the laser on Bob's side will pass through an acousto-optic modulator (AOM) after beam splitting, thereby generating a positive 300.7MHz laser heterodyne frequency shift. Through the frequency shift of the AOM, the adjustment of the laser on Bob's side compensates for the original 300.7M frequency difference, so that before the beam splitter BS4, the optical signals of Alice and Bob achieve zero frequency difference. This process ensures that the two beams of light can be superimposed on each other when interfering and produce an ideal interference effect, thereby improving the performance of the TF-QKD system.

[0049] The high-precision crystal oscillator clock source on the Xilinx ZCU111 development board provides a 37.5875MHz local oscillator (LO) clock signal, which is then multiplied by 8 times by an electrical phase-locked loop and a radio frequency signal amplifier to output a 300.7MHz radio frequency signal to drive an acousto-optic modulator (AOM). The AOM can control the intensity, frequency or direction of light by modulating sound waves. In the present invention, it is used to control the frequency deviation of light and acts as a frequency shifter.

[0050] Step 2, the continuous light at Alice's end passes through a polarization controller PC (Polarization Controller) and is then split by a 50:50 beam splitter BS1 into a first beam and a second beam;

[0051] The continuous light at Bob's end is also split by a 50:50 optical beam splitter BS3 after passing through a phase modulator PM, into a third beam and a fourth beam. The first and third beams are used for subsequent quantum state encoding, and the second and fourth beams are used to generate beat frequency at the 50:50 optical beam splitter BS4 at Bob's end. Here, the role of PC is to make the polarization of the two paths of light that generate beat frequency the same to amplify the beat frequency phenomenon.

[0052] The beat frequency signal is optically converted through a 12GHz bandwidth optical probe PD (Optilab PR-12-BM). The 300.7MHz RF signal is also used as the reference clock signal of the mixer and is sent to the loop filter together with the optically converted signal. The electrical signal is calculated and adjusted accordingly to quickly modulate the wavelength emitted from the laser at the Bob end, so that the optical frequency difference between the two sides is minimized and kept stable, completing the frequency and phase locking function;

[0053] After the frequency locking and phase synchronization are achieved by the optical phase-locked loop OPLL, the phase noise of the signal is still significant, so we added a PI module to further process the signal to reduce the signal noise. The first and third light beams of the optical signals used for quantum state encoding at Alice and Bob are split by 50:50 optical beam splitters BS2 and BS5 respectively, and then one of the output optical signals S1 and S2 is coupled by 50:50 optical beam splitter BS6, and the power value w of any output optical signal is measured using an optical power meter (Thorlabs PM100USB) to obtain the power signal.

[0054] The power signal is input to the Xilinx ZCU111 development board, mixed with the low-frequency sinusoidal signal generated by the FPGA, and then filtered through the low-pass filter LPF to generate the input signal of the proportional-integral PI module. The PI module dynamically adjusts the proportional coefficient gp and the integral coefficient fI according to the real-time changes of the signal to generate the pre-PM modulation signal p(t) loaded to the Bob end. This method greatly reduces the phase noise of the TF-QKD system. Among them, the proportional coefficient gp of the PI module is -3dB, and the integral coefficient fI is 125kHz.

[0055] The present invention combines an integrated solution of an optical phase-locked loop OPLL and a proportional-integral PI control. Frequency locking and phase synchronization are achieved through the optical phase-locked loop OPLL, and a proportional-integral PI control module is further introduced to perform real-time precise feedback adjustment, which effectively enhances the system's ability to suppress complex noise. Compared with traditional methods, the present invention combines these two technologies to not only improve the system's phase control accuracy and response speed, but also significantly improve the system's ability to adapt to changes in the external environment, and is particularly suitable for TF-QKD systems in long-distance transmission. This innovative control scheme effectively overcomes the defects in the prior art and can ensure the stability and efficiency of the system while ensuring the security of key distribution.

[0056] Figure 1 The block diagram of the frequency-locked and phase-locked structure of the independent light source is shown. The master and slave lasers Alice and Bob at the light source end are frequency-beaten through the corresponding optical elements, and the frequency locking and phase synchronization of the laser are achieved through the optical phase-locked loop OPLL. The power signal is received through the FPGA, mixed with the low-frequency sinusoidal signal and then filtered. The phase signal is dynamically modulated by PI and reloaded to the front PM. Through such a structural process, the phase noise at the light source end is reduced.

[0057] Figure 2The Allan variance of the beat signal after being locked by the optical phase-locked loop OPLL is shown in Figure 1. The vertical axis is the Allan variance in logarithmic form, which indicates the instability or noise level of the measured signal. The horizontal axis is the time interval Δt (in seconds). The curve in the figure shows how the Allan variance of the beat signal changes with the time interval. As the time interval increases, the Allan variance gradually decreases, which indicates that the relative stability of the signal increases with time. In the figure, the Allan variance is 10 -12 This indicates that the beat frequency signal has good frequency stability.

[0058] Figure 3 The frequency spectrum characteristics of the beat signal are shown, with the horizontal axis being frequency (unit: MHz) and the vertical axis being amplitude (unit: dBFS). As can be seen from the figure, the beat signal has a significant peak near 300.7MHz, indicating that the signal at this frequency is the strongest. As the frequency moves away from 300MHz, the amplitude of the signal gradually decreases, showing a typical peak shape. This shows that the signal at this frequency is very concentrated and stable. The embedded figure shows in detail the signal distribution in the range of 299MHz to 303MHz, and further describes the specific shape and width of the beat signal through Lorentz fitting, and calculates that there is σ after the PI module. 2 =5.6×10 -3 rad 2 The residual phase noise is about 4.3029°, which means the phase noise at the light source is effectively reduced after passing through the PI module.

[0059] In summary, the present invention proposes a method for reducing the phase noise of the light source in the dual-field quantum key distribution TF-QKD system. By combining the optical phase-locked loop OPLL and the proportional-integral PI feedback control, the synchronous regulation of the optical frequency and phase is achieved, and the phase noise is effectively reduced. The experimental results show that the Allan variance of the beat frequency signal is 10 -12 The residual phase noise is σ 2 =5.6×10^-3rad 2 , indicating that the phase error is small. This method can significantly improve the phase stability and anti-interference ability of the system, which illustrates the feasibility of the present invention.

[0060] The specific embodiments described above further illustrate the objectives, technical solutions and beneficial results of the present invention. It should be understood that the current patent specification only introduces the TF-QKD system as an example. For example, the method used in the specific embodiments of the present invention is also applicable to other QKD systems or free-space QKD systems, and is not used to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for reducing the phase noise of a light source in a dual-field quantum key distribution system, characterized in that: The steps include: The continuous light generated by the laser at Bob's end passes through the phase modulator PM and then is split by the optical beam splitter BS3 into a third light beam and a fourth light beam. One of the light beams generates a beat signal with the light beam at Alice's end after passing through the beam splitter BS4. The beat frequency signal is converted into an electrical signal by an optical detector PD and input into an optical phase-locked loop OPLL for frequency and phase locking, generating a feedback control signal, which is fed back to the Bob-end laser to adjust the frequency and phase of the Bob-end laser.

2. A method for reducing the phase noise of a light source in a dual-field quantum key distribution system according to claim 1, characterized in that: The optical phase-locked loop OPLL includes a local oscillator LO, a mixer and a loop filter Loop filter; The beat frequency signal converted into an electrical signal is mixed with a reference frequency signal generated by a local oscillator LO in a mixer to generate an error signal, which is processed by a loop filter to generate the feedback control signal.

3. A method for reducing the phase noise of a light source in a dual-field quantum key distribution system according to claim 2, characterized in that: The light beam that generates the beat signal with the light beam at the Alice end after passing through the beam splitter BS4 is the light beam modulated by the acousto-optic modulator AOM; As a slave laser, Bob locks its own optical frequency to the optical frequency of the master laser at Alice's end, and regulates the laser at Bob's end to make the optical frequency difference between Alice's and Bob's lasers; The frequency of the third light beam or the fourth light beam is shifted by using an acousto-optic modulator AOM, and the frequency-shifted light beam and the light beam at the Alice end generate a beat frequency signal after passing through the beam splitter BS4; The signal of the local oscillator LO in the optical phase-locked loop OPLL is amplified and used to drive the acousto-optic modulator AOM.

4. A method for reducing the phase noise of a light source in a dual-field quantum key distribution system according to claim 3, characterized in that: The continuous light at the Alice end is split by the optical beam splitter BS1 into a first light beam and a second light beam; One of the first light beam and the second light beam is used to beat the frequency with the frequency-shifted third light beam or the fourth light beam; After the other of the first light beam and the second light beam passes through the beam splitter BS2, an optical signal S1 and another signal for quantum key encoding are obtained; After the light beam in the third light beam or the fourth light beam that has not been frequency shifted passes through the beam splitter BS5, an optical signal S2 and another signal for quantum key encoding are obtained; The optical signal S1 and the optical signal S2 are coupled via the optical beam splitter BS6; Use an optical power meter to measure the power value w of the output optical signal in any optical path after coupling to obtain a power signal; The power signal is input into a proportional-integral PI feedback module to generate a pre-PM modulation signal p(t) which is loaded into the Bob terminal.

5. A method for reducing the phase noise of a light source in a dual-field quantum key distribution system according to claim 4, characterized in that: The proportional-integral PI feedback module includes a low-pass filter LPF and a PI module; The power signal is input into the proportional-integral PI feedback module, mixed with the low-frequency sinusoidal signal, and then filtered through the low-pass filter LPF to generate the input signal of the PI module. The PI module dynamically adjusts the proportional coefficient gp and the integral coefficient fI according to the real-time changes of the signal.

6. A method for reducing the phase noise of a light source in a dual-field quantum key distribution system according to claim 4, characterized in that: The continuous light at Alice's end is split by the beam splitter BS1, specifically: The continuous light at the Alice end first passes through a polarization controller PC and then is split by the beam splitter BS1.

7. A structure for reducing the phase noise of a light source in a dual-field quantum key distribution system, characterized in that: It includes a beat frequency signal generation module and an optical phase-locked loop OPLL; The laser signals of Alice and Bob generate beat frequency signals through the beat frequency signal generation module, which are converted into electrical signals through the optical detector PD and input into the optical phase-locked loop OPLL to generate the feedback control signal, which is fed back to the laser at the Bob end to adjust the frequency and phase of the laser at the Bob end.

8. The structure for reducing the phase noise of the light source in the dual-field quantum key distribution system according to claim 7, characterized in that: The beat frequency signal generation module includes a beam splitter BS1, a beam splitter BS3, a beam splitter BS4 and an AOM; The continuous light at the Alice end is split by the optical beam splitter BS1 into a first light beam and a second light beam; The continuous light generated by the Bob-end laser passes through the phase modulator PM and then is split by the optical beam splitter BS3 into the third light beam and the fourth light beam; The third light beam or the fourth light beam is frequency-shifted by the AOM to obtain a frequency-shifted light beam; One of the first light beam and the second light beam and the frequency-shifted light beam generate a beat signal through the beam splitter BS4.

9. The structure for reducing the phase noise of the light source in the dual-field quantum key distribution system according to claim 7, characterized in that: The optical phase-locked loop OPLL includes a local oscillator LO, a mixer and a loop filter Loop filter; The beat frequency signal is converted into an electrical signal by an optical detector PD and input into an optical phase-locked loop OPLL, mixed with a reference frequency signal generated by a local oscillator LO in a mixer to generate an error signal, and processed by a loop filter to generate the feedback control signal.

10. The structure for reducing the phase noise of the light source in the dual-field quantum key distribution system according to claim 8, characterized in that: Also includes beam splitter BS2, beam splitter BS5, beam splitter BS6 and proportional-integral PI feedback module The proportional-integral PI feedback module includes a low-pass filter LPF and a PI module; After the other of the first light beam and the second light beam passes through the beam splitter BS2, an optical signal S1 and another signal for quantum key encoding are obtained; After the light beam in the third light beam or the fourth light beam that has not been frequency shifted passes through the beam splitter BS5, an optical signal S2 and another signal for quantum key encoding are obtained; The optical signal S1 and the optical signal S2 are coupled via the optical beam splitter BS6, and an optical power meter is used to measure the power value w of the output optical signal in any optical path after coupling to obtain a power signal, and the power signal is input into the proportional-integral PI feedback module; The power signal is input into the proportional-integral PI feedback module, mixed with the low-frequency sinusoidal signal, and then filtered through the low-pass filter LPF to generate the input signal of the PI module. The PI module dynamically adjusts the proportional coefficient gp and the integral coefficient fI according to the real-time changes of the signal.

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