A polarization compensation system and method based on entangled quantum key distribution

Abstract

The present application relates to the field of quantum communication, and particularly relates to a polarization compensation system and method based on entangled quantum key distribution. The polarization compensation system based on entangled quantum key distribution comprises an entangled quantum preparation and sending module, an entangled quantum receiving module, a reference light preparation and sending module, a reference light depolarization module and a polarization compensation driving control module. In the present application, the preparation and sending of reference light, the depolarization detection of reference light, the preparation and sending of signal light, the depolarization detection of signal light and the control of polarization compensation are all performed in real time and synchronously. The real-time polarization information obtained by the depolarization of reference light is fed back to the polarization compensation driving control module, and the polarization state is changed through the action of EPC. The signal photons of the back propagation first experience the polarization change of EPC, and then enter the optical fiber channel, so as to realize polarization compensation. The technical scheme of the present application can effectively improve the rate of polarization compensation, and make the error rate of the system stable at a relatively low level.

Description

Technical Field

[0001] This invention relates to the field of quantum communication, and in particular to a polarization compensation system and method based on entangled quantum key distribution. Background Technology

[0002] Quantum Key Distribution (QKD) is the application of quantum cryptography in quantum communication. Generally, QKD can be divided into two methods: one involves either party generating a random key, which is then encoded into a quantum state and distributed. The other party randomly chooses its measurement basis for measurement; this is called the Prepare-and-Measure (PM) protocol. The other method utilizes entangled states in quantum states. Instead of generating a random key beforehand, either party or a third party prepares sufficient entangled photon pairs, which are then sent to both parties. The receiving parties then measure the received photons. This type of QKD protocol is called an Entanglement-Based (EB) protocol.

[0003] Currently, the main encoding methods used in quantum key distribution (QKD) systems are polarization encoding and phase encoding. Polarization-encoded QKD systems have the advantage of high key generation rates. However, in polarization-encoded QKD, the geometry and birefringence of the optical fiber itself cause changes in the polarization state of photons during propagation. Furthermore, the birefringence of the fiber is extremely susceptible to external environmental influences, making these polarization state changes random and time-dependent. These changes increase the system's bit error rate, ultimately leading to key generation interruption. In addition, the dispersion and polarization mode dispersion of the optical fiber further reduce the time correlation and polarization degree of photons, introducing errors into the system and increasing the bit error rate. Therefore, when using polarization-encoded QKD schemes, polarization compensation must be considered; otherwise, the system cannot achieve the correct key distribution process.

[0004] There are various polarization compensation schemes and devices in fabrication-measurement quantum key distribution (PM-QKD) systems, but there are fewer polarization compensation methods and devices for entanglement-based quantum key distribution (EB-QKD) systems. In order to improve the development of EB-QKD systems, faster and more stable polarization compensation methods and corresponding compensation devices based on EB-QKD are needed. Summary of the Invention

[0005] In view of this, the present invention proposes a wavelength division multiplexing polarization compensation scheme for entangled quantum key distribution systems, which solves the polarization compensation problem in current quantum communication systems based on entangled quantum key distribution.

[0006] First, this invention provides a polarization compensation system based on entangled quantum key distribution, comprising:

[0007] Entangled quantum preparation and transmission module, used to generate entangled photon pairs as signal light for quantum key distribution;

[0008] Entangled quantum receiving module, used to receive photons and measure them;

[0009] Reference light preparation and transmission module, used to output conjugate reference light signal;

[0010] The reference beam depolarization module is used to detect and depolarize the reference beam.

[0011] The polarization compensation drive control module is used to acquire data from the reference light depolarization module and perform polarization compensation on the polarization state of the signal light according to the polarization change of the reference light.

[0012] The entangled quantum preparation and transmission module and the reference light depolarization module are connected to one end of the optical fiber channel via a first dense wavelength division multiplexer and the polarization compensation drive control module. The entangled quantum receiving module and the reference light preparation and transmission module are connected to the other end of the optical fiber channel via a second dense wavelength division multiplexer. The polarization compensation drive control module is electrically connected to the reference light depolarization module.

[0013] Furthermore, the entangled quantum preparation and transmission module includes: a quantum entanglement source, a first beam splitter, a first polarization beam splitter, a first half-wave plate, a second polarization beam splitter, and four single-photon detectors.

[0014] The quantum entanglement source is used to prepare entangled photon pairs and is connected to the input ends of the first dense wavelength division multiplexer and the first beam splitter, respectively. The first output end of the first beam splitter is connected to the input end of the first polarization beam splitter, and the two output ends of the first polarization beam splitter are each connected to a single photon detector.

[0015] The second output of the first beam splitter is connected to the input of the second polarization beam splitter via the first half-wave plate, and the two outputs of the second polarization beam splitter are respectively connected to a single-photon detector.

[0016] Furthermore, the entangled quantum receiving module includes: a second beam splitter, a third polarization beam splitter, a second half-wave plate, a fourth polarization beam splitter, and four single-photon detectors.

[0017] Wherein, the input end of the second beam splitter is connected to the second dense wavelength division multiplexer, the first output end of the second beam splitter is connected to the input end of the third polarization beam splitter, and the two output ends of the third polarization beam splitter are respectively connected to a single-photon detector;

[0018] The second output terminal of the second beam splitter is connected to the input terminal of the fourth polarization beam splitter via the second half-wave plate, and the two output terminals of the fourth polarization beam splitter are respectively connected to a single-photon detector.

[0019] Furthermore, the reference light preparation and transmission module includes: two lasers with different center wavelengths, two optical isolators, a first polarization-maintaining beam splitter, a second polarization-maintaining beam splitter, and a Faraday rotator;

[0020] One laser, one optical isolator, and the first polarization-maintaining beam splitter are sequentially connected to the second dense wavelength division multiplexer, and another laser, another optical isolator, the second polarization-maintaining beam splitter, and the Faraday rotator are sequentially connected to the second dense wavelength division multiplexer.

[0021] Furthermore, the reference optical polarization de-polarization module includes: a polarization controller, a fifth polarization beam splitter, a sixth polarization beam splitter, and a photodetector.

[0022] The input terminal of the fifth polarization beamsplitter is connected to the first dense wavelength division multiplexer, and the input terminal of the sixth polarization beamsplitter is connected to the first dense wavelength division multiplexer via the polarization controller; the two output terminals of the fifth polarization beamsplitter and the two output terminals of the sixth polarization beamsplitter are each connected to a photodetector.

[0023] The polarization controller is used to ensure the polarization relationship between the two reference beams and the accuracy of the polarization reference system. The fifth and sixth polarization beam splitters are used to perform polarization projection measurements on the reference beams. The four photodetectors convert the light intensity obtained after the projection measurement into voltage information.

[0024] Furthermore, the polarization compensation drive control module includes: an electric polarization controller and a drive control circuit unit;

[0025] The electric polarization controller is used to compensate for the influence of the optical fiber channel on the polarization state, and the drive control circuit unit is used to provide a drive voltage that satisfies the driving voltage of the electric polarization controller and to perform polarization compensation calculation on the output voltage information of the four photodetectors.

[0026] Furthermore, the electric polarization controller is any one of a lithium niobate polarization controller, a piezoelectric ceramic electric polarization controller, and a liquid crystal electric polarization controller.

[0027] Furthermore, the drive control circuit unit includes: a power boost circuit, a positive and negative dual voltage generation circuit, a differential amplifier circuit, an FPGA processing circuit, a data acquisition and processing circuit, and an interactive interface circuit;

[0028] The power boost circuit is used to convert an analog voltage with an input voltage of 5V into a 60V drive signal through a boost chip.

[0029] The positive and negative dual voltage generation circuit is used to generate positive and negative 45V voltages from the 60V drive signal through a step-down chip for output.

[0030] The differential amplifier circuit is used to amplify and output the input analog signal;

[0031] The data acquisition and processing circuit includes an A / D conversion chip and a D / A conversion chip, as well as surrounding circuitry. The A / D conversion chip is used to convert the input analog signal into a digital signal that can be read by the FPGA; the D / A conversion chip is used to convert the digital signal output by the FPGA into an analog signal; and the surrounding circuitry is used to drive the A / D conversion chip and the D / A conversion chip.

[0032] The FPGA processing circuit is used to process the acquired voltage information and output a digital voltage signal to the data acquisition and processing circuit.

[0033] The interactive interface circuit is used to connect with the output cable of an external photodetector to obtain the voltage information after reference photodepolarization, and is also used to perform real-time information interaction with external devices such as computers and other host computers.

[0034] Furthermore, when the polarization compensation drive control module is in operation, it performs the following steps:

[0035] S1: After receiving the decoded polarization information transmitted by the four photodetectors, the polarization compensation drive control module converts the analog signal into the corresponding digital signal by the A / D conversion module to obtain the output voltage of the four photodetectors;

[0036] S2: The FPGA processing circuit subtracts the respective reference voltages from the output voltages of the four photodetectors to obtain voltage values ​​V1', V2', V3', and V4'. It then calculates polarization evaluation functions F1 and F2 according to the formulas F1 = V2' / (V1' + V2') and F2 = V4' / (V3' + V4'), and compares them with preset thresholds T1 and T2. If F1 ≥ T1 or F2 ≥ T2, it indicates a change in polarization state, requiring compensation, and step S3 is executed; if F1 < T1 and F2 < T2, step S5 is executed.

[0037] S3: Obtain the voltage variable to be applied to the currently selected axis of the electric polarization controller using the formula (V2'+V4') / n, output a digital signal, convert the digital signal into a voltage signal through the D / A conversion module, and then amplify it through the differential amplifier circuit before outputting it to each pin of the electric polarization controller. Here, n is the control parameter for the increase or decrease gradient of the voltage variable. After each adjustment of the voltage value, F1 and F2 are recalculated. If F1 < T1 and F2 < T2, proceed to step S5; otherwise, execute step S4.

[0038] S4: Calculate the absolute value of the difference between V2' and V4' before and after the voltage adjustment. If both absolute values ​​are less than the preset axis switching value, switch to the next axis of the electric polarization controller for compensation. Otherwise, continue to compensate on the current axis.

[0039] S5: Repeat the above steps until the preset threshold N times are reached. If F1 < T1 and F2 < T2 are satisfied each time, the compensation is considered to be completed. If not, return to step S1 and continue to perform polarization compensation until the compensation is completed.

[0040] S6: Real-time detection of the system bit error rate. When the bit error rate exceeds the preset value, repeat steps S1 to S5 to ensure that the system bit error rate is within the set range.

[0041] Secondly, this invention also proposes a polarization compensation method based on entangled quantum key distribution, comprising the following steps:

[0042] S10: The reference light preparation and transmission module prepares two conjugate reference lights, which are coupled into the optical fiber channel through different channels of the dense wavelength division multiplexer. After being demultiplexed by the polarization compensation drive control module and the dense wavelength division multiplexing module, the polarization state of the conjugate reference light is detected by the four photodetectors of the reference light depolarization module.

[0043] S20: The reference light polarization depolarization module transmits the analog signal of the polarization information of the reference light to the polarization compensation drive control module;

[0044] S30: The A / D conversion module in the polarization compensation drive control module converts the analog signal into the corresponding digital signal to obtain the output voltage of the four photodetectors;

[0045] S40: The FPGA processing circuit subtracts the respective reference voltages from the output voltages of the four photodetectors to obtain voltage values ​​V1', V2', V3', and V4'. It then calculates polarization evaluation functions F1 and F2 according to the formulas F1 = V2' / (V1' + V2') and F2 = V4' / (V3' + V4'), and compares them with preset thresholds T1 and T2. If F1 ≥ T1 or F2 ≥ T2, it indicates a change in polarization state, requiring compensation, and step S50 is executed; if F1 < T1 and F2 < T2, step S70 is executed.

[0046] S50: Obtain the voltage variable to be applied to the currently selected axis of the electric polarization controller using the formula (V2'+V4') / n, output a digital signal, convert the digital signal into a voltage signal through the D / A conversion module, and then amplify it through the differential amplifier circuit before outputting it to each pin of the electric polarization controller. Here, n is the control parameter for the increase or decrease gradient of the voltage variable. After each adjustment of the voltage value, F1 and F2 are recalculated. If F1 < T1 and F2 < T2, proceed to step S5; otherwise, execute step S4.

[0047] S60: Calculate the absolute value of the difference between V2' and V4' before and after the voltage adjustment. If both absolute values ​​are less than the preset axis switching value, switch to the next axis of the electric polarization controller for compensation. Otherwise, continue to compensate on the current axis.

[0048] S70: Repeat the above steps until the preset threshold N times are reached. If F1 < T1 and F2 < T2 are satisfied each time, the compensation is considered to be completed. If not, return to step S30 and continue to perform polarization compensation until the compensation is completed.

[0049] S80: Real-time detection of the system bit error rate. When the bit error rate exceeds the preset value, repeat steps S10 to S70 to ensure that the system bit error rate is within the set range.

[0050] The beneficial effects of this invention are:

[0051] 1. The polarization compensation system and method based on entangled quantum key distribution system proposed in this invention are based on the wavelength division multiplexing polarization compensation method of preparation-measurement (PM) quantum key distribution system. At present, there is no wavelength division multiplexing polarization compensation scheme applied to entangled quantum key distribution system. The proposed method can improve the stability of entangled quantum key distribution system and has better performance than the polarization compensation schemes of entangled quantum key distribution system that have been proposed. It effectively improves the polarization compensation rate and stabilizes the bit error rate of the system at a lower level.

[0052] 2. The polarization compensation control method proposed in this invention is based on the reference light polarization detection module of this system. The polarization information is obtained by detecting the light intensity through four photodetectors and converted into voltage information. When processing in the FPGA processing circuit, a specific blind search algorithm is adopted. After one iteration, the deviation between the target polarization state and the current polarization state is calculated, and the gradient of the optimization algorithm is continuously corrected according to the deviation, that is, the voltage increment is applied. This avoids the problems of local extrema and repeated iteration caused by some optimization algorithms, and greatly reduces the polarization compensation time.

[0053] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of the structures pointed out in the description, claims, and drawings. Attached Figure Description

[0054] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0055] Figure 1 A schematic diagram of the polarization compensation system based on entanglement quantum key distribution of the present invention is shown;

[0056] Figure 2 A schematic diagram of the internal structure of the polarization compensation drive control module of the present invention is shown; Detailed Implementation

[0057] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0058] Figure 1 This is a schematic diagram of the polarization compensation system based on entanglement quantum key distribution of the present invention, as shown below. Figure 1 As shown, the polarization compensation system based on entanglement quantum key distribution includes:

[0059] Entangled quantum preparation and transmission module, used to generate entangled photon pairs as signal light for quantum key distribution;

[0060] Entangled quantum receiving module, used to receive photons and measure them;

[0061] Reference light preparation and transmission module, used to output conjugate reference light signal;

[0062] The reference beam depolarization module is used to detect and depolarize the reference beam.

[0063] The polarization compensation drive control module is used to acquire data from the reference light depolarization module and perform polarization compensation on the polarization state of the signal light according to the polarization change of the reference light.

[0064] In one specific embodiment, the entangled quantum preparation and transmission module includes: a quantum entanglement source, a first beam splitter BS1, a first polarization beam splitter PBS1, a first half-wave plate HWP1, a second polarization beam splitter PBS2, and four single-photon detectors SPD1, SPD2, SPD3, and SPD4.

[0065] In one specific embodiment, the entangled quantum receiving module includes: a second beam splitter BS2, a third polarization beam splitter PBS3, a second half-wave plate HWP2, a fourth polarization beam splitter PBS4, and four single-photon detectors SPD5, SPD6, SPD7, and SPD8.

[0066] In one specific embodiment, the reference light preparation and transmission module includes: two lasers LD with different center wavelengths, two optical isolators ISO, a first polarization-maintaining beam splitter PMPBS1, a second polarization-maintaining beam splitter PMPBS2, and a Faraday rotator FR;

[0067] In one specific embodiment, the reference optical depolarization module includes: a polarization controller PC, a fifth polarization beam splitter PBS5, a sixth polarization beam splitter PBS6, and photodetectors PD1, PD2, PD3, and PD4.

[0068] The polarization controller PC is used to ensure the polarization relationship between the two reference beams and the accuracy of the polarization reference system. The fifth polarization beam splitter PBS5 and the sixth polarization beam splitter PBS6 are used to perform polarization projection measurement on the reference beam. The four photodetectors convert the light intensity obtained after the projection measurement into voltage information.

[0069] In other specific embodiments, in addition to using the polarization beam splitter and photodetector mentioned in this invention to obtain polarization information, other methods can also be used to obtain polarization information, such as using a polarization detector (POD) or a polarization degree meter (DOP) to obtain polarization information.

[0070] In one specific embodiment, the polarization compensation drive control module includes: an electric polarization controller EPC and a drive control circuit unit CM;

[0071] The electric polarization controller (EPC) is used to compensate for the influence of the optical fiber channel on the polarization state, and the drive control circuit unit (CM) is used to provide a drive voltage that satisfies the driving voltage of the electric polarization controller (EPC) and to perform polarization compensation calculations on the output voltage information of the four photodetectors.

[0072] In one specific embodiment, the electric polarization controller EPC is any one of a lithium niobate polarization controller, a piezoelectric ceramic electric polarization controller, and a liquid crystal electric polarization controller.

[0073] In a specific embodiment, such as Figure 2 As shown, the drive control circuit unit CM includes: a power boost circuit, a positive and negative dual voltage generation circuit, a differential amplifier circuit, an FPGA processing circuit, a data acquisition and processing circuit, and an interactive interface circuit.

[0074] The power boost circuit is used to convert an analog voltage with an input voltage of 5V into a 60V drive signal through a boost chip.

[0075] The positive and negative dual voltage generation circuit is used to generate positive and negative 45V voltages from the 60V drive signal through a step-down chip for output.

[0076] The differential amplifier circuit is used to amplify and output the input analog signal;

[0077] The data acquisition and processing circuit includes an A / D conversion chip and a D / A conversion chip, as well as surrounding circuitry. The A / D conversion chip is used to convert the input analog signal into a digital signal that can be read by the FPGA; the D / A conversion chip is used to convert the digital signal output by the FPGA into an analog signal; and the surrounding circuitry is used to drive the A / D conversion chip and the D / A conversion chip.

[0078] The FPGA processing circuit is used to process the acquired voltage information and output a digital voltage signal to the data acquisition and processing circuit.

[0079] The interactive interface circuit is used to connect with the output cable of an external photodetector to obtain the voltage information after reference photodepolarization, and is also used to perform real-time information interaction with external devices such as computers and other host computers.

[0080] The working principle of the entire system is as follows:

[0081] The entangled quantum preparation and transmission module prepares a pair of photons with a center wavelength of 1550 nm in an entangled state using a quantum entanglement source. The first beam splitter BS1 is connected to the quantum entanglement source via a polarization-maintaining fiber. One photon randomly selects either the Z-base or X-base for polarization measurement via the first beam splitter BS1. The X-base measurement end has a first half-wave plate HWP1 for rotating the polarization state by 45 degrees. Two polarization beam splitters are connected to the transmission and reflection exits of the first beam splitter via single-mode fibers, respectively, for projection measurements. A single-photon detector measures the single photon. The other photon is connected to the first dense wavelength division multiplexer DWDM1 via a single-mode fiber and then enters the channel to the entangled quantum receiving module. Similarly, the entangled quantum receiving module has the same polarization detection device as the entangled quantum preparation and transmission module. The photon enters the polarization measurement module after passing through the second dense wavelength division multiplexer DWDM2, and then randomly selects either the Z-base or X-base for measurement after passing through the second beam splitter. Ideally, when both parties use the same measurement basis, the responses of the single-photon detectors will be opposite. For example, if both parties choose to use the Z-basis for measurement, the depolarized polarization states will be horizontal for one party and vertical for the other. However, as mentioned above, in practice, when a single photon is transmitted over a long distance through a single-mode fiber, the polarization state will randomly change due to the influence of the external environment and the fiber's internal structure. This random change in polarization state leads to an increase in the system's bit error rate, affecting the actual performance of key distribution.

[0082] To improve this situation, this invention proposes a polarization compensation system based on entanglement-based quantum key distribution to compensate for perturbations in the polarization state of signal photons in optical fiber channels. A reference light preparation and transmission module, a polarization compensation drive control module, and a reference light depolarization module are added to the original system. To avoid inconsistencies in polarization effects experienced by different wavelengths in the channel, the reference light module uses two continuous beams of different wavelengths with a wavelength interval differing from the signal light interval by 0.8 nm, with the signal light wavelength falling between the two reference light wavelengths. The reference light passes through an optical isolator (ISO) and then enters a polarization-maintaining beam splitter. The polarization-maintaining beam splitter generates slow-axis light with a horizontal polarization state. One beam, after passing through the polarization-maintaining beam splitter to form a slow-axis light with a horizontal polarization state, passes through a Faraday rotator (FR) to form 45-degree polarized light, creating two conjugate reference beams, thus enabling complete control over either polarization state. Two reference beams are coupled into the optical fiber channel through different channels of the second dense wavelength division multiplexer (DWDM2), and are opposite to the transmission direction of the signal photons. The polarization state, which has been affected by the channel, is compensated by the polarization compensation drive control module. After compensation, the two reference beams are demultiplexed by the first dense wavelength division multiplexer. Then, the polarization beam splitter, polarization controller and single-photon detector of the reference beam depolarization module complete the depolarization and detection operations.

[0083] During system operation, the preparation and transmission of reference light, the depolarization detection of reference light, the preparation and transmission of signal light, the depolarization detection of signal light, and the control of polarization compensation are all performed synchronously in real time. After the two conjugate reference beams pass through the fiber optic channel and undergo depolarization detection, the resulting real-time polarization information is fed back to the polarization compensation drive control module. The motorized polarization controller (EPC) then modifies the polarization state to output the desired target polarization state. The back-propagating signal photon first undergoes polarization change by the EPC before entering the fiber optic channel. This reverse polarization change compensates for the polarization effects experienced by the signal photon in the fiber optic channel, thus completing the polarization compensation process.

[0084] The polarization compensation method based on entangled quantum key distribution proposed in this invention includes the following steps:

[0085] S10: The reference light preparation and transmission module prepares two conjugate reference lights, which are coupled into the optical fiber channel through different channels of the dense wavelength division multiplexer. After being demultiplexed by the polarization compensation drive control module and the dense wavelength division multiplexing module, the polarization state of the conjugate reference light is detected by the four photodetectors of the reference light depolarization module.

[0086] S20: The reference light polarization depolarization module transmits the analog signal of the polarization information of the reference light to the polarization compensation drive control module;

[0087] S30: The A / D conversion module in the polarization compensation drive control module converts the analog signal into the corresponding digital signal to obtain the output voltage of the four photodetectors;

[0088] S40: The FPGA processing circuit subtracts the respective reference voltages from the output voltages of the four photodetectors to obtain voltage values ​​V1', V2', V3', and V4'. It then calculates polarization evaluation functions F1 and F2 according to the formulas F1 = V2' / (V1' + V2') and F2 = V4' / (V3' + V4'), and compares them with preset thresholds T1 and T2. If F1 ≥ T1 or F2 ≥ T2, it indicates a change in polarization state, requiring compensation, and step S50 is executed; if F1 < T1 and F2 < T2, step S70 is executed.

[0089] S50: The voltage variable to be applied to the currently selected axis of the electric polarization controller is obtained using the formula (V2'+V4') / n. A digital signal is output, which is then converted to a voltage signal by a D / A converter. This signal is amplified by a differential amplifier circuit and output to the various pins of the electric polarization controller. Here, n is the control parameter for the voltage variable's increase or decrease gradient. F1 and F2 are recalculated after each voltage adjustment. If F1 < T1 and F2 < T2, proceed to step S5; otherwise, execute step S4.

[0090] S60: Calculate the absolute value of the difference between V2' and V4' before and after the voltage adjustment. If both absolute values ​​are less than the preset axis switching value, switch to the next axis of the electric polarization controller for compensation. Otherwise, continue to compensate on the current axis.

[0091] S70: Repeat the above steps until the preset threshold N times are reached. If F1 < T1 and F2 < T2 are satisfied each time, the compensation is considered to be completed. If not, return to step S30 and continue to perform polarization compensation until the compensation is completed.

[0092] S80: Real-time detection of the system bit error rate. When the bit error rate exceeds the preset value, repeat steps S10 to S70 to ensure that the system bit error rate is within the set range.

[0093] In step S50, the voltage variable can be calculated using the collected voltage values ​​V2 and V4. The initial value of the voltage variable is zero, and the voltage variable = (V2' + V4') / n. Here, n is not a fixed value; changing the value of n can change the gradient of voltage variable increase or decrease in each iteration. During the compensation process, the number of iterations is observed. If the number of iterations is too high, it indicates that the compensation time is too long, and n can be appropriately reduced. The value of n needs to be determined based on the actual compensation effect. The default setting in the experiment is n = 128.

[0094] In step S60, the axis-switching accuracy is adjustable, preferably 5mV. Specifically, if the voltage difference between two consecutive voltages is less than 5mV, it indicates that the applied voltage has not caused a change in the polarization state, proving that applying voltage to this axis cannot achieve the compensation effect, and it is necessary to jump to the next axis to apply voltage, i.e., to perform an axis-switching operation. Setting the axis-switching value is also to avoid repeated compensation on this axis without being able to jump out, which would cause excessive compensation time. In addition to adjusting the axis-switching value, the size of n can also be appropriately adjusted according to the actual situation to avoid the time consumption caused by repeated axis-switching.

[0095] During compensation, repeat steps S30-S60 above.

[0096] In step S70, the number of iterations N can be adjusted according to actual needs, preferably 100 times.

[0097] The polarization compensation control method proposed in this invention is based on the reference light polarization detection module of this system. The polarization information is obtained by detecting the light intensity through four photodetectors and converted into voltage information. When processing in the FPGA processing circuit, a specific blind search algorithm is adopted. After one iteration, the deviation between the target polarization state and the current polarization state is calculated, and the gradient of the optimization algorithm is continuously corrected according to the deviation, i.e., voltage increment is applied. This avoids the problems of local extrema and repeated iteration caused by some optimization algorithms, and greatly reduces the polarization compensation time.

[0098] Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A polarization compensation system based on entangled quantum key distribution, characterized in that, include: Entangled quantum preparation and transmission module, used to generate entangled photon pairs as signal light for quantum key distribution; Entangled quantum receiving module, used to receive photons and measure them; Reference light preparation and transmission module, used to output conjugate reference light signal; The reference light depolarization module is used to detect and depolarize the reference light, and converts the light intensity obtained after projection measurement into voltage information through four photodetectors. The polarization compensation drive control module is used to acquire data from the reference light depolarization module and perform polarization compensation on the polarization state of the signal light according to the polarization change of the reference light. The entangled quantum preparation and transmission module and the reference light depolarization module are connected to one end of the optical fiber channel via a first dense wavelength division multiplexer and the polarization compensation drive control module, and the entangled quantum receiving module and the reference light preparation and transmission module are connected to the other end of the optical fiber channel via a second dense wavelength division multiplexer. The polarization compensation drive control module is electrically connected to the reference optical polarization de-polarization module. When the polarization compensation drive control module is working, it performs the following steps: S1: After receiving the decoded polarization information transmitted by the four photodetectors, the polarization compensation drive control module converts the analog signal into the corresponding digital signal by the A / D conversion module to obtain the output voltage of the four photodetectors; S2: The FPGA processing circuit subtracts the respective reference voltages from the output voltages of the four photodetectors to obtain voltage values ​​V1', V2', V3', and V4'. It then calculates polarization evaluation functions F1 and F2 according to the formulas F1 = V2' / (V1' + V2') and F2 = V4' / (V3' + V4'), and compares them with preset thresholds T1 and T2. If F1 ≥ T1 or F2 ≥ T2, it indicates a change in polarization state, requiring compensation, and proceeds to step S3; if F1 < T1 and F2 < T2, proceeds to step S5. S3: Obtain the voltage variable to be applied to the currently selected axis of the electric polarization controller using the formula (V2'+V4') / n, output a digital signal, convert the digital signal into a voltage signal through the D / A conversion module, and then amplify it through the differential amplifier circuit before outputting it to each pin of the electric polarization controller. Here, n is the control parameter for the increase or decrease gradient of the voltage variable. After each adjustment of the voltage value, F1 and F2 are recalculated. If F1 < T1 and F2 < T2, proceed to step S5; otherwise, execute step S4. S4: Calculate the absolute value of the difference between V2' and V4' before and after the voltage adjustment. If both absolute values ​​are less than the preset axis switching value, switch to the next axis of the electric polarization controller for compensation. Otherwise, continue to compensate on the current axis. S5: Repeat the above steps until the preset threshold N times are reached. If F1 < T1 and F2 < T2 are satisfied each time, the compensation is considered to be completed. If not, return to step S1 and continue to perform polarization compensation until the compensation is completed. S6: Real-time detection of the system bit error rate. When the bit error rate exceeds the preset value, repeat steps S1 to S5 to ensure that the system bit error rate is within the set range.

2. The polarization compensation system based on entanglement quantum key distribution according to claim 1, characterized in that, The entangled quantum preparation and transmission module includes: a quantum entanglement source, a first beam splitter, a first polarization beam splitter, a first half-wave plate, a second polarization beam splitter, and four single-photon detectors; The quantum entanglement source is used to prepare entangled photon pairs and is connected to the input ends of the first dense wavelength division multiplexer and the first beam splitter, respectively. The first output end of the first beam splitter is connected to the input end of the first polarization beam splitter, and the two output ends of the first polarization beam splitter are respectively connected to a single photon detector. The second output of the first beam splitter is connected to the input of the second polarization beam splitter via the first half-wave plate, and the two outputs of the second polarization beam splitter are respectively connected to a single-photon detector.

3. The polarization compensation system based on entanglement quantum key distribution according to claim 1, characterized in that, The entangled quantum receiving module includes: a second beam splitter, a third polarization beam splitter, a second half-wave plate, a fourth polarization beam splitter, and four single-photon detectors; The input end of the second beam splitter is connected to the second dense wavelength division multiplexer, the first output end of the second beam splitter is connected to the input end of the third polarization beam splitter, and the two output ends of the third polarization beam splitter are respectively connected to a single-photon detector. The second output terminal of the second beam splitter is connected to the input terminal of the fourth polarization beam splitter via the second half-wave plate, and the two output terminals of the fourth polarization beam splitter are respectively connected to a single-photon detector.

4. The polarization compensation system based on entanglement quantum key distribution according to claim 3, characterized in that, The reference light preparation and transmission module includes: two lasers with different center wavelengths, two optical isolators, a first polarization-maintaining beam splitter, a second polarization-maintaining beam splitter, and a Faraday rotator; One laser, one optical isolator, and the first polarization-maintaining beam splitter are sequentially connected to the second dense wavelength division multiplexer, and another laser, another optical isolator, the second polarization-maintaining beam splitter, and the Faraday rotator are sequentially connected to the second dense wavelength division multiplexer.

5. A polarization compensation system based on entanglement-based quantum key distribution according to claim 2, characterized in that, The reference optical polarization de-polarization module includes: a polarization controller, a fifth polarization beam splitter, a sixth polarization beam splitter, and four photodetectors; The input terminal of the fifth polarization beamsplitter is connected to the first dense wavelength division multiplexer, and the input terminal of the sixth polarization beamsplitter is connected to the first dense wavelength division multiplexer via the polarization controller; the two output terminals of the fifth polarization beamsplitter and the two output terminals of the sixth polarization beamsplitter are each connected to a photodetector. The polarization controller is used to ensure the polarization relationship between the two reference beams and the accuracy of the polarization reference system. The fifth and sixth polarization beam splitters are used to perform polarization projection measurements on the reference beams.

6. A polarization compensation system based on entanglement-based quantum key distribution according to claim 1, characterized in that, The polarization compensation drive control module includes: an electric polarization controller and a drive control circuit unit; The electric polarization controller is used to compensate for the influence of the optical fiber channel on the polarization state, and the drive control circuit unit is used to provide a drive voltage that satisfies the driving voltage of the electric polarization controller and to perform polarization compensation calculation on the output voltage information of the four photodetectors.

7. A polarization compensation system based on entanglement-based quantum key distribution according to claim 6, characterized in that, The electric polarization controller is a lithium niobate polarization controller or a piezoelectric ceramic electric polarization controller.

8. A polarization compensation system based on entanglement-based quantum key distribution according to claim 7, characterized in that, The drive control circuit unit includes: a power boost circuit, a positive and negative dual voltage generation circuit, a differential amplifier circuit, an FPGA processing circuit, a data acquisition and processing circuit, and an interactive interface circuit. The power boost circuit is used to convert an analog voltage with an input voltage of 5V into a 60V drive signal through a boost chip. The positive and negative dual voltage generation circuit is used to generate positive and negative 45V voltages from the 60V drive signal through a step-down chip for output. The differential amplifier circuit is used to amplify and output the input analog signal; The data acquisition and processing circuit includes an A / D conversion chip and a D / A conversion chip, as well as surrounding circuitry. The A / D conversion chip is used to convert the input analog signal into a digital signal that can be read by the FPGA; the D / A conversion chip is used to convert the digital signal output by the FPGA into an analog signal; and the surrounding circuitry is used to drive the A / D conversion chip and the D / A conversion chip. The FPGA processing circuit is used to process the acquired voltage information and output a digital voltage signal to the data acquisition and processing circuit. The interactive interface circuit is used to connect to the output cable of an external photodetector to obtain the voltage information after reference optical polarization, and is also used to perform real-time information interaction with external devices.

9. A polarization compensation method based on entangled quantum key distribution, employing a polarization compensation system based on entangled quantum key distribution as described in any one of claims 1-8, characterized in that, include: S10: The reference light preparation and transmission module prepares two conjugate reference lights, which are coupled into the optical fiber channel through different channels of the dense wavelength division multiplexer. After being demultiplexed by the polarization compensation drive control module and the dense wavelength division multiplexing module, the polarization state of the conjugate reference light is detected by the four photodetectors of the reference light depolarization module. S20: The reference light polarization depolarization module transmits the analog signal of the polarization information of the reference light to the polarization compensation drive control module; S30: The A / D conversion module in the polarization compensation drive control module converts the analog signal into the corresponding digital signal to obtain the output voltage of the four photodetectors; S40: The FPGA processing circuit subtracts the respective reference voltage from the output voltage of the four photodetectors to obtain voltage values ​​V1', V2', V3', and V4'. According to the formulas F1 = V2' / (V1'+V2') and F2 = V4' / (V3'+V4'), the polarization evaluation functions F1 and F2 are calculated and compared with the preset thresholds T1 and T2. If F1 ≥ T1 or F2 ≥ T2, it indicates that the polarization state has changed and compensation is required. Step S50 is then executed. If F1 < T1 and F2 < T2, then proceed to step S70; S50: Obtain the voltage variable to be applied to the currently selected axis of the electric polarization controller using the formula (V2'+V4') / n, output a digital signal, convert the digital signal into a voltage signal through the D / A conversion module, and then amplify it through the differential amplifier circuit before outputting it to each pin of the electric polarization controller. Here, n is the control parameter for the increase or decrease gradient of the voltage variable. After each adjustment of the voltage value, F1 and F2 are recalculated. If F1 < T1 and F2 < T2, proceed to step S5; otherwise, execute step S4. S60: Calculate the absolute value of the difference between V2' and V4' before and after the voltage adjustment. If both absolute values ​​are less than the preset axis switching value, switch to the next axis of the electric polarization controller for compensation. Otherwise, continue to compensate on the current axis. S70: Repeat the above steps until the preset threshold N times are reached. If F1 < T1 and F2 < T2 are satisfied each time, the compensation is considered to be completed. If not, return to step S30 and continue to perform polarization compensation until the compensation is completed. S80: Real-time detection of the system bit error rate. When the bit error rate exceeds the preset value, repeat steps S10 to S70 to ensure that the system bit error rate is within the set range.

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