TF-QKD device for reference light polarization multiplexing and coding method

Abstract

The invention provides a reference light polarization multiplexing TF-QKD device and a coding method, and relates to the technical field of quantum communication. A light source emitted by a single-wavelength laser is subjected to time division multiplexing modulation through a phase modulator, an intensity modulator and a polarization modulator to obtain a strong phase reference light pulse, a weak phase reference light pulse and a quantum signal light pulse; after being attenuated by the optical attenuator, the signals are sent to a detection end through an optical fiber line; after light pulses of an Alice end and a Bob end received by a detection end pass through respective links, the light pulses are modulated by polarization controllers of respective light paths and then enter a light interference module for interference, the interfered light is divided into two beams, and the two beams respectively enter a first polarization beam splitter and a second polarization beam splitter for polarization; the light split by each polarization beam splitter respectively enters two avalanche detectors for detection, and the detection result is sent to a synchronous counter to ensure that the pulse arrival time of the Alice end is consistent with the pulse arrival time of the Bob end.

Description

Technical Field

[0001] The present invention provides a TF-QKD device and an encoding method with reference light polarization multiplexing, relating to the technical field of quantum communication. Background Art

[0002] TF-QKD (Two-Field Quantum Key Distribution) is a new type of quantum key distribution protocol. Compared with the QKD protocol, TF-QKD has the advantage of a higher key generation rate over long distances. The TF-QKD protocol has two sending ends, Alice and Bob, and one detecting end, Charlie. The sending ends Alice and Bob use encoding modules to encode their respective light sources into optical pulses carrying information - by modulating the pulse intensity and phase, the "0" and "1" bit information is randomly encoded into signal optical pulses at the single-photon level of a weak coherent state, and then sent to the detecting end Charlie through an optical fiber line. After the signal optical pulses from both sides reach the optical interference module of Charlie, single-photon interference occurs and is detected by a single-photon detector. The principle is as Figure 1 shown.

[0003] Currently, the commonly used single-photon detectors include superconducting nanowire detectors (SNSPDs) based on superconductors and avalanche detectors based on avalanche semiconductor diodes (such as InGaAs detectors, Si detectors). Compared with SNSPDs, avalanche detectors have the advantages of low power consumption and low cost. Moreover, different from SNSPDs, avalanche detectors do not require a very low operating temperature (for example, InGaAs detectors can operate at temperatures from -30°C to near room temperature). Therefore, compared with SNSPDs, using avalanche detectors does not require a large-volume cryogenic refrigeration device, reducing the complexity and volume of the TF-QKD system and expanding the applicable range of TF-QKD.

[0004] However, compared with SNSPDs, avalanche detectors have a lower detection efficiency and higher noise caused by dark counts. In addition, the most critical factor that has the greatest impact on the TF-QKD protocol is the afterpulse effect of avalanche detectors, that is, after a detection event occurs in the detector, there is a probability of generating additional noise, and the probability of generating noise decays with time. Therefore, when applying semiconductor detectors, a dead time needs to be set, that is, the detection data within a period of time after a detection event is discarded.

[0005] In TF-QKD, in order to achieve stable interference at Charlie's end, Alice's end and Bob's end need to send phase reference light, and Charlie's end feeds back or compensates the phase difference according to the detection result of the phase reference light. The phase reference light needs to accumulate a large count rate in a short time. Therefore, it is necessary to ensure that at the detection end, the count rate of the reference light after the link transmission loss reaches a certain level; while the quantum signal light at the exit of the sending end is less than the single-photon level. Therefore, the phase reference light pulse is stronger than the quantum signal light pulse, and the farther the communication distance is, the larger the intensity ratio of the reference light to the signal light is. In the usual coding method, the phase reference light and the quantum signal light are encoded in a time-division multiplexing manner, as Figure 2 shown. When an avalanche detector is used, there is still a certain probability of afterpulse noise outside the dead time. Since the count rate of the phase reference light is much higher than that of the quantum signal light, even if the dead time is set, the afterpulse noise of the phase reference light will still cause serious interference to the quantum signal light. Moreover, the farther the signal transmission distance is, the more stable the noise count rate generated by the afterpulse of the reference light is, while the lower the signal count rate generated by the signal light is, resulting in a decrease in the signal-to-noise ratio of long-distance communication and the inability to generate codes at long distances. For example, in the experiment, the link length is 200 km. Assuming that the probability of afterpulse noise generated outside the detector dead time is about 1%, and the coding period is 100 ns, at this time, the afterpulse noise generated by the phase reference light can be equivalently regarded as a uniform distribution in the time domain. In the experiment, assuming that the count rate of the phase reference light in the single-photon detector is about 2 MHz, then the afterpulse noise outside the dead time is 20 kHz. We reduce the noise through filtering in the time domain. For a 1 GHz system, the filtering system opens a 200 ps gate in the time domain, and the noise count rate is reduced to 1 / 5, which is 4 kHz. The average photon number of the quantum signal light is 0.1, the system frequency is 1 GHz, the link and system loss is 40 dB, and the signal light count rate is about 10 kHz. This results in a very low signal-to-noise ratio at the detection end, thus unable to generate codes.

[0006] To solve this problem, in existing solutions (such as patent CN116260508A), dual-wavelength feedback is adopted. The phase reference light and the quantum signal light are encoded with different wavelengths. At the detection end, DWDM is used to separate the reference light and the signal light by wavelength to different detectors for detection, so that the noise of the reference light will not affect the detection of the signal light. However, the dual-wavelength feedback scheme has the following disadvantages: (1) Dual-wavelength feedback requires an additional light source for frequency locking. The light source for frequency locking usually needs to be sent from Charlie to Alice and Bob through an idle channel. To ensure the interference quality, additional requirements are imposed on the performance of the laser and the link, which increases the complexity of the overall system including the link; (2) As mentioned above, the dual-wavelength scheme occupies more channels than the single-wavelength. In particular, when using DWDM for wavelength division multiplexing of multiple systems, the dual-wavelength scheme will reduce the number of available multiplexing channels in DWDM to half, increasing the system cost and reducing the number of available quantum channels; (3) The phase differences introduced by different-wavelength lights during link transmission are different. The error caused by the phase difference between the reference light and the signal light will introduce additional errors and reduce the coding rate. Summary of the Invention

[0007] To solve the above technical problems, the present application proposes a TF-QKD device with polarization multiplexing of reference light, including: an Alice end, a Bob end, and a Charlie end. The Alice end and the Bob end serve as the transmitting ends of the TF-QKD device, and the Charlie end serves as the detection end of the TF-QKD device;

[0008] The transmitting end includes: a single-wavelength laser, a phase modulator, an intensity modulator, a polarization modulator, and an optical attenuator;

[0009] The light source emitted by the single-wavelength laser is subjected to time-division multiplexing modulation by the phase modulator, the intensity modulator, and the polarization modulator to obtain strong phase reference light pulses, weak phase reference light pulses, and quantum signal light pulses. After being attenuated by the optical attenuator, they are sent to the detection end through an optical fiber line;

[0010] The detection end includes: a polarization controller, an optical interference module, a polarization beam splitter, multiple avalanche detectors, a synchronous clock, and a synchronous counter;

[0011] After the optical pulses from the Alice end and the Bob end received by the detection end pass through their respective links, they are first modulated by the polarization controllers of their respective optical paths and then enter the optical interference module for interference. The interfered light is divided into two beams, which are respectively polarized by the first polarization beam splitter and the second polarization beam splitter. The light split by each polarization beam splitter enters two avalanche detectors for detection, and the detection results are sent to the synchronous counter; the synchronous clock sends out clock signals for calibration and delay to ensure that the pulse arrival times of the Alice end and the Bob end are the same.

[0012] In a preferred embodiment, the light split by each polarization beam splitter enters two avalanche detectors for detection respectively. One avalanche detector is located at the exit of the horizontal end of the polarization beam splitter, and the other avalanche detector is located at the vertical end interface of the polarization beam splitter.

[0013] The present invention also proposes an encoding method for the TF-QKD device for the above-mentioned reference light polarization multiplexing, which is characterized in that the Alice end and the Bob end encode the phase reference light pulses and quantum signal light pulses with the same wavelength in a time-division multiplexing manner. Among them, the Alice end encodes strong phase reference light pulses in the time periods of T1a and T1c and extinguishes light in the T1b period; Bob encodes strong phase reference light pulses in the time periods of T1b and T1c and extinguishes light in T1a; the weak phase reference light pulses are located in the T2 period, and the quantum signal light pulses are located in the T3 period. The polarization direction of the strong phase reference light pulses is orthogonal to the polarization directions of the weak phase reference light pulses and the quantum signal light pulses.

[0014] In a preferred embodiment, the TF-QKD debugging stage is executed according to the following steps:

[0015] Step 11: The Alice end sends a strong phase reference test optical signal at a selected moment, and the Bob end does not emit light;

[0016] Step 12: Adjust the polarization controller of the optical path where the Alice end is located so that within the strong phase reference time range, the sum of the counting rates of the A1 detector and the A2 detector reaches the maximum value M1, and at this time, the sum of the counting rates of the B1 detector and the B2 detector reaches the lowest;

[0017] Step 13: Statistically analyze the detection results of the A1 detector and the A2 detector to obtain the rising edge moment of the Alice end;

[0018] Step 14: The Alice end stops sending the strong phase reference test optical signal, and Bob sends the strong phase reference test optical signal;

[0019] Step 15: Adjust the polarization controller of the optical path where the Bob end is located so that the sum of the counting rates of the A1 detector and the A2 detector reaches the maximum value M2;

[0020] Step 16: Statistically analyze the detection results of detector A1 and detector A2 to obtain the rising edge moment of the Bob end;

[0021] Step 17: According to the rising edge moment of the Alice end and the rising edge moment of the Bob end, adjust the relative delay between the Alice end and the Bob end so that the rising edges are aligned.

[0022] In a preferred embodiment, during the QKD operation stage, the polarization feedback is performed in the following manner:

[0023] Step 21: Periodically count the detection results of detector A1 and detector A2, and divide them according to the coding period;

[0024] Step 22: Monitor the sum of the counting rates of detector A1 and detector A2 within the time period T1a. If it is less than the first set value, use the polarization feedback algorithm to adjust the state S1 of the polarization controller in the optical path where Alice is located, so that the counting rates of detector A1 and detector A2 within the time period T1a are greater than or equal to the third set value;

[0025] Step 23: Monitor the sum of the counting rates of detector A1 and detector A2 within the time period T1b. If it is less than the second set value, use the polarization feedback algorithm to adjust the state S2 of the polarization controller in the optical path where Bob is located, so that the counting rates of detector A1 and detector A2 within the time period T1a are greater than or equal to the fourth set value.

[0026] In a preferred embodiment, the delay calibration is performed in the following manner:

[0027] Step 31: Periodically count the detection results of detector A1 and detector A2, and divide them according to the coding period;

[0028] Step 32: Accumulate the detection results within the time period T1a into a histogram, and obtain the rising edge moment of the Alice side according to the histogram;

[0029] Step 33: Accumulate the detection results within the time period T1b into a histogram, and obtain the rising edge moment of the Bob side according to the histogram;

[0030] Step 34: Adjust the delay based on the rising edge moment of the Alice side and the rising edge moment of the Bob side to align the rising edges.

[0031] In a preferred embodiment, the first set value is 95% of M1, and the second set value is 95% of M2.

[0032] In a preferred embodiment, the third set value is M1, and the fourth set value is M2.

[0033] Compared with the prior art, the present invention has the following beneficial technical effects:

[0034] The solution of the present application modulates the strong reference light, the weak reference light, and the quantum signal light in a time-division multiplexing manner at the sending end, and uses the polarization multiplexing method to modulate the polarization of the strong reference light to be orthogonal to the polarization of the weak reference light and the quantum signal light. At the detection end, PBS is used to make the polarization of the strong reference light enter different detectors from the polarization of the weak reference light and the quantum signal light for detection, so as to reduce the influence of the after-pulse noise of the strong reference light with a high counting rate on the detection of the quantum signal light.

[0035] The present invention adopts a single-sided light-emitting method with a strong reference light, performs polarization feedback and delay calibration based on the detection results within the single-sided light-emitting time period, reduces the number of single-photon detectors at the detection end, and reduces the complexity of the system.

[0036] Compared with the patent solution CN118972053A, there is no optical switch at the detection end of this application, and it is not necessary to align the operation cycle of the optical switch with each coding cycle of the sending end, thereby reducing the time synchronization requirement between the sending end and the detection end. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0038] Figure 1 It is a schematic diagram of the TF-QKD principle;

[0039] Figure 2 It is a schematic diagram of the coding cycle of the prior art;

[0040] Figure 3 It is a schematic diagram of the device structure at the sending end;

[0041] Figure 4 It is a schematic diagram of the device structure at the detection end;

[0042] Figure 5 It is a schematic diagram of the coding cycle at the Alice end;

[0043] Figure 6 It is a schematic diagram of the coding cycle at the Bob end; DETAILED DESCRIPTION OF THE EMBODIMENTS

[0044] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the following will clearly and completely describe the technical solutions in the embodiments of this application with reference to the drawings in the embodiments of this application. Obviously, the described embodiments are some, but not all, of the embodiments of this application. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of this application without creative efforts belong to the scope of protection of this application.

[0045] (1) Setting of the TF-QKD device

[0046] The TF-QKD device with reference light polarization multiplexing in this application is divided into a sending end and a detection end.

[0047] The structure of the sending end is set as Figure 3As shown in the figure, different intensities and polarizations are modulated during the coding period to achieve time-division multiplexing modulation of the reference light and the signal light. Among them, a single-wavelength laser light source is used to perform time-division multiplexing modulation of strong phase reference light pulses, weak phase reference light pulses, and quantum signal light pulses through a phase modulator PM, an intensity modulator IM, and a polarization modulator PolM. Then, after being attenuated by an optical attenuator ATT, it is sent to the detection end through an optical fiber line.

[0048] The device setting structure at the detection end is as Figure 4 shown. After the optical pulses at the Alice end and the Bob end reach the detection end through the link, the optical pulses at both ends are first modulated in polarization by the polarization controllers (PC1, PC2) in their respective optical paths, and then enter the optical interference module for interference. After interference, the light enters the first polarization beam splitter PBS1 and the second polarization beam splitter PBS2 for polarization respectively. The two beams of light split by the first polarization beam splitter PBS1 enter the A1 detector and the B1 detector for detection respectively, and the two beams of light split by the second polarization beam splitter PBS2 enter the A2 detector and the B2 detector for detection respectively. The polarization directions set by the first polarization beam splitter PBS1 and the second polarization beam splitter PBS2 are the same.

[0049] The synchronization clock issues a clock signal, which is calibrated and delayed before QKD to ensure that the pulse arrival times at the Alice end and the Bob end are the same.

[0050] The A1 detector, the A2 detector, the B1 detector, and the B2 detector are all avalanche detectors. The detected data is collected by a synchronization counter. Among them, the A1 detector and the A2 detector are respectively located at the horizontal end outlets of the first polarization beam splitter PBS1 and the second polarization beam splitter PBS2, while the B1 detector and the B2 detector are respectively located at the vertical end interfaces of the first polarization beam splitter PBS1 and the second polarization beam splitter PBS2. Optionally, the A1 detector and the A2 detector are selected as high-saturation counting detectors; the B1 detector and the B2 detector are selected as low-noise detectors (such as cryogenic avalanche detectors).

[0051] The synchronization counter can synchronously record the detection results of the A1 detector, the A2 detector, the B1 detector, and the B2 detector, and attach a timestamp based on the clock signal. The detection end performs polarization feedback and delay calibration depending on the coding method.

[0052] (2) QKD operation process and feedback execution method

[0053] During the coding period, a time period when only the Alice end or the Bob end emits light unilaterally is set, and polarization feedback and delay calibration are performed according to the detection results of the A1 detector and the A2 detector during the unilateral light emission time period.

[0054] Encoding method: The Alice side and the Bob side encode the phase reference optical pulse and the quantum signal optical pulse with the same wavelength through time-division multiplexing. The encoding periods of the Alice side and the Bob side are as Figure 5 shown in Figure 6 . In this embodiment, the time period T1 is divided into three parts: T1a, T1b, and T1c. The Alice side encodes strong phase reference optical pulses in T1a and T1c and extinguishes light in T1b; Bob encodes strong phase reference optical pulses in T1b and T1c and extinguishes light in T1a.

[0055] TF-QKD debugging stage: The synchronous clock at the Charlie side starts to send clock signals to the Alice side and the Bob side, and then the debugging is performed according to the following steps.

[0056] Step 1: The Alice side sends a strong phase reference test optical signal at a selected moment, and the Bob side does not emit light.

[0057] Step 2: Adjust the polarization controller PC1 in the optical path where the Alice side is located so that within the strong phase reference time range, the sum of the counting rates of A1 and A2 reaches the maximum value M1, and at this time, the sum of the counting rates of B1 and B2 reaches the lowest.

[0058] Step 3: Statistically analyze the detection results of the A1 detector and the A2 detector to obtain the rising edge moment of the Alice side.

[0059] Step 4: The Alice side stops sending the strong phase reference test optical signal, and the Bob side sends the strong phase reference test optical signal.

[0060] Step 5: Adjust the polarization controller PC2 in the optical path where the Bob side is located so that the sum of the counting rates of the A1 detector and the A2 detector reaches the maximum value M2.

[0061] Step 6: Statistically analyze the detection results of the A1 detector and the A2 detector to obtain the rising edge moment of the Bob side.

[0062] Step 7: According to the rising edge moment of the Alice side and the rising edge moment of the Bob side, adjust the relative delay between the Alice side and the Bob side so that the rising edges are aligned.

[0063] QKD operation stage: The sum of the counting rates of the A1 detector and the A2 detector in the T1a time period within the encoding period is fed back to M1 to perform the polarization feedback of the light from the Alice side; the sum of the counting rates of the A1 detector and the A2 detector in the T1b time period within the encoding period is fed back to M2 to perform the polarization feedback of the light from the Bob side.

[0064] Specifically, the polarization feedback is performed in the following manner:

[0065] 1. Periodically count the detection results of detector A1 and detector A2, and divide them according to the coding period.

[0066] 2. Monitor the sum of the counting rates of detector A1 and detector A2 during time period T1a. If it is less than the first set value (preferably, the first set value is 95% of M1), use the polarization feedback algorithm to adjust the state S1 of PC1 so that the counting rates of detector A1 and detector A2 during time period T1a are greater than or equal to the third set value, and the third set value is preferably M1. Or a real-time polarization tracking algorithm can be adopted to ensure that the sum of the counting rates of A1 and A2 is maintained at the highest value during this time period.

[0067] 3. Monitor the sum of the counting rates of detector A1 and detector A2 during time period T1b. If it is less than the second set value (preferably, the second set value is 95% of M2), use the polarization feedback algorithm to adjust the state S2 of PC2 so that the counting rates of detector A1 and detector A2 during time period T1a are greater than or equal to the fourth set value, and the fourth set value is preferably M2.

[0068] Perform delay calibration in the following manner:

[0069] 1. Periodically count the detection results of detector A1 and detector A2, and divide them according to the coding period.

[0070] 2. Accumulate the detection results during time period T1a into a histogram, and obtain the rising edge time at the Alice end according to the histogram (optionally, the rising edge time is the time when the accumulated value is greater than 1 / 2 of the maximum value, and the time before the rising edge is the time when the accumulated value is less than 1 / 2 of the maximum value).

[0071] 3. Accumulate the detection results during time period T1b into a histogram, and obtain the rising edge time at the Bob end according to the histogram.

[0072] 4. Adjust the delay based on the rising edge time at the Alice end and the rising edge time at the Bob end to align the rising edges.

[0073] In the above embodiments, it can be implemented in whole or in part by software, hardware, firmware, or any combination thereof. When implemented using software, it can be implemented in whole or in part in the form of a computer program product. The computer program product includes one or more computer instructions. When the computer program instructions are loaded and executed on a computer, the processes or functions described in the embodiments of the present application are generated in whole or in part. The computer can be a general-purpose computer, a special-purpose computer, a computer network, or other programmable devices. The computer instructions can be stored in a computer-readable storage medium or transmitted through the computer-readable storage medium. The computer-readable storage medium can be any available medium that the computer can access or a data storage device such as a server or a data center that integrates one or more available media. The available medium can be a magnetic medium (e.g., floppy disk, hard disk, magnetic tape), an optical medium (e.g., DVD), or a semiconductor medium (e.g., solid state disk (SSD)), etc.

[0074] The above is only the specific implementation manner of the present application, but the protection scope of the present application is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present application can easily think of various equivalent modifications or substitutions, and these modifications or substitutions should all be covered within the protection scope of the present application. Therefore, the protection scope of the present application shall be subject to the protection scope of the claims.

Claims

1. A TF-QKD device with polarization multiplexing of reference light, characterized in that: include: Alice, Bob and Charlie. Alice and Bob are used as the transmitting end of the TF-QKD device, and Charlie is used as the detecting end of the TF-QKD device. The transmitting end includes: a single-wavelength laser, a phase modulator, an intensity modulator, a polarization modulator and an optical attenuator; The light source emitted by the single-wavelength laser is time-division multiplexed and modulated by a phase modulator, an intensity modulator, and a polarization modulator to obtain a strong phase reference light pulse, a weak phase reference light pulse, and a quantum signal light pulse, which are attenuated by an optical attenuator and then sent to the detection end via an optical fiber line; The detection end includes: a polarization controller, an optical interference module, a polarization beam splitter, a plurality of avalanche detectors, a synchronous clock and a synchronous counter; After passing through their respective links, the optical pulses from Alice and Bob received by the detection end are first modulated by the polarization controllers of their respective optical paths and then enter the optical interference module for interference. The interfered light is divided into two beams, which enter the first polarization beam splitter and the second polarization beam splitter for polarization respectively. The light separated by each polarization beam splitter enters two avalanche detectors for detection respectively, and the detection results are sent to the synchronization counter; the synchronization clock sends a clock signal for calibration and delay to ensure that the pulses at Alice and Bob arrive at the same time.

2. The TF-QKD device with polarization multiplexing of reference light according to claim 1, characterized in that: The light split by each polarization beam splitter enters two avalanche detectors for detection, one of which is located at the horizontal end exit of the polarization beam splitter, and the other is located at the vertical end interface of the polarization beam splitter.

3. A coding method for the TF-QKD device with polarization multiplexing of reference light according to claim 1 or 2, characterized in that: Alice and Bob encode phase reference optical pulses and quantum signal optical pulses with the same wavelength by time division multiplexing, wherein Alice encodes strong phase reference optical pulses in the T1a and T1c time periods and extinguishes them in the T1b time period; Bob encodes strong phase reference optical pulses in the T1b and T1c time periods and extinguishes them in T1a; the weak phase reference optical pulse is located in the T2 time period, and the quantum signal optical pulse is located in the T3 time period, and the polarization direction of the strong phase reference optical pulse is orthogonal to the polarization directions of the weak phase reference optical pulse and the quantum signal optical pulse.

4. The encoding method according to claim 3, characterized in that The TF-QKD debugging phase is performed as follows: Step 11: Alice sends a strong phase reference test optical signal at a selected time, and Bob does not emit light; Step 12: Adjust the polarization controller of the optical path where Alice is located so that within the strong phase reference time range, the sum of the count rates of the A1 detector and the A2 detector reaches a maximum value M1, and at this time, the sum of the count rates of the B1 detector and the B2 detector reaches a minimum; Step 13: Count the detection results of detectors A1 and A2, and calculate the rising edge time of Alice. Step 14: Alice stops sending a strong phase reference test optical signal, and Bob sends a strong phase reference test optical signal; Step 15, adjust the polarization controller of the light path where the Bob end is located so that the sum of the count rates of the A1 detector and the A2 detector reaches a maximum value M2; Step 16, count the detection results of detector A1 and detector A2, and find the rising edge time of Bob end; Step 17: According to the rising edge timing of Alice and Bob, adjust the relative delay between Alice and Bob so that the rising edges are aligned.

5. The encoding method according to claim 3, characterized in that: During the QKD operation phase, polarization feedback is performed as follows: Step 21, periodically counting the detection results of the A1 detector and the A2 detector, and dividing them according to the coding period; Step 22: monitor the sum of the count rates of the A1 detector and the A2 detector in the T1a time period. If the sum is less than the first set value, use the polarization feedback algorithm to adjust the state S1 of the polarization controller of the optical path where the Alice end is located, so that the count rates of the A1 detector and the A2 detector in the T1a time period are greater than or equal to the third set value; Step 23, monitor the sum of the count rates of the A1 detector and the A2 detector in the T1b time period. If it is less than the second set value, use the polarization feedback algorithm to adjust the state S2 of the polarization controller of the optical path where the Bob end is located, so that the count rates of the A1 detector and the A2 detector in the T1a time period are greater than or equal to the fourth set value.

6. The encoding method according to claim 5, characterized in that Perform a time delay calibration as follows: Step 31, periodically counting the detection results of the A1 detector and the A2 detector, and dividing them according to the coding period; Step 32: Accumulate the detection results in the T1a time period into a histogram, and calculate the rising edge time of Alice's terminal according to the histogram; Step 33, accumulate the detection results in the T1b time period into a histogram, and calculate the rising edge time of Bob's terminal according to the histogram; Step 34: Adjust the delay based on the rising edge timing of Alice's end and the rising edge timing of Bob's end to align the rising edges.

7. The encoding method according to claim 5, characterized in that: The first set value is 95% of M1, and the second set value is 95% of M2.

8. The encoding method according to claim 5, characterized in that: The third setting value is M1, and the fourth setting value is M2.

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