A Local Active Phase Compensation Method and System for MDI-QKD
By combining the fiber delay superposition ring and the Faraday interference ring, the control module is used to optimize the function of the optical switch, and the problems of local active phase compensation in the MDI-QKD protocol are solved, and efficient, fast and economical compensation effects are achieved.
Patent Information
- Application Number
- CN202211642584.3
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- Filing Date
- 2022-12-20
- Publication Date
- 2025-06-24
- Estimated Expiration
- 2042-12-20
AI Technical Summary
The existing local active phase compensation method has problems such as low efficiency, high cost and complex logic in the MDI-QKD protocol, especially in the time phase encoding scenario of discrete variables, and lacks an effective compensation scheme.
The method of combining the optical fiber delay superposition ring and the Faraday interference ring is adopted. The reference light and signal light are split through the optical fiber delay superposition ring, and the local active phase compensation work is completed by using the Faraday interference ring and the control module. The structure composition and circuit parameter determination method of the control module make the optical switch control channel turn on and off, improving the compensation efficiency.
It improves compensation efficiency and accuracy, reduces the complexity and application difficulty of FPGA logic processing, and achieves faster and more economical local active phase compensation. It is suitable for discrete variable time phase encoding scenarios in the MDI-QKD protocol.
Smart Images
Figure CN115955304B_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to the technical field of quantum secure communication, and more specifically, to a local active phase compensation method and system applied to MDI-QKD. Background Art
[0002] Quantum cryptographic communication is a communication method that has been proven at the physical law level to achieve absolute information security. Among them, quantum key distribution is the basis of quantum cryptographic communication research. The measurement device independent quantum key distribution protocol (MDI-QKD) and some of its variant protocols (such as the twin-field protocol (TF-QKD)) are the most promising quantum key distribution protocols for practical applications due to their good distance-key generation rate performance. However, in actual quantum key distribution systems, phase drifts often occur due to various factors, ultimately affecting the security and stability of communication.
[0003] Disadvantages and problems of the prior art: The existing local active phase compensation methods have widely used a combination of strong light reference, optical switch switching, and FPGA receiving single-photon detector (SPD) photon counting for feedback control, enabling Alice (abbreviated as A) and Bob (abbreviated as B) to determine the temporary operating point of the electro-optic phase modulator without exchanging information through the classical channel, thereby avoiding eavesdropping from Eve and improving the security of communication. However, the optical switch used in the above method separates the signal light and the compensation light from the path for compensation, that is, the light beam enters the compensation optical path during the compensation stage and enters the channel during the QKD stage, and the compensation efficiency is affected by the switching rate, so the overall operating rate of the system will be reduced; on the other hand, the process time for SPD feedback photon counting to be processed by FPGA is relatively long and the logic is relatively complex. The use of SPD means that the cost of this solution is relatively high. The local active phase compensation scheme using photodetectors has also been used in QKD protocols, but in the case of the no-relay QKD protocol, the communication distance is limited, and there is no detailed design for the physical composition structure of the control module using photodetectors and the setting of relevant circuit parameters. The MDI-QKD protocol can extend the QKD communication distance to twice the original. In particular, there is still a technical gap in the local active phase compensation for the discrete variable time-phase encoded MDI-QKD protocol.
[0004] Therefore, in view of the above problems, a local active phase compensation method and system applied to MDI-QKD are proposed. Summary of the Invention
[0005] 1. Technical Problems to be Solved
[0006] Aiming at the problems existing in the prior art, the purpose of the present invention is to provide a local active phase compensation method and system for a measurement device-independent quantum key distribution protocol for discrete variables. By combining a fiber delay superposition loop and a Faraday interference loop, the reference light and the signal light are split by the fiber delay superposition loop, and the local active phase compensation work is completed by the Faraday interference loop and the control module. Through the composition structure of the control module and the determination method of the important circuit parameters, the function of the optical switch is no longer to switch the compensation optical path and the channel, thereby improving the compensation efficiency and reducing the proportion of the compensation stage in the entire communication cycle. The compensation method of this scheme has high precision, fast speed, relatively simple logic processing of the FPGA, and is more simple to apply.
[0007] 2. Technical solution
[0008] To solve the above problems, the present invention adopts the following technical solutions.
[0009] A local active phase compensation method and system applied to MDI-QKD, the local active phase compensation method applied to MDI-QKD includes the following steps:
[0010] S1: The light emitted from the light source first passes through the fiber delay superposition loop structure. The light emitted from the light source is split into a reference light path and a signal light path by the first optical beam splitter coupler or the fourth optical beam splitter coupler. They are respectively modulated by intensity modulators, and then combined at the second optical beam splitter coupler or the fifth optical beam splitter coupler;
[0011] S2: The light combined at the second optical beam splitter coupler or the fifth optical beam splitter coupler passes through the optical circulator, and then enters from the first incident arm of the third optical beam splitter coupler or the sixth optical beam splitter coupler and is split into two beams of light with uniform intensity and exits from the two exit arms of the third optical beam splitter coupler or the sixth optical beam splitter coupler. One beam of light is modulated by the first phase modulator or the second phase modulator and then reflected by the first Faraday mirror or the third Faraday mirror, and one beam of light is attenuated by the first fiber attenuator or the second fiber attenuator and then reflected by the second Faraday mirror or the fourth Faraday mirror;
[0012] S3: The two reflected light beams interfere at the third beam splitter coupler or the sixth beam splitter coupler, and the interfered light exits from the two incident arms of the third beam splitter coupler or the sixth beam splitter coupler. The interference result of the light exiting from the first incident arm is received by the first photodetector or the second photodetector;
[0013] S4: The light exiting from the Faraday interference loop passes through the first optical switch and the second optical switch. The optical switches block the compensation light from passing through. The control waveforms of the first optical switch and the second optical switch are opposite to those of the second intensity modulator and the fourth intensity modulator respectively;
[0014] S5: The signal light passing through the first optical switch and the second optical switch is attenuated by the first bench-top optical attenuator or the second bench-top optical attenuator and then transmitted in the channel. The signal lights of Alice and Bob generate multi-photon interference at Charlie and the measurement results are announced.
[0015] S6: Alice and Bob screen the raw key and perform post-processing steps such as error detection and privacy amplification.
[0016] Furthermore, the local active phase compensation method is dedicated to the measurement-device-independent quantum key distribution protocol for discrete variable time-phase encoding, and adopts a method combining a fiber delay superposition loop and a Faraday interference loop. The fiber delay superposition loop separates the reference light and the signal light in the time domain.
[0017] Furthermore, the determination method of the control signal of the second intensity modulator and the control signal of the fourth intensity modulator in S4 is as follows: When measuring the conditions for applying passive compensation measures to the Faraday interference loops of both parties, in the QKD stage, when the control module continuously outputs the first temporary working point voltage, the longest time plus the stable time and the scanning time during which the optical intensity received by the photodetector can be stable is recorded as t. Then the frequency of the control signal is f = 1 / t; the control signal is a pulse signal emitted by a signal generator. The pulse width of the pulse signal should be greater than the stable stage time plus the scanning stage time. When the pulse arrives, the control module starts the work of stable reset and scanning.
[0018] Furthermore, in S5 and S6, the optical paths and compensation structures of Alice and Bob adopt a symmetric design, which is for compensating the working point drift of their respective phase modulators. The scheme is applied in MDI-QKD, and the two parties do not need to exchange compensation information through an additional channel.
[0019] Furthermore, a local active phase compensation system applied to MDI-QKD includes an A-end local active phase compensation system, a B-end local active phase compensation system, and a C-end multi-photon interference measurement system connected by optical fibers. The A-end local active phase compensation system includes a first continuous light source module, a first circulator module, a first beam splitter module, a first electro-optic intensity modulator module, a first electro-optic phase modulator module, a first Faraday mirror module, a first fiber optic attenuator module, a first bench-top optical attenuator module, and a first control module.
[0020] Furthermore, the B-end local active phase compensation system includes a second continuous light source module, a second circulator module, a second beam splitter module, a second electro-optic intensity modulator module, a second electro-optic phase modulator module, a second Faraday mirror module, a second fiber optic attenuator module, a second bench-top optical attenuator module, and a second control module. The C-end multi-photon interference measurement system includes a third beam splitter module, a third polarization controller module, and a third single-photon detector module.
[0021] Further, the first control module includes a first signal amplifier, a first analog-to-digital conversion module, a first FPGA development board, a first digital-to-analog conversion module, and a second signal amplifier.
[0022] Further, the second control module includes a third signal amplifier, a second analog-to-digital conversion module, a second FPGA development board, a second digital-to-analog conversion module, and a fourth signal amplifier.
[0023] Further, the method for determining the amplification factors of the first signal amplifier and the third signal amplifier in the first control module and the second control module is as follows: If the compensation time is T, let the frequency F = 1 / T. Input a sine wave signal with a frequency greater than or equal to F and an amplitude approximately equal to the maximum output voltage amplitude of the photodetector into the first signal amplifier and the third signal amplifier. Adjust the amplification factor. When the signal amplifier linearly amplifies the sine wave signal and the amplitude is within the input noise tolerance of the analog-to-digital conversion module, the amplification factor at this time meets the requirements.
[0024] Further, the method for determining the amplification factors of the second signal amplifier and the fourth signal amplifier in the first control module and the second control module is as follows: If the compensation time is T, let the frequency F = 1 / T. Input a sawtooth wave signal with a frequency greater than or equal to F and an amplitude equal to the maximum output voltage amplitude of the digital-to-analog conversion module into the second signal amplifier and the fourth signal amplifier. Adjust the amplification factor. When the sawtooth wave can be linearly amplified and the amplitude of the amplified signal is greater than twice the half-wave voltage of the phase modulator, the amplification factor at this time meets the requirements.
[0025] 3. Beneficial Effects
[0026] Compared with the prior art, the advantages of the present invention are as follows:
[0027] (1) Compared with the local active phase compensation scheme using an optical intensity detection device, through the structural composition of the control module and the method for determining its circuit parameters, the optical switch is not used to switch between the channel and the compensation optical path, but to control the on / off of the channel, saving the blank time for switching between the two optical paths. Therefore, the compensation efficiency is improved, the proportion of the compensation stage in the entire communication cycle is lower, the accuracy of the compensation method of this scheme is high, the speed is fast, the logic processing of the FPGA is relatively simple, and it is more simple to apply. BRIEF DESCRIPTION OF THE DRAWINGS
[0028] Figure 1 It is a schematic diagram of the QKD working process of the present invention;
[0029] Figure 2 It is an optical path diagram of the MDI-QKD using local active phase compensation of the present invention;
[0030] Figure 3 Schematic diagram of the FPGA structure of the present invention;
[0031] Figure 4 Schematic diagram of the expected experimental results of the present invention;
[0032] Figure 5 Schematic diagram when there is a signal signal in the present invention.
[0033] Explanation of the reference numerals in the figure:
[0034] LD1 / 2: 1550nm continuous light source; CIR1 / 2: circulator; BS1-7: optical beam splitting coupler; IM1-4: electro-optic intensity modulator; PM1 / 2: electro-optic phase modulator; FM1-4: Faraday mirror; VOA: fiber optic attenuator; ATT: bench-top optical attenuator; CLK: synchronous clock; SS: signal generator; CMA(CMB): control module Alice(Bob); PC: polarization controller; SPD: single photon detector; OS: optical switch. Specific implementation mode
[0035] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention; obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.
[0036] In the description of the present invention, it should be noted that the orientation or positional relationship indicated by the terms "upper", "lower", "inner", "outer", "top / bottom end", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation of the present invention. In addition, the terms "first", "second" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance.
[0037] In the description of the present invention, it should be noted that unless otherwise clearly defined and limited, the terms "installed", "provided with", "sheathed / connected", "connected", etc. should be understood in a broad sense. For example, "connected" can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, and can be the communication inside two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific situations.
[0038] Example 1:
[0039] Please refer to Figure 1 , a local active phase compensation method and system applied to MDI-QKD. A local active phase compensation method applied to MDI-QKD includes the following steps:
[0040] S1: The light emitted from the light source first passes through a fiber delay and superposition ring structure. The light emitted from the light source is split into a reference light path and a signal light path by the first optical beam splitter coupler or the fourth optical beam splitter coupler. They are respectively modulated by intensity modulators and then combined at the second optical beam splitter coupler or the fifth optical beam splitter coupler;
[0041] S2: The light combined at the second optical beam splitter coupler or the fifth optical beam splitter coupler passes through an optical circulator, and then enters through the first incident arm of the third optical beam splitter coupler or the sixth optical beam splitter coupler and is split into two beams of light with uniform intensity and then exits from the two exit arms of the third optical beam splitter coupler or the sixth optical beam splitter coupler. One beam of light is modulated by the first phase modulator or the second phase modulator and then reflected by the first Faraday mirror or the third Faraday mirror. One beam of light passes through the first fiber attenuator or the second fiber attenuator and then is reflected by the second Faraday mirror or the fourth Faraday mirror.
[0042] S3: The two reflected light beams interfere at the third beam splitter coupler or the sixth beam splitter coupler, and the interfered light exits from the two incident arms of the third beam splitter coupler or the sixth beam splitter coupler. The interference result of the light exiting from the first incident arm is received by the first photodetector or the second photodetector;
[0043] S4: The light exiting from the Faraday interference ring passes through the first optical switch and the second optical switch. The optical switches block the compensation light from passing through. The control waveforms of the first optical switch and the second optical switch are opposite to those of the second intensity modulator and the fourth intensity modulator respectively;
[0044] S5: The signal light passing through the first optical switch and the second optical switch is attenuated by the first bench-top optical attenuator or the second bench-top optical attenuator and then transmitted in the channel. The signal lights of Alice and Bob generate multi-photon interference at Charlie and announce the measurement results;
[0045] S6: Alice and Bob screen the raw keys and perform post-processing steps such as error detection and privacy amplification.
[0046] The local active phase compensation method is dedicated to the measurement device-independent quantum key distribution protocol for discrete variable time phase encoding, and adopts a method combining a fiber delay and superposition ring with a Faraday interference ring. The fiber delay and superposition ring separates the reference light and the signal light in the time domain.
[0047] The method for determining the control signal of the second intensity modulator and the control signal of the fourth intensity modulator in S4 is as follows: Measure the longest time during which the light intensity received by the photodetector can be stable when the control module continuously outputs the first temporary operating point voltage during the QKD phase under the condition that passive compensation measures are applied to the double Faraday interference rings, plus the stable time and the scanning time, denoted as t. Then the frequency of the control signal is f = 1 / t. The control signal is a pulse signal generated by a signal generator. The pulse width of the pulse signal should be greater than the stable stage time plus the scanning stage time. When the pulse arrives, the control module starts the work of stable reset and scanning.
[0048] In S5 and S6, the optical paths and compensation structures of Alice and Bob adopt a symmetric design for compensating the working point drift of their respective phase modulators. The scheme is applied in MDI-QKD, and the two parties do not need to exchange compensation information through an additional channel.
[0049] In view of the above-mentioned problems, this scheme aims to provide a local active phase compensation method with a faster rate, lower cost, and simpler logic, so that the scanning compensation stage time is as short as possible and the proportion is as low as possible. At the same time, it reduces the complexity of the optical system, reduces the influence of the working point drift of the phase modulator on the performance of the communication protocol, and makes the operation of the entire device system more efficient.
[0050] Please refer to Figures 2-5 , a local active phase compensation system applied to MDI-QKD, including an A-end local active phase compensation system, a B-end local active phase compensation system, and a C-end multi-photon interference measurement system connected by optical fibers. The A-end local active phase compensation system includes a first continuous light source module, a first circulator module, a first beam splitter module, a first electro-optic intensity modulator module, a first electro-optic phase modulator module, a first Faraday mirror module, a first fiber optic attenuator module, a first bench-top optical attenuator module, and a first compensation module.
[0051] The B-end local active phase compensation system includes a second continuous light source module, a second circulator module, a second beam splitter module, a second electro-optic intensity modulator module, a second electro-optic phase modulator module, a second Faraday mirror module, a second fiber optic attenuator module, a second bench-top optical attenuator module, and a second compensation module. The C-end multi-photon interference measurement system includes a third beam splitter module, a third polarization controller module, and a third single-photon detector module.
[0052] The first control module includes a first signal amplifier, a first analog-to-digital conversion module, a first FPGA development board, a first digital-to-analog conversion module, and a second signal amplifier.
[0053] The second control module includes a third signal amplifier, a second analog-to-digital conversion module, a second FPGA development board, a second digital-to-analog conversion module, and a fourth signal amplifier.
[0054] The method for determining the amplification factors of the first signal amplifier and the third signal amplifier in the first control module and the second control module is as follows: If the compensation time is T, let the frequency F = 1 / T. Input a sine wave signal with a frequency greater than or equal to F and an amplitude approximately equal to the maximum output voltage amplitude of the photodetector into the first signal amplifier and the third signal amplifier. Adjust the amplification factor. When the signal amplifier linearly amplifies the sine wave signal and the amplitude is within the input noise tolerance of the analog-to-digital conversion module, the amplification factor at this time meets the requirements.
[0055] The method for determining the amplification factors of the second signal amplifier and the fourth signal amplifier in the first control module and the second control module is as follows: If the compensation time is T, let the frequency F = 1 / T. Input a sawtooth wave signal with a frequency greater than or equal to F and an amplitude equal to the maximum output voltage amplitude of the digital-to-analog conversion module into the second signal amplifier and the fourth signal amplifier. Adjust the amplification factor. When the sawtooth wave can be linearly amplified and the amplitude of the amplified signal is greater than twice the half-wave voltage of the phase modulator, the amplification factor at this time meets the requirements.
[0056] Each control module in this solution consists of an FPGA development board main body, an analog-to-digital conversion module, a digital-to-analog conversion module, and an operational amplifier (the models used in the experiment are ALINX Altera FPGA development board AX515 / AX530, AN926, AN9767, and OPA177 respectively). The inputs of this module are the analog signals from PD1 / PD2, the wide pulse signal from SS, and the 0 / 1 signal from the host computer; the output is the compensation electrical signal sent to the phase modulator. (The operational amplifier 1 is placed before AN926 to amplify the electrical signal from PD (the amplitude of 0 - 20 mV cannot be directly processed by the FPGA because AN926 itself has a 20 mV background noise); the operational amplifier 2 is placed between AN9767 and the phase modulator to make the amplitude of the emitted electrical signal basically greater than twice the half-wave voltage of the phase modulator). Its working process cycle includes three stages: the stable stage, the scanning stage, and the QKD stage. The working cycle of the FPGA is equal to the cycle of the wide pulse. Therefore, the rising edge of the wide pulse signal indicates the start of the FPGA's work.
[0057] Stable stage: After the FPGA receives the rising edge of the wide pulse, it first outputs a low level with a duration of 100 us to the phase modulator to stabilize its working state. At this time, the electrical signal output by the PD is ideally a straight line (the stable duration can be adjusted according to the experimental conditions of the specific circuit module);
[0058] Scanning stage: The FPGA outputs a scanning voltage that increases linearly to the phase modulator within 33 us. The scanning voltage U 扫描 The amplitude ΔU after being linearly amplified by the operational amplifier 2扫描放大后 It can cover the half-wave voltage U of the double phase modulator 半波 (i.e., ΔU 扫描放大后＞ 2U 半波 ). In this way, the phase modulator is equivalent to continuously modulating from 0 phase to 2π phase within 33 us. Therefore, the interference result presented at the PD is in the form of a sine wave. The FPGA records the voltage U1 output to the phase modulator when the sine wave appears at the lowest point and the voltage U0 output to the phase modulator when it appears at the highest point (U0 = U1 ± U 半波 ). U0 and U1 are the operating points of the phase modulator during this communication cycle (the scanning duration can be adjusted according to the specific optical path experimental conditions. The shorter the scanning duration, the faster the compensation speed and the lower the accuracy; the longer the scanning duration, the slower the compensation speed and the higher the accuracy. The two are in an inverse proportion relationship). During the period from the end of the scanning to the start of the QKD phase, the FPGA continuously outputs the voltage of this operating point. This period is called the waiting time;
[0059] QKD phase: When the falling edge of the wide pulse arrives, the QKD phase starts. During the QKD phase, the FPGA outputs U0 or U1 according to the 0 or 1 signal provided by the host computer.
[0060] Compared with the local active phase compensation scheme using an optical intensity detection device, this scheme elaborates in detail on the structural composition of the control module and the determination method of its circuit parameters. Compared with the local active phase compensation scheme using an optical intensity detection device, in this scheme, the optical switch is not used to switch between the channel and the compensation optical path, but to control the on / off of the channel. In this way, there is no blank time for switching between the two optical paths, thus improving the compensation efficiency.
[0061] The above is only the preferred specific implementation manner of the present invention; however, the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution of the present invention and its improved concept, makes equivalent substitutions or changes, and all should be covered within the protection scope of the present invention.
Claims
1. A local active phase compensation method applied to MDI-QKD, characterized in that: The local active phase compensation method applied to MDI-QKD includes the following steps: S1: The light emitted from the light source first passes through the fiber delay superposition ring structure. The light emitted from the light source is split into a reference light beam and a signal light beam by the first optical beam splitter coupler or the fourth optical beam splitter coupler. They are respectively modulated by intensity modulators, and then combined at the second optical beam splitter coupler or the fifth optical beam splitter coupler; S2: The light combined at the second optical beam splitter coupler or the fifth optical beam splitter coupler passes through the optical circulator, and then enters through the first incident arm of the third optical beam splitter coupler or the sixth optical beam splitter coupler and is split into two beams of light with uniform intensity and then exits from the two exit arms of the third optical beam splitter coupler or the sixth optical beam splitter coupler. One beam of light is modulated by the first phase modulator or the second phase modulator and then reflected by the first Faraday mirror or the third Faraday mirror. One beam of light is attenuated by the first fiber optic attenuator or the second fiber optic attenuator and then reflected by the second Faraday mirror or the fourth Faraday mirror; S3: The two reflected light beams interfere at the third beam splitter coupler or the sixth beam splitter coupler. The interfered light exits from the two incident arms of the third beam splitter coupler or the sixth beam splitter coupler. The interference result of the light exiting from the first incident arm is received by the first photodetector or the second photodetector; S4: The light exiting from the Faraday interference ring passes through the first optical switch and the second optical switch. The optical switches block the compensation light from passing through. The control waveforms of the first optical switch and the second optical switch are respectively opposite to those of the second intensity modulator and the fourth intensity modulator; S5: The signal light passing through the first optical switch and the second optical switch is attenuated by the first bench-top optical attenuator or the second bench-top optical attenuator and then transmitted in the channel. The signal lights of Alice and Bob generate multi-photon interference at Charlie and announce the measurement results; S6: Alice and Bob screen the raw keys and perform error detection and privacy amplification; The local active phase compensation method is dedicated to the measurement device-independent quantum key distribution protocol for discrete variable time-phase encoding, and adopts a method combining a fiber delay superposition ring and a Faraday interference ring. The fiber delay superposition ring separates the reference light and the signal light in the time domain.
2. The local active phase compensation method applied to MDI-QKD according to claim 1, wherein: The determination method of the control signals of the second intensity modulator and the fourth intensity modulator in S4 is as follows: When measuring the conditions for applying passive compensation measures to the Faraday interference rings of both parties, during the QKD phase, when the control module continuously outputs the first temporary working point voltage, the longest stable time of the light intensity received by the photodetector plus the stable time and the scanning time are recorded as t. Then the frequency of the control signal is f = 1 / t; The control signal is a pulse signal generated by a signal generator. The pulse width of the pulse signal should be greater than the stable stage time plus the scanning stage time. When the pulse arrives, the control module starts the work of stable reset and scanning.
3. The local active phase compensation method applied to MDI-QKD according to claim 1, characterized in that: In S5 and S6, the optical paths and compensation structures of Alice and Bob adopt a symmetric design, which is for compensating the working point drift of their respective phase modulators. Applied in MDI-QKD, the two parties do not need to exchange compensation information through an additional channel.
4. The local active phase compensation system applied to MDI-QKD according to claim 1, wherein: It includes an A - end local active phase compensation system, a B - end local active phase compensation system, and a C - end multi - photon interference measurement system connected by optical fibers. The A - end local active phase compensation system includes a first continuous light source module, a first circulator module, a first beam splitter module, a first electro - optic intensity modulator module, a first electro - optic phase modulator module, a first Faraday mirror module, a first fiber optic attenuator module, a first bench - top optical attenuator module, and a first control module.
5. The local active phase compensation system applied to MDI-QKD according to claim 4, wherein: The B - end local active phase compensation system includes a second continuous light source module, a second circulator module, a second beam splitter module, a second electro - optic intensity modulator module, a second electro - optic phase modulator module, a second Faraday mirror module, a second fiber optic attenuator module, a second bench - top optical attenuator module, and a second control module. The C - end multi - photon interference measurement system includes a third beam splitter module, a third polarization controller module, and a third single - photon detector module.
6. The local active phase compensation system for MDI-QKD according to claim 4, wherein: The first control module includes a first signal amplifier, a first analog - to - digital conversion module, a first FPGA development board, a first digital - to - analog conversion module, and a second signal amplifier.
7. The local active phase compensation system applied to MDI-QKD according to claim 5, wherein: The second control module includes a third signal amplifier, a second analog - to - digital conversion module, a second FPGA development board, a second digital - to - analog conversion module, and a fourth signal amplifier.
8. A local active phase compensation system applied to MDI-QKD according to claim 7, characterized in that: The method for determining the amplification factors of the first signal amplifier and the third signal amplifier in the first control module and the second control module is as follows: If the compensation time is T, let the frequency F = 1 / T. Input a sine - wave signal with a frequency greater than or equal to F and an amplitude approximately equal to the maximum output voltage amplitude of the photodetector into the first signal amplifier and the third signal amplifier. Adjust the amplification factor. When the signal amplifier linearly amplifies the above - mentioned sine - wave signal and the amplitude is within the input noise tolerance of the analog - to - digital conversion module, the amplification factor at this time meets the requirements.
9. The local active phase compensation system for MDI-QKD according to claim 8, characterized in that: The method for determining the amplification factors of the second signal amplifier and the fourth signal amplifier in the first control module and the second control module is as follows: If the compensation time is T, let the frequency F = 1 / T. Input a saw - tooth wave signal with a frequency greater than or equal to F and an amplitude equal to the maximum output voltage amplitude of the digital - to - analog conversion module into the second signal amplifier and the fourth signal amplifier. Adjust the amplification factor. When the saw - tooth wave can be linearly amplified and the amplitude of the amplified signal is greater than twice the half - wave voltage of the phase modulator, the amplification factor at this time meets the requirements.