Quantum key distribution passive decoding device and distribution system

Abstract

The application belongs to the technical field of quantum secure communication, and discloses a quantum key distribution passive decoding device and distribution system, which comprises an optical chip of an integrated passive base selection module and an interferometer module group, a beam combination module and a single photon detector connected in sequence; the interferometer module group comprises a Z base interferometer module and an X base interferometer module, which are used for decoding phase encoding quantum signals of Z base vectors and X base vectors respectively; the output ends of the Z base interferometer module and the X base interferometer module are connected to the beam combination module through unequal length single mode optical fibers, so that four-way interference results form a time difference, enter the single photon detector in a time division multiplexing mode, and the single detector realizes the differential detection of four-way signals. Compared with the prior art, the number of relatively expensive detectors is reduced, the cost of the detectors and the cost of the supporting circuit are greatly reduced, the total cost of the system is reduced by more than 50%, and the commercial threshold is reduced; the system realizes polarization-independent decoding and does not need compensation.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of quantum secure communication, in particular to a quantum key distribution passive decoding device and distribution system. BACKGROUND

[0002] Quantum key distribution (QKD) is one of the core technologies in the field of quantum communication, which can provide theoretically unconditional secure encryption keys for both parties of communication based on the basic principles of quantum mechanics. Phase-encoding QKD protocol is widely used in commercial systems due to its strong anti-interference ability and adaptability to long-distance fiber transmission. The core of the protocol lies in the efficient decoding of phase-encoding quantum signals.

[0003] The existing phase-encoding QKD decoding device has many defects: first, the structure is complex and the cost is high. The traditional passive decoding device needs to configure independent interferometers and corresponding single-photon detectors for Z-basis and X-basis. Usually, four detectors are needed to complete the detection of four interference results, and the cost of detectors accounts for more than 60% of the total system cost. Second, it is highly sensitive to polarization. The polarization state fluctuation of the input signal will cause the decoding performance to deteriorate, and an additional polarization compensation module is needed, which increases the system complexity and cost. Third, active decoding relies on active phase modulation. To meet the need of high-speed switching between Z-basis measurement and X-basis measurement, the phase needs to be adjusted in real time, which not only increases the energy consumption but also reduces the reliability of long-term operation of the system. Fourth, the beam combining loss is high. If a traditional optical beam splitter is used for multi-channel signal beam combining, it will introduce inherent loss, resulting in a decrease in key rate. Fifth, the phase difference stability is poor. Z-basis and X-basis decoding need to maintain specific long-short arm phase differences, and the existing active phase modulation method cannot balance precision and stability.

[0004] To solve the above problems, the existing technology attempts to use a single detector architecture, but there are problems such as polarization dependence, the need for active phase modulation, or unstable phase difference, which makes it difficult to balance the needs of simplifying the structure, reducing the cost, and ensuring the performance. Therefore, it is urgent to design a polarization-independent, active phase-free, and phase-stable single-detector passive decoding device that simplifies the structure while improving the stability and practicality of the system. SUMMARY

[0005] In view of the above defects of the prior art, the present application provides a quantum key distribution passive decoding device and distribution system to solve the problems of high cost, large size, poor environmental adaptability, and difficulty in popularization for civilian use of existing QKD systems, and to realize high-security key distribution in consumer-level scenarios.

[0006] The technical scheme of the present application is as follows:

[0007] A quantum key distribution passive decoding device, comprising an integrated passive basis selection module and an interferometer module group optical chip connected in sequence, a beam combining module, and a single-photon detector.

[0008] The interferometer module group comprises a Z-based interferometer module and an X-based interferometer module, which are used for decoding phase-encoded quantum signals of Z-based vectors and X-based vectors, respectively;

[0009] The beam combining module adopts a lossless beam combining structure and is used for integrating multiple interference results into a single signal;

[0010] The passive base selection module realizes passive selection of Z-based vectors and X-based vectors without active control components;

[0011] The Z-based interferometer module is provided with a first phase shifter, which is used for maintaining a phase difference between long and short arms of an internal unequal arm interferometer as 0, and the X-based interferometer module is provided with a second phase shifter, which is used for maintaining a phase difference between long and short arms of an internal unequal arm interferometer as π / 2;

[0012] Output ends of the Z-based interferometer module and the X-based interferometer module are connected to the beam combining module through unequal length single mode optical fibers, so that four interference results form a time difference, enter a single photon detector in a time division multiplexing manner, and realize differentiated detection of four signals by the single detector,

[0013] The device has polarization-independent characteristics, can adapt to input quantum signals of any polarization state without polarization compensation components, and ensures consistent decoding performance of signals of different polarization states.

[0014] Preferably, the Z-based interferometer module comprises a first polarization beam splitter PBS1, a third polarization beam splitter PBS3, a fourth polarization beam splitter PBS4 and two unequal arm interferometers; the PBS1 polarizes an input signal into a horizontal polarization component and a vertical polarization component, which are input into the two unequal arm interferometers, and a first phase shifter is integrated on a short arm of the unequal arm interferometer to maintain a phase difference between long and short arms as 0; one-way outputs of the two unequal arm interferometers are polarized and combined by the PBS3 into a first interference result, and the other way outputs are polarized and combined by the PBS4 into a second interference result; the X-based interferometer module comprises a second polarization beam splitter PBS2, a fifth polarization beam splitter PBS5, a sixth polarization beam splitter PBS6 and two unequal arm interferometers; the PBS2 polarizes an input signal into a horizontal polarization component and a vertical polarization component, which are input into the two unequal arm interferometers, and a second phase shifter is integrated on a short arm of the unequal arm interferometer to maintain a phase difference between long and short arms as π / 2; one-way outputs of the two unequal arm interferometers are polarized and combined by the PBS5 into a third interference result, and the other way outputs are polarized and combined by the PBS6 into a fourth interference result;

[0015] All unequal arm interferometer structures are the same, composed of a first beam splitter BS1, a second beam splitter BS2 and a delay waveguide DLW, the two output ports of BS1 are connected to the two input ports of BS2 through waveguides, and DLW is located on one of the waveguides as a long arm, and the other waveguide as a short arm.

[0016] Preferably, the length difference of the single-mode optical fiber connecting the four-way interference result and the beam combining module satisfies: the time interval of adjacent interference results reaching the single-photon detector is not less than the full instrument timing response half-width of the single-photon detector, ensuring that the four-way signals can be distinguished in the time dimension and avoiding crosstalk.

[0017] Preferably, the beam combining module is a photon lantern structure, the input end contains four interfaces for adapting single-mode optical fibers, and the output end is a single multi-mode optical fiber interface, the number of modes supported by the output end is not less than the sum of the number of modes of the input end, and the insertion loss is controlled within 0.5dB.

[0018] Preferably, the first phase shifter and the second phase shifter are thermo-optic phase shifters, which compensate for slow phase drift by controlling electrode voltage, and the phase compensation accuracy error is not more than ±0.01π.

[0019] Preferably, the passive base selection module is a Y-branch waveguide structure, and the two output ends of the Y-branch waveguide are connected to the Z-base interferometer module and the X-base interferometer module respectively, realizing the equal-probability passive selection of Z-base vector and X-base vector, and the branch loss consistency error is not more than 0.2dB.

[0020] Preferably, the passive base selection module includes a depolarization module and a seventh polarization beam splitter PBS7; the depolarization module randomizes the polarization state of the input quantum optical signal, PBS7 performs polarization beam splitting on the random polarization state signal, and the two output signals are connected to the Z-base interferometer module and the X-base interferometer module respectively, realizing the passive selection of Z-base vector and X-base vector.

[0021] The Z-base interferometer module and the X-base interferometer module are both an unequal arm interferometer, and the structures are the same, composed of a first beam splitter BS1, BS2 and a delay waveguide DLW, the two output ports of BS1 are connected to the two input ports of BS2 through waveguides, and DLW is located on one of the waveguides as a long arm, and the other waveguide as a short arm.

[0022] Preferably, the single-photon detector is configured with a time gating unit, the time gating unit is synchronized with the time delay of the four-way interference result, and the preset gating window further suppresses dark count and signal crosstalk, and the gating window width is 1 / 3~1 / 2 of the time interval of adjacent signals.

[0023] Preferably, a signal processing unit is further included, which is connected with the single-photon detector, used for recording a time stamp of the detection signal, distinguishing the decoding results of the Z basis and the X basis in combination with a preset time-interference result correspondence, completing quantum bit information extraction and key generation, and simultaneously monitoring a decoding error rate in real time and feeding back phase compensation parameters of the first phase shifter and the second phase shifter for optimization.

[0024] The application further discloses a quantum key distribution system comprising a sending end and a receiving end connected through an optical fiber channel, wherein the sending end comprises a laser, an intensity modulator, an unequal-arm interferometer chip and an adjustable attenuator, the high-speed phase shifter is arranged on the short arm of the unequal-arm interferometer chip and used for randomly generating four phases of 0, π / 2, π and 3π / 2 to emit four corresponding phase-encoding quantum states; and the receiving end comprises the decoding device in any one of the preceding aspects.

[0025] Compared with the prior art, the application has the following beneficial effects:

[0026] 1. The cost is significantly reduced: only a single single-photon detector is needed to replace four traditional detectors, the cost of the detector and the supporting circuit is greatly reduced, the total cost of the system is reduced by more than 50%, and the commercial threshold is reduced.

[0027] 2. Polarization-independent and no compensation: polarization-independent decoding is realized through polarization-elimination design or polarization beam splitting / combining optimization, and any polarization state input signal is adapted without the need of an additional polarization compensation module, and the system structure is simplified.

[0028] 3. Low-loss beam combining and efficient detection: a photonic lantern or a fused fiber combiner is used as a beam combining module to realize nearly lossless combining of four signals with an insertion loss controlled within 0.5 dB; time division multiplexing design ensures that a single detector can accurately distinguish four signals, and the decoding error rate is equivalent to that of the traditional multi-detector scheme.

[0029] 5. Strong applicability: the phase-encoding QKD protocol is adapted, long-distance optical fiber transmission and free space transmission scenarios are supported, the decoding module in the existing commercial system can be directly replaced, and the compatibility is strong. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 It is a principle block diagram of the quantum key distribution passive decoding device of the application;

[0031] Figure 2 It is a principle block diagram of the quantum key distribution passive decoding device embodiment one of the application;

[0032] Figure 3 It is a principle block diagram of the quantum key distribution passive decoding device embodiment two of the application. DETAILED DESCRIPTION

[0033] The application will be described in detail below with reference to the accompanying drawings.

[0034] As shown in the drawings, a quantum key distribution passive decoding device is characterized in that it comprises an optical chip of an integrated passive basis selection module and an interferometer module group connected in sequence, a beam combination module and a single photon detector. Figure 1

[0035] The interferometer module group comprises a Z-basis interferometer module and an X-basis interferometer module for decoding phase-encoding quantum signals of Z-basis and X-basis respectively.

[0036] The beam combination module adopts a lossless beam combination structure for integrating multiple interference results into a single signal.

[0037] The passive basis selection module realizes passive selection of Z-basis and X-basis without active control components.

[0038] The Z-basis interferometer module is provided with a first phase shifter for keeping the phase difference between long and short arms of an internal unequal arm interferometer stable at 0, and the X-basis interferometer module is provided with a second phase shifter for keeping the phase difference between long and short arms of an internal unequal arm interferometer stable at π / 2.

[0039] The output ends of the Z-basis interferometer module and the X-basis interferometer module are connected to the beam combination module through unequal length single mode fibers, so that the four interference results form a time difference and enter the single photon detector in a time division multiplexing manner, realizing the differentiated detection of four signals by a single detector.

[0040] The device has polarization independent characteristics and can adapt to input quantum signals of any polarization state without polarization compensation components, ensuring consistent decoding performance of signals of different polarization states.

[0041] The specific working process is as follows:

[0042] The phase-encoding quantum optical signal enters the input end of the decoding device, first enters the passive basis selection module of the optical chip, is divided into two quantum optical signal components for passive basis selection, and randomly selects Z-basis or X-basis without active control. Subsequently, the two quantum optical signal components enter the Z-basis interferometer module and the X-basis interferometer module respectively for interference decoding, wherein the phase difference between the long and short arms of the Z-basis interferometer module is 0, and the decoding produces two interference results; the phase difference between the long and short arms of the X-basis interferometer module is π / 2, and the decoding produces another two interference results.

[0043] ​The four-way interference results are output from the optical chip simultaneously, and pass through single-mode optical fibers with preset length differences, that is, pass through single-mode optical fibers with lengths of L, L+AL, L+2AL and L+3AL in sequence and are connected to four input ports of the beam combining module, to form stable time differences (such as adjacent intervals of 1 ns) and realize time division multiplexing preprocessing. The four-way interference results enter the beam combining module (such as a photon lantern) and are coupled to an output multi-mode optical fiber in an independent mode, to realize near-lossless beam combining.

[0044] The interference results after beam combining enter a single single-photon detector in time sequence, and a time gating unit is triggered synchronously, to accurately detect signals at each time point.

[0045] Finally, a signal processing unit distinguishes the four-way interference results according to detection time stamps, extracts quantum bit information, completes key generation and error rate monitoring, and feeds back and adjusts the parameters of the phase shifters as necessary to maintain phase difference accuracy.

[0046] As shown in FIG. 1, an embodiment one includes a Z-based interferometer module and an X-based interferometer module. Figure 2

[0047] The Z-based interferometer module includes PBS1, PBS3, PBS4, two unequal-arm interferometers and two first phase shifters; PBS1 polarizes and splits an input signal into horizontal polarization components and vertical polarization components, which are input into the two unequal-arm interferometers, the first phase shifters are integrated in the unequal-arm interferometers and maintain a phase difference between long and short arms as 0; one-way outputs of the two unequal-arm interferometers are polarized and combined by PBS3 as a first interference result, and the other way outputs are polarized and combined by PBS4 as a second interference result; the X-based interferometer module includes PBS2, PBS5, PBS6, two unequal-arm interferometers and two second phase shifters; PBS2 polarizes and splits an input signal into horizontal polarization components and vertical polarization components, which are input into the two unequal-arm interferometers, the second phase shifters are integrated in the unequal-arm interferometers and maintain a phase difference between long and short arms as π / 2; one-way outputs of the two unequal-arm interferometers are polarized and combined by PBS5 as a third interference result, and the other way outputs are polarized and combined by PBS6 as a fourth interference result.

[0048] All the unequal-arm interferometers have the same structure, which is composed of first beam splitters BS1 and BS2 and a delay waveguide DLW, two output ports of BS1 are connected to two input ports of BS2 through waveguides, DLW is located on one of the waveguides as a long arm, and the other waveguide is a short arm.

[0049] The length difference of the single-mode optical fibers connecting the four-way interference results and the beam combining module satisfies that the time interval of adjacent interference results reaching the single-photon detector is not less than the full-instrument timing response half-width of the single-photon detector, to ensure that the four-way signals can be distinguished in the time dimension and to avoid crosstalk.

[0050] ​The beam combining module is a photon lantern structure, the input end comprises four interfaces for adapting single-mode optical fibers, the output end is a single multi-mode optical fiber interface, the number of modes supported by the output end is not less than the sum of the number of modes of the input end, near-lossless beam combining of four interference results is realized, and the insertion loss is controlled within 0.5dB.

[0051] The first phase shifter and the second phase shifter are thermo-optic phase shifters, compensation for slow phase drift is realized by controlling the electrode voltage, and the phase compensation accuracy error is not more than ±0.01π.

[0052] The passive base selection module is a Y-branch waveguide structure, two output ends of the Y-branch waveguide are connected with the Z-base interferometer module and the X-base interferometer module respectively, equal-probability passive selection of the Z-base vector and the X-base vector is realized, and the branch loss consistency error is not more than 0.2dB.

[0053] The working process is as follows:

[0054] The phase-encoded quantum light signal enters the input end of the decoding device, first enters the passive base selection module of the optical chip, that is, the Y-branch waveguide, is divided into two quantum light signal components for passive base vector selection, and randomly selects the Z base or the X base without active control.

[0055] One of the quantum light signal components enters the Z-base interferometer module, is polarized by PBS1 into a horizontal polarization component and a vertical polarization component, and is input into two unequal-arm interferometers for interference. The first phase shifter is integrated in the two unequal-arm interferometers, and the phase difference between the long and short arms is maintained at 0. The horizontal polarization component generates a first interference signal and a second interference signal through the unequal-arm interferometer, and the vertical polarization component generates a third interference signal and a fourth interference signal through the unequal-arm interferometer; the first interference signal is in phase with the third interference signal, and the polarization beam combining is performed through PBS3 to obtain a first interference result; the second interference signal is in phase with the fourth interference signal, and the polarization beam combining is performed through PBS4 to obtain a second interference result.

[0056] As can be seen, by polarizing the quantum light signal component, respectively performing interference, and then polarizing and beam combining the in-phase interference signals through a polarization beam splitter, when the polarization state of the input quantum light signal changes, the intensity of the interference result does not change, so polarization-independent decoding can be realized.

[0057] Another quantum optical signal component enters the X base interferometer module, and is polarized by PBS2 into a horizontal polarization component and a vertical polarization component, which are input into two unequal arm interferometers for interference. The second phase shifter is integrated in the two unequal arm interferometers, and the phase difference between the long arm and the short arm is maintained at π / 2. The horizontal polarization component generates a fifth interference signal and a sixth interference signal through the unequal arm interferometer, and the vertical polarization component generates a seventh interference signal and an eighth interference signal through the unequal arm interferometer; the fifth interference signal and the seventh interference signal are in phase, and the third interference result is obtained by polarization beam combining through PBS5; the sixth interference signal and the eighth interference signal are in phase, and the fourth interference result is obtained by polarization beam combining through PBS6.

[0058] The four interference results are simultaneously output from the optical chip, and pass through single-mode optical fibers with different lengths, i.e., sequentially pass through single-mode optical fibers with lengths of L, L+ΔL, L+2ΔL, and L+3ΔL, and are connected to four input ports of the photon lantern, to form stable time differences (adjacent intervals of 1 ns) and realize time division multiplexing preprocessing. A 4-input 1-output photon lantern structure is adopted, the four input interfaces are connected to the single-mode optical fibers of the four interference results, the output end is a multi-mode optical fiber, the number of supported modes is ≥4, and the beam combining insertion loss is ≤0.5 dB. Therefore, the four interference results enter the photon lantern and are coupled to the output multi-mode optical fiber in independent modes, realizing near-lossless beam combining.

[0059] The interference results after beam combining enter a single single-photon detector in time sequence, and a time gating unit is synchronously triggered to accurately detect the signals at each time point.

[0060] An FPGA chip + data processing module is adopted to record the detection signal timestamp, distinguish the four interference results through the timestamp (e.g., t0 corresponds to the first, t0+1 ns corresponds to the second, t0+2 ns corresponds to the third, and t0+3 ns corresponds to the fourth), complete quantum bit extraction and key generation, and simultaneously monitor the bit error rate in real time to feedback and optimize the phase compensation accuracy of the phase shifter.

[0061] As shown in FIG. 2, embodiment two includes a passive selection base module, a Z base interferometer module, an X base interferometer module, a time gating unit, and a data processing module. Figure 3

[0062] The passive selection base module includes a depolarization module and PBS7; the depolarization module randomizes the polarization state of the input quantum optical signal, PBS7 polarizes and splits the signal with random polarization state, and two output signals are connected to the Z base interferometer module and the X base interferometer module respectively to realize passive selection of the Z base vector and the X base vector.

[0063] ​The Z base interferometer module and the X base interferometer module are both unequal-arm interferometers and have the same structure, and are composed of first beam splitters BS1 and BS2 and a delay waveguide DLW, two output ports of the BS1 are connected with two input ports of the BS2 through waveguides, the DLW is located on one of the waveguides and serves as a long arm, and the other waveguide serves as a short arm.

[0064] The single-photon detector is configured with a time gating unit, the time gating unit is synchronized with time delay of the four-way interference results, dark counts and signal crosstalk are further suppressed through a preset gating window, and a width of the gating window is 1 / 3-1 / 2 of a time interval of adjacent signals.

[0065] The device further includes a signal processing unit connected with the single-photon detector, for recording a time stamp of a detection signal, distinguishing a decoding result of the Z base and the X base in combination with a preset time-interference result corresponding relationship, completing quantum bit information extraction and key generation, and simultaneously monitoring a decoding error rate in real time and feeding back phase compensation parameters of the first phase shifter and the second phase shifter.

[0066] The working process is as follows:

[0067] The phase-encoding quantum optical signal enters an input end of the decoding device, and first passes through a depolarization module to randomize a polarization state of the input quantum optical signal. The depolarization module can be built by devices or integrated on an optical chip, but its function must be polarization-independent, and can be formed by cascading two unequal-arm polarization interferometers with different arm lengths, wherein a difference between the two arm lengths is much larger than a coherence time of the optical signal, and a difference between the two arm length delays is also much larger than the coherence time of the optical signal.

[0068] After polarization randomization, the quantum optical signal enters the PBS7 and is split into two quantum optical signal components by polarization splitting to select a passive basis vector, and the Z base or the X base is randomly selected with equal probability without active control. In addition, since polarization randomization and polarization splitting are performed at the receiving end, no matter how a polarization state of the input quantum optical signal is changed by channel disturbance or even an eavesdropper, the passive selected basis will not be affected, so that polarization-independent decoding can be realized, and the security of the system can be improved to resist polarization attacks.

[0069] One of the quantum optical signal components after polarization splitting enters the Z base interferometer module, that is, an unequal-arm interferometer, for interference, wherein the first phase shifter is integrated to maintain a phase difference between long and short arms as 0, to generate a first interference result and a second interference result.

[0070] The other quantum optical signal component enters the X base interferometer module, that is, another unequal-arm interferometer, for interference, wherein the second phase shifter is integrated to maintain a phase difference between long and short arms as π / 2, to generate a third interference result and a fourth interference result.

[0071] The four-way interference results are output simultaneously from the optical chip, pass through single-mode optical fibers with preset length differences in turn, that is, the four-way interference results are connected with four input ports of the photon lantern through single-mode optical fibers with lengths of L, L+AL, L+2AL and L+3AL respectively, to form stable time differences (adjacent intervals of 1ns) and realize time division multiplexing preprocessing.

[0072] The interference results after beam combination enter a single single-photon detector in time sequence, and a time gating unit is synchronously triggered to accurately detect signals at each time point.

[0073] An FPGA chip and a data processing module are used to record timestamps of the detection signals, the four-way interference results are distinguished through the timestamps (for example, t0 corresponds to the first way, t0+1ns corresponds to the second way, t0+2ns corresponds to the third way, and t0+3ns corresponds to the fourth way), quantum bit extraction and key generation are completed, and the bit error rate is monitored in real time to feed back the phase compensation accuracy of the phase shifter.

[0074] According to the principle and the embodiment of the present application, the present application only needs a single single-photon detector to replace four traditional detectors, greatly reduces the cost of the detector and the cost of the supporting circuit, reduces the total cost of the system by more than 50%, significantly reduces the cost of commercialization, realizes polarization-independent decoding through polarization elimination design or polarization beam splitting / combining optimization, adapts to any polarization state input signal without the need for additional polarization compensation modules, simplifies the system structure, uses a photon lantern or a fused fiber beam combiner as a beam combining module to realize near-lossless beam combination of four-way signals with an insertion loss controlled within 0.5dB, ensures that a single detector can accurately distinguish four-way signals through time division multiplexing design, and the decoding bit error rate is equivalent to that of the traditional multi-detector scheme, adapts to phase encoding QKD protocol, supports long-distance optical fiber transmission and free space transmission scenarios, can directly replace the decoding module in the existing commercial system, and has strong compatibility.

Claims

1. A passive decoding device for quantum key distribution, characterized in that, It includes an optical chip, a beam combiner module, and a single single-photon detector, which are connected in sequence to form an integrated passive basis selection module and an interferometer module. The interferometer module group includes a Z-basis interferometer module and an X-basis interferometer module, which are used to decode the phase-encoded quantum signals of the Z-basis vector and the X-basis vector, respectively. The beam combining module adopts a lossless beam combining structure, which is used to integrate multi-channel interference results into a single signal; The passive basis selection module enables the passive selection of Z basis vectors and X basis vectors without the need for active control components; The Z-based interferometer module is equipped with a first phase shifter to keep the phase difference between the long and short arms of its internal unequal-arm interferometer stable at 0, and the X-based interferometer module is equipped with a second phase shifter to keep the phase difference between the long and short arms of its internal unequal-arm interferometer stable at π / 2. The outputs of the Z-based interferometer module and the X-based interferometer module are connected to a beam combiner module via unequal-length single-mode optical fibers. This allows the four interference results to form a time difference, which is then time-division multiplexed into a single-photon detector, enabling a single detector to distinguish and detect the four signals. The Z-based interferometer module includes a first polarization beamsplitter PBS1, a third polarization beamsplitter PBS3, a fourth polarization beamsplitter PBS4, and two unequal-arm interferometers. PBS1 splits the input signal polarization beam into horizontal and vertical polarization components, which are then input to the two unequal-arm interferometers. A first phase shifter is integrated on the short arm of each unequal-arm interferometer to maintain a zero phase difference between the long and short arms. One output from the two unequal-arm interferometers is combined by polarization through PBS3 to obtain the first interference result, and the other output is combined by polarization through PBS4 to obtain the second interference result. The X-based interferometer module includes a second polarization beamsplitter PBS2, a fifth polarization beamsplitter PBS5, a sixth polarization beamsplitter PBS6, and two unequal-arm interferometers. PBS2 splits the input signal polarization beam into horizontal and vertical polarization components, which are then input into the two unequal-arm interferometers respectively. A second phase shifter is integrated on the short arm of each unequal-arm interferometer to maintain a phase difference of π / 2 between the long and short arms. One output of the two unequal-arm interferometers is combined by polarization through PBS5 to obtain a third interference result, and the other output is combined by polarization through PBS6 to obtain a fourth interference result. All unequal-arm interferometers have the same structure, consisting of a first beam splitter BS1, a second beam splitter BS2, and a time-delay waveguide DLW. The two output ports of BS1 are connected to the two input ports of BS2 through waveguides. DLW is located on one of the waveguides as the long arm, and the other waveguide as the short arm.

2. The quantum key distribution passive decoding device according to claim 1, characterized in that, The length difference of the single-mode fiber connecting the four interference results and the beam combining module satisfies the following: the time interval between adjacent interference results arriving at the single-photon detector is not less than the full instrument timing response half-width of the single-photon detector, in order to ensure that the four signals are distinguishable in the time dimension.

3. The quantum key distribution passive decoding device according to claim 1, characterized in that, The beam combining module is a photonic lantern structure. The input end includes four interfaces that adapt to single-mode optical fibers, and the output end is a single multimode optical fiber interface. The number of modes supported by the output end is not less than the sum of the number of modes at the input end, and the insertion loss is controlled within 0.5dB.

4. The quantum key distribution passive decoding device according to claim 1, characterized in that, The first and second phase shifters are thermo-optical phase shifters, which compensate for slow phase drift by controlling the electrode voltage, and the phase compensation accuracy error does not exceed ±0.01π.

5. The quantum key distribution passive decoding device according to claim 3, characterized in that, The passive basis selection module is a Y-branch waveguide structure. The two output terminals of the Y-branch waveguide are connected to the Z-basis interferometer module and the X-basis interferometer module, respectively, to achieve equal-probability passive selection of the Z basis and the X basis, with a branch loss consistency error of no more than 0.2dB.

6. The quantum key distribution passive decoding device according to claim 1, characterized in that, The passive basis selection module includes a polarization depolarization module and a seventh polarization beam splitter PBS7. The polarization depolarization module randomizes the polarization state of the input quantum light signal, and PBS7 performs polarization beam splitting on the random polarization state signal. The two output signals are respectively connected to the Z-basis interferometer module and the X-basis interferometer module to realize the passive selection of the Z basis vector and the X basis vector. Both the Z-based interferometer module and the X-based interferometer module are unequal-arm interferometers with the same structure, consisting of first beam splitters BS1 and BS2 and a time-delay waveguide DLW. The two output ports of BS1 are connected to the two input ports of BS2 through waveguides. DLW is located on one of the waveguides as the long arm, and the other waveguide as the short arm.

7. The quantum key distribution passive decoding device according to claim 1, characterized in that, The single-photon detector is equipped with a time gating unit, which is synchronized with the time delay of the four-way interference results. The dark count and signal crosstalk are further suppressed by a preset gating window, and the width of the gating window is 1 / 3 to 1 / 2 of the time interval between adjacent signals.

8. The quantum key distribution passive decoding device according to claim 1, characterized in that, It also includes a signal processing unit, which is connected to a single-photon detector to record the timestamp of the detection signal, and, in conjunction with a preset time-interference result correspondence, distinguishes the decoding results of Z-based and X-based signals, completes the extraction of quantum bit information and key generation, and simultaneously monitors the decoding error rate in real time and feeds back to optimize the phase compensation parameters of the first phase shifter and the second phase shifter.

9. A quantum key distribution system, comprising a transmitter and a receiver connected via an optical fiber channel, wherein the transmitter includes a laser, an intensity modulator, an unequal-arm interferometer chip, and an adjustable attenuator, wherein a high-speed phase modulator is provided on the short arm of the unequal-arm interferometer chip for randomly generating four phases: 0, π / 2, π, and 3π / 2, and transmitting four corresponding phase-encoded quantum states. Its features are, The receiving end includes the decoding device according to any one of claims 1-8.

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