Quantum key distribution passive decoding device and distribution system

Abstract

The invention belongs to the technical field of quantum security communication, and discloses a quantum key distribution passive decoding device and a distribution system, and the device comprises an optical chip integrating a passive base selection module and an interferometer module group, a beam combining module and a single single photon detector which are sequentially connected. The interferometer module group comprises a Z-base interferometer module and an X-base interferometer module which are used for respectively decoding the phase coding quantum signals of the Z-base vector and the X-base vector; and the output ends of the Z-based interferometer module and the X-based interferometer module are connected with the beam combining module through unequal-length single-mode optical fibers, so that four paths of interference results form a time difference and enter a single photon detector in a time division multiplexing manner, and the distinguishing detection of four paths of signals by the single detector is realized. Compared with the prior art, the number of expensive detectors is reduced, the cost of the detectors and the cost of matched circuits 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 without 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 is 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 the 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 a specific long-short arm phase difference, 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 solution of the present application is as follows: 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 single-photon detector. 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. The beam combining module adopts a lossless beam combining structure and is used for integrating multiple interference results into a single signal. The passive base selection module realizes passive selection of Z-based vectors and X-based vectors without active control components. The Z-based interferometer module is provided with a first phase shifter, which is used for keeping the phase difference between long and short arms of an internal unequal-arm interferometer stable at 0, and the X-based interferometer module is provided with a second phase shifter, which is used for keeping the phase difference between long and short arms of an internal unequal-arm interferometer stable at π / 2. The 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 and enter a single-photon detector in a time-division multiplexing manner, and the single detector realizes differentiated detection of the four signals. The device has polarization-independent characteristics and can adapt to input quantum signals of any polarization state without polarization compensation components, thereby ensuring consistent decoding performance of signals of different polarization states.

[0007] 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, respectively; the first phase shifter is integrated on the short arm of the unequal-arm interferometer to maintain the phase difference between the long and short arms at 0; one output of the two unequal-arm interferometers is polarized and combined by the PBS3 into a first interference result, and the other output is 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, respectively; the second phase shifter is integrated on the short arm of the unequal-arm interferometer to maintain the phase difference between the long and short arms at π / 2; one output of the two unequal-arm interferometers is polarized and combined by the PBS5 into a third interference result, and the other output is polarized and combined by the PBS6 into a fourth interference result. All the unequal-arm interferometers have the same structure, which is composed of a first beam splitter BS1, a second beam splitter BS2 and a delay waveguide DLW; the two output ports of the BS1 are connected to the two input ports of the BS2 through waveguides, and the DLW is located on one of the waveguides as a long arm, and the other waveguide is a short arm.

[0008] Preferably, the connection of the four-path interference results and the single-mode fiber length difference of the beam combination 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-path signals can be distinguished in the time dimension and avoiding crosstalk.

[0009] Preferably, the beam combination module is a photon lantern structure, the input end includes four interfaces for adapting single-mode optical fibers, and the output end is a single multi-mode optical fiber interface, the number of supported modes of the output end is not less than the sum of the number of modes of the input ends, and the insertion loss is controlled within 0.5 dB.

[0010] Preferably, the first phase shifter and the second phase shifter are thermo-optic phase shifters, and the compensation for slow phase drift is realized by controlling the electrode voltage, and the phase compensation accuracy error is not more than ±0.01π.

[0011] 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 the Z-base vector and the X-base vector, and the branch loss consistency error is not more than 0.2 dB.

[0012] 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, the 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 the Z-base vector and the X-base vector. The Z-base interferometer module and the X-base interferometer module are both unequal-arm interferometers and have the same structure, which is composed of first beam splitters BS1 and BS2 and a delay waveguide DLW, the two output ports of BS1 are connected to the two input ports of BS2 through waveguides, and the DLW is located on one of the waveguides as a long arm, and the other waveguide is a short arm.

[0013] 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-path interference results, and the preset gating window further suppresses dark counts and signal crosstalk, and the width of the gating window is 1 / 3~1 / 2 of the time interval of adjacent signals.

[0014] Preferably, it further includes a signal processing unit connected with the single-photon detector, for recording the time stamp of the detection signal, distinguishing the decoding results of the Z-base and the X-base in combination with the preset time-interference result correspondence, completing the extraction of quantum bit information and the generation of keys, and simultaneously monitoring the decoding error rate in real time and feeding back the phase compensation parameters of the first phase shifter and the second phase shifter for optimization.

[0015] The present invention also discloses a quantum key distribution system, including a transmitter and a receiver connected via an optical fiber channel. The transmitter includes a laser, an intensity modulator, an unequal-arm interferometer chip, and an adjustable attenuator. The short arm of the unequal-arm interferometer chip is equipped with a high-speed phase modulator for randomly generating four phases: 0, π / 2, π, and 3π / 2, and transmitting four corresponding phase-encoded quantum states. The receiver includes any of the above-mentioned decoding devices.

[0016] Compared with the prior art, the present invention has the following beneficial effects: 1. Significantly reduced costs: Only a single single-photon detector is needed to replace the traditional four detectors, greatly reducing detector costs and associated circuit costs, reducing the total system cost by more than 50%, and lowering the commercialization threshold.

[0017] 2. Polarization independent and no compensation required: Polarization-independent decoding is achieved through depolarization design or polarization beam splitting / combining optimization, adapting to input signals with arbitrary polarization states, without the need for additional polarization compensation modules, thus simplifying the system structure.

[0018] 3. Low-loss combining and efficient detection: Photonic lanterns or fused fiber combiners are used as combining modules to achieve near-lossless combining of four signals, with insertion loss controlled within 0.5dB; the time-division multiplexing design ensures that a single detector can accurately distinguish four signals, and the decoding error rate is comparable to that of traditional multi-detector schemes.

[0019] 5. Strong applicability: It is compatible with the phase coding QKD protocol, supports long-distance fiber optic transmission and free space transmission scenarios, and can directly replace the decoding module in the existing commercial system, with strong compatibility. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the passive decoding device for quantum key distribution according to the present invention. Figure 2 This is a schematic diagram of a first embodiment of the quantum key distribution passive decoding device of the present invention; Figure 3 This is a schematic diagram of a second embodiment of the quantum key distribution passive decoding device of the present invention. Detailed Implementation

[0021] The present invention will now be clearly and completely described with reference to the accompanying drawings in the embodiments of the present invention.

[0022] like Figure 1 As shown, a passive decoding device for quantum key distribution is characterized by comprising an optical chip, a beam combining module, and a single single-photon detector, which are connected in sequence to 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 the bundle combining module through unequal-length single-mode optical fibers, so that the four interference results form a time difference and enter the single-photon detector in a time-division multiplexing manner, so as to realize the single detector to distinguish and detect the four signals. The device is polarization-independent and can adapt to input quantum signals of any polarization state without the need for polarization compensation components, ensuring consistent decoding performance for signals of different polarization states.

[0023] The specific work process is as follows: The phase-encoded quantum optical signal enters the input of the decoding device and first enters the passive basis selection module of the optical chip. There, it is split into two quantum optical signal components for passive basis vector selection, randomly choosing either the 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. In the Z-basis interferometer module, the phase difference between the long and short arms is 0, resulting in two interference results. In the X-basis interferometer module, the phase difference between the long and short arms is π / 2, resulting in two other interference results.

[0024] The four interference results are simultaneously output from the optical chip and connected to the four input ports of the combining module via single-mode fibers of preset length differences, i.e., sequentially through single-mode fibers of lengths L, L+ΔL, L+2ΔL, and L+3ΔL, respectively. This forms a stable time difference (e.g., an adjacent interval of 1 ns), achieving time-division multiplexing preprocessing. The four interference results then enter the combining module (e.g., a photonic lantern) and are coupled to the output multimode fiber in an independent mode, achieving near-lossless combining.

[0025] The interference results after beam combining are fed into individual single-photon detectors in chronological order, and the time gating unit is triggered synchronously to accurately detect the signals at each time point.

[0026] Finally, the signal processing unit distinguishes the four interference results based on the detection timestamp, extracts the quantum bit information, completes key generation and bit error rate monitoring, and, if necessary, feeds back to adjust the phase shifter parameters to maintain phase difference accuracy.

[0027] likeFigure 2 As shown, Example 1: The Z-based interferometer module includes PBS1, PBS3, PBS4, two unequal-arm interferometers, and two first phase shifters. PBS1 splits the input signal polarization into horizontal and vertical polarization components, which are then input into the two unequal-arm interferometers. The first phase shifters are integrated into the unequal-arm interferometers to maintain a phase difference of 0 between the long and short arms. One output of the two unequal-arm interferometers is combined through PBS3 to obtain the first interference result, and the other output is combined through PBS4 to obtain the second interference result. The X-based interferometer module includes PBS2, PBS5, PBS6, two unequal-arm interferometers, and two second phase shifters. PBS2 splits the input signal polarization into horizontal and vertical polarization components, which are then input into the two unequal-arm interferometers. The second phase shifters are integrated into the unequal-arm interferometers to maintain a phase difference of π / 2 between the long and short arms. One output of the two unequal-arm interferometers is combined through PBS5 to obtain the third interference result, and the other output is combined through PBS6 to obtain the fourth interference result. All unequal-arm interferometers have 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.

[0028] The length difference of the single-mode fiber connecting the four interference results and the beam combiner 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, ensuring that the four signals are distinguishable in the time dimension and avoiding crosstalk.

[0029] 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, realizing near-lossless beam combining of four interference results, with insertion loss controlled within 0.5dB.

[0030] 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.

[0031] 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.

[0032] The work process is as follows: When the phase-encoded quantum optical signal enters the input end of the decoding device, it first enters the passive basis selection module of the optical chip, namely the Y-branch waveguide, where it is split into two quantum optical signal components for passive basis selection, randomly selecting either the Z-basis or the X-basis without active control.

[0033] One quantum light signal component enters the Z-based interferometer module, where it is polarized and split into horizontal and vertical polarization components by PBS1. These components are then input into two unequal-arm interferometers for interference. Each unequal-arm interferometer integrates a first phase shifter to maintain a zero phase difference between the long and short arms. The horizontal polarization component generates a first and second interference signal through the unequal-arm interferometer, while the vertical polarization component generates a third and fourth interference signal. The first and third interference signals are in phase and are combined by PBS3 to obtain the first interference result. The second and fourth interference signals are in phase and are combined by PBS4 to obtain the second interference result.

[0034] It can be seen that by polarizing the quantum light signal components, interfering them separately, and then using a polarization beam splitter to combine the in-phase interference signals, the intensity of the interference result does not change when the polarization state of the input quantum light signal changes. Therefore, polarization-independent decoding can be achieved.

[0035] Another quantum light signal component enters the X-based interferometer module, where it is polarized and split into horizontal and vertical polarization components by PBS2. These components are then input into two unequal-arm interferometers for interference. Each unequal-arm interferometer integrates a second phase shifter to maintain a phase difference of π / 2 between the long and short arms. The horizontal polarization component generates the fifth and sixth interference signals through the unequal-arm interferometer, while the vertical polarization component generates the seventh and eighth interference signals. The fifth and seventh interference signals are in phase and are combined by PBS5 to obtain the third interference result. The sixth and eighth interference signals are in phase and are combined by PBS6 to obtain the fourth interference result.

[0036] The four interference results are simultaneously output from the optical chip and connected to the four input ports of the photonic lantern via single-mode fibers of predetermined length differences, i.e., sequentially through single-mode fibers of lengths L, L+ΔL, L+2ΔL, and L+3ΔL, forming a stable time difference (1 ns between adjacent fibers) for time-division multiplexing preprocessing. A 4-input, 1-output photonic lantern structure is adopted. The four input ports are connected to the single-mode fibers of the four interference results, while the output is a multimode fiber supporting ≥4 modes and a bundle insertion loss ≤0.5dB. Therefore, the four interference results enter the photonic lantern and are coupled to the output multimode fiber in independent modes, achieving near-lossless bundle combining.

[0037] The interference results after beam combining are fed into individual single-photon detectors in chronological order, and the time gating unit is triggered synchronously to accurately detect the signals at each time point.

[0038] An FPGA chip and a data processing module are used to record the timestamps of the detection signals. The timestamps are used to distinguish the four interference results (e.g., t0 corresponds to the first path, t0+1ns corresponds to the second path, t0+2ns corresponds to the third path, and t0+3ns corresponds to the fourth path). This completes the extraction of qubits and key generation. At the same time, the bit error rate is monitored in real time, and feedback is used to optimize the phase compensation accuracy of the phase shifter.

[0039] like Figure 3 As shown, Example 2: The passive basis selection module includes a polarization depolarization module and a PBS7. The polarization depolarization module randomizes the polarization state of the input quantum light signal, and the PBS7 performs polarization beam splitting on the random polarization state signal. The two output signals are connected to the Z-basis interferometer module and the X-basis interferometer module, respectively, to realize the passive selection of the Z-basis and the X-basis. 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.

[0040] 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.

[0041] The device also includes a signal processing unit connected to a single-photon detector. The signal processing unit records the timestamp of the detected signal, distinguishes the decoding results of Z-based and X-based signals by combining the preset time-interference result correspondence, completes the extraction of quantum bit information and key generation, and 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.

[0042] The work process is as follows: The phase-encoded quantum optical signal enters the input of the decoding device and first passes through a depolarization module to randomize the polarization state of the input quantum optical signal. The depolarization module can be constructed from components or integrated into an optical chip, but its function must be polarization-independent. It can be constructed by cascading two unequal-arm polarization interferometers with unequal arm lengths, where the delay difference of one arm is much greater than the coherence time of the optical signal, and the difference between the delay differences of the two arms is also much greater than the coherence time of the optical signal.

[0043] After polarization randomization, the quantum optical signal enters the PBS7 and is split into two quantum optical signal components for passive basis selection. The Z-basis or X-basis is randomly selected with equal probability, requiring no active control. Furthermore, because polarization randomization and polarization beam splitting occur at the receiver, passive basis selection remains unaffected by channel disturbances or even changes in the polarization state of the input quantum optical signal by eavesdroppers. This enables polarization-independent decoding and improves system security, making it immune to polarization attacks.

[0044] One quantum light signal component after polarization beam splitting enters the Z-based interferometer module, which is an unequal-arm interferometer for interference. It integrates a first phase shifter to maintain the phase difference between the long and short arms at 0, generating the first interference result and the second interference result.

[0045] Another quantum light signal component enters the X-based interferometer module, which is another unequal-arm interferometer, to perform interference. It integrates a second phase shifter to maintain the phase difference between the long and short arms at π / 2, producing a third and a fourth interference result.

[0046] The four interference results are simultaneously output from the optical chip and connected to the four input ports of the photonic lantern via single-mode fibers of predetermined length differences, i.e., sequentially through single-mode fibers of lengths L, L+ΔL, L+2ΔL, and L+3ΔL, forming a stable time difference (1 ns between adjacent fibers) for time-division multiplexing preprocessing. A 4-input, 1-output photonic lantern structure is adopted. The four input ports are connected to the single-mode fibers of the four interference results, while the output is a multimode fiber supporting ≥4 modes and a bundle insertion loss ≤0.5dB. Therefore, the four interference results enter the photonic lantern and are coupled to the output multimode fiber in independent modes, achieving near-lossless bundle combining.

[0047] The interference results after beam combining are fed into individual single-photon detectors in chronological order, and the time gating unit is triggered synchronously to accurately detect the signals at each time point.

[0048] An FPGA chip and a data processing module are used to record the timestamps of the detection signals. The timestamps are used to distinguish the four interference results (e.g., t0 corresponds to the first path, t0+1ns corresponds to the second path, t0+2ns corresponds to the third path, and t0+3ns corresponds to the fourth path). This completes the extraction of qubits and key generation. At the same time, the bit error rate is monitored in real time, and feedback is used to optimize the phase compensation accuracy of the phase shifter.

[0049] Based on the principles and embodiments of this invention, it can be seen that this invention requires only a single single-photon detector to replace four traditional detectors, significantly reducing detector costs and associated circuit costs, lowering the total system cost by more than 50%, and significantly reducing the commercialization threshold. Through polarization depolarization design or polarization beam splitting / combining optimization, polarization-independent decoding is achieved, adapting to input signals with arbitrary polarization states without the need for additional polarization compensation modules, simplifying the system structure. Using photonic lanterns or fused fiber combiners as the combining module, near-lossless combining of four signals is achieved, with insertion loss controlled within 0.5dB. Time-division multiplexing design ensures that a single detector can accurately distinguish four signals, with a decoding error rate comparable to traditional multi-detector schemes. Adapting to the phase-coded QKD protocol, it supports long-distance fiber optic transmission and free-space transmission scenarios, and can directly replace the decoding module in existing commercial systems, exhibiting 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 the beam combiner module through unequal-length single-mode optical fibers, so that the four interference results form a time difference and enter the single-photon detector in a time-division multiplexing manner, so that a single detector can distinguish and detect the four signals.

2. The quantum key distribution passive decoding device according to claim 1, characterized in that, 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.

3. 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.

4. 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.

5. 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.

6. The quantum key distribution passive decoding device according to claim 4, 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.

7. 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.

8. 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.

9. 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.

10. 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 1-9.

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