Discrete and continuous variable adaptive coding module for quantum key distribution system

Abstract

The invention belongs to the technical field of optical secret communication, and discloses a discrete and continuous variable adaptive coding module for a quantum key distribution system, and when an optical switch enables an optical path between a polarization beam splitter and a phase modulator to be conducted, the coding module can realize continuous variable coding; when the optical switch enables an optical path between the polarization beam splitter and the reflection module to be conducted, the encoding module can realize phase encoding or polarization encoding; when the reflection module reflects the light signal and the polarization state of the light signal is not changed, the encoding module realizes phase encoding; when the reflection module reflects the light signal and enables the polarization state of the light signal to rotate by 90 degrees, the coding module realizes polarization coding. Compared with the prior art, switching between continuous variable coding and discrete variable coding is achieved, switching between polarization coding and phase coding in discrete variables can be achieved, only one phase modulator is needed, and adaptive coding mode switching can be achieved according to actual scene requirements when the method is applied to the quantum key distribution network; the method has good compatibility and adaptability.

Description

Technical Field

[0001] The present invention relates to the technical field of optical secure communication, and in particular to a discrete and continuous variable adaptive coding module for a quantum key distribution system. Background Art

[0002] Quantum key distribution (QKD) can resist the threat from quantum computers and plays an important role in confidential communications. QKD encoding methods can be divided into two categories: discrete variables (DV) and continuous variables (CV). Among them, DV encoding has a high tolerance for channel loss, can ensure stable information transmission over long distances, and is suitable for long-distance communications. In short-distance communication scenarios, CV encoding shows unique advantages. It can achieve a higher key rate than DV encoding and meet the needs of short-distance, high-data-volume transmission. Therefore, for different communication scenarios, different communication distances require different encoding methods.

[0003] Since the basic structure of QKD is point-to-point communication, it is necessary to deploy the transmitter and receiver at different nodes in the network construction. Considering the network construction and cost optimization, the communication network often contains multiple types of links. Some links are close and may need to coexist with a large number of classical communication data channels. CV coding can easily co-transmit with classical communication in the same optical fiber through wavelength division multiplexing technology. At the same time, its hardware cost may be lower than DV coding hardware, especially the fact that single photon detectors are not required, which saves costs for network construction. However, other links may pay more attention to the ability to resist channel loss, such as long-distance scenarios, in which case DV coding is more suitable. A switchable coding module can flexibly select the coding method according to the characteristics of different links, avoiding the high cost of equipping different links with different coding hardware.

[0004] Paper Grande IHL et al. "Adaptable transmitter for discrete and continuous variable quantum key distribution". Optics express, 2021, 29(10):14815-14827. Three amplitude modulators and one intensity modulator can be used to implement DV phase encoding and CV encoding respectively, but polarization encoding cannot be implemented, and a large number of modulators are required, which makes the modulation circuit complex and increases the complexity of the system. Patent CN116155495B proposes a QKD encoding module with switchable encoding mode, but it can only realize the switching between polarization or phase encoding in DV encoding, and cannot realize CV encoding, and its adaptability in networking applications is not high. Summary of the invention

[0005] In view of the above defects in the prior art, the present invention proposes a discrete and continuous variable adaptive encoding module for a quantum key distribution system.

[0006] The technical solution of the present invention is achieved in this way: A discrete and continuous variable adaptive encoding module for a quantum key distribution system, comprising a first circulator CIR1, a polarization beam splitter PBS, a phase modulator PM, a first optical switch OS1 and a reflection module, The polarization-maintaining fiber at the first port of CIR1 is spliced ​​at 45°; The second port of CIR1 is connected to an input port of PBS; the third port is used as the output port of the encoding module; The two output ports of PBS are connected to one port of PM and one input port of OS1 respectively; The two output ports of OS1 are connected to the other port of PM and the reflection module respectively; Another input port of OS1 is connected to another input port of PBS; The length of the optical fiber connecting PBS and PM is equal to the sum of the lengths of the optical fiber connecting PBS and OS1 and the lengths of the optical fiber connecting OS1 and PM; The reflection module is used to reflect the light pulse and can keep the polarization state of the light pulse unchanged or rotate it by 90°; When OS1 makes the optical path between PBS and PM conductive, the encoding module can realize continuous variable encoding; When OS1 makes the optical path between PBS and the reflection module conductive, the encoding module can realize phase encoding or polarization encoding; when the reflection module reflects the light signal and does not change its polarization state, the encoding module realizes phase encoding; when the reflection module reflects the light signal and rotates its polarization state by 90°, the encoding module realizes polarization encoding.

[0007] Preferably, the reflection module comprises a first reflector M1 and an adjustable quarter wave plate VQWP, and OS1 is connected to M1 after being connected to VQWP; The angle between the main axis direction of the VQWP and the slow axis of the polarization-maintaining fiber can be adjusted to 0° and 45°.

[0008] Preferably, the reflection module comprises a second optical switch OS2, a second reflector M2, a third reflector M3 and a first polarization rotation module. The first polarization rotation module is used to rotate the polarization state of the light pulse passing through it by 90°; OS1 is connected to the input port of OS2; One output port of OS2 is directly connected to M2; Another output port of OS2 is connected to the first polarization rotation module and then connected to M3.

[0009] Preferably, the reflection module includes a third optical switch OS3, a fourth reflector M4 and a first polarization rotation module. The first polarization rotation module is used to rotate the polarization state of the light pulse passing through it by 90°; OS1 is connected to an input port of OS3; One output port of OS3 is directly connected to M4; Another output port of OS3 is connected to the first polarization rotation module and then connected to another input port of OS3.

[0010] Preferably, the first polarization rotation module is a quarter wave plate QWP, and the angle between the main axis direction of the first polarization rotation module and the slow axis of the polarization-maintaining optical fiber is 45°.

[0011] Preferably, the first polarization rotation module is a Faraday rotator FR, and its polarization rotation angle is 45°.

[0012] Preferably, the reflection module comprises a second circulator CIR2, a fourth optical switch OS4 and a second polarization rotation module. OS1 is connected to the second port of CIR2; The third port and the first port of CIR2 are connected to an output port and an output port of OS4 respectively; Another input port of OS4 is connected to the second polarization rotation module and then connected to another output port thereof.

[0013] Preferably, the second polarization rotation module is a half-wave plate HWP, and the angle between the main axis direction of the second polarization rotation module and the slow axis of the polarization-maintaining optical fiber is 45°.

[0014] Preferably, the second polarization rotation module is a 90° fusion splicing of polarization-maintaining optical fibers.

[0015] Compared with the prior art, the present invention has the following beneficial effects: The present invention proposes a discrete and continuous variable adaptive coding module for a quantum key distribution system, which can not only realize the switching between continuous variable coding and discrete variable coding, but also realize the switching between polarization coding and phase coding in discrete variables. It only requires one phase modulator and has a relatively simple structure. It can be applied to a quantum key distribution network to realize adaptive coding mode switching according to actual scenario requirements and has good compatibility and adaptability. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a schematic diagram of the principle of discrete and continuous variable adaptive encoding modules of the quantum key distribution system of the present invention; Figure 2 This is a schematic diagram of the principle of Embodiment 1 of the discrete and continuous variable adaptive encoding module of the quantum key distribution system of the present invention; Figure 3 This is a schematic diagram of the principle of Embodiment 2 of the discrete and continuous variable adaptive encoding module of the quantum key distribution system of the present invention; Figure 4 This is a schematic diagram of the principle of Embodiment 3 of the discrete and continuous variable adaptive encoding module of the quantum key distribution system of the present invention; Figure 5 This is a schematic diagram of the principle of Embodiment 4 of the discrete and continuous variable adaptive encoding module of the quantum key distribution system of the present invention. DETAILED DESCRIPTION

[0017] The present invention will be described clearly and completely below with reference to the accompanying drawings in the embodiments of the present invention.

[0018] like Figure 1 As shown, a discrete and continuous variable adaptive encoding module for a quantum key distribution system includes a first circulator CIR1, a polarization beam splitter PBS, a phase modulator PM, a first optical switch OS1 and a reflection module. The polarization-maintaining fiber at the first port of CIR1 is spliced ​​at 45°; The second port of CIR1 is connected to an input port of PBS; the third port is used as the output port of the encoding module; The two output ports of PBS are connected to one port of PM and one input port of OS1 respectively; The two output ports of OS1 are connected to the other port of PM and the reflection module respectively; Another input port of OS1 is connected to another input port of PBS; The length of the optical fiber connecting PBS and PM is equal to the sum of the lengths of the optical fiber connecting PBS and OS1 and the lengths of the optical fiber connecting OS1 and PM; The reflection module is used to reflect the light pulse and can keep the polarization state of the light pulse unchanged or rotate it by 90°; When OS1 makes the optical path between PBS and PM conductive, the encoding module can realize continuous variable encoding; When OS1 makes the optical path between PBS and the reflection module conductive, the encoding module can realize phase encoding or polarization encoding; when the reflection module reflects the light signal and does not change its polarization state, the encoding module realizes phase encoding; when the reflection module reflects the light signal and rotates its polarization state by 90°, the encoding module realizes polarization encoding.

[0019] The specific working process is as follows: The optical pulse enters the encoding module and passes through the 45° fiber fusion point. The polarization state is rotated from horizontal polarization to 45° polarization. Then, it passes through CIR1 and reaches PBS, where it is polarization-split into horizontal polarization component and vertical polarization component.

[0020] When OS1 makes the optical path between PBS and PM conductive, it is equivalent to connecting the two output ports of PBS through PM to form a Sagnac loop. At this time, the optical pulse will not pass through the reflection module. The horizontal polarization component is transmitted from PBS to PM through OS1; at the same time, since the length of the optical fiber connecting PBS and PM is equal to the sum of the length of the optical fiber connecting PBS and OS1 and the length of the optical fiber connecting OS1 and PM, the vertical polarization component is reflected by PBS and reaches PM at the same time as the horizontal polarization component, so it is modulated with the same phase j. Subsequently, the two return to PBS at the same time for polarization beam combining. Since the optical path experienced by the two is exactly the same, the horizontal component and the vertical component of the optical pulse generated after beam combining are exchanged, and the overall phase of the optical pulse is modulated to j. When j=-π or π, continuous variable BPSK encoding can be achieved. When j=π / 4, 3π / 4, 5π / 4 or 7π / 4, continuous variable QPSK encoding can be achieved.

[0021] When OS1 connects the optical path between the PBS and the reflection module, it is equivalent to connecting one output port of the PBS to the reflection module, and connecting another output port of the PBS to another input port through PM, forming a first ring structure.

[0022] When the reflection module reflects the optical signal without changing its polarization state, the horizontal polarization component is transmitted from PBS through OS1 to the reflection module. After being reflected by it, the polarization remains unchanged. After passing through OS1 again, it reaches PBS and is directly transmitted, still with horizontal polarization. After being reflected by PBS, the vertical polarization component propagates in the first ring structure in a clockwise direction, passes through PM and is modulated by phase θ, then passes through OS1 to reach PBS, is reflected by it and reaches the reflection module through OS1, and its polarization remains unchanged after being reflected. It returns along the original path and passes through PM again and is emitted from PBS, becoming vertically polarized. Since the vertical polarization component is transmitted for a path twice the length of the first ring structure than the horizontal polarization component, when it is emitted from PBS, the former lags behind the latter by the time τ transmitted by the optical fiber length of the first ring structure, which is the arm length delay difference of the unequal arm interferometer. Finally, the quantum state output by CIR1 is , in, They represent the previous time mode and the next time mode in phase encoding, and the phase difference between the two time modes is θ. When θ=0, π, π / 2, 3π / 2, four phase encoding states can be obtained, which can realize the phase encoding of discrete variables.

[0023] When the reflection module reflects the optical signal and rotates its polarization state by 90°, the horizontal polarization component is transmitted from PBS through OS1 to the reflection module. After being reflected by it, the polarization state is rotated by 90° to become vertical polarization. After passing through OS1 again, it reaches PBS and is reflected. It propagates along the counterclockwise optical fiber in the first ring structure, passes through PM and is modulated by phase φ1, and then is emitted from PBS and becomes vertical polarization; the vertical polarization component is reflected by PBS and propagates along the clockwise direction in the first ring structure, passes through PM and is modulated by phase φ2, then passes through OS1 to reach PBS, is reflected by it and reaches the reflection module through OS1, and the polarization state is rotated by 90° after being reflected. After being reflected, it reaches PBS and is directly transmitted and becomes horizontal polarization. Since the optical path of the horizontal polarization and vertical polarization components is equal, the difference is that the time they pass through PM is different, so they are emitted from PBS at the same time, with a phase difference of φ2-φ1, and the quantum state output by CIR1 is , When φ2-φ1=0, π, π / 2, 3π / 2, four polarization coding states can be obtained, thus realizing polarization coding of discrete variables.

[0024] like Figure 2 As shown, the first embodiment of the present invention: The reflection module includes a first reflector M1 and an adjustable quarter wave plate VQWP, and OS1 is connected to M1 after being connected to VQWP; The angle between the main axis direction of the VQWP and the slow axis of the polarization-maintaining fiber can be adjusted to 0° and 45°.

[0025] The specific working process of embodiment 1 is as follows: The optical pulse enters the encoding module and passes through the 45° fiber fusion point. The polarization state is rotated from horizontal polarization to 45° polarization. Then, it passes through CIR1 and reaches PBS, where it is polarization-split into horizontal polarization component and vertical polarization component.

[0026] When OS1 makes the optical path between PBS and PM conductive, it is equivalent to connecting the two output ports of PBS through PM to form a Sagnac loop. At this time, the optical pulse will not pass through the reflection module. The horizontal polarization component is transmitted from PBS to PM through OS1; at the same time, since the length of the optical fiber connecting PBS and PM is equal to the sum of the length of the optical fiber connecting PBS and OS1 and the length of the optical fiber connecting OS1 and PM, the vertical polarization component is reflected by PBS and reaches PM at the same time as the horizontal polarization component, so it is modulated with the same phase j. Subsequently, the two return to PBS at the same time for polarization beam combining. Since the optical path experienced by the two is exactly the same, the horizontal component and the vertical component of the optical pulse generated after beam combining are exchanged, and the overall phase of the optical pulse is modulated to j. When j=-π or π, continuous variable BPSK encoding can be achieved. When j=π / 4, 3π / 4, 5π / 4 or 7π / 4, continuous variable QPSK encoding can be achieved.

[0027] When OS1 connects the optical path between the PBS and the reflection module, it is equivalent to connecting one output port of the PBS to the reflection module, and connecting another output port of the PBS to another input port through PM, forming a first ring structure.

[0028] The reflection module consists of the first reflector M1 and the adjustable quarter-wave plate VQWP. When the angle between the main axis direction of the VQWP and the slow axis of the polarization-maintaining fiber is 0°, the overall effect can be expressed by the Jones matrix as follows: , That is, it is equivalent to the function of a single reflector without changing the polarization state of the light pulse. When the angle between the main axis direction of the VQWP and the slow axis of the polarization-maintaining fiber is 45°, its overall effect can be expressed by the Jones matrix: , That is, the horizontal polarization and the vertical polarization can be rotated 90° respectively.

[0029] When the reflection module reflects the optical signal without changing its polarization state, the horizontal polarization component is transmitted from PBS through OS1 to the reflection module. After being reflected by it, the polarization remains unchanged. After passing through OS1 again, it reaches PBS and is directly transmitted, still with horizontal polarization. After being reflected by PBS, the vertical polarization component propagates in the first ring structure in a clockwise direction, passes through PM and is modulated by phase θ, then passes through OS1 to reach PBS, is reflected by it and reaches the reflection module through OS1, and its polarization remains unchanged after being reflected. It returns along the original path and passes through PM again and is emitted from PBS, becoming vertically polarized. Since the vertical polarization component is transmitted for a path twice the length of the first ring structure than the horizontal polarization component, when it is emitted from PBS, the former lags behind the latter by the time τ transmitted by the optical fiber length of the first ring structure, which is the arm length delay difference of the unequal arm interferometer. Finally, the quantum state output by CIR1 is , in, They represent the previous time mode and the next time mode in phase encoding, and the phase difference between the two time modes is θ. When θ=0, π, π / 2, 3π / 2, four phase encoding states can be obtained, which can realize the phase encoding of discrete variables.

[0030] When the reflection module reflects the optical signal and rotates its polarization state by 90°, the horizontal polarization component is transmitted from PBS through OS1 to the reflection module. After being reflected by it, the polarization state is rotated by 90° to become vertical polarization. After passing through OS1 again, it reaches PBS and is reflected. It propagates along the counterclockwise optical fiber in the first ring structure, passes through PM and is modulated by phase φ1, and then is emitted from PBS and becomes vertical polarization; the vertical polarization component is reflected by PBS and propagates along the clockwise direction in the first ring structure, passes through PM and is modulated by phase φ2, then passes through OS1 to reach PBS, is reflected by it and reaches the reflection module through OS1, and the polarization state is rotated by 90° after being reflected. After being reflected, it reaches PBS and is directly transmitted and becomes horizontal polarization. Since the optical path of the horizontal polarization and vertical polarization components is equal, the difference is that the time they pass through PM is different, so they are emitted from PBS at the same time, with a phase difference of φ2-φ1, and the quantum state output by CIR1 is , When φ2-φ1=0, π, π / 2, 3π / 2, four polarization coding states can be obtained, thus realizing polarization coding of discrete variables.

[0031] like Figure 3 As shown, the second embodiment of the present invention: The reflection module includes a second optical switch OS2, a second reflector M2, a third reflector M3 and a first polarization rotation module. The first polarization rotation module is used to rotate the polarization state of the light pulse passing through it by 90°; OS1 is connected to the input port of OS2; One output port of OS2 is directly connected to M2; Another output port of OS2 is connected to the first polarization rotation module and then connected to M3.

[0032] The first polarization rotation module is a quarter wave plate QWP, and the angle between the main axis direction and the slow axis of the polarization-maintaining optical fiber is 45°.

[0033] The specific working process of the second embodiment is as follows: The optical pulse enters the encoding module and passes through the 45° fiber fusion point. The polarization state is rotated from horizontal polarization to 45° polarization. Then, it passes through CIR1 and reaches PBS, where it is polarization-split into horizontal polarization component and vertical polarization component.

[0034] When OS1 makes the optical path between PBS and PM conductive, it is equivalent to connecting the two output ports of PBS through PM to form a Sagnac loop. At this time, the optical pulse will not pass through the reflection module. The horizontal polarization component is transmitted from PBS to PM through OS1; at the same time, since the length of the optical fiber connecting PBS and PM is equal to the sum of the length of the optical fiber connecting PBS and OS1 and the length of the optical fiber connecting OS1 and PM, the vertical polarization component is reflected by PBS and reaches PM at the same time as the horizontal polarization component, so it is modulated with the same phase j. Subsequently, the two return to PBS at the same time for polarization beam combining. Since the optical path experienced by the two is exactly the same, the horizontal component and the vertical component of the optical pulse generated after beam combining are exchanged, and the overall phase of the optical pulse is modulated to j. When j=-π or π, continuous variable BPSK encoding can be achieved. When j=π / 4, 3π / 4, 5π / 4 or 7π / 4, continuous variable QPSK encoding can be achieved.

[0035] When OS1 connects the optical path between the PBS and the reflection module, it is equivalent to connecting one output port of the PBS to the reflection module, and connecting another output port of the PBS to another input port through PM, forming a first ring structure.

[0036] The reflector module can switch between two paths through OS2. The first path is to make the optical path from OS1 to M2 conductive. The effect of M2 can be expressed by the Jones matrix as , That is, the polarization state of the optical pulse is not changed. The second path is to make the optical path from OS1 to QWP and M3 conductive, and its overall effect can be expressed by the Jones matrix as follows: , That is, the horizontal polarization and the vertical polarization can be rotated 90° respectively.

[0037] When OS2 is switched on to make the optical path where M2 is located conductive, the horizontal polarization component is transmitted from PBS through OS1 to M2. After being reflected by it, the polarization remains unchanged. After passing through OS1 again, it reaches PBS and is directly transmitted, still with horizontal polarization. After being reflected by PBS, the vertical polarization component propagates in the first ring structure in a clockwise direction, passes through PM and is modulated by phase θ, then passes through OS1 to PBS, is reflected by it, and reaches M2 through OS1. After being reflected, the polarization remains unchanged, returns along the original path, passes through PM again, and is emitted from PBS, becoming vertically polarized. Since the vertical polarization component is transmitted for a path twice the length of the first ring structure than the horizontal polarization component, when it is emitted from PBS, the former lags behind the latter by the time τ transmitted by the optical fiber length of the first ring structure, which is the arm length delay difference of the unequal arm interferometer. Finally, the quantum state output by CIR1 is , in, They represent the previous time mode and the next time mode in phase encoding, and the phase difference between the two time modes is θ. When θ=0, π, π / 2, 3π / 2, four phase encoding states can be obtained, which can realize the phase encoding of discrete variables.

[0038] When OS2 is switched on to make the optical path where M3 and QWP are located conductive, the horizontal polarization component is transmitted from PBS through OS1 to QWP and M3. After being reflected by them, the polarization state is rotated 90° to become vertical polarization. After passing through OS1 again, it reaches PBS and is reflected. It propagates along the counterclockwise optical fiber in the first ring structure, passes through PM and is modulated by phase φ1, and then is emitted from PBS and becomes vertical polarization; the vertical polarization component is reflected by PBS and propagates along the clockwise direction in the first ring structure, passes through PM and is modulated by phase φ2, then passes through OS1 to PBS, is reflected by it and reaches QWP and M3 through OS1, and the polarization state is rotated 90° after being reflected. After being reflected, it reaches PBS and is directly transmitted and becomes horizontal polarization. Since the optical path of the horizontal polarization and vertical polarization components is equal, the difference is that the time they pass through PM is different, so they are emitted from PBS at the same time, with a phase difference of φ2-φ1, and the quantum state output by CIR1 is , When φ2-φ1=0, π, π / 2, 3π / 2, four polarization coding states can be obtained, thus realizing polarization coding of discrete variables.

[0039] like Figure 4 As shown, the third embodiment of the present invention: The reflection module includes a third optical switch OS3, a fourth reflector M4 and a first polarization rotation module. The first polarization rotation module is used to rotate the polarization state of the light pulse passing through it by 90°; OS1 is connected to an input port of OS3; One output port of OS3 is directly connected to M4; Another output port of OS3 is connected to the first polarization rotation module and then connected to another input port of OS3.

[0040] The first polarization rotation module is a Faraday rotator FR, and its polarization rotation angle is 45°.

[0041] The specific working process of the third embodiment is as follows: The optical pulse enters the encoding module and passes through the 45° fiber fusion point. The polarization state is rotated from horizontal polarization to 45° polarization. Then, it passes through CIR1 and reaches PBS, where it is polarization-split into horizontal polarization component and vertical polarization component.

[0042] When OS1 makes the optical path between PBS and PM conductive, it is equivalent to connecting the two output ports of PBS through PM to form a Sagnac loop. At this time, the optical pulse will not pass through the reflection module. The horizontal polarization component is transmitted from PBS to PM through OS1; at the same time, since the length of the optical fiber connecting PBS and PM is equal to the sum of the length of the optical fiber connecting PBS and OS1 and the length of the optical fiber connecting OS1 and PM, the vertical polarization component is reflected by PBS and reaches PM at the same time as the horizontal polarization component, so it is modulated with the same phase j. Subsequently, the two return to PBS at the same time for polarization beam combining. Since the optical path experienced by the two is exactly the same, the horizontal component and the vertical component of the optical pulse generated after beam combining are exchanged, and the overall phase of the optical pulse is modulated to j. When j=-π or π, continuous variable BPSK encoding can be achieved. When j=π / 4, 3π / 4, 5π / 4 or 7π / 4, continuous variable QPSK encoding can be achieved.

[0043] When OS1 connects the optical path between the PBS and the reflection module, it is equivalent to connecting one output port of the PBS to the reflection module, and connecting another output port of the PBS to another input port through PM, forming a first ring structure.

[0044] The reflector module can switch between two paths through OS3. The first path is to make the optical path from OS1 to M4 conductive. The effect of M4 can be expressed by the Jones matrix as follows: , That is, the polarization state of the optical pulse is not changed. The second path is to make the optical path from OS1 to FR and M4 conductive. The overall effect can be expressed by the Jones matrix as follows: , That is, the horizontal polarization and the vertical polarization can be rotated 90° respectively.

[0045] When OS3 is switched on to make the optical path where M4 is located conductive, the horizontal polarization component is transmitted from PBS through OS1 to M4. After being reflected by it, the polarization remains unchanged. After passing through OS1 again, it reaches PBS and is directly transmitted, still with horizontal polarization. After being reflected by PBS, the vertical polarization component propagates in the first ring structure in a clockwise direction, passes through PM and is modulated by phase θ, then passes through OS1 to PBS, is reflected by it, and reaches M4 through OS1. After being reflected, the polarization remains unchanged, returns along the original path, passes through PM again, and is emitted from PBS, becoming vertically polarized. Since the vertical polarization component is transmitted for a path twice the length of the first ring structure than the horizontal polarization component, when it is emitted from PBS, the former lags behind the latter by the time τ transmitted by the optical fiber length of the first ring structure, which is the arm length delay difference of the unequal arm interferometer. Finally, the quantum state output by CIR1 is , in, They represent the previous time mode and the next time mode in phase encoding, and the phase difference between the two time modes is θ. When θ=0, π, π / 2, 3π / 2, four phase encoding states can be obtained, which can realize the phase encoding of discrete variables.

[0046] When OS3 is switched on to make the optical path where FR and M4 are located conductive, the horizontal polarization component is transmitted from PBS through OS1 to FR and M4. After being reflected by them, the polarization state is rotated 90° to become vertical polarization. After passing through OS1 again, it reaches PBS and is reflected. It propagates along the counterclockwise optical fiber in the first ring structure, passes through PM and is modulated by phase φ1, and then is emitted from PBS and becomes vertical polarization; the vertical polarization component is reflected by PBS and propagates along the clockwise direction in the first ring structure, passes through PM and is modulated by phase φ2, then passes through OS1 to PBS, and after being reflected by them, reaches FR and M4 through OS1. After being reflected, the polarization state is rotated 90°, and after being reflected, it reaches PBS and is directly transmitted and becomes horizontal polarization. Since the optical path of the horizontal polarization and vertical polarization components is equal, the difference is that the time they pass through PM is different, so they are emitted from PBS at the same time, with a phase difference of φ2-φ1, and the quantum state output by CIR1 is , When φ2-φ1=0, π, π / 2, 3π / 2, four polarization coding states can be obtained, thus realizing polarization coding of discrete variables.

[0047] like Figure 5 As shown, the fourth embodiment of the present invention: The reflection module includes a second circulator CIR2, a fourth optical switch OS4 and a second polarization rotation module, OS1 is connected to the second port of CIR2; The third port and the first port of CIR2 are connected to an output port and an output port of OS4 respectively; Another input port of OS4 is connected to the second polarization rotation module and then connected to another output port thereof.

[0048] The second polarization rotation module is a half-wave plate HWP, and the angle between the main axis direction and the slow axis of the polarization-maintaining optical fiber is 45°.

[0049] The specific working process of the fourth embodiment is as follows: The optical pulse enters the encoding module and passes through the 45° fiber fusion point. The polarization state is rotated from horizontal polarization to 45° polarization. Then, it passes through CIR1 and reaches PBS, where it is polarization-split into horizontal polarization component and vertical polarization component.

[0050] When OS1 makes the optical path between PBS and PM conductive, it is equivalent to connecting the two output ports of PBS through PM to form a Sagnac loop. At this time, the optical pulse will not pass through the reflection module. The horizontal polarization component is transmitted from PBS to PM through OS1; at the same time, since the length of the optical fiber connecting PBS and PM is equal to the sum of the length of the optical fiber connecting PBS and OS1 and the length of the optical fiber connecting OS1 and PM, the vertical polarization component is reflected by PBS and reaches PM at the same time as the horizontal polarization component, so it is modulated with the same phase j. Subsequently, the two return to PBS at the same time for polarization beam combining. Since the optical path experienced by the two is exactly the same, the horizontal component and the vertical component of the optical pulse generated after beam combining are exchanged, and the overall phase of the optical pulse is modulated to j. When j=-π or π, continuous variable BPSK encoding can be achieved. When j=π / 4, 3π / 4, 5π / 4 or 7π / 4, continuous variable QPSK encoding can be achieved.

[0051] When OS1 connects the optical path between the PBS and the reflection module, it is equivalent to connecting one output port of the PBS to the reflection module, and connecting another output port of the PBS to another input port through PM, forming a first ring structure.

[0052] The second port and the third port of CIR2 are connected through OS4 to form a second ring structure; an input port and an output port of OS4 are connected through HWP to form a third ring structure. The reflection module can switch two paths through OS4. The first path is to make the second ring structure directly conductive without passing through the third ring structure, so that the optical pulse is emitted from the second port of CIR2 without changing the polarization state of the optical pulse. The second path is to make the second ring structure conductive through the third ring structure. When passing through the third ring structure, it will pass through HWP, and its Jones matrix is , That is, the horizontal polarization and the vertical polarization can be rotated 90° respectively.

[0053] When OS4 is switched so that the optical pulse only passes through the second ring structure, the horizontal polarization component is transmitted from PBS through OS1 to CIR2, and then emitted from CIR2 after being transmitted through the second ring structure, with the polarization unchanged, and then directly transmitted through OS1 to PBS again, still with horizontal polarization; the vertical polarization component is reflected by PBS and propagates in the clockwise direction in the first ring structure, and is modulated by the phase θ through PM, and then passes through OS1 to PBS, and then is reflected by it and reaches CIR2 through OS1, and then is emitted from CIR2 after being transmitted through the second ring structure, with the polarization unchanged, and returns along the original path, passes through PM again, and is emitted from PBS, becoming vertically polarized. Since the vertical polarization component is transmitted for a path twice the length of the first ring structure than the horizontal polarization component, when it is emitted from PBS, the former lags behind the latter by the time τ transmitted by the optical fiber length of the first ring structure, which is the arm length delay difference of the unequal arm interferometer, and the quantum state output by CIR1 is finally , in, They represent the previous time mode and the next time mode in phase encoding, and the phase difference between the two time modes is θ. When θ=0, π, π / 2, 3π / 2, four phase encoding states can be obtained, which can realize the phase encoding of discrete variables.

[0054] When OS4 is switched to make the optical pulse pass through the second and third ring structures, the horizontal polarization component is transmitted from PBS through OS1 to CIR2. After passing through the second and third ring structures and HWP, the polarization state rotates 90° to become vertical polarization. After passing through OS1 again, it reaches PBS and is reflected. It propagates along the counterclockwise optical fiber in the first ring structure, is modulated by the phase φ1 through PM, and then is emitted from PBS and becomes vertical polarization. The vertical polarization component is reflected by PBS and propagates along the clockwise direction in the first ring structure. It is modulated by the phase φ2 through PM, and then reaches PBS through OS1. After being reflected by it, it reaches CIR2 through OS1. After passing through the second and third ring structures and HWP, the polarization state rotates 90°. After being reflected, it reaches PBS and is directly transmitted and becomes horizontal polarization. Since the optical path of the horizontal polarization and vertical polarization components is equal, the difference is that the time they pass through PM is different. Therefore, they are emitted from PBS at the same time with a phase difference of φ2-φ1. Finally, the quantum state output through CIR1 is , When φ2-φ1=0, π, π / 2, 3π / 2, four polarization coding states can be obtained, thus realizing polarization coding of discrete variables.

[0055] From the various embodiments of the present invention, it can be seen that the present invention proposes a discrete and continuous variable adaptive coding module for a quantum key distribution system, which can not only realize the switching between continuous variable coding and discrete variable coding, but also realize the switching between polarization coding and phase coding in discrete variables. It only requires one phase modulator and has a relatively simple structure. It can be applied to a quantum key distribution network to realize adaptive coding mode switching according to actual scenario requirements, and has good compatibility and adaptability.

Claims

1. A discrete and continuous variable adaptive encoding module for a quantum key distribution system, characterized in that: It includes a first circulator CIR1, a polarization beam splitter PBS, a phase modulator PM, a first optical switch OS1 and a reflection module, The polarization-maintaining fiber at the first port of CIR1 is spliced ​​at 45°; The second port of CIR1 is connected to an input port of PBS; the third port is used as the output port of the encoding module; The two output ports of PBS are connected to one port of PM and one input port of OS1 respectively; The two output ports of OS1 are connected to the other port of PM and the reflection module respectively; Another input port of OS1 is connected to another input port of PBS; The length of the optical fiber connecting PBS and PM is equal to the sum of the lengths of the optical fiber connecting PBS and OS1 and the lengths of the optical fiber connecting OS1 and PM; The reflection module is used to reflect the light pulse and can keep the polarization state of the light pulse unchanged or rotate it by 90°; When OS1 makes the optical path between PBS and PM conductive, the encoding module can realize continuous variable encoding; When OS1 makes the optical path between PBS and the reflection module conductive, the encoding module can realize phase encoding or polarization encoding; when the reflection module reflects the light signal and does not change its polarization state, the encoding module realizes phase encoding; when the reflection module reflects the light signal and rotates its polarization state by 90°, the encoding module realizes polarization encoding.

2. The discrete and continuous variable adaptive encoding module for a quantum key distribution system according to claim 1, characterized in that: The reflection module includes a first reflector M1 and an adjustable quarter wave plate VQWP, and OS1 is connected to M1 after being connected to VQWP; The angle between the main axis direction of the VQWP and the slow axis of the polarization-maintaining fiber can be adjusted to 0° and 45°.

3. The discrete and continuous variable adaptive encoding module for a quantum key distribution system according to claim 1, characterized in that: The reflection module includes a second optical switch OS2, a second reflector M2, a third reflector M3 and a first polarization rotation module. The first polarization rotation module is used to rotate the polarization state of the light pulse passing through it by 90°; OS1 is connected to the input port of OS2; One output port of OS2 is directly connected to M2; Another output port of OS2 is connected to the first polarization rotation module and then connected to M3.

4. The discrete and continuous variable adaptive encoding module for a quantum key distribution system according to claim 1, characterized in that: The reflection module includes a third optical switch OS3, a fourth reflector M4 and a first polarization rotation module. The first polarization rotation module is used to rotate the polarization state of the light pulse passing through it by 90°; OS1 is connected to an input port of OS3; One output port of OS3 is directly connected to M4; Another output port of OS3 is connected to the first polarization rotation module and then connected to another input port of OS3.

5. The discrete and continuous variable adaptive encoding module for a quantum key distribution system according to claim 3 or 4, characterized in that: The first polarization rotation module is a quarter wave plate QWP, and the angle between the main axis direction and the slow axis of the polarization-maintaining optical fiber is 45°.

6. The discrete and continuous variable adaptive encoding module for a quantum key distribution system according to claim 3, characterized in that: The first polarization rotation module is a Faraday rotator FR, and its polarization rotation angle is 45°.

7. The discrete and continuous variable adaptive encoding module for a quantum key distribution system according to claim 1, characterized in that: The reflection module includes a second circulator CIR2, a fourth optical switch OS4 and a second polarization rotation module, OS1 is connected to the second port of CIR2; The third port and the first port of CIR2 are connected to an output port and an output port of OS4 respectively; Another input port of OS4 is connected to the second polarization rotation module and then connected to another output port thereof.

8. The discrete and continuous variable adaptive encoding module for a quantum key distribution system according to claim 7, characterized in that: The second polarization rotation module is a half-wave plate HWP, and the angle between the main axis direction and the slow axis of the polarization-maintaining optical fiber is 45°.

9. The discrete and continuous variable adaptive encoding module for a quantum key distribution system according to claim 7, characterized in that: The second polarization rotation module is a 90° fusion splicing of polarization-maintaining optical fibers.

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