Discrete and Continuous Variable Adaptive Coding Module for Quantum Key Distribution System

Abstract

The present invention belongs to the technical field of optical secure communication, and discloses a discrete and continuous variable adaptive encoding module for a quantum key distribution system. When the optical switch makes the optical path between the polarization beam splitter and the phase modulator conductive, the encoding module can achieve continuous variable encoding; when the optical switch makes the optical path between the polarization beam splitter and the reflection module conductive, the encoding module can achieve phase encoding or polarization encoding; when the reflection module reflects the optical signal without changing its polarization state, the encoding module achieves phase encoding; when the reflection module reflects the optical signal and rotates its polarization state by 90°, the encoding module achieves polarization encoding. Compared with the prior art, the present invention realizes the switching between continuous variable encoding and discrete variable encoding, and can also realize the switching between polarization encoding and phase encoding in discrete variables. Only one phase modulator is required, and it can be applied to a quantum key distribution network to achieve adaptive encoding mode switching according to the actual scenario requirements, and has good compatibility and adaptability.

Description

Technical Field

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

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

[0003] Since the basic structure of QKD is point-to-point communication, it is necessary to deploy transmitters and receivers at different nodes in network construction. Considering network construction and cost optimization, communication networks often include various types of links. Some links are relatively short and may need to coexist with a large number of classical communication data channels. CV encoding can easily co-transmit with classical communication in the same optical fiber through wavelength division multiplexing technology, and its hardware cost may be lower than that of DV encoding hardware. In particular, the fact that it does not require single-photon detectors saves costs for network construction. However, other links may pay more attention to the ability to resist channel loss, such as in long-distance scenarios, where DV encoding is more suitable. A switchable encoding module can flexibly select the encoding method according to different link characteristics, avoiding the high cost of separately equipping different encoding hardware for different links.

[0004] The paper "Adaptable transmitter for discrete and continuous variable quantum key distribution" by Grande I H L et al. in Optics express, 2021, 29(10): 14815-14827 can respectively implement DV phase encoding and CV encoding using three amplitude modulators and one intensity modulator. However, it cannot implement polarization encoding, and the required number of modulators is relatively large, making the modulation circuit complex and increasing the complexity of the system. Patent CN116155495B proposes a QKD encoding module with a switchable encoding method. However, it can only switch between polarization or phase encoding in DV encoding and cannot implement CV encoding, resulting in low adaptability in network applications. 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 coding module for a quantum key distribution system.

[0006] The technical solution of the present invention is implemented as follows:

[0007] A discrete and continuous variable adaptive coding 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.

[0008] The polarization-maintaining optical fiber at the first port of CIR1 is fusion-spliced at 45°.

[0009] The second port of CIR1 is connected to an input port of PBS; the third port serves as the output port of the coding module.

[0010] The two output ports of PBS are respectively connected to one port of PM and one input port of OS1.

[0011] The two output ports of OS1 are respectively connected to the other port of PM and the reflection module.

[0012] The other input port of OS1 is connected to the other input port of PBS.

[0013] The optical fiber length connecting PBS and PM is equal to the sum of the optical fiber lengths connecting PBS and OS1 and connecting OS1 and PM.

[0014] The reflection module is used to reflect optical pulses and can keep the polarization state of the optical pulses unchanged or rotate it by 90°.

[0015] When OS1 conducts the optical path between PBS and PM, the coding module can achieve continuous variable coding.

[0016] When OS1 conducts the optical path between PBS and the reflection module, the coding module can achieve phase coding or polarization coding; among them, when the reflection module reflects the optical signal and does not change its polarization state, the coding module achieves phase coding; when the reflection module reflects the optical signal and rotates its polarization state by 90°, the coding module achieves polarization coding.

[0017] Preferably, the reflection module includes a first mirror M1 and an adjustable quarter-wave plate VQWP, and OS1 is connected to VQWP and then connected to M1.

[0018] The included angle between the main axis direction of VQWP and the slow axis of the polarization-maintaining optical fiber can be adjusted to 0° and 45°.

[0019] Preferably, the reflection module includes a second optical switch OS2, a second mirror M2, a third mirror M3, and a first polarization rotation module.

[0020] The first polarization rotation module is used to rotate the polarization state of the optical pulse passing through it back and forth by 90°;

[0021] The input port of OS1 is connected to the input port of OS2;

[0022] One output port of OS2 is directly connected to M2;

[0023] The other output port of OS2 is connected to the first polarization rotation module and then connected to M3.

[0024] Preferably, the reflection module includes the third optical switch OS3, the fourth mirror M4 and the first polarization rotation module.

[0025] The first polarization rotation module is used to rotate the polarization state of the optical pulse passing through it back and forth by 90°;

[0026] The input port of OS1 is connected to one input port of OS3;

[0027] One output port of OS3 is directly connected to M4;

[0028] The other output port of OS3 is connected to the first polarization rotation module and then connected to the other input port of OS3.

[0029] Preferably, the first polarization rotation module is a quarter-wave plate QWP, and the included angle between its main axis direction and the slow axis of the polarization-maintaining fiber is 45°.

[0030] Preferably, the first polarization rotation module is a Faraday rotator FR, and its polarization rotation angle is 45°.

[0031] Preferably, the reflection module includes the second circulator CIR2, the fourth optical switch OS4 and the second polarization rotation module.

[0032] The input port of OS1 is connected to the second port of CIR2;

[0033] The third port and the first port of CIR2 are respectively connected to one output port and one input port of OS4;

[0034] The other input port of OS4 is connected to the second polarization rotation module and then connected to its other output port.

[0035] Preferably, the second polarization rotation module is a half-wave plate HWP, and the included angle between its main axis direction and the slow axis of the polarization-maintaining fiber is 45°.

[0036] Preferably, the second polarization rotation module is a 90° fusion splicing of polarization-maintaining fibers.

[0037] Compared with the prior art, the present invention has the following beneficial effects:

[0038] The present invention provides a discrete and continuous variable adaptive encoding module for a quantum key distribution system, which can not only realize the switching between continuous variable encoding and discrete variable encoding, but also realize the switching between polarization encoding and phase encoding in discrete variables. Only one phase modulator is required, and the structure is relatively simple. When applied to a quantum key distribution network, it can realize the adaptive encoding mode switching according to the actual scenario requirements, and has good compatibility and adaptability. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 It is a schematic diagram of the principle of the discrete and continuous variable adaptive encoding module of the quantum key distribution system of the present invention;

[0040] Figure 2 It is a schematic diagram of the principle of the first embodiment of the discrete and continuous variable adaptive encoding module of the quantum key distribution system of the present invention;

[0041] Figure 3 It is a schematic diagram of the principle of the second embodiment of the discrete and continuous variable adaptive encoding module of the quantum key distribution system of the present invention;

[0042] Figure 4 It is a schematic diagram of the principle of the third embodiment of the discrete and continuous variable adaptive encoding module of the quantum key distribution system of the present invention;

[0043] Figure 5 It is a schematic diagram of the principle of the fourth embodiment of the discrete and continuous variable adaptive encoding module of the quantum key distribution system of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0044] The present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.

[0045] As Figure 1 shown, the 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.

[0046] The polarization-maintaining fiber at the first port of CIR1 is fusion-spliced at 45°.

[0047] The second port of CIR1 is connected to an input port of PBS; the third port serves as the output port of the encoding module.

[0048] The two output ports of PBS are respectively connected to one port of PM and one input port of OS1.

[0049] The two output ports of OS1 are respectively connected to the other port of PM and the reflection module.

[0050] Another input port of OS1 is connected to another input port of PBS;

[0051] The optical fiber length connecting PBS and PM is equal to the sum of the optical fiber lengths connecting PBS and OS1 and connecting OS1 and PM;

[0052] The reflection module is used to reflect optical pulses and can keep the polarization state of the optical pulses unchanged or rotate it by 90°;

[0053] When OS1 makes the optical path between PBS and PM conductive, the encoding module can achieve continuous variable encoding;

[0054] When OS1 makes the optical path between PBS and the reflection module conductive, the encoding module can achieve phase encoding or polarization encoding; among them, when the reflection module reflects the optical signal and does not change its polarization state, the encoding module achieves phase encoding; when the reflection module reflects the optical signal and rotates its polarization state by 90°, the encoding module achieves polarization encoding.

[0055] The specific working process is as follows:

[0056] The optical pulse enters the encoding module. After passing through the 45° optical fiber fusion joint, the polarization state rotates from horizontal polarization to 45° polarization, and then passes through CIR1 to reach PBS, where it is polarization split into a horizontal polarization component and a vertical polarization component.

[0057] 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 through OS1 to PM; at the same time, since the optical fiber length connecting PBS and PM is equal to the sum of the optical fiber lengths connecting PBS and OS1 and 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 by the same phase j. Subsequently, both return to PBS for polarization beam combination. Since the optical paths they experience are exactly the same, the horizontal and vertical components of the optical pulse generated after beam combination 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.

[0058] When OS1 makes the optical path between PBS and the reflection module conductive, it is equivalent to connecting one output port of PBS to the reflection module, and connecting the other output port of PBS to another input port through PM to form a first ring structure.

[0059] When the reflection module reflects the optical signal without changing its polarization state, the horizontally polarized component is transmitted through the PBS, passes through the OS1, reaches the reflection module, is reflected by it with unchanged polarization, passes through the OS1 again, reaches the PBS and is directly transmitted, still being horizontally polarized; the vertically polarized component is reflected by the PBS and propagates clockwise in the first annular structure, passes through the PM and is phase-modulated by θ, then passes through the OS1, reaches the PBS, is reflected by it, passes through the OS1, reaches the reflection module, is reflected by it with unchanged polarization, returns along the original path, passes through the PM again and exits from the PBS, becoming vertically polarized. Since the vertically polarized component travels a path that is 2 times the length of the first annular structure longer than the horizontally polarized component, when exiting from the PBS, the former lags behind the latter by the time τ transmitted by the fiber length of the first annular structure, which is the arm length delay difference of the unbalanced interferometer. Finally, the quantum state output by the CIR1 is

[0060] ,

[0061] wherein, respectively represent the previous time mode and the subsequent time mode in the phase encoding, and the phase difference between the two time modes is θ. When θ = 0, π, π / 2, 3π / 2, 4 kinds of phase encoding states can be obtained, that is, discrete variable phase encoding can be realized.

[0062] When the reflection module reflects the optical signal and rotates its polarization state by 90°, the horizontally polarized component is transmitted through the PBS, passes through the OS1, reaches the reflection module, is reflected by it and its polarization state rotates by 90° to become vertically polarized, passes through the OS1 again, reaches the PBS and is reflected, and propagates counterclockwise in the fiber in the first annular structure, passes through the PM and is phase-modulated by φ1, and then exits from the PBS, becoming vertically polarized; the vertically polarized component is reflected by the PBS and propagates clockwise in the first annular structure, passes through the PM and is phase-modulated by φ2, then passes through the OS1, reaches the PBS, is reflected by it, passes through the OS1, reaches the reflection module, is reflected by it and its polarization state rotates by 90°, and after being reflected, reaches the PBS and is directly transmitted, becoming horizontally polarized. Since the optical paths traveled by the horizontally polarized and vertically polarized components are equal, the difference is that the time they pass through the PM is different. Therefore, they exit from the PBS simultaneously, with a phase difference of φ2 - φ1. Finally, the quantum state output by the CIR1 is

[0063] ,

[0064] When φ2 - φ1 = 0, π, π / 2, 3π / 2, 4 kinds of polarization encoding states can be obtained, that is, discrete variable polarization encoding can be realized.

[0065] As Figure 2 shown, Embodiment 1 of the present invention:

[0066] The reflection module includes a first mirror M1 and an adjustable quarter-wave plate VQWP. OS1 is connected to VQWP and then to M1.

[0067] The included 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°.

[0068] The specific working process of the first embodiment is as follows:

[0069] The optical pulse enters the encoding module. After passing through the 45° fiber fusion joint, the polarization state rotates from horizontal polarization to 45° polarization, and then passes through CIR1 to reach PBS, where it is polarization-split into a horizontal polarization component and a vertical polarization component.

[0070] 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 does not pass through the reflection module. The horizontal polarization component is transmitted from PBS through OS1 to PM. At the same time, since the length of the fiber connecting PBS and PM is equal to the sum of the lengths of the fiber connecting PBS and OS1 and the 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 by the same phase j. Subsequently, both return to PBS for polarization beam combination. Since the optical paths they experience are exactly the same, the horizontal and vertical components of the optical pulse generated after beam combination 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.

[0071] When OS1 makes the optical path between PBS and the reflection module conductive, it is equivalent to connecting one output port of PBS to the reflection module, and the other output port of PBS is connected to the other input port through PM to form a first ring structure.

[0072] The reflection module is composed of a first mirror M1 and an adjustable quarter-wave plate VQWP. When the included angle between the main axis direction of the VQWP and the slow axis of the polarization-maintaining fiber is 0°, its overall effect can be represented by the Jones matrix as

[0073] ,

[0074] That is, it is equivalent to the function of a single mirror without changing the polarization state of the optical pulse. When the included 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 represented by the Jones matrix as

[0075] ,

[0076] That is, it can rotate the horizontal polarization and the vertical polarization by 90° respectively.

[0077] When the reflection module reflects the optical signal without changing its polarization state, the horizontally polarized component is transmitted through the PBS to the OS1 and then reaches the reflection module. After being reflected by it, its polarization remains unchanged. After passing through the OS1 again, it reaches the PBS and is directly transmitted, still being horizontally polarized. The vertically polarized component is reflected by the PBS and propagates clockwise in the first annular structure. After passing through the PM, its phase is modulated by θ. Then, it passes through the OS1 to reach the PBS, is reflected by it, passes through the OS1 to reach the reflection module, is reflected with unchanged polarization, returns along the original path, passes through the PM again, and exits from the PBS, becoming vertically polarized. Since the vertically polarized component travels a path that is 2 times the length of the first annular structure more than the horizontally polarized component, when exiting from the PBS, the former lags behind the latter by the time τ transmitted by the fiber length of the first annular structure, which is the arm length delay difference of the unbalanced interferometer. Finally, the quantum state output by the CIR1 is

[0078] ,

[0079] where respectively represent the previous time mode and the subsequent time mode in the phase encoding, and the phase difference between the two time modes is θ. When θ = 0, π, π / 2, 3π / 2, 4 kinds of phase encoding states can be obtained, that is, discrete variable phase encoding can be realized.

[0080] When the reflection module reflects the optical signal and rotates its polarization state by 90°, the horizontally polarized component is transmitted through the PBS to the OS1 and then reaches the reflection module. After being reflected by it, its polarization state rotates by 90° and becomes vertically polarized. After passing through the OS1 again, it reaches the PBS and is reflected, and propagates counterclockwise in the fiber in the first annular structure. After passing through the PM, its phase is modulated by φ1. Then, it exits from the PBS and becomes vertically polarized. The vertically polarized component is reflected by the PBS and propagates clockwise in the first annular structure. After passing through the PM, its phase is modulated by φ2. Then, it passes through the OS1 to reach the PBS, is reflected by it, passes through the OS1 to reach the reflection module, is reflected with its polarization state rotated by 90°, and after being reflected, it reaches the PBS and is directly transmitted, becoming horizontally polarized. Since the optical paths traveled by the horizontally polarized and vertically polarized components are equal, the difference is that their times passing through the PM are different. Therefore, they exit from the PBS simultaneously and have a phase difference of φ2 - φ1. Finally, the quantum state output by the CIR1 is

[0081] ,

[0082] When φ2 - φ1 = 0, π, π / 2, 3π / 2, 4 kinds of polarization encoding states can be obtained, that is, discrete variable polarization encoding can be realized.

[0083] As Figure 3 shown, Embodiment 2 of the present invention:[[]]

[0084] The reflection module includes a second optical switch OS2, a second mirror M2, a third mirror M3, and a first polarization rotation module.

[0085] The first polarization rotation module is used to rotate the polarization state of the optical pulse passing through it back and forth by 90°.

[0086] The input port of OS1 is connected to the input port of OS2.

[0087] One output port of OS2 is directly connected to M2.

[0088] The other output port of OS2 is connected to the first polarization rotation module and then to M3.

[0089] The first polarization rotation module is a quarter-wave plate QWP, and the angle between its main axis direction and the slow axis of the polarization-maintaining fiber is 45°.

[0090] The specific working process of the second embodiment is as follows:

[0091] The optical pulse enters the encoding module. After passing through the 45° fiber fusion joint, the polarization state rotates from horizontal polarization to 45° polarization, and then passes through CIR1 to reach PBS, where it is polarization-split into a horizontal polarization component and a vertical polarization component.

[0092] 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 does not pass through the reflection module. The horizontal polarization component is transmitted through PBS, passes through OS1, and reaches PM. At the same time, since the length of the fiber connecting PBS and PM is equal to the sum of the lengths of the fiber connecting PBS and OS1 and the 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 by the same phase j. Subsequently, both return to PBS for polarization beam combination. Since the optical paths they have experienced are exactly the same, the horizontal and vertical components of the optical pulse generated after beam combination 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.

[0093] When OS1 makes the optical path between PBS and the reflection module conductive, it is equivalent to connecting one output port of PBS to the reflection module, and the other output port of PBS is connected to the other input port through PM to form a first ring structure.

[0094] The reflection module can switch between two paths through OS2. The first path is to make the optical path between OS1 and M2 conductive. The effect of M2 can be represented by the Jones matrix as

[0095] ,

[0096] That is, the polarization state of the optical pulse is not changed. The second path is to turn on the optical path from OS1 to the QWP and M3, and its overall effect can be represented by the Jones matrix as

[0097] ,

[0098] That is, it can rotate the horizontal polarization and the vertical polarization by 90° respectively.

[0099] When OS2 is switched to turn on the optical path where M2 is located, the horizontal polarization component is transmitted through PBS, passes through OS1, and reaches M2. After being reflected by it, the polarization remains unchanged. After passing through OS1 again, it reaches PBS and is directly transmitted, still being horizontally polarized; the vertical polarization component is reflected by PBS and propagates clockwise in the first annular structure. After passing through PM, the phase θ is modulated. Then it passes through OS1, reaches PBS, is reflected by it, passes through OS1, reaches M2, is reflected, the polarization remains unchanged, returns along the original path, passes through PM again, and exits from PBS, becoming vertically polarized. Since the vertical polarization component travels a path that is 2 times the length of the first annular structure more than the horizontal polarization component, when exiting from PBS, the former lags behind the latter by the time τ transmitted by the fiber length of the first annular structure, which is the arm length delay difference of the unbalanced interferometer. Finally, the quantum state output by CIR1 is

[0100] ,

[0101] Among them, respectively represent the previous time mode and the subsequent time mode in the phase encoding, and the phase difference between the two time modes is θ. When θ = 0, π, π / 2, 3π / 2, 4 phase encoding states can be obtained, that is, discrete variable phase encoding can be realized.

[0102] When OS2 is switched to turn on the optical path where M3 and QWP are located, the horizontal polarization component is transmitted through PBS, passes through OS1, reaches QWP and M3, and after being reflected by it, the polarization state rotates 90° and becomes vertically polarized. After passing through OS1 again, it reaches PBS and is reflected, and propagates counterclockwise in the first annular structure. After passing through PM, the phase φ1 is modulated. Then it exits from PBS and becomes vertically polarized; the vertical polarization component is reflected by PBS and propagates clockwise in the first annular structure. After passing through PM, the phase φ2 is modulated. Then it passes through OS1, reaches PBS, is reflected by it, passes through OS1, reaches QWP and M3, is reflected, the polarization state rotates 90°, and after being reflected, it reaches PBS and is directly transmitted, becoming horizontally polarized. Since the optical paths traveled by the horizontal polarization and the vertical polarization components are equal, the difference is that the time they pass through PM is different. Therefore, they exit from PBS at the same time and have a phase difference of φ2 - φ1. Finally, the quantum state output by CIR1 is

[0103] ,

[0104] When φ2 - φ1 = 0, π, π / 2, 3π / 2, four polarization encoding states can be obtained, and thus discrete variable polarization encoding can be achieved.

[0105] As Figure 4 shown, Embodiment III of the present invention:

[0106] The reflection module includes a third optical switch OS3, a fourth mirror M4, and a first polarization rotation module.

[0107] The first polarization rotation module is used to rotate the polarization state of the optical pulse passing through it back and forth by 90°.

[0108] One input port of OS1 is connected to one input port of OS3.

[0109] One output port of OS3 is directly connected to M4.

[0110] The other output port of OS3 is connected to the first polarization rotation module and then connected to the other input port of OS3.

[0111] The first polarization rotation module is a Faraday rotator FR with a polarization rotation angle of 45°.

[0112] The specific working process of Embodiment III is as follows:

[0113] The optical pulse enters the encoding module. After passing through the 45° fiber fusion point, the polarization state rotates from horizontal polarization to 45° polarization, and then passes through CIR1 to reach PBS, where it is polarization split into a horizontal polarization component and a vertical polarization component.

[0114] 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 does not pass through the reflection module. The horizontal polarization component is transmitted through PBS, passes through OS1, and reaches PM. At the same time, since the length of the optical fiber connecting PBS and PM is equal to the sum of the lengths of the optical fibers connecting PBS and OS1 and 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 by the same phase j. Subsequently, both return to PBS for polarization beam combination. Since the optical paths they experience are exactly the same, the horizontal and vertical components of the optical pulse generated after beam combination 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.

[0115] When the optical path between the PBS and the reflection module is turned on by the OS1, it is equivalent to connecting an output port of the PBS to the reflection module, and the other output port of the PBS is connected to another input port through the PM, forming a first ring structure.

[0116] The reflection module can switch between two paths through the OS3. The first path is to turn on the optical path between the OS1 and the M4. The effect of the M4 can be represented by the Jones matrix as

[0117] ,

[0118] That is, the polarization state of the optical pulse is not changed. The second path is to turn on the optical paths from the OS1 to the FR and the M4. The overall effect can be represented by the Jones matrix as

[0119] ,

[0120] That is, it can rotate the horizontal polarization and the vertical polarization by 90° respectively.

[0121] When the OS3 is switched to turn on the optical path where the M4 is located, the horizontal polarization component is transmitted through the PBS, passes through the OS1, reaches the M4, is reflected by it with the polarization unchanged, and then reaches the PBS directly through the OS1 and is still horizontally polarized; the vertical polarization component is reflected by the PBS and propagates clockwise in the first ring structure, passes through the PM and is phase-modulated by θ, then passes through the OS1 to reach the PBS, is reflected by it, passes through the OS1 to reach the M4, is reflected by it with the polarization unchanged, returns along the original path, passes through the PM again and exits from the PBS, becoming vertically polarized. Since the vertical polarization component travels a path that is 2 times the length of the first ring structure more than the horizontal polarization component, when exiting from the PBS, the former lags behind the latter by the time τ transmitted by the fiber length of the first ring structure, which is the arm length delay difference of the unbalanced interferometer. Finally, the quantum state output by the CIR1 is

[0122] ,

[0123] Among them, respectively represent the previous time mode and the next time mode in the phase encoding, and the phase difference between the two time modes is θ. When θ = 0, π, π / 2, 3π / 2, 4 kinds of phase encoding states can be obtained, that is, the phase encoding of discrete variables can be realized.

[0124] When switching the OS3 to make the optical paths where the FR and M4 are located conductive, the horizontally polarized component is transmitted through the PBS, passes through the OS1, reaches the FR and M4, and after being reflected by them, the polarization state rotates by 90° and becomes vertically polarized. After passing through the OS1 again, it reaches the PBS and is reflected, and then propagates along the optical fiber in the counterclockwise direction in the first ring structure. After passing through the PM, the phase φ1 is modulated, and then it exits from the PBS and becomes vertically polarized; the vertically polarized component is reflected by the PBS and propagates in the clockwise direction in the first ring structure. After passing through the PM, the phase φ2 is modulated, and then it passes through the OS1 to reach the PBS, is reflected by it, passes through the OS1 to reach the FR and M4, and after being reflected, the polarization state rotates by 90°. After being reflected, it reaches the PBS and is directly transmitted, becoming horizontally polarized. Since the optical paths traveled by the horizontally polarized and vertically polarized components are equal, and the difference lies in the different times they pass through the PM, they both exit from the PBS simultaneously, with a phase difference of φ2 - φ1. Finally, the quantum state output by the CIR1 is

[0125] ,

[0126] When φ2 - φ1 = 0, π, π / 2, 3π / 2, four polarization encoding states can be obtained, that is, the polarization encoding of discrete variables can be realized.

[0127] As Figure 5 shown, Embodiment 4 of the present invention:

[0128] The reflection module includes a second circulator CIR2, a fourth optical switch OS4, and a second polarization rotation module.

[0129] The OS1 is connected to the second port of the CIR2;

[0130] The third port and the first port of the CIR2 are respectively connected to an output port and an output port of the OS4;

[0131] The other input port of the OS4 is connected to the second polarization rotation module and then connected to its other output port.

[0132] The second polarization rotation module is a half-wave plate HWP, and the included angle between its main axis direction and the slow axis of the polarization-maintaining optical fiber is 45°.

[0133] The specific working process of Embodiment 4 is as follows:

[0134] The optical pulse enters the encoding module. After passing through the 45° optical fiber fusion point, the polarization state rotates from horizontal polarization to 45° polarization, and then passes through the CIR1 to reach the PBS, where it is polarization-split into a horizontally polarized component and a vertically polarized component.

[0135] When OS1 enables the optical path between PBS and PM to conduct, it is equivalent to connecting the two output ports of PBS through PM, forming a Sagnac loop. At this time, the optical pulse will not pass through the reflection module. The horizontally polarized component is transmitted through PBS, passes through OS1, and reaches PM. At the same time, since 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 optical fiber connecting OS1 and PM, the vertically polarized component is reflected by PBS and reaches PM at the same time as the horizontally polarized component, so the same phase j is modulated. Subsequently, both return to PBS for polarization beam combination. Since the optical paths they have experienced are exactly the same, the horizontal and vertical components of the optical pulse generated after beam combination are exchanged, and the overall phase of the optical pulse is modulated to j. When j = -π or π, continuous variable BPSK coding can be achieved. When j = π / 4, 3π / 4, 5π / 4 or 7π / 4, continuous variable QPSK coding can be achieved.

[0136] When OS1 enables the optical path between PBS and the reflection module to conduct, it is equivalent to connecting one output port of PBS to the reflection module, and connecting the other output port of PBS to the other input port through PM, forming a first ring structure.

[0137] The second port and the third port of CIR2 are connected through OS4, forming a second ring structure; one input port and one output port of OS4 are connected through HWP, forming a third ring structure. The reflection module can switch between two paths through OS4. The first path is to directly conduct the second ring structure without passing through the third ring structure, so that the optical pulse exits from the second port of CIR2 without changing the polarization state of the optical pulse. The second path is to conduct the second ring structure through the third ring structure. When passing through the third ring structure, it will pass through HWP, and its Jones matrix is

[0138] ,

[0139] that is, it can rotate the horizontal polarization and the vertical polarization by 90° respectively.

[0140] When switching OS4 to make the optical pulse pass only through the second ring structure, the horizontally polarized component is transmitted through PBS, passes through OS1 to reach CIR2, exits from CIR2 after transmission through the second ring structure with unchanged polarization, reaches PBS directly after passing through OS1 again and is still horizontally polarized; the vertically polarized component is reflected by PBS and propagates clockwise in the first ring structure, the phase is modulated by PM by θ, then passes through OS1 to reach PBS, is reflected by it, passes through OS1 to reach CIR2, exits from CIR2 after transmission through the second ring structure with unchanged polarization, returns along the original path, passes through PM again and exits from PBS, becoming vertically polarized. Since the vertically polarized component travels a path that is 2 times the length of the first ring structure more than the horizontally polarized component, when exiting from PBS, the former lags behind the latter by the time τ transmitted by the fiber length of the first ring structure, which is the arm length delay difference of the unbalanced interferometer. Finally, the quantum state output by CIR1 is

[0141] ,

[0142] where, respectively represent the previous time mode and the next time mode in the phase encoding, and the phase difference between the two time modes is θ. When θ = 0, π, π / 2, 3π / 2, 4 kinds of phase encoding states can be obtained, that is, discrete variable phase encoding can be realized.

[0143] When switching OS4 to make the optical pulse pass through the second ring structure and the third ring structure, the horizontally polarized component is transmitted through PBS, passes through OS1 to reach CIR2, and after passing through the second ring structure, the third ring structure and HWP, the polarization state rotates 90° and becomes vertically polarized. After passing through OS1 again, it reaches PBS and is reflected, and propagates counterclockwise in the first ring structure, the phase is modulated by PM by φ1, and then exits from PBS, becoming vertically polarized; the vertically polarized component is reflected by PBS and propagates clockwise in the first ring structure, the phase is modulated by PM by φ2, then passes through OS1 to reach PBS, is reflected by it, passes through OS1 to reach CIR2, and after passing through the second ring structure, the third ring structure and HWP, the polarization state rotates 90°, is reflected and reaches PBS directly and is transmitted, becoming horizontally polarized. Since the optical paths traveled by the horizontally polarized and vertically polarized components are equal, the difference is that the time they pass through PM is different. Therefore, they exit from PBS at the same time with a phase difference of φ2 - φ1. Finally, the quantum state output by CIR1 is

[0144] ,

[0145] When φ2 - φ1 = 0, π, π / 2, 3π / 2, 4 kinds of polarization encoding states can be obtained, that is, discrete variable polarization encoding can be realized.

[0146] As can be seen from various embodiments of the present invention, the present invention proposes a discrete and continuous variable adaptive coding module for a quantum key distribution system, which can not only achieve the switching between continuous variable coding and discrete variable coding, but also achieve the switching between polarization coding and phase coding in discrete variables. Only one phase modulator is required, and the structure is relatively simple. When applied to a quantum key distribution network, it can realize the switching of adaptive coding modes according to the 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 connects the optical path between PBS and the reflection module, the encoding module can realize phase encoding or polarization encoding. When the reflection module reflects the optical signal without changing its polarization state, the encoding module realizes phase encoding. When the reflection module reflects the optical signal and rotates its polarization state by 90°, the encoding module realizes polarization encoding. 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, and 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 lengths of the optical fiber connecting PBS and OS1 and 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 is realized, BPSK is binary phase shift keying, and when j=π / 4, 3π / 4, 5π / 4 or 7π / 4, continuous variable QPSK encoding is realized, QPSK is quadrature phase shift keying. When OS1 connects the optical path between PBS and the reflection module, it is equivalent to connecting one output port of PBS to the reflection module, and connecting another output port of PBS to another input port through PM, forming a first ring structure. 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 horizontally polarized. After being reflected by PBS, the vertical polarization component propagates in the clockwise direction in the first ring structure, 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, and finally outputs the quantum state through CIR1. The quantum state includes a time mode before and after, and the phase difference between the two time modes is θ. When θ=0, π, π / 2, 3π / 2, four phase-encoded states are obtained, that is, the phase encoding of discrete variables is realized. 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. After being reflected, the polarization state is rotated by 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 φ2-φ1, and finally the quantum state output by CIR1, When φ2-φ1=0, π, π / 2, 3π / 2, four polarization coding states are obtained, that is, polarization coding of discrete variables is realized.

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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