Polarization encoding apparatus and method, and quantum communication system

Abstract

Provided in the present disclosure are a polarization encoding apparatus and method, and a quantum communication system. The apparatus comprises: a first polarization operation module and a second polarization operation module, which are connected in series, wherein the first polarization operation module and the second polarization operation module each comprise a polarization-control optical path and a phase modulator, each polarization-control optical path is used for performing polarization beam splitting on an input optical pulse to obtain two optical sub-pulses, and each phase modulator performs phase modulation on one of the two optical sub-pulses, or performs different phase modulations on the two optical sub-pulses, respectively. The included angle between eigen polarization states of the two polarization-control optical paths can be set according to encoding requirements of quantum communication protocols, and a plurality of two-basis or three-basis polarization-encoded quantum states are obtained by means of combined modulation performed by the two polarization operation modules, such that encoding requirements of different quantum communication protocols are met.

Description

Polarization coding device, method and quantum communication system Technical Field

[0001] This disclosure relates to the fields of quantum communication and optical quantum coding technology, and in particular to a polarization coding device, method and quantum communication system. Background Technology

[0002] Quantum communication technology is a cutting-edge and hotly debated field combining quantum physics and information science. Current applications primarily include quantum key distribution and quantum direct communication. Based on the Heisenberg uncertainty principle and the no-cloning theorem in quantum mechanics, quantum key distribution enables two communicating parties to securely share keys in real time, while quantum direct communication ensures secure information transmission from the outset. Quantum communication can detect potential eavesdropping on communication channels and can be applied to fields with high-security information transmission requirements, such as national defense, government affairs, finance, and power.

[0003] The physical implementation of quantum communication systems, such as quantum key distribution and quantum direct communication, requires the encoding and decoding of quantum states. Different quantum communication protocols have different requirements for the encoding of quantum states. For example, the BB84 quantum communication protocol requires the encoding of two sets of basis states and four quantum states, while reference frame-independent quantum communication systems require the encoding of three sets of basis states and six or three sets of basis states and four quantum states, and so on.

[0004] How to make multiple quantum communication protocols compatible in the same optical quantum coding device and flexibly realize the coding requirements of multiple quantum states is an important problem in the current application of quantum communication.

[0005] Summary of the Invention

[0006] This disclosure provides a polarization encoding device, method, and quantum communication system to solve the technical problems mentioned in the prior art.

[0007] According to a first aspect of this disclosure, a polarization encoding device is provided, comprising: a first polarization operation module and a second polarization operation module connected in series, wherein the first polarization operation module includes: a first polarization control optical path and a first phase modulator disposed in a transmission optical path of the first polarization control optical path; the first polarization control optical path is used to polarize and split an input optical pulse into two sub-optical pulses, namely a first sub-optical pulse and a second sub-optical pulse; the first phase modulator is used to perform phase modulation on one of the first sub-optical pulses and the second sub-optical pulse, or to perform different phase modulations on the first sub-optical pulse and the second sub-optical pulse, so that a phase difference φ is generated between the two sub-optical pulses; the first sub-optical pulse after passing through the first phase modulator... The first optical pulse and the second sub-optical pulse are combined and output through the first polarization control optical path. The second polarization operation module includes: a second polarization control optical path and a second phase modulator disposed in the transmission optical path of the second polarization control optical path. The second polarization control optical path is connected in series with the first polarization control optical path. The second polarization control optical path is used to input the optical pulse output from the combined optical pulse of the first polarization control optical path from the input port, and split its polarization into two sub-optical pulses, namely a third sub-optical pulse and a fourth sub-optical pulse. The second phase modulator is used to perform phase modulation on one of the third sub-optical pulses and the fourth sub-optical pulse, or to perform different phase modulations on the third sub-optical pulse and the fourth sub-optical pulse, so that a phase difference is generated between the two sub-optical pulses. The third and fourth sub-light pulses after passing through the second phase modulator are combined and output through the second polarization control optical path; wherein, the angle between the intrinsic polarization state of the first polarization control optical path and the intrinsic polarization state of the second polarization control optical path is set according to the requirements of the quantum communication protocol.

[0008] In some embodiments, the first phase modulator includes: a first port, a second port, and a third port. The first port and the second port are respectively the forward input optical port and the reverse input optical port of the first phase modulator. The first sub-optical pulse is input to the first phase modulator through the first port, and output from the second port after passing through the first phase modulator. The second sub-optical pulse is input to the first phase modulator through the second port, and output from the first port after passing through the first phase modulator. The third port is an electrical port used to apply a modulation electrical signal. The second phase modulator includes: a fourth port, a fifth port, and a sixth port. The fourth port and the fifth port are respectively the forward input optical port and the reverse input optical port of the second phase modulator. The third sub-optical pulse is input to the second phase modulator through the fourth port, and output from the fifth port after passing through the second phase modulator. The fourth sub-optical pulse is input to the second phase modulator through the fifth port, and output from the fourth port after passing through the second phase modulator. The sixth port is an electrical port used to apply a modulation electrical signal.

[0009] In some embodiments, after the first phase modulator applies a high-frequency modulation electrical signal with a frequency higher than a specified threshold, the first phase modulator operates in a non-reciprocal state, and the ratio of the modulation efficiency of the first sub-optical pulse input from the forward input optical port to the modulation efficiency of the second sub-optical pulse input from the reverse input optical port is not less than a preset threshold; after the second phase modulator applies a high-frequency modulation electrical signal with a frequency higher than a specified threshold, the second phase modulator operates in a non-reciprocal state, and the ratio of the modulation efficiency of the third sub-optical pulse input from the forward input optical port to the modulation efficiency of the fourth sub-optical pulse input from the reverse input optical port is not less than a preset threshold.

[0010] In some embodiments, the first optical pulse and the second optical pulse pass through the first phase modulator simultaneously; and / or the third optical pulse and the fourth optical pulse pass through the second phase modulator simultaneously.

[0011] In some implementations, when the quantum communication protocol requires two bases, the angle between the intrinsic polarization state of the first polarization control optical path and the intrinsic polarization state of the second polarization control optical path is n·90°, or 22.5°±n·45°, or 45°±n·90°, where n is an integer.

[0012] In some embodiments, when the angle between the intrinsic polarization state of the first polarization control optical path and the intrinsic polarization state of the second polarization control optical path is n·90°, where n is an integer, the polarization state of the optical pulse input to the first polarization control optical path is... Alternatively, the polarization state of the optical pulse input to the first polarization control optical path is... Where k is an integer, and Let β be any value from 0 to 2π, representing the two intrinsic polarization states of the first polarization control optical path. When the angle between the intrinsic polarization states of the first and second polarization control optical paths is 22.5°±n·45° or 45°±n·90°, where n is an integer, the polarization state of the optical pulse input to the first polarization control optical path is... Alternatively, the polarization state of the optical pulse input to the first polarization control optical path is... Alternatively, the polarization state of the optical pulse input to the first polarization control optical path is... or Where k is an integer, and These are the two intrinsic polarization states of the first polarization control optical path, where β is any value from 0 to 2π.

[0013] In some implementations, when the quantum communication protocol requires three bases, the angle between the intrinsic polarization state of the first polarization control optical path and the intrinsic polarization state of the second polarization control optical path is 22.5°±n·45°, or 45°±n·90°, where n is an integer.

[0014] In some embodiments, the polarization state of the optical pulse input to the first polarization control optical path is Alternatively, the polarization state of the optical pulse input to the first polarization control optical path is... Alternatively, the polarization state of the optical pulse input to the first polarization control optical path is... or Where k is an integer, and These are the two intrinsic polarization states of the first polarization control optical path, where β is any value from 0 to 2π.

[0015] In some embodiments, the first polarization operation module further includes: a third phase modulator, which is disposed in the transmission optical path of the first polarization control optical path and connected in series with the first phase modulator, for performing phase modulation on one of the first sub-optical pulses and the second sub-optical pulses, or performing different phase modulations on the first sub-optical pulses and the second sub-optical pulses, so that a phase difference α is generated between the first sub-optical pulses and the second sub-optical pulses; or the second polarization operation module further includes: a third phase modulator, which is disposed in the transmission optical path of the second polarization control optical path and connected in series with the second phase modulator, for performing phase modulation on one of the third sub-optical pulses and the fourth sub-optical pulses, or performing different phase modulations on the third sub-optical pulses and the fourth sub-optical pulses, so that a phase difference α is generated between the third sub-optical pulses and the fourth sub-optical pulses.

[0016] In some embodiments, the third phase modulator includes a seventh port, an eighth port, and a ninth port. The seventh port and the eighth port are the forward input optical port and the reverse input optical port of the third phase modulator, respectively, and the ninth port is an electrical port used to apply a modulation electrical signal. After applying a high-frequency modulation electrical signal with a frequency higher than a specified threshold, the third phase modulator operates in a non-reciprocal state, and the ratio of the modulation efficiency of the optical pulse input from the forward input optical port to the modulation efficiency of the optical pulse input from the reverse input optical port is not less than a preset threshold.

[0017] In some embodiments, the polarization state of the optical pulse input to the first polarization control optical path is 45° linear polarization, -45° linear polarization, left-hand circular polarization, or right-hand circular polarization.

[0018] In some embodiments, the angle between the intrinsic polarization state of the first polarization control optical path and the intrinsic polarization state of the second polarization control optical path is achieved by rotating at least one of the two polarization control optical paths; and / or

[0019] The device further includes a polarization state rotator, which can be a half-wave plate or a Faraday rotator. The polarization state rotator is disposed between two polarization control optical paths, and the included angle between the intrinsic polarization states of the two polarization control optical paths is adjusted by the polarization state rotator.

[0020] In some embodiments, the device further includes an optical isolation unit, which is an optical isolator or an optical circulator, and is disposed between the first polarization operation module and the second polarization operation module.

[0021] In some implementations, a phase difference φ is generated between the first and second sub-optical pulses, and a phase difference is generated between the third and fourth sub-optical pulses. Determined based on the polarization coding requirements of the quantum communication protocol.

[0022] In some embodiments, the first polarization control optical path and / or the second polarization control optical path include: a polarization beam splitter unit and a transmission optical path; the polarization beam splitter unit includes at least three ports, namely port A, port B, and port C; the intrinsic polarization state of the polarization beam splitter unit is... and The polarization state of the polarization beam splitter is the intrinsic polarization state of the first polarization control optical path and / or the second polarization control optical path; the polarization beam splitter is used to polarize and split the optical pulse input from port A into two sub-optical pulses, which are output from port B and port C respectively; the transmission optical path is used to connect port B and port C of the polarization beam splitter.

[0023] In some embodiments, the polarization beam splitter unit is a polarization beam splitter, and the intrinsic polarization state of the polarization beam splitter is the intrinsic polarization state of the polarization beam splitter unit.

[0024] In some embodiments, the polarization beam splitter unit includes: a polarization beam splitter, a first polarizer, and a second polarizer; the polarization beam splitter unit further includes port D; the polarization beam splitter includes four ports, namely a first port, a second port, a third port, and a fourth port, wherein the first port of the polarization beam splitter is port A of the polarization beam splitter unit, and the fourth port of the polarization beam splitter is port D of the polarization beam splitter unit; the second port and the third port of the polarization beam splitter are respectively connected to the port on the first side of the first polarizer and the port on the first side of the second polarizer; the first polarizer... The ports on both sides are ports B of the polarization beam splitter unit, and the port on the second side of the second polarizer is port C of the polarization beam splitter unit; the ports on the second side of the first polarizer and the ports on the second side of the second polarizer are connected through the transmission optical path; the angle between the polarization direction of the first polarizer and one intrinsic polarization state of the polarization beam splitter is θ, and the angle between the polarization direction of the second polarizer and another intrinsic polarization state of the polarization beam splitter is δ; where θ, δ ≠ n·90°, and n is an integer; the intrinsic polarization state of the polarization beam splitter is the intrinsic polarization state of the polarization beam splitter unit.

[0025] In some embodiments, the polarization beamsplitter unit includes: an optical beamsplitter, a first polarizer, and a second polarizer; the optical beamsplitter includes at least three ports, namely a first port, a second port, and a third port, wherein the first port of the optical beamsplitter is port A of the polarization beamsplitter unit; the second port and the third port of the optical beamsplitter are respectively connected to a port on a first side of the first polarizer and a port on a first side of the second polarizer; the port on a second side of the first polarizer is port B of the polarization beamsplitter unit, and the port on a second side of the second polarizer is port C of the polarization beamsplitter unit; the port on the second side of the first polarizer and the port on the second side of the second polarizer are connected through the transmission optical path. The polarization direction of the first polarizer is... The polarization direction of the second polarizer is The polarization direction of the first polarizer The deflection direction of the second deflector Mutually orthogonal; the deflection direction of the first deflector The deflection direction of the second deflector The intrinsic polarization state of the polarization beam splitter unit.

[0026] In some embodiments, the optical beamsplitter further includes a fourth port; the polarization beamsplitter further includes a port D; and the fourth port of the optical beamsplitter is the port D of the polarization beamsplitter.

[0027] In some implementations, the transmission optical path is a free-space optical path or a polarization-maintaining fiber.

[0028] In some embodiments, ports B and C of the polarization beam splitter are both coupled to the slow axis of the polarization-maintaining fiber or both are coupled to the fast axis of the polarization-maintaining fiber.

[0029] In some embodiments, the device further includes a half-wave plate or a 90-degree Faraday rotator disposed in the transmission optical path.

[0030] According to a second aspect of this disclosure, a polarization encoding method is provided, comprising: inputting a single optical pulse into a first polarization control optical path and polarizing it into two sub-optical pulses, namely a first sub-optical pulse and a second sub-optical pulse; performing phase modulation on at least one of the first sub-optical pulse and the second sub-optical pulse by a first phase modulator, or performing different phase modulations on the first sub-optical pulse and the second sub-optical pulse, thereby generating a phase difference φ between the two sub-optical pulses; combining the first sub-optical pulse and the second sub-optical pulse after passing through the first phase modulator and outputting them through the first polarization control optical path; inputting the combined output optical pulse into a second polarization control optical path connected in series with the first polarization control optical path, and polarizing it into two sub-optical pulses, namely a third sub-optical pulse and a fourth sub-optical pulse by the second polarization control optical path; performing phase modulation on at least one of the third sub-optical pulse and the fourth sub-optical pulse by a second phase modulator, or performing different phase modulations on the third sub-optical pulse and the fourth sub-optical pulse, thereby generating a phase difference between the two sub-optical pulses. The third and fourth sub-light pulses after passing through the second phase modulator are combined and output through the second polarization control optical path; wherein, the angle between the intrinsic polarization state of the first polarization control optical path and the intrinsic polarization state of the second polarization control optical path is set according to the requirements of the quantum communication protocol.

[0031] In some implementations, when the quantum communication protocol requires two bases, the angle between the intrinsic polarization state of the first polarization control optical path and the intrinsic polarization state of the second polarization control optical path is n·90°, or 22.5°±n·45°, or 45°±n·90°, where n is an integer.

[0032] In some implementations, when the quantum communication protocol requires three bases, the angle between the intrinsic polarization state of the first polarization control optical path and the intrinsic polarization state of the second polarization control optical path is 22.5°±n·45°, or 45°±n·90°, where n is an integer.

[0033] In some embodiments, when the angle between the intrinsic polarization state of the first polarization control optical path and the intrinsic polarization state of the second polarization control optical path is n·90°, the polarization state of the optical pulse input to the first polarization control optical path is... Alternatively, the polarization state of the optical pulse input to the first polarization control optical path is... Where k is an integer, and Let β be any value from 0 to 2π, representing the two intrinsic polarization states of the first polarization control optical path. When the angle between the intrinsic polarization state of the first polarization control optical path and the intrinsic polarization state of the second polarization control optical path is 22.5°±n·45° or 45°±n·90°, the polarization state of the optical pulse input to the first polarization control optical path is... Alternatively, the polarization state of the optical pulse input to the first polarization control optical path is... Alternatively, the polarization state of the optical pulse input to the first polarization control optical path is... or Where k is an integer, and These are the two intrinsic polarization states of the first polarization control optical path, where β is any value from 0 to 2π.

[0034] According to a third aspect of this disclosure, a quantum communication system is provided, including the polarization encoding device described above.

[0035] In summary, the polarization coding device, method, and quantum communication system provided in this disclosure have at least the following beneficial effects:

[0036] The polarization encoding device disclosed herein includes a first polarization operation module and a second polarization operation module connected in series. Each of the first or second polarization operation modules includes a polarization control optical path and a phase modulator. The polarization control optical path is used to polarize and split the input optical pulse into two sub-optical pulses. The phase modulators in the two polarization operation modules modulate the phase of one of the two sub-optical pulses, or modulate the two sub-optical pulses differently, so that the two sub-optical pulses generate phase differences φ and φ, respectively. Furthermore, the angle between the intrinsic polarization states of the two polarization control optical paths is set according to the requirements of the quantum communication protocol. With the aforementioned phase differences φ and φ, multiple polarization-encoded quantum states with two or three bases can be modulated to meet the requirements of different quantum communication protocols and reduce the complexity of the modulation phase. Attached Figure Description

[0037] To more clearly illustrate the technical solutions in the specific embodiments of this disclosure or the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this disclosure. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0038] Figure 1 is a structural block diagram of a polarization encoding device provided in an embodiment of this disclosure;

[0039] Figure 2 is a structural diagram of an embodiment of a polarization encoding device provided in this disclosure;

[0040] Figure 3 is a structural diagram of a first embodiment of a polarization control optical path provided by the present disclosure;

[0041] Figure 4 is a structural diagram of a second embodiment of a polarization control optical path provided by the present disclosure;

[0042] Figure 5 is a structural diagram of a third embodiment of a polarization control optical path provided by the present disclosure;

[0043] Figure 6 is a structural diagram of a fourth embodiment of a polarization control optical path provided by the present disclosure;

[0044] Figure 7 is a flowchart of a polarization encoding method provided by an embodiment of this disclosure. Detailed Implementation

[0045] To make the above and other features and advantages of this disclosure clearer, the disclosure is further described below with reference to the accompanying drawings. It should be understood that the specific embodiments given herein are for the purpose of explanation to those skilled in the art and are exemplary only, not restrictive.

[0046] In the following description, numerous specific details are set forth to provide a thorough understanding of this disclosure. However, it will be apparent to those skilled in the art that the specific details are not required to practice this disclosure. In other instances, well-known steps or operations have not been described in detail to avoid obscuring this disclosure.

[0047] This disclosure provides a polarization encoding device, as shown in Figures 1 and 2. Figure 1 is a structural block diagram of the polarization encoding device according to an embodiment of this disclosure, and Figure 2 is a structural diagram of one embodiment of the polarization encoding device according to an embodiment of this disclosure. The polarization encoding device includes a first polarization operation module 100 and a second polarization operation module 200 connected in series.

[0048] In some embodiments, the first polarization operation module 100 includes: a first polarization control optical path and a first phase modulator 205 disposed in the transmission optical path of the first polarization control optical path; the first polarization control optical path is used to polarize and split an input optical pulse into two sub-optical pulses, namely a first sub-optical pulse and a second sub-optical pulse; the first phase modulator 205 is used to perform phase modulation on one of the first sub-optical pulses and the second sub-optical pulse, or to perform different phase modulations on the first sub-optical pulse and the second sub-optical pulse, so that a phase difference φ is generated between the two sub-optical pulses; the first sub-optical pulse and the second sub-optical pulse after passing through the first phase modulator are combined and output through the first polarization control optical path.

[0049] In one exemplary embodiment, the first phase modulator 205 includes a first port E, a second port F, and a third port N. The first port E and the second port F are the forward input optical port and the reverse input optical port of the first phase modulator 205, respectively. A first sub-optical pulse is input to the first phase modulator 205 via the first port E, and is output from the second port F after passing through the first phase modulator 205. A second sub-optical pulse is input to the first phase modulator 205 via the second port F, and is output from the first port E after passing through the first phase modulator 205. The third port N is an electrical port connected to the first driver 212 and is used to receive the modulation electrical signal applied by the first driver 212.

[0050] The structures of the first and second polarization control optical paths in the polarization encoding device shown in Figure 2 are only one embodiment. The first and second polarization control optical paths described in this disclosure can each adopt the structures shown in Figures 3 to 6. Furthermore, the structure of the second polarization control optical path can be the same as or different from the structure of the first polarization control optical path; that is, the two polarization control optical paths can each adopt any one of the four structures shown in Figures 3 to 6 provided in the embodiments of this disclosure.

[0051] In one exemplary embodiment, as shown in FIG3, a first embodiment of the first polarization control optical path is provided. In this embodiment, the first polarization control optical path includes: a first polarization beam splitter unit and a first transmission optical path 204; the first polarization beam splitter unit includes at least three ports, namely port A, port B, and port C; the intrinsic polarization state of the first polarization beam splitter unit is... and The intrinsic polarization state of the first polarization beamsplitter unit is the intrinsic polarization state of the first polarization control optical path. The first polarization beamsplitter unit polarizes the optical pulse input from port A into two sub-optical pulses, namely the first sub-optical pulse and the second sub-optical pulse, which are output from ports B and C, respectively. The first transmission optical path 204 is used to connect ports B and C of the first polarization beamsplitter unit and transmit the first sub-optical pulse and the second sub-optical pulse in opposite directions. In this embodiment, the first polarization beamsplitter unit is a polarization beamsplitter, and the intrinsic polarization state of the polarization beamsplitter is the intrinsic polarization state of the first polarization beamsplitter unit.

[0052] In one exemplary embodiment, as shown in FIG4, a second embodiment of the first polarization control optical path is presented. In this embodiment, the first polarization control optical path includes: a first polarization beam splitter unit and a first transmission optical path 204. The intrinsic polarization state of the first polarization beam splitter unit is... and The intrinsic polarization state of the first polarization beamsplitter unit is the intrinsic polarization state of the first polarization control optical path. The first polarization beamsplitter unit includes: a first polarization beamsplitter 201, a first polarizer 202, and a second polarizer 203. The first polarization beamsplitter unit has four ports: port A, port B, port C, and port D. The first polarization beamsplitter 201 has four ports: a first port, a second port, a third port, and a fourth port. The first port of the first polarization beamsplitter 201 is port A of the first polarization beamsplitter unit, and the fourth port of the first polarization beamsplitter 201 is port D of the first polarization beamsplitter unit. The second and third ports of the first polarization beamsplitter 201 are respectively... The first polarizer 202 and the second polarizer 203 are connected to the first side of the first polarizer 202 and the second polarizer 203. The second side of the first polarizer 202 is port B of the first polarization beam splitter unit, and the second side of the second polarizer 203 is port C of the first polarization beam splitter unit. The second side of the first polarizer 202 and the second side of the second polarizer 203 are connected through the first transmission optical path 204. The angle between the polarization direction of the first polarizer 202 and one intrinsic polarization state of the first polarization beam splitter 201 is α, and the angle between the polarization direction of the second polarizer 203 and another intrinsic polarization state of the first polarization beam splitter 201 is γ, where α and γ ≠ n·90°, and n is an integer. The angle between the polarization direction of the first polarizer 202 and the polarization direction of the second polarizer 203 is ω, where 0 ≤ ω ≤ 2π. In this embodiment, the intrinsic polarization state of the first polarization beam splitter 201 is the intrinsic polarization state of the first polarization beam splitter unit.

[0053] In one exemplary embodiment, as shown in FIG5, a third embodiment of the first polarization control optical path is presented. In this embodiment, the first polarization control optical path includes: a first polarization beam splitter unit and a first transmission optical path 204. The intrinsic polarization state of the first polarization beam splitter unit is... and The intrinsic polarization state of the first polarization beamsplitter unit is the intrinsic polarization state of the first polarization control optical path. The first polarization beamsplitter unit includes: a first optical beamsplitter 201, a first polarizer 202, and a second polarizer 203. The first polarization beamsplitter unit includes at least three ports: port A, port B, and port C. The first optical beamsplitter 201 includes at least three ports: a first port, a second port, and a third port. The first port of the first optical beamsplitter 201 is port A of the first polarization beamsplitter unit. The second and third ports of the first optical beamsplitter 201 are connected to ports on the first side of the first polarizer 202 and the first side of the second polarizer 203, respectively. The port on the second side of the first polarizer 202 is port B of the first polarization beamsplitter unit, and the port on the second side of the second polarizer 203 is port C of the first polarization beamsplitter unit. The ports on the second side of the first polarizer 202 and the second side of the second polarizer 203 are connected through a first transmission optical path 204. The polarization direction of the first polarizer 202 is The polarization direction of the second polarizer 203 is The direction of the first polarizer 202 The polarization direction of the second polarizer 203 Mutually orthogonal; the deflection direction of the first deflector 202 in this embodiment The polarization direction of the second polarizer 203 This represents the intrinsic polarization state of the first polarization beam splitter unit.

[0054] In one exemplary embodiment, as shown in FIG6, a fourth embodiment of the first polarization control optical path is presented. In this embodiment, the first polarization control optical path includes: a first polarization beam splitter unit and a first transmission optical path 204. The intrinsic polarization state of the first polarization beam splitter unit is... and The polarization state of the first polarization beamsplitter unit is the intrinsic polarization state of the first polarization control optical path. The first polarization beamsplitter unit includes: a first optical beamsplitter 201, a first polarizer 202, and a second polarizer 203. The first polarization beamsplitter unit includes four ports: port A, port B, port C, and port D. The first optical beamsplitter 201 includes four ports: a first port, a second port, a third port, and a fourth port. The first port of the first optical beamsplitter 201 is port A of the first polarization beamsplitter unit. The fourth port of the optical beamsplitter 201 is port D of the first polarization beamsplitter unit; the second and third ports of the first optical beamsplitter 201 are connected to the ports on the first side of the first polarizer 202 and the first side of the second polarizer 203, respectively; the port on the second side of the first polarizer 202 is port B of the first polarization beamsplitter unit, and the port on the second side of the second polarizer 203 is port C of the first polarization beamsplitter unit; the ports on the second side of the first polarizer 202 and the second side of the second polarizer 203 are connected through the first transmission optical path 204. The polarization direction of the first polarizer 202 is... The polarization direction of the second polarizer 203 is The direction of the first polarizer 202 The polarization direction of the second polarizer 203 Mutually orthogonal; the deflection direction of the first deflector 202 in this embodiment The polarization direction of the second polarizer 203 This represents the intrinsic polarization state of the first polarization beam splitter unit.

[0055] In one embodiment, a first phase modulator 205 is disposed in a first transmission optical path 204, wherein the first transmission optical path 204 is coupled to the slow axis of the first port E and the second port F of the first phase modulator, or is coupled to the fast axis of the first port E and the second port F of the first phase modulator. The first phase modulator 205 modulates one optical pulse input to one of the first ports E and the second port F, or modulates optical pulses input to one port E and the second port F with different phases.

[0056] In one exemplary embodiment, after applying a high-frequency modulation electrical signal with a frequency higher than a specified threshold, the first phase modulator 205 operates in a non-reciprocal state, where the ratio of the modulation efficiency of the first sub-optical pulse input from the forward input optical port to the modulation efficiency of the second sub-optical pulse input from the reverse input optical port is not less than a preset threshold. In one embodiment, the preset threshold can be 10 dB. Non-reciprocity is a physical concept describing whether a physical process and its inverse process are equivalent. If they are equivalent, it is called reciprocal; if they are not equivalent, it is called non-reciprocal. In this disclosure, non-reciprocity refers to the different loss, phase shift, and other characteristics that light waves exhibit when propagating in opposite directions in an optical device. The high-frequency modulation signal is, for example, a modulation signal of not less than 10 GHz.

[0057] Understandably, when a high-frequency modulation signal is applied to the first phase modulator 205, the modulation efficiency of the first phase modulator 205 for the reverse-propagating optical pulse (i.e., the second sub-optical pulse) input from the second port F is much lower than the modulation efficiency for the forward-propagating optical pulse (i.e., the first sub-optical pulse) input from the first port E. That is, in this case, the first phase modulator 205 can effectively perform phase modulation on the forward-propagating optical pulse passing through it, but cannot effectively perform phase modulation on the reverse-propagating optical pulse. When both sub-optical pulses pass through the high-frequency modulation first phase modulator 205 simultaneously, if the same modulation signal pulse is applied to both the forward and reverse-propagating sub-optical pulses, a phase difference can be formed between the first and second sub-optical pulses. If different modulation signals are applied to the first phase modulator 205, different phase differences will be formed between the first and second sub-optical pulses, thus generating optical pulses with different polarization states during beam combining.

[0058] In one exemplary embodiment, the first polarization operation module further includes a third phase modulator, which is disposed in the transmission optical path of the first polarization control optical path and connected in series with the first phase modulator 205. The third phase modulator is used to perform phase modulation on one of the first sub-light pulses and the second sub-light pulses, or to perform different phase modulations on the first light pulses and the second light pulses, so that a phase difference α is generated between the first light pulses and the second light pulses.

[0059] In one exemplary embodiment, the third phase modulator includes a seventh port, an eighth port, and a ninth port. The seventh port and the eighth port are the forward input optical port and the reverse input optical port of the third phase modulator, respectively. The ninth port is an electrical port used to apply a modulated electrical signal. After applying a high-frequency modulated electrical signal with a frequency higher than a specified threshold, the third phase modulator operates in a non-reciprocal state, and the ratio of the modulation efficiency of the optical pulse input from the forward input optical port to the modulation efficiency of the optical pulse input from the reverse input optical port is not less than a preset threshold.

[0060] In some embodiments, the second polarization operation module 200 includes: a second polarization control optical path and a second phase modulator 210 disposed in the transmission optical path of the second polarization control optical path. The second polarization control optical path is connected in series with the first polarization control optical path. The second polarization control optical path is used to input the optical pulse output from the beam combining of the first polarization control optical path from the input port, and to split its polarization into two sub-optical pulses, namely a third sub-optical pulse and a fourth sub-optical pulse. The second phase modulator 210 is used to perform phase modulation on one of the third sub-optical pulse and the fourth sub-optical pulse, or to perform different phase modulations on the third sub-optical pulse and the fourth sub-optical pulse, so that a phase difference φ is generated between the two sub-optical pulses. The third sub-optical pulse and the fourth sub-optical pulse after passing through the second phase modulator are combined and output through the second polarization control optical path.

[0061] In one exemplary embodiment, the second phase modulator 210 includes a fourth port K, a fifth port L, and a sixth port M. The fourth port K and the fifth port L are the forward input optical port and the reverse input optical port of the second phase modulator, respectively. The third sub-optical pulse is input to the second phase modulator 210 in the forward direction via the fourth port K, and is output from the fifth port L after passing through the second phase modulator 210. The fourth sub-optical pulse is input to the second phase modulator 210 in the reverse direction via the fifth port L, and is output from the fourth port K after passing through the second phase modulator 210. The third port is an electrical port connected to the second driver 213 and is used to receive the modulated electrical signal applied by the second driver 213.

[0062] In one exemplary embodiment, the second polarization control optical path includes: a second polarization beam splitter unit and a second transmission optical path 209; the second polarization beam splitter unit includes at least three ports, namely port G, port H and port I (port G, port H and port I function as corresponding ports A, port B and port C in this disclosure); the intrinsic polarization state of the second polarization beam splitter unit is and The intrinsic polarization state of the second polarization beamsplitter unit is the intrinsic polarization state of the second polarization control optical path. The second polarization beamsplitter unit polarizes the optical pulse input from port G into two sub-optical pulses, namely the third sub-optical pulse and the fourth sub-optical pulse, which are output from ports H and I, respectively. The second transmission optical path 209 is used to connect ports H and I of the second polarization beamsplitter unit and transmit the third and fourth sub-optical pulses in opposite directions. In this embodiment, the second polarization beamsplitter unit is a polarization beamsplitter, and the intrinsic polarization state of the polarization beamsplitter is the intrinsic polarization state of the second polarization beamsplitter unit.

[0063] In one exemplary embodiment, the second polarization beamsplitter unit includes: a second polarization beamsplitter 206, a third polarizer 207, and a fourth polarizer 208; the polarization beamsplitter unit further includes port J; the second polarization beamsplitter 206 includes four ports, namely a first port, a second port, a third port, and a fourth port, the first port of the second polarization beamsplitter 206 is port G of the second polarization beamsplitter unit, and the fourth port of the second polarization beamsplitter 206 is port J of the second polarization beamsplitter unit; the second port and the third port of the second polarization beamsplitter 206 are respectively connected to the first side port of the third polarizer 207 and the first side port of the fourth polarizer 208; the second side port of the third polarizer 207 is port H of the second polarization beamsplitter unit. The port on the second side of the fourth polarizer 208 is port I of the second polarization beam splitter unit; the ports on the second side of the third polarizer 207 and the second side of the fourth polarizer 208 are connected through the second transmission optical path 209; the angle between the polarization direction of the third polarizer 207 and one intrinsic polarization state of the second polarization beam splitter 206 is θ, and the angle between the polarization direction of the fourth polarizer 208 and another intrinsic polarization state of the second polarization beam splitter 206 is δ; where θ, δ ≠ n·90°, n is an integer, and the angle between the polarization directions of the third polarizer 207 and the fourth polarizer 208 is ω, 0 ≤ ω ≤ 2π. In this embodiment, the intrinsic polarization state of the second polarization beam splitter 206 is the intrinsic polarization state of the second polarization beam splitter unit.

[0064] In one exemplary embodiment, the second polarization beamsplitter unit includes: a second optical beamsplitter, a third polarizer, and a fourth polarizer; the second optical beamsplitter includes at least three ports, namely a fifth port, a sixth port, and a seventh port, the fifth port of the second optical beamsplitter being port G of the second polarization beamsplitter unit; the sixth and seventh ports of the second optical beamsplitter are respectively connected to ports on the first side of the third polarizer and ports on the first side of the fourth polarizer; the port on the second side of the third polarizer is port H of the second polarization beamsplitter unit, and the port on the second side of the fourth polarizer is port I of the second polarization beamsplitter unit; the ports on the second side of the third polarizer and the ports on the second side of the fourth polarizer are connected through a second transmission optical path. The polarization direction of the third polarizer is... The direction of the fourth polarizer is The direction of the third polarizer The direction of the fourth polarizer Mutually orthogonal; the deflection direction of the third deflector in this embodiment The direction of the fourth polarizer This represents the intrinsic polarization state of the second polarization beam splitter unit.

[0065] In one exemplary embodiment, the second polarization beamsplitter unit includes: a second optical beamsplitter, a third polarizer, and a fourth polarizer; the second polarization beamsplitter unit further includes port J; the second optical beamsplitter includes four ports, namely a fifth port, a sixth port, a seventh port, and an eighth port, the fifth port of the second optical beamsplitter being port G of the second polarization beamsplitter unit, and the eighth port of the second optical beamsplitter being port J of the second polarization beamsplitter unit; the sixth and seventh ports of the second optical beamsplitter are respectively connected to the ports on the first side of the third polarizer and the ports on the first side of the fourth polarizer; the ports on the second side of the third polarizer are ports H of the second polarization beamsplitter unit, and the ports on the second side of the fourth polarizer are ports I of the second polarization beamsplitter unit; the ports on the second side of the third polarizer and the ports on the second side of the fourth polarizer are connected through a second transmission optical path. The polarization direction of the third polarizer is... The direction of the fourth polarizer is The direction of the third polarizer The direction of the fourth polarizer Mutually orthogonal; the deflection direction of the third deflector in this embodiment The direction of the fourth polarizer This represents the intrinsic polarization state of the second polarization beam splitter unit.

[0066] For example, a second phase modulator 210 is configured in the second transmission optical path 209. The second transmission optical path 209 is coupled to the slow axis of the fourth port K and the fifth port L of the second phase modulator 210, or it is coupled to the fast axis of the fourth port K and the fifth port L of the second phase modulator 210. The second phase modulator 210 modulates one optical pulse input from one of the fourth port K and the fifth port L, or modulates two optical pulses input from the fourth port K and the fifth port L with different phases.

[0067] In one exemplary embodiment, after applying a high-frequency modulated electrical signal with a frequency higher than a specified threshold, the second phase modulator 210 operates in a non-reciprocal state, and the ratio of the modulation efficiency of the third sub-optical pulse input from the forward input optical port (i.e., the fourth port K) to the modulation efficiency of the fourth sub-optical pulse input from the reverse input optical port (i.e., the fifth port L) is not less than a preset threshold.

[0068] Understandably, when a high-frequency modulation signal is applied to the second phase modulator 210, the modulation efficiency of the second phase modulator 210 for the reverse-propagating optical pulse (i.e., the fourth sub-optical pulse) input from the fifth port L is much lower than the modulation efficiency for the forward-propagating optical pulse (i.e., the third sub-optical pulse) input from the fourth port K. That is, the second phase modulator 210 can effectively perform phase modulation on the forward-propagating optical pulse passing through it each time it modulates, but cannot effectively perform phase modulation on the reverse-propagating optical pulse passing through it. When both sub-optical pulses pass through the high-frequency modulated second phase modulator 210 simultaneously, if the same modulation signal pulse is applied to both the forward and reverse-propagating sub-optical pulses, a phase difference can be formed between the third and fourth sub-optical pulses; if different modulation signals are applied to the second phase modulator 210, different phase differences will be formed between the third and fourth sub-optical pulses, thus generating optical pulses with different polarization states during beam combining.

[0069] In one exemplary embodiment, the second polarization operation module further includes a third phase modulator, which is disposed in the transmission optical path of the second polarization control optical path and connected in series with the second phase modulator 210. The third phase modulator is used to perform phase modulation on one of the third sub-light pulses and the fourth sub-light pulses, or to perform different phase modulations on the third sub-light pulses and the fourth sub-light pulses, so that a phase difference α is generated between the third sub-light pulses and the fourth sub-light pulses.

[0070] As can be seen from the above description, in some embodiments, for example, when the quantum communication protocol requires three bases, two phase modulators connected in series can be set simultaneously in the first polarization operation module or the second polarization operation module. Then, in the polarization encoding device, the optical pulses are combined and modulated by three phase modulators to meet the requirements of the three bases quantum communication protocol.

[0071] It should be noted that, regardless of whether it is the first phase modulator 205, the second phase modulator 210, or the third phase modulator, when a high-frequency modulation signal with a frequency higher than a specified threshold is applied, there can be multiple sub-optical pulses input from the forward input optical port and multiple sub-optical pulses input from the reverse input optical port at the same time.

[0072] In one exemplary embodiment, the first polarization control optical path and the second polarization control optical path are connected in series.

[0073] In one exemplary embodiment, a phase difference φ is generated between the first sub-optical pulse and the second sub-optical pulse, and a phase difference is generated between the third sub-optical pulse and the fourth sub-optical pulse. Determined based on the polarization coding requirements of the quantum communication protocol.

[0074] In some implementations, the angle between the intrinsic polarization state of the first polarization control optical path and the intrinsic polarization state of the second polarization control optical path is set according to the requirements of the quantum communication protocol.

[0075] In one exemplary embodiment, when the quantum communication protocol requires two bases, the angle between the intrinsic polarization state of the first polarization control optical path and the intrinsic polarization state of the second polarization control optical path is n·90°, or 22.5°±n·45°, or 45°±n·90°, where n is an integer.

[0076] In one exemplary embodiment, when the angle between the intrinsic polarization state of the first polarization control optical path and the intrinsic polarization state of the second polarization control optical path is n·90°, the polarization state of the light pulse input to the first polarization control optical path is... Alternatively, the polarization state of the optical pulse input to the first polarization control optical path is... Where k is an integer, and These are the two intrinsic polarization states of the first polarization control optical path, where β is any value from 0 to 2π.

[0077] In one exemplary embodiment, when the angle between the intrinsic polarization state of the first polarization control optical path and the intrinsic polarization state of the second polarization control optical path is 22.5°±n·45°, or 45°±n·90°, the polarization state of the light pulse input to the first polarization control optical path is... Alternatively, the polarization state of the optical pulse input to the first polarization control optical path is... Alternatively, the polarization state of the optical pulse input to the first polarization control optical path is... or Where k is an integer, and These are the two intrinsic polarization states of the first polarization control optical path, where β is any value from 0 to 2π.

[0078] In one exemplary embodiment, when the quantum communication protocol requires three bases, the angle between the intrinsic polarization state of the first polarization control optical path and the intrinsic polarization state of the second polarization control optical path is 22.5°±n·45°, or 45°±n·90°, where n is an integer.

[0079] In one exemplary embodiment, the polarization state of the optical pulse input to the first polarization control optical path is: Alternatively, the polarization state of the optical pulse input to the first polarization control optical path is... Alternatively, the polarization state of the optical pulse input to the first polarization control optical path is... or Where k is an integer, and These are the two intrinsic polarization states of the first polarization control optical path, where β is any value from 0 to 2π.

[0080] In one exemplary embodiment, the polarization state of the optical pulse input to the first polarization control optical path is 45° linear polarization, -45° linear polarization, left-hand circular polarization, or right-hand circular polarization.

[0081] When the angle between the intrinsic polarization state of the first polarization control optical path and the intrinsic polarization state of the second polarization control optical path is n·90°, or 22.5°±n·45°, or 45°±n·90°, phase modulation with the following effect can be achieved, resulting in a phase difference φ of 0°, 45°, 90°, or 135°, or 45°, 90°, 135°, or 180°, or 0° or 180°, or 0° or 90°, or 90° or 270°, or 0°, 90°, 180°, or 270°; phase difference It can be 0° or 180°, or 0° or 90°, or 90° or 270°, or 0°, 90°, 180° or 270°.

[0082] In some embodiments, the angle between the intrinsic polarization state of the first polarization control optical path and the intrinsic polarization state of the second polarization control optical path can be achieved by rotating at least one of the two polarization control optical paths; and / or

[0083] The device further includes a polarization state rotator, which can be a half-wave plate or a Faraday rotator. The polarization state rotator is disposed between two polarization control optical paths (that is, between two polarization operation modules) and the included angle between the intrinsic polarization states of the two polarization control optical paths is adjusted by the polarization state rotator.

[0084] In some embodiments, the device further includes an optical isolation unit, which is an optical isolator or an optical circulator. The optical isolation unit is disposed between the first polarization operation module and the second polarization operation module to isolate the optical signal transmitted back from the second polarization operation module to the first polarization operation module, thereby reducing signal interference and crosstalk.

[0085] In some implementations, the transmission optical path is a free-space optical path or a polarization-maintaining fiber.

[0086] In one exemplary embodiment, both port B and port C of the polarization beam splitter are coupled to the slow axis of the polarization-maintaining fiber or both are coupled to the fast axis of the polarization-maintaining fiber.

[0087] For example, the first transmission optical path 204 is a polarization-maintaining fiber. In this case, the second port and the third port of the first polarization beam splitter 201 are both coupled to the slow axis of the polarization-maintaining fiber or both are coupled to the fast axis of the polarization-maintaining fiber.

[0088] In one exemplary embodiment, the first polarization control optical path and / or the second polarization control optical path further include a half-wave plate or a 90-degree Faraday rotator disposed in the transmission optical path, for rotating the polarization state of the passing sub-light pulse by 90°, so that both sub-light pulses are coupled to the slow axis of the polarization-maintaining fiber or both are coupled to the fast axis of the polarization-maintaining fiber.

[0089] In some implementations, the first polarization control optical path and / or the second polarization control optical path is a Sagnac interferometer.

[0090] In some implementations, the first phase modulator 205, the second phase modulator 210, and / or the third phase modulator may be a single-polarization phase modulator or a birefringent phase modulator.

[0091] In some embodiments, the first polarizer 202 may be bonded to the second port B of the polarization beam splitter 201, and the second polarizer 203 may be bonded to the third port C of the polarization beam splitter 201.

[0092] In some embodiments, the directions of the two intrinsic polarization states of the first polarization beam splitter 201 and These are the horizontal and vertical directions, respectively.

[0093] According to the applied polarization encoding device, when the angle between the intrinsic polarization state of the first polarization control optical path and the intrinsic polarization state of the second polarization control optical path is n·90°, the polarization encoding device can prepare a ±45° polarization base by combining and modulating the first phase modulator configured in the first polarization control optical path and the second phase modulator configured in the second polarization control optical path. and left-hand and right-hand circular polarization bases Two bases and four polarization states.

[0094] For example, assuming that both the first and second polarization control optical paths use a polarization beam splitter unit consisting of a four-port polarization beam splitter and two polarizers, the first polarization control optical path will input... The intrinsic polarization state remains unchanged at the output, and the input is converted to a non-intrinsic polarization state. The intrinsic polarization state also remains unchanged at the output, meaning the relationship between the input and output of the first polarization control optical path is as follows: (The negative sign indicates that it was taken into consideration) (Due to the polarization state being reflected twice by the polarization beam splitter); the second polarization control optical path will also maintain the intrinsic polarization state of the input light at the output. When the polarization state of the light pulse input to the first polarization control optical path is 45° linearly polarized (i.e., ... When the first phase modulator randomly modulates 0° or 180°, i.e., the phase difference φ is modulated to 0° or 180°, the second phase modulator randomly modulates 0° or 90°, i.e., the phase difference φ is modulated to 0° or 180°. By modulating to 0° or 90°, ±45° polarization bases can be prepared. and left-hand and right-hand circular polarization bases Two bases and four polarization states.

[0095] When the angle between the intrinsic polarization state of the first polarization control optical path and the intrinsic polarization state of the second polarization control optical path is 45°±n·90°, the polarization encoding device can prepare a horizontal and vertical polarization base by combining and modulating the first phase modulator configured in the first polarization control optical path and the second phase modulator configured in the second polarization control optical path. and ±45° polarization base Two sets of bases with four polarization states; or horizontal and vertical polarization bases can be prepared. and left-hand and right-hand circular polarization bases Two bases with four polarization states; or ±45° polarization bases can be prepared. and left-hand and right-hand circular polarization bases Two sets of bases with four polarization states; or horizontal and vertical polarization bases can be prepared. ±45° polarization base and left-hand and right-hand circular polarization bases Three bases and six polarization states; or a polarization encoding device can prepare horizontal and vertical polarization bases. ±45° polarization base and left-hand and right-hand circular polarization bases Three bases and four polarization states, each base containing at least one polarization state, and so on.

[0096] For example, assuming that both the first and second polarization control optical paths use a polarization beam splitter unit consisting of a four-port polarization beam splitter and two polarizers, the first polarization control optical path will input... The intrinsic polarization state remains unchanged at the output, and the input is converted to a non-intrinsic polarization state. The intrinsic polarization state also remains unchanged at the output, meaning the relationship between the input and output of the first polarization control optical path is as follows: (The negative sign indicates that it was taken into consideration) (Due to the polarization state being reflected twice by the polarization beam splitter); the second polarization control optical path will also maintain the intrinsic polarization state of the input light at the output. When the polarization state of the light pulse input to the first polarization control optical path is 45° linearly polarized (i.e., ... When the first phase modulator is randomly modulated at 0°, 90°, 180°, or 270° (i.e., the phase difference φ is modulated to 0°, 90°, 180°, or 270°), and the second phase modulator is modulated at 0° (i.e., not modulated), a ±45° polarization base can be prepared. and left-hand and right-hand circular polarization bases Two sets of four polarization states are used; if the first phase modulator randomly modulates 0°, 90°, or 180°, that is, the phase difference φ is modulated to 0°, 90°, or 180°, and the second phase modulator randomly modulates 90° or 270°, that is, the phase difference φ is modulated to 0°, 90°, or 180°. By modulating the polarization to 90° or 270°, horizontal and vertical polarization bases can be prepared. and ±45° polarization base Two sets of bases with four polarization states; if the first phase modulator is randomly modulated at 90° or 270°, and the second phase modulator is modulated at 0° or 90°, then a horizontally and vertically polarized base can be prepared. and left-hand and right-hand circular polarization bases Two sets of bases with four polarization states; if the first phase modulator is randomly modulated at 0°, 90°, 180°, or 270°, and the second phase modulator is randomly modulated at 0° or 90°, then a horizontally and vertically polarized base can be prepared. ±45° polarization base and left-hand and right-hand circular polarization bases Three bases and six polarization states; if the first phase modulator is randomly modulated to 0°, 90°, or 180°, and the second phase modulator is randomly modulated to 0° or 90°, then a horizontal and vertical polarization base can be prepared. ±45° polarization base and left-hand and right-hand circular polarization bases Three bases and four polarization states.

[0097] According to the polarization encoding device of this disclosure, when the angle between the intrinsic polarization state of the first polarization control optical path and the intrinsic polarization state of the second polarization control optical path is 22.5°±n·45°, the polarization encoding device can prepare a horizontal and vertical polarization base by combining and modulating the first phase modulator disposed in the first polarization control optical path and the second phase modulator disposed in the second polarization control optical path. and ±45° polarization base Two sets of bases with four polarization states; or horizontal and vertical polarization bases can be prepared. and left-hand and right-hand circular polarization bases Two bases with four polarization states; or ±45° polarization bases can be prepared. and left-hand and right-hand circular polarization bases Two sets of bases with four polarization states; or horizontal and vertical polarization bases can be prepared. ±45° polarization base and left-hand and right-hand circular polarization bases Three bases and six polarization states; or a polarization encoding device can prepare horizontal and vertical polarization bases. ±45° polarization base and left-hand and right-hand circular polarization bases Three bases and four polarization states, each base containing at least one polarization state, and so on.

[0098] For example, assuming that both the first and second polarization control optical paths use a polarization beam splitter unit consisting of a four-port polarization beam splitter and two polarizers, the first polarization control optical path will input... The intrinsic polarization state remains unchanged at the output, and the input is converted to a non-intrinsic polarization state. The intrinsic polarization state also remains unchanged at the output, meaning the relationship between the input and output of the first polarization control optical path is as follows: (The negative sign indicates that it was taken into consideration) (Due to the polarization state being reflected twice by the polarization beam splitter); the second polarization control optical path will also maintain the intrinsic polarization state of the input light at the output. When the polarization state of the light pulse input to the first polarization control optical path is 45° linearly polarized (i.e., ... If the first phase modulator is randomly modulated at 0°, 90°, 180°, or 270°, and the second phase modulator is modulated at 0° (i.e., not modulated), then a ±45° polarization base can be prepared. and left-hand and right-hand circular polarization bases Two bases and four polarization states; if the first phase modulator is randomly modulated at 0° or 180°, and the second phase modulator is randomly modulated at 0° or 180°, then a horizontal and vertical polarization base can be prepared. and ±45° polarization base Two sets of bases with four polarization states; if the first phase modulator is randomly modulated at 0°, 90°, or 180°, and the second phase modulator is randomly modulated at 0° or 180°, then a horizontally and vertically polarized base can be prepared. and left-hand and right-hand circular polarization bases Two sets of bases with four polarization states; if the first phase modulator is randomly modulated to 0°, 90°, or 180°, and the second phase modulator is randomly modulated to 0° or 180°, then a horizontally and vertically polarized base can be prepared. ±45° polarization base and left-hand and right-hand circular polarization bases Three bases and six polarization states; if the first phase modulator is randomly modulated to 0°, 90°, or 180°, and the second phase modulator is randomly modulated to 0° or 180°, then a horizontal and vertical polarization base can be prepared. ±45° polarization base and left-hand and right-hand circular polarization bases Three bases and four polarization states. If three phase modulators are set in this polarization encoding device (for example, two phase modulators are connected in series in the first polarization operation module), then if the first phase modulator is randomly modulated to 0° or 90°, the third phase modulator is randomly modulated to 0° or 180°, and the second phase modulator is randomly modulated to 0° or 180°, then a horizontal and vertical polarization base can be prepared. ±45° polarization base and left-hand and right-hand circular polarization bases Three bases and six polarization states.

[0099] As described above, the polarization encoding device of this embodiment, when the quantum communication protocol requires two bases, includes two phase modulators in the first and second polarization operation modules. These modulators can be combined to achieve four polarization states digitally. When the quantum communication protocol requires three bases, six polarization states can be digitally modulated by combining the three phase modulators.

[0100] In some embodiments, the polarization encoding device disclosed herein can also be used in the polarization decoding process, depending on the requirements of quantum communication.

[0101] The polarization encoding device disclosed herein includes a first polarization operation module and a second polarization operation module connected in series. Each of the first or second polarization operation modules includes a polarization control optical path and a phase modulator. The polarization control optical path is used to polarize and split the input optical pulse into two sub-optical pulses. The phase modulators in the two polarization operation modules modulate the phase of one of the two sub-optical pulses, or modulate the two sub-optical pulses differently, so that the two sub-optical pulses generate phase differences φ and φ, respectively. Furthermore, the angle between the intrinsic polarization states of the two polarization control optical paths is set according to the requirements of the quantum communication protocol. With the aforementioned phase differences φ and φ, multiple polarization-encoded quantum states with two or three bases can be modulated to meet the requirements of different quantum communication protocols and reduce the complexity of the modulation phase.

[0102] Furthermore, this disclosure applies a high-frequency modulation electrical signal with a frequency higher than a specified threshold to the first phase modulator and / or the second phase modulator, making the modulation efficiency of the first phase modulator and / or the second phase modulator for reverse-propagating optical pulses much lower than that for forward-propagating optical pulses. The first phase modulator and / or the second phase modulator can effectively perform phase modulation on forward-propagating optical pulses, but cannot effectively perform phase modulation on reverse-propagating optical pulses. Therefore, when forward-propagating optical pulses and reverse-propagating optical pulses pass through the high-frequency modulation first phase modulator and / or the second phase modulator, only the same modulation electrical signal needs to be applied to the two optical pulses once to form a phase difference between the two optical pulses. Applying modulation electrical signals of different magnitudes to the first phase modulator and / or the second phase modulator results in different phase differences between the two sub-optical pulses, enabling polarization quantum state encoding at speeds of 10 GHz or higher.

[0103] Furthermore, in the polarization control optical path, the two sub-light pulses of polarization beam splitting travel through identical optical paths from polarization beam splitting to beam combining, exhibiting self-compensation against environmental interference and possessing the advantages of high stability and anti-interference capabilities. This disclosure provides an implementation scheme for a high-speed polarization quantum state encoding device that is easy to implement and apply, and compatible with multiple quantum communication protocols.

[0104] The polarization encoding method provided in this disclosure can be executed by the polarization encoding device and system provided in this disclosure.

[0105] Referring to Figure 7, this disclosure provides a polarization encoding method, which can be implemented by the polarization encoding device of the above embodiments, and the method includes steps 710-720.

[0106] Step 710: Input one optical pulse into the first polarization control optical path and split it into two sub-optical pulses, namely the first sub-optical pulse and the second sub-optical pulse. The first phase modulator modulates the phase of one of the first sub-optical pulse and the second sub-optical pulse, or modulates the first sub-optical pulse and the second sub-optical pulse differently, so that a phase difference φ is generated between the two sub-optical pulses. The first sub-optical pulse and the second sub-optical pulse after passing through the first phase modulator are combined and output through the first polarization control optical path.

[0107] For example, referring to the structure shown in FIG2, a single optical pulse is input to the polarization encoding device through the first port A of the first polarization beam splitter 201. The first polarization beam splitter 201 polarizes and splits the input single optical pulse into a first sub-optical pulse and a second sub-optical pulse, which are output from ports B and C respectively. The first sub-optical pulse is output from port B of the first polarization beamsplitter 201 and then transmitted clockwise through the first polarizer 202, the first polarization-maintaining fiber 204, the first phase modulator 205, and the second polarizer 203 to port C of the first polarization beamsplitter 201. After being reflected by the first polarization beamsplitter 201, it is output from port D. The second sub-optical pulse is output from port C of the first polarization beamsplitter 201 and then transmitted counterclockwise through the second polarizer 203, the first polarization-maintaining fiber 204, the first phase modulator 205, and the first polarizer 202 to port B of the first polarization beamsplitter 201. After being transmitted through the first polarization beamsplitter 201, it is output from port D. The first and second sub-optical pulses are combined at port D of the first polarization beamsplitter 201 and output to the second polarization control optical path. During operation, the first phase modulator 205 modulates the optical pulses input from either port E or port F; or modulates the optical pulses input from ports E and F with different phases; or when the first sub-optical pulse and the second sub-optical pulse pass through the first phase modulator 205 simultaneously, a high-frequency modulation electrical signal, such as a 10GHz high-frequency modulation electrical signal pulse, is applied to the first phase modulator 205. Although the first sub-optical pulse and the second sub-optical pulse are subjected to the same modulation electrical signal when passing through the first phase modulator 205, the first phase modulator 205 modulates the optical pulses at high frequencies. At that time, the first sub-optical pulse transmitted forward from port E through the first phase modulator 205 is effectively phase modulated, while the second sub-optical pulse transmitted backward from port F through the first phase modulator 205 is not effectively phase modulated, thus forming a phase difference between the first and second sub-optical pulses. When different modulation electrical signals are applied to the first phase modulator 205, the phase difference formed between the two sub-optical pulses is different, thus causing the two sub-optical pulses to generate different polarization states with high-speed modulation when they are combined and output by the first polarization beam splitter 201. The first phase modulator 205 can have multiple modulation methods, such as randomly modulating four phase states: 0°, 45°, 90°, or 135°; or randomly modulating four phase states: 45°, 90°, 135°, or 180°; or randomly modulating two phase states: 0° or 180°; or randomly modulating two phase states: 0° or 90°; or randomly modulating two phase states: 90° or 270°; or randomly modulating four phase states: 0°, 90°, 180°, or 270°.

[0108] Step 720: The optical pulse output from the beam combining is input into the second polarization control optical path connected in series with the first polarization control optical path. The second polarization control optical path then splits the beam into two sub-optical pulses, namely the third sub-optical pulse and the fourth sub-optical pulse. A second phase modulator modulates the phase of one of the third and fourth sub-optical pulses, or modulates the third and fourth sub-optical pulses differently, thereby creating a phase difference between the two sub-optical pulses. The third and fourth sub-light pulses after passing through the second phase modulator are combined and output through the second polarization control optical path. The angle between the intrinsic polarization state of the first polarization control optical path and the intrinsic polarization state of the second polarization control optical path is set according to the requirements of the quantum communication protocol.

[0109] For example, referring to the structure shown in Figure 2, the optical pulse output from port D of the first polarization beamsplitter 201 is input to the second polarization control optical path via port G of the second polarization beamsplitter 206. The second polarization beamsplitter 206 splits the input optical pulse into a third sub-optical pulse and a fourth sub-optical pulse, which are output from ports H and I, respectively. The third sub-optical pulse is output from port H of the second polarization beam splitter 206 and then transmitted clockwise through the third polarizer 207, the second polarization-maintaining fiber 209, the second phase modulator 210, and the fourth polarizer 208 to port I of the second polarization beam splitter 206. After being reflected by the second polarization beam splitter 206, it is output from port J. The fourth sub-optical pulse is output from port I of the second polarization beam splitter 206 and then transmitted counterclockwise through the fourth polarizer 208, the second polarization-maintaining fiber 209, the second phase modulator 210, and the third polarizer 207 to port H of the second polarization beam splitter 206. After being transmitted through the second polarization beam splitter 206, it is output from port J. The third and fourth sub-optical pulses are combined and output from port J of the second polarization beam splitter 206. During operation, the second phase modulator 210 modulates the optical pulses input from either port K or port L; or modulates the optical pulses input from ports K and L with different phases; or, when the third and fourth sub-optical pulses pass through the second phase modulator 210, a high-frequency modulation signal, such as a 10 GHz high-frequency modulation signal, is applied to the second phase modulator 210. Although the third and fourth sub-optical pulses are subjected to the same modulation signal when passing through the second phase modulator 210, the second phase modulator 210 modulates at high frequencies... The third sub-optical pulse, transmitted forward from port K through the second phase modulator 210, undergoes effective phase modulation, while the fourth sub-optical pulse, transmitted backward from port L through the second phase modulator 210, does not undergo effective phase modulation, thus creating a phase difference between the third and fourth sub-optical pulses. Applying different modulation signals to the second phase modulator 210 results in different phase differences between the two sub-optical pulses, causing them to generate different polarization states during high-speed modulation when combined at the output of the second polarization beamsplitter 201. The second phase modulator 210 can have various modulation methods, such as randomly modulating 0° or 180° phase states; or randomly modulating 0° or 90° phase states; or randomly modulating 90° or 270° phase states; or randomly modulating 0°, 90°, 180°, or 270° phase states.

[0110] In one exemplary embodiment, when the quantum communication protocol requires two bases, the angle between the intrinsic polarization state of the first polarization control optical path and the intrinsic polarization state of the second polarization control optical path is n·90°, or 22.5°±n·45°, or 45°±n·90°, where n is an integer.

[0111] In one exemplary embodiment, when the quantum communication protocol requires three bases, the angle between the intrinsic polarization state of the first polarization control optical path and the intrinsic polarization state of the second polarization control optical path is 22.5°±n·45°, or 45°±n·90°, where n is an integer.

[0112] In one exemplary embodiment, when the angle between the intrinsic polarization state of the first polarization control optical path and the intrinsic polarization state of the second polarization control optical path is n·90°, the polarization state of the light pulse input to the first polarization control optical path is... Alternatively, the polarization state of the optical pulse input to the first polarization control optical path is... Where k is an integer, and These are the two intrinsic polarization states of the first polarization control optical path, where β is any value from 0 to 2π.

[0113] In one exemplary embodiment, when the angle between the intrinsic polarization state of the first polarization control optical path and the intrinsic polarization state of the second polarization control optical path is 22.5°±n·45°, or 45°±n·90°, the polarization state of the light pulse input to the first polarization control optical path is... Alternatively, the polarization state of the optical pulse input to the first polarization control optical path is... Alternatively, the polarization state of the optical pulse input to the first polarization control optical path is... or Where k is an integer, and These are the two intrinsic polarization states of the first polarization control optical path, where β is any value from 0 to 2π.

[0114] The polarization encoding method disclosed herein is based on phase modulators in two cascaded polarization operation modules. By modulating one of the two sub-optical pulses, or by modulating the two sub-optical pulses with different phases, the two sub-optical pulses generate phase differences φ and φ, respectively. Furthermore, the angle between the intrinsic polarization states of the two polarization control optical paths is set according to the requirements of the quantum communication protocol. Combined with the aforementioned phase differences φ and φ, the two phase modulators are used to modulate multiple polarization encoded quantum states with two or three bases to meet the requirements of different quantum communication protocols and reduce the complexity of the modulation phase.

[0115] According to this disclosure, a quantum communication system is provided, including the polarization encoding device described above.

[0116] It should be understood that the specific features, operations, and details described herein with respect to the apparatus of this disclosure can also be similarly applied to the methods and systems of this disclosure, or vice versa. Furthermore, each step of the method of this disclosure described above can be performed by a corresponding component or unit of the apparatus or system of this disclosure.

[0117] The technical features described above can be combined arbitrarily. Although not all possible combinations of these technical features are described, any combination of these technical features should be considered to be covered by this specification, provided that such combination does not contain contradictions.

[0118] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this disclosure, and are not intended to limit them. Although this disclosure has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this disclosure.

Claims

1. A polarization encoding device, wherein, The device comprises: a first polarization operation module and a second polarization operation module connected in series, wherein the first polarization operation module comprises a first polarization control optical path and a first phase modulator arranged in a transmission light path of the first polarization control optical path; the first polarization control optical path is used for polarizing and splitting an input light pulse into two sub light pulses, i.e., a first sub light pulse and a second sub light pulse; the first phase modulator is used for phase modulating one of the first sub light pulse and the second sub light pulse, or performing different phase modulations on the first sub light pulse and the second sub light pulse, so that a phase difference φ is generated between the two sub light pulses, and the first sub light pulse and the second sub light pulse after passing through the first phase modulator are combined and output through the first polarization control optical path; the second polarization operation module comprises a second polarization control optical path and a second phase modulator arranged in a transmission light path of the second polarization control optical path, the second polarization control optical path is connected in series with the first polarization control optical path, and the second polarization control optical path is used for inputting the light pulse combined and output by the first polarization control optical path from an input port, polarizing and splitting the light pulse into two sub light pulses, i.e., a third sub light pulse and a fourth sub light pulse; the second phase modulator is used for phase modulating one of the third sub light pulse and the fourth sub light pulse, or performing different phase modulations on the third sub light pulse and the fourth sub light pulse, so that a phase difference φ is generated between the two sub light pulses, and the third sub light pulse and the fourth sub light pulse after passing through the second phase modulator are combined and output through the second polarization control optical path; wherein the angle between the eigenpolarization state of the first polarization control optical path and the eigenpolarization state of the second polarization control optical path is set according to the requirements of a quantum communication protocol.

2. The device according to claim 1, wherein the first phase modulator comprises a first port, a second port and a third port, the first port and the second port are a forward input optical port and a reverse input optical port of the first phase modulator respectively, the first sub light pulse is input into the first phase modulator through the first port, and is output by the second port after passing through the first phase modulator; the second sub light pulse is input into the first phase modulator through the second port, and is output by the first port after passing through the first phase modulator; and the third port is an electrical port for applying a modulation electrical signal; the second phase modulator comprises a fourth port, a fifth port and a sixth port, the fourth port and the fifth port are a forward input optical port and a reverse input optical port of the second phase modulator respectively, the third sub light pulse is input into the second phase modulator through the fourth port, and is output by the fifth port after passing through the second phase modulator; the fourth sub light pulse is input into the second phase modulator through the fifth port, and is output by the fourth port after passing through the second phase modulator; and the sixth port is an electrical port for applying a modulation electrical signal.

3. The device according to claim 2, wherein The first phase modulator, when a high-frequency modulation electrical signal with a frequency higher than a specified threshold is applied, works in a non-reciprocal state, and a ratio of a modulation efficiency on a first sub-light pulse input by a forward input optical port to a modulation efficiency on a second sub-light pulse input by a backward input optical port is not less than a preset threshold; The second phase modulator, when a high-frequency modulation electrical signal with a frequency higher than a specified threshold is applied, works in a non-reciprocal state, and a ratio of a modulation efficiency on a third sub-light pulse input by a forward input optical port to a modulation efficiency on a fourth sub-light pulse input by a backward input optical port is not less than a preset threshold.

4. The apparatus of claim 3, wherein, The first sub-light pulse and the second sub-light pulse pass through the first phase modulator at the same time; And / or The third sub-light pulse and the fourth sub-light pulse pass through the second phase modulator at the same time.

5. The device of any one of claims 1 to 4, wherein, When the quantum communication protocol requires two groups of bases, an angle between an eigenpolarization state of the first polarization control optical path and an eigenpolarization state of the second polarization control optical path is n*90°, or 22.5°±n*45°, or 45°±n*90°, where n is an integer.

6. The apparatus of claim 5, wherein, When the angle between the eigenpolarization state of the first polarization control optical path and the eigenpolarization state of the second polarization control optical path is n*90°, where n is an integer, The polarization state of the light pulses input to the first polarization control light path is Or, The polarization state of the light pulses input to the first polarization control light path is wherein k is an integer, and The two eigenpolarization states of the first polarization control optical path, and β is an arbitrary value in the range of 0-2π; When the angle between the eigenpolarization state of the first polarization control optical path and the eigenpolarization state of the second polarization control optical path is 22.5°±n*45°, or 45°±n*90°, where n is an integer, The polarization state of the light pulses input to the first polarization control light path is Or, The polarization state of the light pulses input to the first polarization control light path is Or, The polarization state of the light pulses input to the first polarization control light path is or wherein k is an integer, and The two eigenpolarization states of the first polarization control optical path, and β is an arbitrary value in the range of 0-2π.

7. The device of any one of claims 1 to 4, wherein, When the quantum communication protocol requires three groups of bases, an angle between an eigenpolarization state of the first polarization control optical path and an eigenpolarization state of the second polarization control optical path is 22.5°±n*45°, or 45°±n*90°, where n is an integer.

8. The apparatus of claim 7, wherein, The polarization state of the light pulses input to the first polarization control light path is Or, The polarization state of the light pulses input to the first polarization control light path is Or, The polarization state of the light pulses input to the first polarization control light path is or wherein k is an integer, and The two eigenpolarization states of the first polarization control optical path, and β is an arbitrary value in the range of 0-2π.

9. The apparatus of claim 8, wherein, The polarization state of the light pulse input into the first polarization control optical path is 45° linear polarization, -45° linear polarization, left-handed circular polarization, or right-handed circular polarization.

10. The apparatus of claim 7, wherein, The first polarization operation module further comprises: A third phase modulator, which is arranged in series with the first phase modulator in the transmission optical path of the first polarization control optical path, and is used to perform phase modulation on one of the first sub-light pulse and the second sub-light pulse, or perform different phase modulations on the first sub-light pulse and the second sub-light pulse, so that a phase difference α is generated between the first sub-light pulse and the second sub-light pulse; or The second polarization operation module further comprises: A third phase modulator, which is arranged in series with the second phase modulator in the transmission optical path of the second polarization control optical path, and is used to perform phase modulation on one of the third sub-light pulse and the fourth sub-light pulse, or perform different phase modulations on the third sub-light pulse and the fourth sub-light pulse, so that a phase difference α is generated between the third sub-light pulse and the fourth sub-light pulse. A transmission optical path in series with the second phase modulator, for phase modulating one of the third and fourth sub-light pulses, or for phase modulating the third and fourth sub-light pulses differently, so that a phase difference α is generated between the third and fourth sub-light pulses.

11. The apparatus of claim 10, wherein, The third phase modulator comprises a seventh port, an eighth port and a ninth port, the seventh and eighth ports being forward and reverse input optical ports of the third phase modulator respectively, and the ninth port being an electrical port for applying a modulation electrical signal; when a high-frequency modulation electrical signal with a frequency higher than a specified threshold is applied, the third phase modulator works in a non-reciprocal state, and a ratio of a modulation efficiency of an optical pulse input from the forward input optical port to a modulation efficiency of an optical pulse input from the reverse input optical port is not less than a preset threshold.

12. The device of any one of claims 1 to 4, wherein, An angle between the eigenpolarization state of the first polarization control optical path and the eigenpolarization state of the second polarization control optical path is realized by rotating at least one of the two polarization control optical paths; and / or The apparatus further comprises a polarization state rotator, which is arranged between the two polarization control optical paths and adjusts the angle between the eigenpolarization states of the two polarization control optical paths.

13. The device of any one of claims 1 to 4, wherein, The apparatus further comprises an optical isolation unit, which is an optical isolator or an optical circulator, and is arranged between the first polarization operation module and the second polarization operation module.

14. The apparatus of claim 1, wherein, The first polarization control optical path and / or the second polarization control optical path comprises a polarization beam splitting unit and a transmission optical path. The polarization beam splitting unit comprises at least three ports, port A, port B and port C respectively; the eigenpolarization state of the polarization beam splitting unit is and The polarization beam splitting unit splits the polarization of the optical pulse input from the port A into two sub-light pulses, which are output from the ports B and C respectively. The transmission optical path is used to connect the ports B and C of the polarization beam splitting unit.

15. The apparatus of claim 14, wherein, The polarization beam splitting unit is a polarization beam splitter, and the eigenpolarization state of the polarization beam splitter is the eigenpolarization state of the polarization beam splitting unit.

16. The apparatus of claim 14, wherein, The polarization beam splitting unit comprises a polarization beam splitter, a first polarizer and a second polarizer. The polarization beam splitting unit further comprises a port D. The polarization beam splitter comprises four ports, namely a first port, a second port, a third port and a fourth port, the first port of the polarization beam splitter is port A of the polarization beam splitting unit, and the fourth port of the polarization beam splitter is port D of the polarization beam splitting unit; the second port and the third port of the polarization beam splitter are connected with the port of the first side of the first polarizer and the port of the first side of the second polarizer respectively; the port of the second side of the first polarizer is port B of the polarization beam splitting unit, and the port of the second side of the second polarizer is port C of the polarization beam splitting unit; the port of the second side of the first polarizer and the port of the second side of the second polarizer are connected through the transmission optical path; the angle between the polarization direction of the first polarizer and one eigenpolarization state of the polarization beam splitter is θ, and the angle between the polarization direction of the second polarizer and the other eigenpolarization state of the polarization beam splitter is δ; wherein θ, δ ≠ n·90°, n is an integer; the eigenpolarization state of the polarization beam splitter is the eigenpolarization state of the polarization beam splitting unit.

17. The apparatus of claim 14, wherein The polarization beam splitting unit comprises an optical beam splitter, a first polarizer and a second polarizer. The polarization beam splitting unit comprises an optical beam splitter, a first polarizer and a second polarizer. The optical beam splitter comprises at least three ports, respectively a first port, a second port and a third port, the first port of the optical beam splitter is port A of the polarization beam splitting unit; the second port and the third port of the optical beam splitter are connected with the port of the first side of the first polarizer and the port of the first side of the second polarizer respectively; the port of the second side of the first polarizer is port B of the polarization beam splitting unit, and the port of the second side of the second polarizer is port C of the polarization beam splitting unit; the port of the second side of the first polarizer and the port of the second side of the second polarizer are connected through the transmission optical path, and the polarization direction of the first polarizer is The second polarizer has a polarization direction that is a polarization direction of the first polarizer a polarizing direction of the second polarizer mutually orthogonal; the polarizing direction of the first polarizer a polarizing direction of the second polarizer 18. The apparatus of claim 17, wherein The optical beam splitter further comprises a fourth port; The polarization beam splitting unit further comprises port D; The fourth port of the optical beam splitter is port D of the polarization beam splitting unit.

19. The apparatus of claim 14, wherein The transmission optical path is a free-space optical path or a polarization-maintaining optical fiber.

20. The apparatus of claim 19, wherein Port B and port C of the polarization beam splitting unit are both coupled to the slow axis of the polarization-maintaining optical fiber or both coupled to the fast axis of the polarization-maintaining optical fiber.

21. The apparatus of claim 19, wherein The apparatus further comprises a half-wave plate or a 90-degree Faraday rotator arranged in the transmission optical path. including:

22. A polarization encoding method, wherein, inputting a light pulse into a first polarization control optical path to split the light pulse into two sub-light pulses, namely a first sub-light pulse and a second sub-light pulse, performing phase modulation on at least one of the first sub-light pulse and the second sub-light pulse by a first phase modulator, or performing different phase modulations on the first sub-light pulse and the second sub-light pulse, so that a phase difference φ is generated between the two sub-light pulses, and the first sub-light pulse and the second sub-light pulse after the first phase modulator are combined and output through the first polarization control optical path; the third sub-light pulse and the fourth sub-light pulse after the second phase modulator are combined and output through the second polarization control optical path; The light pulse input of the combined output is input into a second polarization control optical path in series with the first polarization control optical path, the light pulse is split into two sub light pulses by the second polarization control optical path, and the two sub light pulses are a third sub light pulse and a fourth sub light pulse; at least one of the third sub light pulse and the fourth sub light pulse is phase modulated by a second phase modulator, or the third sub light pulse and the fourth sub light pulse are phase modulated differently, so that a phase difference is generated between the two sub light pulses wherein the angle between the eigenpolarization state of the first polarization control optical path and the eigenpolarization state of the second polarization control optical path is set according to the requirements of a quantum communication protocol. When the quantum communication protocol requirements are two groups of bases, the angle between the eigenpolarization state of the first polarization control optical path and the eigenpolarization state of the second polarization control optical path is n·90°, or 22.5°±n·45°, or 45°±n·90°, wherein n is an integer.

23. The method of claim 22, wherein, ​ 24. The method of claim 22, wherein, When the quantum communication protocol requirement is three groups of bases, the angle between the eigenpolarization state of the first polarization control optical path and the eigenpolarization state of the second polarization control optical path is 22.5°±n·45°, or 45°±n·90°, wherein n is an integer.

25. The method of any one of claims 22 to 24, wherein, When the angle between the eigenpolarization state of the first polarization control optical path and the eigenpolarization state of the second polarization control optical path is n·90°, wherein n is an integer, The polarization state of the light pulses input to the first polarization control light path is or, The polarization state of the light pulses input to the first polarization control light path is wherein k is an integer, and for the two eigenpolarization states of the first polarization control optical path, β is an arbitrary value of 0-2π; When the angle between the eigenpolarization state of the first polarization control optical path and the eigenpolarization state of the second polarization control optical path is 22.5°±n·45°, or 45°±n·90°, wherein n is an integer, The polarization state of the light pulses input to the first polarization control light path is or, The polarization state of the light pulses input to the first polarization control light path is or, The polarization state of the light pulses input to the first polarization control light path is or wherein k is an integer, and for the two eigenpolarization states of the first polarization control optical path, β is an arbitrary value of 0-2π.

26. A quantum communication system, wherein, A polarization encoding device comprising any one of claims 1 to 21.

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