Quantum key distribution phase codec, corresponding codec device and system

Abstract

The present invention proposes a quantum key distribution phase codec and a corresponding coding and decoding device and system. The phase codec includes: a beam splitter, two polarization orthogonal rotation reflection devices optically coupled to the beam splitter via two arms, and a phase modulator arranged on one of the two arms, wherein one or each of the two reflection devices includes a polarization beam splitter having an input port and two output ports, and is coupled to the corresponding arm via the input port of the polarization beam splitter, and the two output ports of each polarization beam splitter are optically coupled to each other via a transmission optical path, and a half-wave plate is arranged on the transmission optical path, and the polarization direction of the optical pulse input to the half-wave plate is at an angle of 45 degrees to the fast axis or slow axis of the half-wave plate. The present invention enables stable coding and decoding interference of input optical pulses of arbitrary polarization states, and solves the problem that the system cannot work stably due to polarization-induced fading in phase coding and time bit-phase coding quantum key distribution applications.

Description

Technical Field

[0001] The present invention relates to the technical field of optical transmission secure communication, and in particular to a quantum key distribution phase codec based on polarization orthogonal rotation reflection, a corresponding coding and decoding device including the phase codec, and a quantum key distribution system. Background Art

[0002] Quantum secure communication technology is a frontier hotspot combining quantum physics and information science. Based on quantum key distribution technology and the principle of one-time-one-pad cryptography, quantum secure communication can achieve secure information transmission in public channels. Quantum key distribution is based on physical principles such as the Heisenberg uncertainty relation in quantum mechanics and the quantum no-cloning theorem. It can achieve secure key sharing between users and detect potential eavesdropping. It can be applied to fields with high-security information transmission requirements such as national defense, government affairs, finance, and electricity.

[0003] Ground-based quantum key distribution is mainly based on fiber channel transmission. Since phase coding uses the phase difference between the front and rear optical pulses to encode information, it can be stably maintained during long-distance fiber channel transmission. Therefore, phase coding based on unequal-arm interferometers and time bit-phase coding are the main coding schemes for quantum key distribution applications. However, there are non-ideal conditions in optical fiber production, such as non-circular symmetry of the cross section and uneven distribution of the core refractive index along the radial direction. In addition, optical fibers are affected by temperature, strain, bending, etc. in actual environments, which will produce random birefringence effects. Therefore, after the optical pulse is transmitted over a long distance optical fiber and transmitted through the two arms of the unequal-arm interferometer, there is a problem of polarization-induced fading when the phase decoding interference is performed through the unequal-arm interferometer, which leads to unstable decoding interference and increased bit error rate. If a correction device is used, the system complexity and cost will be increased, and it is difficult to achieve stable application in strong interference conditions such as overhead optical cables and road and bridge optical cables.

[0004] For quantum key distribution phase encoding and time bit-phase encoding schemes, how to stably and efficiently perform interference decoding is a hot topic and difficult problem in the application of quantum secure communication based on the existing optical cable infrastructure. Summary of the invention

[0005] The main purpose of the present invention is to propose a quantum key distribution phase codec based on polarization orthogonal rotation reflection, a corresponding encoding and decoding device including the phase codec, and a quantum key distribution system, so as to solve the problem of phase decoding interference instability caused by polarization induced fading in phase coding and time bit-phase coding quantum key distribution applications.

[0006] The present invention provides at least the following technical solutions:

[0007] 1. A quantum key distribution phase codec, comprising: a beam splitter, two reflecting devices optically coupled to the beam splitter via two arms respectively, each of the reflecting devices being a polarization orthogonal rotation reflecting device, one of the two reflecting devices or each of the reflecting devices comprising a polarization beam splitter having an input port and two output ports, and coupled to corresponding arms of the two arms via the input port of the polarization beam splitter, wherein the two output ports of each polarization beam splitter are optically coupled to each other via a transmission optical path, at least one reflecting device comprising a polarization beam splitter is provided with a half-wave plate on its transmission optical path, and the angle between the polarization direction of the optical pulse input to the half-wave plate and the fast axis or slow axis of the half-wave plate is 45 degrees.

[0008] 2. The phase codec according to Scheme 1, wherein the two reflection devices are polarization orthogonal rotation reflection devices of the same structure, or polarization orthogonal rotation reflection devices of different structures.

[0009] 3. The phase codec according to solution 1, wherein the transmission optical path is a polarization maintaining optical path.

[0010] 4. The phase codec according to claim 1, wherein the beam splitter is a polarization-maintaining beam splitter.

[0011] 5. The phase codec according to solution 1, wherein each of the two arms is a polarization-maintaining optical path, and the optical devices on the two arms are polarization-maintaining optical devices and / or non-birefringent optical devices.

[0012] 6. A phase codec according to any one of schemes 1-5, wherein the phase codec further comprises a phase modulator, and the phase modulator is arranged at the front end of the beam splitter or on at least one of the two arms.

[0013] 7. A DC modulated quantum key distribution phase encoding and decoding device, comprising a pre-beam splitter and two phase codecs according to any one of Schemes 1-6, wherein the two phase codecs are optically coupled to the pre-beam splitter via two sub-optical paths respectively, wherein one of the ports of the beam splitter of each phase codec that is not coupled to the two arms of the phase codec is optically coupled to a corresponding sub-path of the two sub-optical paths, and an optical circulator is arranged on each of the sub-optical paths.

[0014] 8. A quantum key distribution time bit-phase encoding and decoding device, comprising a pre-beam splitter and a phase codec according to any one of schemes 1-6, wherein the phase codec is optically coupled to the pre-beam splitter via a sub-optical path, wherein one of the ports of the beam splitter of the phase codec that is not coupled to the two arms is optically coupled to the sub-optical path.

[0015] 9. A DC modulated quantum key distribution time bit-phase encoding and decoding device, comprising a pre-beam splitter and a phase codec according to any one of schemes 1-6, wherein the phase codec is optically coupled to the pre-beam splitter via a sub-optical path, wherein one of the ports of the beam splitter of the phase codec that is not coupled to the two arms is optically coupled to the sub-optical path, wherein an optical circulator is arranged on the sub-optical path.

[0016] 10. The encoding and decoding device according to solution 8 or 9 further includes a beam splitter coupled to the pre-beam splitter via another sub-optical path.

[0017] 11. A quantum key distribution system, comprising:

[0018] The phase codec according to any one of solutions 1 to 6 or the codec device according to any one of solutions 7 to 10, which is arranged at the receiving end of the quantum key distribution system for decoding; and / or

[0019] The phase codec according to any one of schemes 1 to 6 or the encoding and decoding device according to any one of schemes 7 to 10 is arranged at the transmitting end of the quantum key distribution system for encoding.

[0020] The present invention, through a creative structure, enables stable encoding and decoding interference of input optical pulses of any polarization state, thereby achieving unexpected beneficial effects. Using the scheme of the present invention, a stable interference output can be achieved at the phase decoding interferometer for input optical pulses of any polarization state, solving the problem that polarization-induced fading in phase coding and time bit-phase coding quantum key distribution applications causes the system to be unable to work stably. The present invention provides a phase coding and time bit-phase coding quantum key distribution decoding scheme that is resistant to polarization-induced fading and easy to implement and apply. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a schematic diagram of the structure of a phase codec for quantum key distribution based on polarization orthogonal rotation reflection according to a preferred embodiment of the present invention;

[0022] Figure 2 This is a schematic diagram of the composition structure of a phase codec for quantum key distribution based on polarization orthogonal rotation reflection according to another preferred embodiment of the present invention;

[0023] Figure 3 A schematic diagram of the composition structure of a polarization orthogonal rotation reflection device that can be used in the phase codec of the present invention;

[0024] Figure 4 It is a schematic diagram of the composition structure of another polarization orthogonal rotation reflection device that can be used for the phase codec of the present invention;

[0025] Figure 5 It is a schematic diagram of the composition structure of another polarization orthogonal rotation reflection device that can be used for the phase codec of the present invention;

[0026] Figure 6 A schematic diagram of the composition structure of a phase encoding and decoding device for direct current modulation quantum key distribution based on polarization orthogonal rotation reflection according to a preferred embodiment of the present invention;

[0027] Figure 7 A schematic diagram of the composition structure of a quantum key distribution time bit-phase encoding and decoding device based on polarization orthogonal rotation reflection according to a preferred embodiment of the present invention;

[0028] Figure 8 The present invention is a schematic diagram of the composition structure of a DC modulated quantum key distribution time bit-phase encoding and decoding device based on polarization orthogonal rotation reflection according to a preferred embodiment of the present invention. DETAILED DESCRIPTION

[0029] The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of this application and are used to explain the principles of the present invention together with the embodiments of the present invention. For the purpose of clarity and simplicity, detailed descriptions of known functions and structures of the devices described herein will be omitted when they may obscure the subject matter of the present invention.

[0030] A quantum key distribution phase codec based on polarization orthogonal rotation reflection according to a preferred embodiment of the present invention is as follows Figure 1 As shown, the following components are included: a beam splitter 101 , a phase modulator 102 , and two reflection devices 103 and 104 .

[0031] The two reflection devices 103 and 104 are respectively connected through two arms ( Figure 1 The phase modulator 102 is inserted into one of the two arms (in Figure 1 Middle, for the upper arm).

[0032] According to the present invention, both reflection devices 103 and 104 are polarization orthogonal rotation reflection devices.

[0033] Here, the polarization orthogonal rotation reflection device refers to a reflection device that can perform polarization orthogonal rotation reflection on two orthogonal polarization states of the reflected light pulse, that is, when reflecting the incident light pulse, each orthogonal polarization state of the light pulse is converted into a polarization state orthogonal thereto. For example, assuming that the two orthogonal polarization states are respectively an x ​​polarization state and a y polarization state, the x polarization state transmitted along the optical path to a polarization orthogonal rotation reflection device is converted into a polarization state orthogonal thereto, namely, a y polarization state, after polarization orthogonal rotation reflection at the reflection device, and the y polarization state transmitted along the optical path to the reflection device is converted into a polarization state orthogonal thereto, namely, an x ​​polarization state, after polarization orthogonal rotation reflection at the reflection device.

[0034] The beam splitter 101 is used to split an incident optical pulse of any polarization state into two optical pulses to be transmitted along two arms respectively.

[0035] The two arms are used to transmit the two optical pulses respectively.

[0036] The phase modulator 102 is used to perform phase modulation on the optical pulse (i.e., one of the two optical pulses) transmitted by the arm in which it is located according to the quantum key distribution protocol. The phase modulation performed by the phase modulator 102 is determined by the quantum key distribution protocol and depends on the specific application. For example, in one possible application, the phase modulator 102 can randomly modulate the 0 degree phase or the 90 degree phase.

[0037] The phase modulator 102 may be a polarization-independent phase modulator or a birefringent phase modulator. The birefringent phase modulator is suitable for applying different adjustable phase modulations to two orthogonal polarization states passing through it. For example, the birefringent phase modulator may be a lithium niobate phase modulator, and by controlling the voltage applied to the lithium niobate crystal, the phase modulations experienced by each of the two orthogonal polarization states passing through the lithium niobate phase modulator may be controlled and adjusted.

[0038] The reflection devices 103 and 104 are respectively used to reflect the two optical pulses transmitted from the beam splitter 101 via the two arms back to the beam splitter 101 so as to be combined and output by the beam splitter 101 .

[0039] Since both reflection devices 103 and 104 are polarization orthogonal rotation reflection devices, for each of the two optical pulses: when the optical pulse is reflected by the corresponding reflection device of the two reflection devices, the two orthogonal polarization states of the optical pulse are polarization orthogonally rotated and reflected, so that after being reflected by the corresponding reflection device, each orthogonal polarization state of the optical pulse is transformed into a polarization state orthogonal to it. In this way, for Figure 1The phase codec utilizes the polarization orthogonal rotation reflection at the polarization orthogonal rotation reflection device, and the phase difference of the x polarization state of the input light pulse transmitted through the two arms during the process from beam splitting by the beam splitter to beam combining by the beam splitter is exactly equal to the phase difference of the y polarization state of the light pulse transmitted through the two arms during the process from beam splitting by the beam splitter to beam combining by the beam splitter.

[0040] although Figure 1 In the figure, only one phase modulator 102 is shown as being arranged on one of the two arms, but it is also possible to arrange one phase modulator on each of the two arms. In the case where two phase modulators are arranged in this way, the difference between the phases modulated by the two phase modulators is determined by the quantum key distribution protocol, depending on the specific application. In addition, instead of arranging a phase modulator on one or both of the two arms, a phase modulator can be arranged before the beam splitter 101 to phase modulate the input optical pulse before beam splitting according to the quantum key distribution protocol, or to phase modulate the output optical pulse after beam combining according to the quantum key distribution protocol.

[0041] The present invention proposes three creative polarization orthogonal rotation reflection device structures, namely Structure 1, Structure 2 and Structure 3 described below.

[0042] According to configuration 1, the polarization orthogonal rotation reflection device includes a polarization beam splitter, the polarization beam splitter has an input port and two output ports, the two output ports of the polarization beam splitter are optically coupled to each other via a transmission optical path, the transmission optical path is formed by a polarization-maintaining optical fiber, a half-wave plate is arranged on the transmission optical path, and the angle between the polarization direction of the optical pulse input to the half-wave plate and the fast axis or the slow axis of the half-wave plate is 45 degrees. When the polarization orthogonal rotation reflection device with configuration 1 is used in the phase codec of the present invention, the reflection device can be coupled to the arm by coupling the input port of its polarization beam splitter to one arm of the phase codec.

[0043] According to construction 2, the polarization orthogonal rotation reflection device includes a polarization beam splitter, the polarization beam splitter has an input port and two output ports, the two output ports of the polarization beam splitter are optically coupled to each other via a transmission optical path, the transmission optical path is formed by a polarization-maintaining optical fiber, the slow axis and the fast axis of the polarization-maintaining optical fiber respectively maintain the two orthogonal polarization states of the light pulse input into the polarization-maintaining optical fiber for stable transmission, that is, the polarization state remains unchanged, and the two output ports of the polarization beam splitter and the polarization-maintaining optical fiber are constructed so that the light pulses output by the two output ports of the polarization beam splitter are both coupled to the slow axis of the polarization-maintaining optical fiber for transmission or are both coupled to the fast axis of the polarization-maintaining optical fiber for transmission. Here, the light pulses output by the two output ports of the polarization beam splitter are both coupled to the slow axis of the polarization-maintaining optical fiber for transmission or are both coupled to the fast axis of the polarization-maintaining optical fiber for transmission can be achieved by twisting the polarization-maintaining optical fiber by 90 degrees or (90+n*180) degrees, where n is an integer. Regardless of whether the polarization-maintaining fiber is twisted or not, the optical pulse input from the slow axis of the polarization-maintaining fiber is always transmitted along the slow axis (stable transmission along the slow axis), and the optical pulse input from the fast axis of the polarization-maintaining fiber is always transmitted along the fast axis (stable transmission along the fast axis). When the polarization orthogonal rotation reflection device with structure 2 is used in the phase codec of the present invention, the reflection device can be coupled to one arm of the phase codec by coupling the input port of its polarization beam splitter to the arm.

[0044] According to configuration 3, the polarization orthogonal rotation reflection device comprises a polarization beam splitter having an input port and two output ports, the two output ports of the polarization beam splitter being optically coupled to each other via a transmission optical path, the transmission optical path being formed by a polarization-maintaining optical fiber having an odd number of 90-degree fusion points, each 90-degree fusion point being formed by aligning and fusion-joining the slow axis of the polarization-maintaining optical fiber with the fast axis of the polarization-maintaining optical fiber. When the polarization orthogonal rotation reflection device having configuration 3 is used in the phase codec of the present invention, the reflection device can be coupled to the arm by coupling the input port of its polarization beam splitter to the arm of the phase codec.

[0045] Back to Figure 1In the phase codec, at least one of the reflection devices 103 and 104 can be a polarization orthogonal rotation reflection device adopting one of the above-mentioned structures 1, 2 and 3. When one of the reflection devices 103 and 104 is a polarization orthogonal rotation reflection device adopting one of the above-mentioned structures 1, 2 and 3, the other reflection device can be a polarization orthogonal rotation reflection device adopting one of the above-mentioned structures 1, 2 and 3, or a polarization orthogonal rotation reflection device of other structures. The polarization orthogonal rotation reflection device of other structures can be, for example, a quarter-wave plate reflector. The "quarter-wave plate reflector" includes a reflector and a quarter-wave plate, wherein the reflector is integrally formed with the quarter-wave plate at the rear end of the quarter-wave plate, wherein the polarization direction of one of the two orthogonal polarization states of the light pulse input to the quarter-wave plate is at an angle of 45 degrees with the fast axis or the slow axis of the quarter-wave plate. The quarter-wave plate reflector can be realized by plating a reflector on the surface of a quarter-wave plate crystal, or by plating a reflector on the end face of a polarization-maintaining optical fiber whose transmission phases of the fast and slow axes differ by 90 degrees.

[0046] for Figure 1 The phase codec can achieve the relative delay of the above-mentioned two optical pulses by adjusting the length of the two arms and / or adjusting the transmission light path in one or two reflecting devices of the two reflecting devices 103 and 104 using structures selected from Structure 1, Structure 2 and Structure 3.

[0047] In the case where the reflecting device adopts a structure selected from Structure 1, Structure 2 and Structure 3, the two arms of the phase codec can be configured as a polarization-maintaining optical path, such as a polarization-maintaining fiber optical path, and the optical devices on the two arms can be configured as polarization-maintaining optical devices and / or non-birefringent optical devices. In this way, for each of the two optical pulses obtained by splitting: the two orthogonal polarization states of the optical pulse can be kept unchanged during the period from the beam splitter to the corresponding reflecting device, and can be kept unchanged during the period from the corresponding reflecting device to the beam splitter. Generally, the polarization-maintaining optical path can be a free space optical path or a polarization-maintaining fiber optical path. In this article, "non-birefringent optical device" refers to an optical device that has the same refractive index for different polarization states (for example, two orthogonal polarization states). In addition, a polarization-maintaining optical device can also be called a polarization-maintaining optical device.

[0048] In addition, the beam splitter 101 of the phase codec may be a polarization-maintaining beam splitter.

[0049] A phase codec according to another preferred embodiment of the present invention is as follows Figure 2 As shown, the following components are included: a polarization-maintaining beam splitter 203 , a phase modulator 204 , and polarization orthogonal rotation reflection devices 205 and 206 .

[0050] One of the two ports 201 and 202 on one side of the polarization-maintaining beam splitter 203 serves as an input port of the phase codec. The polarization-maintaining beam splitter 203 and the polarization orthogonal rotation reflection devices 205 and 206 constitute an unequal-arm Michelson interferometer, and the two arms therebetween are polarization-maintaining fiber optical paths. The phase modulator 204 is inserted into any one of the two arms of the unequal-arm Michelson interferometer. The port 201 or 202 of the polarization-maintaining beam splitter 203 can serve as an output port of the phase codec.

[0051] During operation, the optical pulse enters the polarization-maintaining beam splitter 203 through the port 201 or 202 of the polarization-maintaining beam splitter 203 and is split into two optical pulses by the polarization-maintaining beam splitter 203. One optical pulse from the polarization-maintaining beam splitter 203 is phase-modulated by the phase modulator 204 and then reflected back by the polarization orthogonal rotation reflection device 205, and the other optical pulse is directly transmitted to the polarization orthogonal rotation reflection device 206 through the polarization-maintaining optical fiber and then reflected back by the polarization orthogonal rotation reflection device 206. The two optical pulses reflected back after a relative delay are combined by the polarization-maintaining beam splitter 203 and then output from the port 201 or 202.

[0052] In the case where the input port and one of the output ports of the polarization-maintaining beam splitter 203 are the same port, the phase codec may further include an optical circulator. The optical circulator may be located at the front end of the polarization-maintaining beam splitter 203. An incident input optical pulse of any polarization state may be input from the first port of the optical circulator and output from the second port of the optical circulator to the polarization-maintaining beam splitter 203, and the combined beam output from the polarization-maintaining beam splitter 203 is input to the second port of the optical circulator and output from the third port of the optical circulator.

[0053] Figure 3 A schematic diagram of the composition structure of a polarization orthogonal rotation reflection device that can be used in the phase codec of the present invention is shown.

[0054] Figure 3 The polarization orthogonal rotation reflection device shown includes the following components: a polarization beam splitter 302 and a polarization-maintaining optical fiber 303 .

[0055] The polarization beam splitter 302 includes three ports, namely, port A, port B, and port C. Port A, port B, and port C may be referred to as an input port, a first output port, and a second output port, respectively. Port 301 connected to port A of the polarization beam splitter 302 serves as both an input port and an output port of the reflection device. Port B and port C of the polarization beam splitter 302 are connected via a polarization-maintaining optical fiber 303. The optical pulses outputted from ports B and port C of the polarization beam splitter 302 are coupled to the slow axis transmission of the polarization-maintaining optical fiber 303 or are coupled to the fast axis transmission of the polarization-maintaining optical fiber.

[0056] During operation, an input optical pulse is input to the polarization beam splitter 302 via port 301, i.e., port A of the polarization beam splitter 302. The input optical pulse can be regarded as being composed of two orthogonal polarization states, which can be respectively recorded as x polarization state and y polarization state. The polarization beam splitter 302 polarizes the input optical pulse into a first optical pulse of x polarization state and a second optical pulse of y polarization state, which are outputted from ports B and C of the polarization beam splitter 302, respectively. The first optical pulse of x polarization state outputted from port B of the polarization beam splitter 302 is coupled to the slow axis of the polarization-maintaining optical fiber 303 for transmission, and is transmitted to port C of the polarization beam splitter 302 along the slow axis of the polarization-maintaining optical fiber 303. At port C, the first optical pulse is coupled to the polarization beam splitter 302 by the slow axis of the polarization-maintaining optical fiber 303, and the polarization state of the first optical pulse coupled to port C of the polarization beam splitter 302 is y polarization state; the first optical pulse of y polarization state is outputted from port A of the polarization beam splitter 302. That is, the x polarization state component of the input light pulse input from port A is transformed into the y polarization state when it is output from port A after being reflected by the reflection device. The second light pulse of the y polarization state output from port C of the polarization beam splitter 302 is coupled to the slow axis of the polarization-maintaining fiber 303 for transmission, and is transmitted to port B of the polarization beam splitter 302 along the slow axis of the polarization-maintaining fiber 303. At port B, the second light pulse is coupled to the polarization beam splitter 302 by the slow axis of the polarization-maintaining fiber 303, and the polarization state of the second light pulse coupled to port B of the polarization beam splitter 302 is the x polarization state; the second light pulse of the x polarization state is output from port A of the polarization beam splitter 302. That is, the y polarization state component of the input light pulse input from port A is transformed into the x polarization state when it is output from port A after being reflected by the reflection device. The reflection device realizes that when the two orthogonal polarization states of the input light pulse are reflected and output by the reflection device, each orthogonal polarization state is transformed into a polarization state orthogonal to it. The polarization-maintaining optical fiber 303 is used to perform polarization orthogonal rotation on the two orthogonal polarization states, so that the phase between the x polarization state and the y polarization state of the input optical pulse remains the same as the phase between the y polarization state and the x polarization state of the output optical pulse.

[0057] Port B and port C of the polarization beam splitter 302 can both be coupled to the fast axis of the polarization-maintaining fiber 303 , and the above results are not affected.

[0058] Figure 4 A schematic diagram of the composition structure of another polarization orthogonal rotation reflection device that can be used in the phase codec of the present invention is shown.

[0059] Figure 4 The polarization orthogonal rotation reflection device shown includes the following components: a polarization beam splitter 402 , a polarization-maintaining optical fiber 403 , and a 90-degree fusion point 404 .

[0060] The polarization beam splitter 402 includes three ports, namely, port A, port B, and port C. Port A, port B, and port C may be referred to as an input port, a first output port, and a second output port, respectively. Port 401 connected to port A of the polarization beam splitter 402 serves as both an input port and an output port of the device. Port B and port C of the polarization beam splitter 402 are connected via a polarization-maintaining fiber 403. An optical pulse outputted from port B of the polarization beam splitter 402 is coupled to the slow axis of the polarization-maintaining fiber 403 and an optical pulse outputted from port C of the polarization beam splitter 402 is coupled to the fast axis of the polarization-maintaining fiber 403, or an optical pulse outputted from port B of the polarization beam splitter 402 is coupled to the fast axis of the polarization-maintaining fiber 403 and an optical pulse outputted from port C of the polarization beam splitter 402 is coupled to the slow axis of the polarization-maintaining fiber 403. The polarization-maintaining fiber 403 includes a 90-degree fusion splice 404, which is formed by aligning and fusion splicing the slow axis of the polarization-maintaining fiber with the fast axis of the polarization-maintaining fiber.

[0061] During operation, an input optical pulse is input to the polarization beam splitter 402 via port 401, i.e., port A of the polarization beam splitter 402. The input optical pulse can be regarded as being composed of two orthogonal polarization states, which can be respectively recorded as an x ​​polarization state and a y polarization state. The polarization beam splitter 402 polarizes the input optical pulse into a first optical pulse of an x ​​polarization state and a second optical pulse of a y polarization state, and outputs them from ports B and C of the polarization beam splitter 402, respectively. The first optical pulse in the x polarization state outputted from port B of the polarization beam splitter 402 is coupled to the slow axis of the polarization-maintaining optical fiber 403 and transmitted to the 90-degree fusion point 404. After passing through the 90-degree fusion point 404, it is transmitted along the fast axis of the polarization-maintaining optical fiber 403 to port C of the polarization beam splitter 402. At port C, the first optical pulse is coupled to the polarization beam splitter 402 by the fast axis of the polarization-maintaining optical fiber 403. The polarization state of the first optical pulse coupled to port C of the polarization beam splitter 402 is the y polarization state, and the first optical pulse in the y polarization state is outputted from port A of the polarization beam splitter 402. That is, the x polarization state component of the input optical pulse inputted from port A is transformed into the y polarization state when it is outputted from port A after being reflected by the device. The second optical pulse of the y polarization state outputted from port C of the polarization beam splitter 402 is coupled to the fast axis of the polarization-maintaining optical fiber 403 and transmitted to the 90-degree fusion point 404. After passing through the 90-degree fusion point 404, it is transmitted along the slow axis of the polarization-maintaining optical fiber 403 to port B of the polarization beam splitter 402. At port B, the second optical pulse is coupled to the polarization beam splitter 402 by the slow axis of the polarization-maintaining optical fiber 403. The polarization state of the second optical pulse coupled to port B of the polarization beam splitter 402 is the x polarization state, and the second optical pulse of the x polarization state is outputted from port A of the polarization beam splitter 402. That is, the y polarization state component of the input optical pulse inputted from port A is transformed into the x polarization state when it is outputted from port A after being reflected by the device. The polarization orthogonal rotation reflection device realizes that when the two orthogonal polarization states of the input optical pulse are reflected and outputted by the device, each orthogonal polarization state is transformed into a polarization state orthogonal thereto.

[0062] although Figure 4 Only one 90-degree fusion point 404 is shown in the figure, but this is only exemplary, and the polarization-maintaining fiber 403 may include any odd number of 90-degree fusion points. Each 90-degree fusion point is formed by aligning and fusion-joining the slow axis of the polarization-maintaining fiber with the fast axis of the polarization-maintaining fiber. In the case where the polarization-maintaining fiber 403 includes more than one odd number of 90-degree fusion points, the above results are not affected, except that the first optical pulse and the second optical pulse outputted from the port B and the port C of the polarization beam splitter 402 are respectively switched between the transmission along the slow axis of the polarization-maintaining fiber and the transmission along the fast axis of the polarization-maintaining fiber more times when they are transmitted along the polarization-maintaining fiber 403, and the number of switching is equal to the number of 90-degree fusion points.

[0063] The polarization-maintaining optical fiber 403 including an odd number of 90-degree fusion points is used to perform polarization orthogonal rotation on two orthogonal polarization states, so that the phase between the x polarization state and the y polarization state of the input optical pulse remains the same as the phase between the y polarization state and the x polarization state of the output optical pulse.

[0064] When port B of the polarization beam splitter 402 is coupled to the fast axis of the polarization-maintaining fiber 403 and port C of the polarization beam splitter 402 is coupled to the slow axis of the polarization-maintaining fiber 403 , the above results are not affected.

[0065] Figure 5 A schematic diagram of the composition structure of another polarization orthogonal rotation reflection device that can be used in the phase codec of the present invention is shown.

[0066] Figure 5 The polarization orthogonal rotation reflection device shown includes the following components: a polarization beam splitter 502 and a half-wave plate 503 .

[0067] The polarization beam splitter 502 includes three ports, namely, port A, port B, and port C. Port A, port B, and port C may be referred to as an input port, a first output port, and a second output port, respectively. Port 501 connected to port A of the polarization beam splitter 502 serves as both an input port and an output port of the device. Port B of the polarization beam splitter 502 is connected to port D of the half-wave plate 503 through a transmission optical path, and port C of the polarization beam splitter 502 is connected to port E of the half-wave plate 503 through a transmission optical path. The transmission optical path connecting port B of the polarization beam splitter 502 to port D of the half-wave plate 503 and the transmission optical path connecting port C of the polarization beam splitter 502 to port E of the half-wave plate 503 are both polarization-maintaining optical paths, such as polarization-maintaining optical fiber paths. The angle between the polarization direction of the polarization state of the light pulse input to the half-wave plate 503 from ports D and E of the half-wave plate 503 and the slow axis or fast axis of the half-wave plate 503 is 45 degrees.

[0068] During operation, the input light pulse is input into the polarization beam splitter 502 via port 501, i.e., port A of the polarization beam splitter 502. The input light pulse can be regarded as being composed of two orthogonal polarization states, which can be respectively recorded as the x polarization state and the y polarization state. The polarization beam splitter 502 polarizes the input light pulse into a first light pulse in the x polarization state and a second light pulse in the y polarization state, which are outputted from ports B and C of the polarization beam splitter 502, respectively. The first light pulse in the x polarization state outputted from port B of the polarization beam splitter 502 is transmitted to the half-wave plate 503, and the polarization state of the first light pulse is transformed into the y polarization state after being orthogonally rotated by the half-wave plate 503. The first light pulse in the y polarization state outputted from port E of the half-wave plate 503 is transmitted to port C of the polarization beam splitter and inputted into the polarization beam splitter 502, and is outputted from port A of the polarization beam splitter 502. In this way, the x polarization state component of the input light pulse input from port A is transformed into the y polarization state when it is output from port A after being reflected by the device. The second light pulse in the y polarization state output from port C of the polarization beam splitter 502 is transmitted to the half-wave plate 503, and the polarization state of the second light pulse is transformed into the x polarization state after being orthogonally rotated by the half-wave plate 503. The second light pulse in the x polarization state output from port D of the half-wave plate 503 is transmitted to port B of the polarization beam splitter and input to the polarization beam splitter 502, and is output from port A of the polarization beam splitter 502. In this way, the y polarization state component of the input light pulse input from port A is transformed into the x polarization state when it is output from port A after being reflected by the device. The polarization orthogonal rotation reflection device realizes that when the two orthogonal polarization states of the input light pulse are reflected and output by the device, each orthogonal polarization state is transformed into a polarization state orthogonal thereto. The half-wave plate 503 is used to perform polarization orthogonal rotation on the two orthogonal polarization states, so that the phase between the x polarization state and the y polarization state of the input light pulse and the phase between the y polarization state and the x polarization state of the output light pulse remain the same.

[0069] The phase codec of the present invention can be used as a component of a DC modulation quantum key distribution phase codec device, can be used as a component of a quantum key distribution time bit-phase codec device, and can also be used as a component of a DC modulation quantum key distribution time bit-phase codec device.

[0070] A phase encoding and decoding device for direct current modulation quantum key distribution based on polarization orthogonal rotation reflection using the phase encoder and decoder of the present invention is as follows Figure 6 As shown, the following components are included: a pre-beam splitter 603, optical circulators 604 and 611, polarization-maintaining beam splitters 605 and 612, DC phase modulators 606 and 613, and polarization orthogonal rotation reflection devices 607, 608, 614 and 615.

[0071] The polarization-maintaining beam splitter 605, two polarization orthogonal rotating reflection devices 607 and 608, and two arms between the polarization-maintaining beam splitter 605 and the two polarization orthogonal rotating reflection devices constitute a first polarization-maintaining unequal-arm Michelson interferometer, i.e., the first phase codec according to the present invention. The two arms of the first phase codec are polarization-maintaining optical fiber paths. The DC phase modulator 606 is located on either arm of the two arms of the first phase codec.

[0072] Similarly, the polarization-maintaining beam splitter 612, the two polarization orthogonal rotating reflection devices 614 and 615, and the two arms between the polarization-maintaining beam splitter 612 and the two polarization orthogonal rotating reflection devices constitute a second polarization-maintaining unequal-arm Michelson interferometer, i.e., the second phase codec according to the present invention. The two arms of the second phase codec are polarization-maintaining optical fiber paths. The DC phase modulator 613 is located on any one of the two arms of the second phase codec.

[0073] Below, Figure 6 An example is given by taking the encoding and decoding device as an example for decoding.

[0074] On the front beam splitter 603 side ( Figure 6 One of the two ports 601 and 602 of the optical circulator 604 (left side in the figure) is used as the input port of the device. The first port A and the second port B of the optical circulator 604 are respectively connected to an output port of the pre-beam splitter 603 and an input port of the polarization-maintaining beam splitter 605. The optical pulse input to the first phase codec is decoded and output by an output port 609 of the polarization-maintaining beam splitter 605, or transmitted to the port B of the optical circulator 604 through another output port of the polarization-maintaining beam splitter 605 (i.e., the one input port of the polarization-maintaining beam splitter 605) and output from the third port C of the optical circulator 604. The first port A and the second port B of the optical circulator 611 are respectively connected to another output port of the pre-beam splitter 603 and an input port of the polarization-maintaining beam splitter 612. The optical pulse input to the second phase codec is decoded and output from an output port 616 of the polarization-maintaining beam splitter 612, or is transmitted to port B of the optical circulator 611 through another output port of the polarization-maintaining beam splitter 612 (i.e., the one input port of the polarization-maintaining beam splitter 612) and output from the third port C of the optical circulator 611.

[0075] During operation, the optical pulse enters the beam splitter 603 through the port 601 or 602 of the beam splitter 603 and is split into a first optical pulse and a second optical pulse by the beam splitter 603. The first optical pulse is input through the port A of the optical circulator 604 and output to the polarization-maintaining beam splitter 605 through the port B of the optical circulator 604. The polarization-maintaining beam splitter 605 splits the input first optical pulse into two first sub-optical pulses. One first sub-optical pulse is phase modulated by the DC phase modulator 606 and reflected back by the polarization orthogonal rotation reflection device 607, and the other first sub-optical pulse is directly transmitted to the polarization orthogonal rotation reflection device 608 through the polarization-maintaining optical fiber and reflected back by the polarization orthogonal rotation reflection device 608. The two first sub-optical pulses reflected back after relative delay are combined by the polarization-maintaining beam splitter 605 and output from the port 609, or are output to the port B of the optical circulator 604 and transmitted to the port C and output from the port 610. The second optical pulse is input through port A of the optical circulator 611 and output from port B of the optical circulator 611 to the polarization-maintaining beam splitter 612. The polarization-maintaining beam splitter 612 splits the input second optical pulse into two second sub-optical pulses. One second sub-optical pulse is phase-modulated by the DC phase modulator 613 and then reflected back by the polarization orthogonal rotation reflection device 614, and the other second sub-optical pulse is directly transmitted to the polarization orthogonal rotation reflection device 615 through the polarization-maintaining optical fiber and then reflected back by the polarization orthogonal rotation reflection device 615. The two second sub-optical pulses reflected back after relative delay are combined by the polarization-maintaining beam splitter 612 and then output from port 616, or are output to port B of the optical circulator 611 and transmitted to port C and output from port 617. The DC phase modulators 606 and 613 cause the DC phase modulation made by one of the first phase codec and the second phase codec to differ by 90 degrees relative to the DC phase modulation made by the other.

[0076] Next, Figure 6 An example is given by taking the encoding and decoding device used for encoding as an example to describe it.

[0077] A port 609 of the polarization-maintaining beam splitter 605, a third port C of the optical circulator 604, a port 616 of the polarization-maintaining beam splitter 612, and a third port C of the optical circulator 611 are used as input ports of the device. A first port A and a second port B of the optical circulator 604 are respectively connected to a port of the pre-beam splitter 603 and another port of the polarization-maintaining beam splitter 605. The optical pulse input from the third port C of the optical circulator 604 is input to the first phase codec via the second port B of the optical circulator 604. The optical pulse input from the one port 609 of the polarization-maintaining beam splitter 605 and the third port C of the optical circulator 604 is encoded by the first phase codec and output by the polarization-maintaining beam splitter 605 to the second port B of the optical circulator 604 and transmitted from the first port A of the optical circulator 604 to the pre-beam splitter 603. The first port A and the second port B of the optical circulator 611 are respectively connected to another port of the pre-beam splitter 603 and another port of the polarization-maintaining beam splitter 612. The optical pulse input from the third port C of the optical circulator 611 is input to the second phase codec through the second port B of the optical circulator 611. The optical pulse input from the one port 616 of the polarization-maintaining beam splitter 612 and the third port C of the optical circulator 612 is encoded by the second phase codec and then output from the polarization-maintaining beam splitter 612 to the second port B of the optical circulator 611 and transmitted from the first port A of the optical circulator 611 to the pre-beam splitter 603. One side of the pre-beam splitter 603 ( Figure 6 One of the two ports 601 and 602 (left side in the figure) is used as the output port of the device. The optical pulses inputted from the one port 609 of the polarization-maintaining beam splitter 605, the third port C of the optical circulator 604, the one port 616 of the polarization-maintaining beam splitter 612, and the third port C of the optical circulator 611 are encoded to realize four types of phase encoding respectively, and the encoded optical pulses are combined by the beam splitter 603 and outputted from the port 601 or 602.

[0078] A quantum key distribution time bit-phase encoding and decoding device based on polarization orthogonal rotation reflection using the phase encoder and decoder of the present invention is as follows Figure 7 As shown, the following components are included: beam splitters 703 and 704, a polarization-maintaining beam splitter 707, a phase modulator 708, and polarization orthogonal rotation reflection devices 709 and 710.

[0079] The polarization-maintaining beam splitter 707, two polarization orthogonal rotating reflection devices 709 and 710, and two arms between the polarization-maintaining beam splitter 707 and the two polarization orthogonal rotating reflection devices constitute a polarization-maintaining unequal-arm Michelson interferometer, i.e., a phase codec according to the present invention. The two arms are polarization-maintaining optical fiber paths. The phase modulator 708 is located on either arm of the two arms of the phase codec.

[0080] Below, Figure 7An example is given by taking the encoding and decoding device as an example for decoding.

[0081] The beam splitter 703 is used as a pre-beam splitter, and one of the two ports 701 and 702 on one side thereof is used as the input port of the device. The beam splitter 704 splits a light pulse from the beam splitter 703 and outputs it through port 705 or 706. The light pulse input to the polarization-maintaining unequal-arm Michelson interferometer is decoded and output from port 711.

[0082] During operation, the input optical pulse enters the beam splitter 703 through the port 701 or 702 of the beam splitter 703, and is split into two optical pulses by the beam splitter 703 for transmission. One optical pulse from the beam splitter 703 is input to the beam splitter 704, and is split by the beam splitter 704 and output through the port 705 or 706 for time bit decoding. Another optical pulse from the beam splitter 703 is input to the polarization-maintaining beam splitter 707, and is split into two sub-optical pulses by the polarization-maintaining beam splitter 707. One sub-optical pulse is randomly modulated by the phase modulator 708 to 0 degrees or 180 degrees and then reflected by the polarization orthogonal rotation reflection device 709, and the other optical pulse is directly transmitted to the polarization orthogonal rotation reflection device 710 through the polarization-maintaining optical fiber and reflected by the polarization orthogonal rotation reflection device 710. The two sub-optical pulses reflected back after relative delay are combined by the polarization-maintaining beam splitter 707 and output from the port 711.

[0083] Here, it should be noted that the beam splitter 704 is optional. It is possible for the pre-beam splitter 703 to directly output the above optical pulse for time bit decoding.

[0084] Next, Figure 7 An example is given by taking the encoding and decoding device used for encoding as an example to describe it.

[0085] Ports 705 and 706 of beam splitter 704 and port 711 of polarization-maintaining beam splitter 707 serve as input ports of the device. Optical pulses input from ports 705 and 706 are combined by beam splitter 704 and output to pre-beam splitter 703 to achieve time bit encoding. Optical pulses input from port 711 are encoded by polarization-maintaining unequal-arm Michelson interferometer and output to pre-beam splitter 703 by polarization-maintaining beam splitter 707, during which two types of phase encoding are achieved by modulating phase modulator 708. One of ports 701 and 702 of pre-beam splitter 703 serves as an output port of the device. Beam splitter 703 combines the optical pulses output by beam splitter 704 and the optical pulses output by polarization-maintaining beam splitter 707 and outputs them from port 701 or 702.

[0086] The beam splitter 704 is optional, and it is possible to directly use the port of the beam splitter 703 connected to the beam splitter 704 as an input port for time bit encoding.

[0087] A time bit-phase encoding and decoding device for direct current modulation quantum key distribution based on polarization orthogonal rotation reflection using the phase encoder and decoder of the present invention is as follows Figure 8 As shown, the following components are included: beam splitters 803 and 804, an optical circulator 807, a polarization-maintaining beam splitter 808, a DC phase modulator 809, and polarization orthogonal rotation reflection devices 810 and 811.

[0088] The polarization-maintaining beam splitter 808, two polarization orthogonal rotating reflection devices 810 and 811, and two arms between the polarization-maintaining beam splitter 808 and the two polarization orthogonal rotating reflection devices constitute a polarization-maintaining unequal-arm Michelson interferometer, i.e., a phase codec according to the present invention. The two arms are polarization-maintaining optical fiber paths. The DC phase modulator 809 is located on either arm of the two arms of the phase codec.

[0089] Below, Figure 8 An example is given by taking the encoding and decoding device as an example for decoding.

[0090] The beam splitter 803 is used as a pre-beam splitter, and one of the two ports 801 and 802 on one side thereof is used as the input port of the device. The beam splitter 804 splits a path of optical pulses from the beam splitter 803 and outputs them from the port 805 or 806. The optical pulse input from the first port A of the optical circulator 807 is output from the second port B of the optical circulator 807, and the optical pulse input from the second port B of the optical circulator 807 is output from the third port C of the optical circulator 807. The optical pulse input to the polarization-maintaining unequal-arm Michelson interferometer is output from the port 812 after being decoded, or is transmitted to the second port B of the optical circulator 807 through another output port of the polarization-maintaining beam splitter 808 and output from the third port C of the optical circulator 807 and then output from the port 813.

[0091] During operation, the input optical pulse enters the beam splitter 803 through the port 801 or 802 of the beam splitter 803, and is split into two optical pulses by the beam splitter 803 for transmission. One optical pulse from the beam splitter 803 is input to the beam splitter 804, and is split by the beam splitter 804 and output from the port 805 or 806 for time bit decoding. Another optical pulse from the beam splitter 803 is input through the first port A of the optical circulator 807 and is output from the second port B of the optical circulator 807 to the polarization-maintaining beam splitter 808. The polarization-maintaining beam splitter 808 splits the other optical pulse into two sub-optical pulses. One sub-optical pulse is modulated by the DC phase modulator 809 to a 0-degree phase or a 180-degree phase and then reflected back by the polarization orthogonal rotation reflection device 810, and the other optical pulse is directly transmitted to the polarization orthogonal rotation reflection device 811 through the polarization-maintaining optical fiber and reflected back by the polarization orthogonal rotation reflection device 811. The two reflected light pulses with relative delay are combined by the polarization-maintaining beam splitter 808 and output from the port 812 , or are transmitted to the second port B of the optical circulator 807 and output from the third port C of the optical circulator 807 and then output from the port 813 .

[0092] Here, it should be noted that the beam splitter 804 is optional. It is possible for the pre-beam splitter 803 to directly output the above optical pulse for time bit decoding.

[0093] Next, Figure 8 An example is given by taking the encoding and decoding device used for encoding as an example to describe it.

[0094] Ports 805 and 806 of the beam splitter 804, port 812 of the polarization-maintaining beam splitter 808, and the third port C of the optical circulator 807 serve as input ports of the device. The optical pulse input from the third port C of the optical circulator 807 is output from the second port B of the optical circulator 807, and the optical pulse input from the second port B of the optical circulator 807 is output from the first port A of the optical circulator 807. The optical pulses input from ports 805 and 806 are combined by the beam splitter 804 and output to the pre-beam splitter 803 to achieve time bit encoding. The optical pulses input from port 812 and the optical pulses input from the third port C of the optical circulator 807 and output from the second port B of the optical circulator 807 to the polarization-maintaining beam splitter 808 are encoded by the polarization-maintaining unequal-arm Michelson interferometer, and are output from the polarization-maintaining beam splitter 808 to the second port B of the optical circulator 807 and transmitted to the pre-beam splitter 803 through the first port A of the optical circulator 807. The optical pulses inputted from the port 812 of the polarization-maintaining beam splitter 808 and the third port C of the optical circulator 807 are encoded to realize two phase encodings respectively. One of the ports 801 and 802 of the beam splitter 803 is used as the output port of the device. The beam splitter 803 combines the optical pulse outputted from the beam splitter 804 and the optical pulse outputted from the first port A of the optical circulator 807 and outputs them from the port 801 or 802.

[0095] The beam splitter 804 is optional, and it is possible to directly use the port of the beam splitter 803 connected to the beam splitter 804 as an input port for time bit encoding.

[0096] although Figure 1-2 and Figure 6-8 A phase modulator is shown in FIG. 1 , but it is possible that the phase codec and the codec device of the present invention do not include a phase modulator.

[0097] In this article, the terms "beam splitter" and "beam combiner" are used interchangeably, and a beam splitter can also be called and used as a beam combiner, and vice versa. In this article, "polarization-maintaining fiber optical path" refers to an optical path that uses polarization-maintaining optical fibers to transmit optical pulses or an optical path formed by connecting polarization-maintaining optical fibers.

[0098] The phase codec based on polarization orthogonal rotation reflection or the corresponding codec device of the present invention as described above can be configured at the receiving end of the quantum key distribution system for decoding. In addition, the phase codec based on polarization orthogonal rotation reflection or the corresponding codec device of the present invention as described above can also be configured at the transmitting end of the quantum key distribution system for encoding. When the phase codec or the corresponding codec device of the present invention is used at the receiving end or the transmitting end of the quantum key distribution system, it can include the above-mentioned combination of Figure 1-2 and Figure 6-8 The phase modulator described exemplarily may not include a phase modulator. In addition, when both the receiving end and the transmitting end of the quantum key distribution system adopt the phase codec or the codec device of the present invention, the phase codec or the codec device used for at least one of the receiving end and the transmitting end may include a phase modulator.

[0099] Through the description of the specific implementation methods, one should be able to have a more in-depth and specific understanding of the technical means and effects adopted by the present invention to achieve the predetermined purpose. However, the accompanying drawings are only for reference and illustration purposes and are not intended to limit the present invention.

Claims

1. A quantum key distribution phase codec, comprising: A beam splitter, two reflecting devices optically coupled to the beam splitter via two arms respectively, each of the reflecting devices being a polarization orthogonal rotation reflecting device, each of the reflecting devices comprising a polarization beam splitter having an input port and two output ports, and coupled to corresponding arms of the two arms via the input port of the polarization beam splitter, wherein the two output ports of each polarization beam splitter are optically coupled to each other via a transmission optical path, a reflecting device comprising a polarization beam splitter is provided with a half-wave plate on its transmission optical path, the polarization direction of the optical pulse input to the half-wave plate and the angle between the fast axis or the slow axis of the half-wave plate is 45 degrees, and for another reflecting device comprising a polarization beam splitter: its transmission optical path is formed by a polarization-maintaining optical fiber, and the two output ports of the polarization beam splitter and the polarization-maintaining optical fiber included therein are constructed so that the optical pulses outputted from the two output ports of the polarization beam splitter are both coupled to the slow axis of the polarization-maintaining optical fiber for transmission or are both coupled to the fast axis of the polarization-maintaining optical fiber for transmission, wherein the polarization beam splitter is provided with a half-wave plate on its transmission optical path, and the polarization direction of the optical pulse input to the half-wave plate is at an angle of 45 degrees to the fast axis or the slow axis of the half-wave plate, and the polarization beam splitter is provided with a half-wave plate on its transmission optical path, and the polarization-maintaining optical fiber is provided with a polarization-maintaining optical fiber ... The optical pulses output from the two output ports of the beam splitter are both coupled to the slow axis of the polarization-maintaining optical fiber for transmission or are both coupled to the fast axis of the polarization-maintaining optical fiber for transmission, which is achieved by twisting the polarization-maintaining optical fiber by 90 degrees or (90+n*180) degrees, wherein n is an integer, the optical pulse output from one output port of the polarization beam splitter is a first optical pulse in an x-polarization state, the first optical pulse in an x-polarization state output from the one output port is coupled to the other output port of the polarization beam splitter, the polarization state of the first optical pulse coupled to the other output port is a y-polarization state, the optical pulse output from the other output port of the polarization beam splitter is a second optical pulse in a y-polarization state, the second optical pulse in a y-polarization state output from the other output port is coupled to the one output port of the polarization beam splitter, the polarization state of the second optical pulse coupled to the one output port is an x-polarization state, and the x-polarization state and the y-polarization state are two orthogonal polarization states.

2. The phase codec according to claim 1, wherein: The transmission optical path is a polarization maintaining optical path.

3. The phase codec according to claim 1, wherein: The beam splitter is a polarization-maintaining beam splitter.

4. The phase codec according to claim 1, wherein: Each of the two arms is a polarization-maintaining optical path, and the optical devices on the two arms are polarization-maintaining optical devices and / or non-birefringent optical devices.

5. The phase codec according to any one of claims 1 to 4, wherein: The phase codec further comprises a phase modulator, which is arranged at the front end of the beam splitter or on at least one of the two arms.

6. A DC modulated quantum key distribution phase encoding and decoding device, comprising a pre-beam splitter and two phase codecs according to any one of claims 1 to 5, wherein the two phase codecs are optically coupled to the pre-beam splitter via two sub-optical paths respectively, wherein one of the ports of the beam splitter of each phase codec that is not coupled to the two arms of the phase codec is optically coupled to a corresponding sub-optical path of the two sub-optical paths, and an optical circulator is arranged on each of the sub-optical paths.

7. A quantum key distribution time bit-phase encoding and decoding device, comprising a pre-beam splitter and a phase codec according to any one of claims 1-5, wherein the phase codec is optically coupled to the pre-beam splitter via a sub-optical path, wherein one of the ports of the beam splitter of the phase codec that is not coupled to the two arms is optically coupled to the sub-optical path.

8. The encoding and decoding device according to claim 7, further comprising a beam splitter coupled to the pre-beam splitter via another optical sub-path.

9. A DC modulated quantum key distribution time bit-phase encoding and decoding device, comprising a pre-beam splitter and a phase codec according to any one of claims 1-5, wherein the phase codec is optically coupled to the pre-beam splitter via a sub-optical path, wherein one of the ports of the beam splitter of the phase codec that is not coupled to the two arms is optically coupled to the sub-optical path, wherein an optical circulator is arranged on the sub-optical path.

10. The encoding and decoding device according to claim 9, further comprising a beam splitter coupled to the pre-beam splitter via another optical sub-path.

11. A quantum key distribution system, comprising: The phase codec according to any one of claims 1 to 5 or the codec device according to any one of claims 6 to 10, which is arranged at the receiving end of the quantum key distribution system for decoding; and / or The phase codec according to any one of claims 1 to 5 or according to any one of claims 6 to 10 The encoding and decoding device described in any one of the above is arranged at the transmitting end of the quantum key distribution system, Used for encoding.

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