Quantum key distribution phase codec, corresponding coding and decoding 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. 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. The two output ports of each polarization beam splitter are optically coupled to each other via a polarization-maintaining fiber twisted 90 degrees, so that the light pulses output by the two output ports are both coupled to the slow axis transmission of the polarization-maintaining fiber or to the fast axis transmission of the polarization-maintaining fiber. The present invention enables stable encoding and decoding interference of input light pulses of arbitrary polarization states, solving the problem of polarization-induced fading causing the system to be unable to operate stably in phase-encoded and time-bit-phase-encoded 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 encoding and decoding device including the phase codec, and a quantum key distribution system. Background Art

[0002] Quantum secure communication technology is a cutting-edge field at the intersection of quantum physics and information science. Based on quantum key distribution (QKD) and the principle of one-time pad cryptography, QKD enables secure information transmission over public channels. Based on physical principles such as the Heisenberg uncertainty relation in quantum mechanics and the quantum no-cloning theorem, QKD enables secure key sharing between users and can detect potential eavesdropping. It has applications in sectors requiring high-security information transmission, such as defense, government affairs, finance, and the power sector.

[0003] Ground-based quantum key distribution is mainly based on fiber optic channel transmission. Since phase coding uses the phase difference between the previous and next light pulses to encode information, it can be stably maintained during long-distance fiber optic channel transmission. Therefore, phase coding and time bit-phase coding based on unequal-arm interferometers 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 the actual environment, which will produce random birefringence effects. Therefore, after the light pulse is transmitted through a long-distance optical fiber and transmitted through the two-arm optical fiber 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, resulting in unstable decoding interference and an increase in the bit error rate. If a correction device is used, the system complexity and cost will increase, 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, 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, and for at least one reflecting device including a polarization beam splitter: its transmission optical path is formed by a polarization-maintaining optical fiber twisted 90 degrees, so that the optical pulses output by the two output ports of its 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.

[0008] 2. The phase codec according to solution 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 polarization-maintaining fiber twisted by 90 degrees comprises a polarization-maintaining fiber twisted by 90 degrees or twisted by (90+n*180) degrees, where n is an integer.

[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, wherein 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 provided on each sub-optical path.

[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 provided 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 encoding and decoding device according to any one of solutions 7 to 10 is provided at the receiving end of the quantum key distribution system for decoding; and / or

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

[0020] Through its innovative construction, the present invention enables stable encoding and decoding interferometry for input optical pulses of arbitrary polarization states, thereby achieving unexpected beneficial effects. Utilizing the present invention's solution, stable interferometry output can be achieved at the phase decoding interferometer for input optical pulses of arbitrary polarization states, resolving the issue of polarization-induced fading causing system instability in phase-encoding and time-bit-phase-encoding quantum key distribution applications. The present invention provides a phase-encoding and time-bit-phase-encoding quantum key distribution decoding scheme that is resistant to polarization-induced fading and is 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 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 structure of a polarization orthogonal rotation reflection device that can be used in the phase codec of the present invention;

[0024] Figure 4 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;

[0025] Figure 5 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;

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

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

[0028] Figure 8 This 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 in a preferred embodiment of the present invention. DETAILED DESCRIPTION

[0029] The preferred embodiments of the present invention are described below in detail with reference to the accompanying drawings, which constitute a part of this application and, together with the embodiments of the present invention, serve to illustrate the principles 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 reflecting devices 103 and 104 .

[0031] The two reflecting devices 103 and 104 are respectively connected through two arms ( Figure 1 The upper and lower arms in the beam splitter 101 are optically coupled to the beam splitter 101. The phase modulator 102 is inserted into one of the two arms (in Figure 1 Middle, upper arm).

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

[0033] Here, a polarization orthogonal rotation reflector refers to a reflector capable of performing polarization orthogonal rotation reflection on two orthogonal polarization states of a reflected light pulse, that is, when reflecting an incident light pulse, converting each orthogonal polarization state of the light pulse into an orthogonal polarization state. For example, assuming the two orthogonal polarization states are x-polarization state and y-polarization state, the x-polarization state transmitted along an optical path to a polarization orthogonal rotation reflector is converted to an orthogonal polarization state, i.e., the y-polarization state, after undergoing polarization orthogonal rotation reflection at the reflector. The y-polarization state transmitted along an optical path to the reflector is converted to an orthogonal polarization state, i.e., the x-polarization state, after undergoing polarization orthogonal rotation reflection at the reflector.

[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] Phase modulator 102 is used to phase-modulate the optical pulse transmitted by its arm (i.e., one of the two optical pulses) according to the quantum key distribution protocol. The phase modulation performed by phase modulator 102 is determined by the quantum key distribution protocol and depends on the specific application. For example, in one possible application, phase modulator 102 may randomly modulate the phase to 0 degrees or 90 degrees.

[0037] Phase modulator 102 may be a polarization-independent phase modulator or a birefringent phase modulator. A 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. 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 can 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 through the two arms back to the beam splitter 101 so as to be combined and output by the beam splitter 101 .

[0039] Since both reflecting devices 103 and 104 are polarization orthogonal rotation reflecting devices, for each of the two optical pulses: when the optical pulse is reflected by the corresponding reflecting device of the two reflecting devices, the two orthogonal polarization states of the optical pulse are polarization orthogonally rotated and reflected, so that after being reflected by the corresponding reflecting 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 polarization orthogonal rotation reflection at the polarization orthogonal rotation reflection device, so that 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 provided on one of the two arms, but it is also possible to provide a phase modulator on each arm. When two phase modulators are provided, the difference in phase modulated by the two phase modulators is determined by the quantum key distribution protocol, depending on the specific application. Alternatively, instead of providing a phase modulator on one or both of the two arms, a phase modulator can be provided before the beam splitter 101 to phase modulate the input optical pulses before beam splitting according to the quantum key distribution protocol, or to phase modulate the output optical pulses 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 and reflection device includes a polarization beam splitter having 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 formed by polarization-maintaining fiber. A half-wave plate is disposed on the transmission optical path. The polarization direction of the optical pulse input to the half-wave plate forms an angle of 45 degrees with the fast axis or slow axis of the half-wave plate. When the polarization orthogonal rotation and reflection device having Configuration 1 is used in the phase codec of the present invention, the device can be coupled to one arm of the phase codec by coupling the input port of the polarization beam splitter to the arm.

[0043] According to construction 2, the polarization orthogonal rotation and reflection device includes a polarization beam splitter, which 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 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. The two output ports of the polarization beam splitter and the polarization-maintaining optical fiber are configured such that the light pulses output 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. Here, the light pulses output 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 by twisting the polarization-maintaining optical fiber by 90 degrees or by twisting it by (90+n*180) degrees, where n is an integer. Regardless of whether the polarization-maintaining fiber is twisted or untwisted, optical pulses input from the slow axis of the polarization-maintaining fiber are always transmitted along the slow axis (stable transmission along the slow axis), and optical pulses input from the fast axis of the polarization-maintaining fiber are always transmitted along the fast axis (stable transmission along the fast axis). When the polarization-orthogonal rotating reflector having Configuration 2 is used in the phase codec of the present invention, the reflector 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 and reflection device includes a polarization beam splitter having 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 formed by a polarization-maintaining fiber containing an odd number of 90-degree splices, each 90-degree splice being formed by aligning the slow axis of the polarization-maintaining fiber with the fast axis of the polarization-maintaining fiber. When the polarization-orthogonal rotation and reflection device having Configuration 3 is used in the phase codec of the present invention, the device can be coupled to one arm of the phase codec by coupling the input port of the polarization beam splitter to the arm.

[0045] Back to Figure 1In the phase codec, at least one of the reflectors 103 and 104 may be a polarization orthogonal rotation reflector using one of the above-mentioned structures 1, 2, and 3. When one of the reflectors 103 and 104 is a polarization orthogonal rotation reflector using one of the above-mentioned structures 1, 2, and 3, the other reflector may be a polarization orthogonal rotation reflector using one of the above-mentioned structures 1, 2, and 3, or may be a polarization orthogonal rotation reflector using another structure. The polarization orthogonal rotation reflector using another structure may be, for example, a quarter-wave plate reflector. The "quarter-wave plate reflector" comprises 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 to the fast axis or slow axis of the quarter-wave plate. The quarter-wave plate reflector may be realized by coating a reflector on the surface of a quarter-wave plate crystal, or by coating a reflector on the end face of a polarization-maintaining optical fiber having a 90-degree difference in transmission phase between the fast and slow axes.

[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 of one or two reflecting devices in 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 polarization-maintaining optical paths, such as polarization-maintaining fiber optical paths, 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 reflected 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, the 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 the input port of the phase codec. The polarization-maintaining beam splitter 203, along with polarization orthogonal rotation and reflection devices 205 and 206, forms an unequal-arm Michelson interferometer, with the two arms between them serving as polarization-maintaining fiber optical paths. The phase modulator 204 is inserted into either arm of the unequal-arm Michelson interferometer. Either port 201 or 202 of the polarization-maintaining beam splitter 203 can serve as the output port of the phase codec.

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

[0052] When 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 in front of the polarization-maintaining beam splitter 203. An incident 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. The combined 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 The figure shows a schematic structural diagram of a polarization orthogonal rotation reflection device that can be used in the phase codec of the present invention.

[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] Polarization beam splitter 302 includes three ports: port A, port B, and port C. Port A, port B, and port C can be referred to as the input port, the first output port, and the second output port, respectively. Port 301, connected to port A of polarization beam splitter 302, serves as both the input port and the output port of the reflector. Ports B and C of polarization beam splitter 302 are connected via polarization-maintaining fiber 303. The optical pulses output from ports B and C of polarization beam splitter 302 are coupled to the slow-axis transmission of polarization-maintaining fiber 303 or to the fast-axis transmission of the same polarization-maintaining 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 considered to consist of two orthogonal polarization states, which can be denoted as the x polarization state and the y polarization state, respectively. The polarization beam splitter 302 polarization-splits the input optical pulse into a first optical pulse with an x ​​polarization state and a second optical pulse with a y polarization state, which are output from ports B and C of the polarization beam splitter 302, respectively. The first optical pulse with an x ​​polarization state output from port B of the polarization beam splitter 302 is coupled to the slow axis of the polarization-maintaining fiber 303 for transmission and is transmitted along the slow axis of the polarization-maintaining fiber 303 to port C of the polarization beam splitter 302. At port C, the first optical pulse is coupled to the polarization beam splitter 302 by the slow axis of the polarization-maintaining fiber 303. The polarization state of the first optical pulse coupled to port C of the polarization beam splitter 302 is the y polarization state; the first optical pulse with a y polarization state is output from port A of the polarization beam splitter 302. That is, the x-polarization state component of the input light pulse input through port A is converted to the y-polarization state when it is reflected by the reflector and output from port A. The second light pulse with 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 along the slow axis of the polarization-maintaining fiber 303 to port B of the polarization beam splitter 302. 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. 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 with 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 through port A is converted to the x-polarization state when it is reflected by the reflector and output from port A. The reflector ensures that each of the two orthogonal polarization states of the input light pulse is converted to an orthogonal polarization state when it is reflected and output by the reflector. 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 will not be affected.

[0058] Figure 4 A schematic structural diagram 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 splice 404 .

[0060] The polarization beam splitter 402 includes three ports: port A, port B, and port C. Port A, port B, and port C can 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. The optical pulse output from port B of the polarization beam splitter 402 is coupled to the slow axis of the polarization-maintaining fiber 403, and the optical pulse output from port C of the polarization beam splitter 402 is coupled to the fast axis of the polarization-maintaining fiber 403, or the optical pulse output from port B of the polarization beam splitter 402 is coupled to the fast axis of the polarization-maintaining fiber 403, and the optical pulse output 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 polarization beam splitter 402 via port 401, also known as port A of polarization beam splitter 402. The input optical pulse can be considered to consist of two orthogonal polarization states, which can be denoted as x-polarization state and y-polarization state, respectively. Polarization beam splitter 402 polarization-splits the input optical pulse into a first optical pulse in the x-polarization state and a second optical pulse in the y-polarization state, which are output from ports B and C of polarization beam splitter 402, respectively. The first optical pulse in the x-polarization state output from port B of the polarization beam splitter 402 is coupled to the slow axis of the polarization-maintaining fiber 403 and transmitted to the 90-degree fusion splice 404. After passing through the 90-degree fusion splice 404, it is transmitted along the fast axis of the polarization-maintaining fiber 404 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 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 output from port A of the polarization beam splitter 402. In other words, the x-polarization state component of the input optical pulse input from port A is converted to the y-polarization state when it is reflected by the device and output from port A. The second optical pulse in the y-polarization state output from port C of the polarization beam splitter 402 is coupled to the fast axis of the polarization-maintaining 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 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 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 in the x-polarization state is output from port A of the polarization beam splitter 402. In other words, the y-polarization state component of the input optical pulse input from port A is converted to the x-polarization state when it is reflected by the device and output from port A. This polarization orthogonal rotation and reflection device converts the two orthogonal polarization states of the input optical pulse into a polarization state orthogonal to it when it is reflected and output by the device.

[0062] although Figure 4 The figure shows only one 90-degree splice 404, but this is merely exemplary. The polarization-maintaining fiber 403 can include any odd number of 90-degree splices. Each 90-degree splice is formed by aligning and splicing the slow axis of the polarization-maintaining fiber with the fast axis of the polarization-maintaining fiber. If the polarization-maintaining fiber 403 includes more than one odd number of 90-degree splices, the above results remain unchanged. However, the first and second optical pulses output from ports B and C of the polarization beam splitter 402 will each alternate between transmitting along the slow axis and transmitting along the fast axis of the polarization-maintaining fiber more times during transmission along the polarization-maintaining fiber 403, with the number of alternations being equal to the number of 90-degree splices.

[0063] The polarization-maintaining optical fiber 403 with 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 light pulse remains the same as the phase between the y polarization state and the x polarization state of the output light 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 structural diagram 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: port A, port B, and port C. Port A, port B, and port C can be referred to as the input port, the first output port, and the second output port, respectively. Port 501 connected to port A of the polarization beam splitter 502 serves as both the input port and the output port of the device. Port B of the polarization beam splitter 502 is connected to port D of the half-wave plate 503 via a transmission optical path, and port C of the polarization beam splitter 502 is connected to port E of the half-wave plate 503 via 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 fiber optical paths. 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 has an angle of 45 degrees with the slow axis or fast axis of the half-wave plate 503.

[0068] During operation, an input light pulse is input to the polarization beam splitter 502 via port 501, which is also port A of the polarization beam splitter 502. The input light pulse can be regarded as consisting 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 output from ports B and C of the polarization beam splitter 502, respectively. The first light pulse in the x polarization state output 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 converted to the y polarization state after being orthogonally rotated by the half-wave plate 503. The first light pulse in the y polarization state output from port E of the half-wave plate 503 is transmitted to port C 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 x-polarization state component of the input light pulse input from port A is converted to 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. After the polarization is orthogonally rotated by the half-wave plate 503, the polarization state of the second light pulse is converted to the x-polarization state. 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 converted to the x-polarization state when it is output from port A after being reflected by the device. This polarization orthogonal rotation and reflection device converts the two orthogonal polarization states of the input light pulse into a polarization state orthogonal to it when they are reflected and output by the device. 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 remains the same as the phase between the y polarization state and the x polarization state of the output light pulse.

[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, it includes the following components: 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 reflectors 607 and 608, and the two arms between the polarization-maintaining beam splitter 605 and the two polarization-orthogonal rotating reflectors 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 fiber optical paths. A DC phase modulator 606 is located on either arm of the first phase codec.

[0072] Similarly, the polarization-maintaining beam splitter 612, two polarization-orthogonal rotating reflectors 614 and 615, and the two arms between the polarization-maintaining beam splitter 612 and the two polarization-orthogonal rotating reflectors 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 fiber optical paths. A DC phase modulator 613 is located on either arm of the second phase codec.

[0073] Below, Figure 6 The decoding is described by taking the encoding and decoding device as an example.

[0074] On the front beam splitter 603 side ( Figure 6 One of the two ports 601 and 602 (left side in the figure) serves as the input port of the device. The first port A and the second port B of the optical circulator 604 are connected to an output port of the pre-beam splitter 603 and an input port of the polarization-maintaining beam splitter 605, respectively. The optical pulses input to the first phase codec are decoded and output from an output port 609 of the polarization-maintaining beam splitter 605, or transmitted to port B of the optical circulator 604 via 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 connected to the other output port of the pre-beam splitter 603 and an input port of the polarization-maintaining beam splitter 612, respectively. 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, an optical pulse enters beam splitter 603 through port 601 or 602 of beam splitter 603 and is split by beam splitter 603 into a first optical pulse and a second optical pulse. The first optical pulse is input through port A of optical circulator 604 and output from port B of optical circulator 604 to polarization-maintaining beam splitter 605. 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 DC phase modulator 606 and then reflected from polarization orthogonal rotation and reflection device 607. The other first sub-optical pulse is directly transmitted through polarization-maintaining fiber to polarization orthogonal rotation and reflection device 608 and reflected from polarization orthogonal rotation and reflection device 608. The two first sub-optical pulses, which have been reflected after a relatively delayed time, are combined by polarization-maintaining beam splitter 605 and output from port 609. Alternatively, they are output to port B of optical circulator 604 and transmitted to port C for output from port 610. The second optical pulse is input through port A of the optical circulator 611 and output through 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 from the polarization orthogonal rotation reflector 614. The other second sub-optical pulse is directly transmitted through the polarization-maintaining fiber to the polarization orthogonal rotation reflector 615 and then reflected back from the polarization orthogonal rotation reflector 615. The two second sub-optical pulses, which have been reflected after a relatively delayed time, are combined by the polarization-maintaining beam splitter 612 and output from port 616. Alternatively, they are output to port B of the optical circulator 611 and transmitted to port C for output from port 617. The DC phase modulators 606 and 613 cause the DC phase modulation performed by one of the first phase codec and the second phase codec to differ by 90 degrees from the DC phase modulation performed by the other.

[0076] Next, Figure 6 The encoding and decoding device is used for encoding as an example to describe it.

[0077] One port 609 of the polarization-maintaining beam splitter 605, the third port C of the optical circulator 604, one port 616 of the polarization-maintaining beam splitter 612, and the third port C of the optical circulator 611 serve as input ports of the device. The first port A and the second port B of the optical circulator 604 are connected to one port of the pre-beam splitter 603 and another port of the polarization-maintaining beam splitter 605, respectively. Optical pulses input from the third port C of the optical circulator 604 are input to the first phase codec via the second port B of the optical circulator 604. Optical pulses input from the one port 609 of the polarization-maintaining beam splitter 605 and the third port C of the optical circulator 604 are encoded by the first phase codec, output from 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 connected to the other port of the pre-beam splitter 603 and another port of the polarization-maintaining beam splitter 612, respectively. The optical pulse input from the third port C of the optical circulator 611 is input to the second phase codec via 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 then transmitted from the first port A of the optical circulator 611 to the pre-beam splitter 603. Figure 6 One of the two ports 601 and 602 (left in the center) serves as the output port of the device. Optical pulses input from the port 609 of the polarization-maintaining beam splitter 605, the third port C of the optical circulator 604, the port 616 of the polarization-maintaining beam splitter 612, and the third port C of the optical circulator 611 are encoded to achieve four phase encodings. The encoded optical pulses are then combined by the beam splitter 603 and output from either 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] A polarization-maintaining beam splitter 707, two polarization-orthogonal rotating reflectors 709 and 710, and two arms between the polarization-maintaining beam splitter 707 and the two polarization-orthogonal rotating reflectors constitute a polarization-maintaining unequal-arm Michelson interferometer, i.e., a phase codec according to the present invention. The two arms are polarization-maintaining fiber optical paths. A phase modulator 708 is located in either arm of the phase codec.

[0080] Below, Figure 7The decoding is described by taking the encoding and decoding device as an example.

[0081] Beam splitter 703 serves as a pre-beam splitter, with one of its two ports 701 and 702 serving as the device's input port. Beam splitter 704 splits the optical pulse from beam splitter 703 and outputs it through port 705 or 706. The optical pulse input to the polarization-maintaining unequal-arm Michelson interferometer is decoded and output through port 711.

[0082] During operation, an input optical pulse enters beam splitter 703 through port 701 or 702 of beam splitter 703 and is split into two optical pulses for transmission. One optical pulse from beam splitter 703 is input to beam splitter 704, where it is split and then output through port 705 or 706 for time bit decoding. The other optical pulse from beam splitter 703 is input to polarization-maintaining beam splitter 707, where it is split into two sub-optical pulses. One sub-optical pulse is randomly phase-modulated by phase modulator 708 to 0 or 180 degrees and then reflected from polarization-rotating reflector 709. The other optical pulse is directly transmitted through polarization-maintaining fiber to polarization-rotating reflector 710 and then reflected from polarization-rotating reflector 710. The two sub-optical pulses, which have been reflected after a relative delay, are combined by polarization-maintaining beam splitter 707 and output from 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 The encoding and decoding device is 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 the device's input ports. Optical pulses input from ports 705 and 706 are combined by beam splitter 704 and output to pre-beam splitter 703, achieving temporal bit encoding. Optical pulses input from port 711 are encoded by a polarization-maintaining unequal-arm Michelson interferometer and output from polarization-maintaining beam splitter 707 to pre-beam splitter 703, where two phase encodings are achieved by phase modulator 708. Ports 701 and 702 of pre-beam splitter 703 serve as the device's output port. 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 the combined pulses 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 based on polarization orthogonal rotation reflection of DC modulation quantum key distribution using the phase encoder and decoder of the present invention is as follows Figure 8 As shown, the optical circulator 807 includes the following components: 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] A polarization-maintaining beam splitter 808, two polarization-orthogonal rotating reflectors 810 and 811, and two arms between the polarization-maintaining beam splitter 808 and the two polarization-orthogonal rotating reflectors constitute a polarization-maintaining unequal-arm Michelson interferometer, i.e., a phase codec according to the present invention. The two arms are polarization-maintaining fiber optical paths. A DC phase modulator 809 is located in either arm of the phase codec.

[0089] Below, Figure 8 The decoding is described by taking the encoding and decoding device as an example.

[0090] Beam splitter 803 serves as a pre-beam splitter, with one of its two ports 801 and 802 serving as the device's input port. Beam splitter 804 splits one optical pulse from beam splitter 803 and outputs it through port 805 or 806. Optical pulses input from port A of optical circulator 807 are output from port B of optical circulator 807, while optical pulses input from port B of optical circulator 807 are output from port C of optical circulator 807. Optical pulses input to the polarization-maintaining unequal-arm Michelson interferometer are decoded and output from port 812, or transmitted through the other output port of polarization-maintaining beam splitter 808 to port B of optical circulator 807, where they are output from port 813.

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

[0094] Ports 805 and 806 of beam splitter 804, port 812 of polarization-maintaining beam splitter 808, and third port C of optical circulator 807 serve as the device's input ports. Optical pulses input from third port C of optical circulator 807 are output from second port B of optical circulator 807, while optical pulses input from second port B of optical circulator 807 are output from first port A of optical circulator 807. Optical pulses input from ports 805 and 806 are combined by beam splitter 804 and output to pre-beam splitter 803, achieving temporal bit encoding. Optical pulses input from port 812 and optical pulses input from third port C of optical circulator 807 and output from second port B of optical circulator 807 to polarization-maintaining beam splitter 808 are encoded by a polarization-maintaining unequal-arm Michelson interferometer, then output from polarization-maintaining beam splitter 808 to second port B of optical circulator 807. These pulses are then transmitted to pre-beam splitter 803 via first port A of optical circulator 807. Optical pulses input from port 812 of polarization-maintaining beam splitter 808 and port C of optical circulator 807 are encoded to achieve two phase encodings. Ports 801 and 802 of beam splitter 803 serve as the device's output port. Beam splitter 803 combines the optical pulses output from beam splitter 804 and the optical pulses output from port A of optical circulator 807, and then outputs the combined beams from 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, but it is possible that the phase codec and the codec apparatus of the present invention do not include a phase modulator.

[0097] In this article, the terms "beam splitter" and "beam combiner" are used interchangeably. 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 fiber to transmit light pulses, or an optical path formed by connecting polarization-maintaining 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 the phase codec and the corresponding codec device. Figure 1-2 and Figure 6-8 The phase modulator described exemplarily may or 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 codec device of the present invention, the phase codec or 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 deeper and more specific understanding of the technical means and effects adopted by the present invention to achieve the intended purpose. However, the accompanying illustrations 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, 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 the corresponding arm 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, and for at least one reflecting device comprising a polarization beam splitter: its transmission optical path is formed by a polarization-maintaining optical fiber twisted 90 degrees, so that the light pulses outputted from the two output ports of its 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, the slow axis and the fast axis of the polarization-maintaining optical fiber respectively maintain the two orthogonal polarization states of the light pulses input into the polarization-maintaining optical fiber for stable transmission - that is, the polarization state remains unchanged, 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, and the non-birefringent optical device refers to an optical device having the same refractive index for different polarization states.

2. The phase codec according to claim 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.

3. The phase codec according to claim 1, wherein: The polarization-maintaining optical fiber twisted at 90 degrees includes a polarization-maintaining optical fiber twisted at 90 degrees or twisted at (90+n*180) degrees, wherein n is an integer.

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

5. The phase codec according to any one of claims 1 to 4, wherein: The phase codec further includes a phase modulator, wherein the phase modulator is disposed 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-path of the two sub-optical paths, and an optical circulator is provided on each sub-optical path.

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 to 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 to 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 provided 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 encoding and decoding device according to any one of claims 6 to 10, which is provided 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 the encoding and decoding device according to any one of claims 6 to 10, which is provided at the transmitting end of the quantum key distribution system for encoding.

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