CN110460428B

CN110460428B - Quantum key distribution phase codec, corresponding codec device and system

Info

Publication number: CN110460428B
Application number: CN201910176940.9A
Authority: CN
Inventors: 许华醒
Original assignee: China Academy of Electronic and Information Technology of CETC
Current assignee: China Academy of Electronic and Information Technology of CETC
Priority date: 2019-03-08
Filing date: 2019-03-08
Publication date: 2024-06-25
Anticipated expiration: 2039-03-08
Also published as: CN110460428A

Abstract

The invention provides a quantum key distribution phase codec and a corresponding codec device and system. The phase codec includes a beam splitter and two polarization quadrature rotating reflecting devices optically coupled to the beam splitter via two arms, respectively. At least one reflecting device includes a polarizing beam splitter having a first port, a second port, a third port, and a fourth port, a quarter wave plate mirror, and a phase modulator, and is coupled to a respective arm via the first port. The second and fourth ports of the polarizing beam splitter are coupled by a first transmission optical path, and the phase modulator is disposed on the first transmission optical path. The third port of the polarizing beam splitter is coupled to the quarter wave plate mirror through a second transmission optical path. The invention can stably encode and decode the input light pulse with any polarization state, solves the problem that the system can not stably work due to polarization induced fading in the phase encoding and time bit-phase encoding quantum key distribution application, and reduces the loss at the interferometer.

Description

Quantum key distribution phase codec, corresponding codec device and system

Technical Field

The invention relates to the technical field of optical transmission secret communication, in particular to a quantum key distribution phase codec based on polarization orthogonal rotation reflection, a corresponding codec device comprising the phase codec and a quantum key distribution system.

Background

Quantum secret communication technology is the leading-edge hotspot field combining quantum physics and information science. Based on the quantum key distribution technology and the one-time secret code principle, the quantum secret communication can realize the safe transmission of information in a public channel. The quantum key distribution is based on the physical principles of quantum mechanics Hessenberg uncertainty relation, quantum unclonable theorem and the like, can realize safe sharing of keys among users, can detect potential eavesdropping behaviors, and can be applied to the fields of national defense, government affairs, finance, electric power and other high-safety information transmission requirements.

The ground quantum key distribution is mainly based on fiber channel transmission, and because phase coding adopts the phase difference of front and rear light pulses to code information, the phase coding based on the unequal-arm interferometer and time bit-phase coding are main coding schemes for quantum key distribution application. However, the optical fiber manufacturing has non-ideal conditions such as non-circular symmetry of a section, non-uniform distribution of refractive index of a fiber core along radial direction, and the like, and the optical fiber is influenced by temperature, strain, bending, and the like in an actual environment, so that random birefringence effect can be generated. Therefore, after the light pulse is transmitted through the long-distance optical fiber and the two-arm optical fiber of the unequal-arm interferometer, the problem of polarization induced fading exists when the unequal-arm interferometer is used for phase decoding interference, so that the decoding interference is unstable, and the error rate is increased. If the correction equipment is used, the complexity and cost of the system are increased, and stable application is difficult to realize for strong interference situations such as aerial optical cables, road bridge optical cables and the like.

In the prior art, an unequal arm faraday-michelson interferometer is proposed, which can make the optical pulse still keep the interference result output stable when the optical pulse is affected by the random birefringence of the optical fiber channel and the polarization state change caused by the random birefringence. However, such interferometers are large in loss, and the insertion loss of the phase modulator is one of the main factors causing the large loss. Specifically, when the phase modulator is placed on an arm of the interferometer, the light pulse passes through the phase modulator twice due to back and forth transmission, which results in a larger loss of the interferometer and lower system efficiency.

For quantum key distribution phase encoding and time bit-phase encoding schemes, how to perform interference decoding stably and efficiently is a hotspot and a difficulty in quantum secret communication applications based on existing optical cable infrastructure.

Disclosure of Invention

The invention mainly aims to provide a quantum key distribution phase codec based on polarization quadrature rotation reflection, a corresponding codec device comprising the phase codec and a quantum key distribution system, so as to solve the problem of unstable phase decoding interference caused by polarization induced fading in phase coding and time bit-phase coding quantum key distribution application, and reduce loss at an interferometer.

The invention provides at least the following technical scheme:

1. A quantum key distribution phase codec comprising: the polarization beam splitter comprises a beam splitter, two polarization orthogonal rotation reflecting devices respectively optically coupled with the beam splitter through two arms, wherein one or each polarization orthogonal rotation reflecting device comprises a polarization beam splitter with a first port, a second port, a third port and a fourth port, a quarter wave plate reflecting mirror and a phase modulator, the polarization beam splitter is coupled with the corresponding arm in the two arms through the first port of the polarization beam splitter, the second port and the fourth port of the polarization beam splitter are optically coupled through a first transmission light path, the phase modulator is arranged on the first transmission light path, the third port of the polarization beam splitter is coupled to the quarter wave plate reflecting mirror through a second transmission light path, and the quarter wave plate reflecting mirror comprises a quarter wave plate and a reflecting mirror integrally formed with the quarter wave plate at the rear end of the quarter wave plate, and an included angle between the polarization direction of an optical pulse input into the quarter wave plate and a fast axis or a slow axis of the quarter wave plate is 45 degrees.

2. The phase codec of claim 1, wherein the two polarization quadrature rotated reflecting devices are polarization quadrature rotated reflecting devices of the same configuration or polarization quadrature rotated reflecting devices of different configurations.

3. The phase codec of claim 1, wherein the first transmission optical path and/or the second transmission optical path is a polarization maintaining optical path.

4. The phase codec of claim 1, wherein the beam splitter is a polarization maintaining beam splitter.

5. The phase codec of claim 1, wherein the two arms are each polarization maintaining optical paths, and the optical devices on the two arms are polarization maintaining optical devices and/or non-birefringent optical devices.

6. A direct current modulated quantum key distribution phase codec device comprising a pre-splitter and two phase codecs according to any of schemes 1-5, the two phase codecs being optically coupled to the pre-splitter via two sub-optical paths, respectively, wherein one of the ports of the splitter of each phase codec not coupled to the two arms of the phase codec is optically coupled to a respective one of the two sub-optical paths, each of the sub-optical paths being provided with an optical circulator, wherein the phase modulator is a direct current phase modulator.

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

8. A direct current modulated quantum key distribution time bit-phase codec device comprising a pre-splitter and a phase codec according to any one of schemes 1-5, the phase codec being optically coupled to the pre-splitter via a sub-optical path, wherein one of the ports of the splitter of the phase codec not coupled to the two arms is optically coupled to the sub-optical path, wherein the sub-optical path is provided with an optical circulator, the phase modulator being a direct current phase modulator.

9. The codec of claim 7 or 8, further comprising a beam splitter coupled to the front beam splitter via another sub-optical path.

10. A quantum key distribution system comprising:

The phase codec according to any one of the schemes 1 to 5 or the codec device according to any one of the schemes 6 to 9, which is provided at a receiving end of the quantum key distribution system, for decoding; and/or

The phase codec according to any one of the schemes 1 to 5 or the codec device according to any one of the schemes 6 to 9 is provided at a transmitting end of the quantum key distribution system for encoding.

The invention can stably encode and decode the input light pulse with any polarization state by creative construction, thereby realizing unexpected beneficial effects. By utilizing the scheme of the invention, stable interference output at the phase decoding interferometer can be realized for the input light pulse with any polarization state, and the problem that the system cannot work stably due to polarization induced fading in phase coding and time bit-phase coding quantum key distribution application is solved. In addition, since the phase modulator is placed in the reflective device of the interferometer, the optical pulses need only be transmitted once through the phase modulator, thereby reducing losses at the interferometer. The invention provides an efficient polarization induced fading resistant phase encoding and time bit-phase encoding quantum key distribution decoding scheme which is easy to implement and apply.

Drawings

FIG. 1 is a schematic diagram of the structure of a quantum key distribution phase codec based on polarization quadrature rotation reflection according to a preferred embodiment of the present invention;

FIG. 2 is a schematic diagram of a polarization quadrature rotating reflection device that can be used in the phase codec of the present invention;

FIG. 3 is a schematic diagram of the structure of another polarization quadrature rotating reflection device that can be used in the phase codec of the present invention;

FIG. 4 is a schematic diagram of the structure of another polarization quadrature rotating reflection device that can be used in the phase codec of the present invention;

FIG. 5 is a schematic diagram of the structure of another polarization quadrature rotating reflection device that can be used in the phase codec of the present invention;

FIG. 6 is a schematic diagram of the structure of a DC-modulated quantum key distribution phase encoding and decoding device based on polarization quadrature rotation reflection according to a preferred embodiment of the present invention;

FIG. 7 is a schematic diagram showing the constitution of a quantum key distribution time bit-phase encoding/decoding apparatus based on polarization quadrature rotation reflection according to a preferred embodiment of the present invention;

FIG. 8 is a schematic diagram of the structure of a DC modulated quantum key distribution time bit-phase encoding and decoding device based on polarization quadrature rotation reflection according to a preferred embodiment of the present invention;

fig. 9 is a schematic diagram of the composition structure of a quantum key distribution phase codec based on polarization quadrature rotation reflection according to a preferred embodiment of the present invention.

Detailed Description

Preferred embodiments of the present application are described in detail below with reference to the attached drawing figures, which form a part of the present application and are used in conjunction with embodiments of the present application to illustrate the principles of the present application. For the purposes of clarity and simplicity, detailed descriptions of known functions and configurations incorporated herein will be omitted when it may obscure the subject matter of the present application.

A quantum key distribution phase codec based on polarization quadrature rotation reflection according to a preferred embodiment of the present invention is shown in fig. 1, and includes the following components: a beam splitter 101, two reflecting means 102 and 103.

The two reflecting means 102 and 103 are optically coupled to the beam splitter 101 via two arms (upper and lower arms in fig. 1), respectively.

According to the invention, both reflecting means 102 and 103 are polarization quadrature rotating reflecting means and at least one of the reflecting means comprises a phase modulator.

Here, the polarization orthogonal rotation reflecting means refers to a reflecting means capable of polarization orthogonal rotation reflecting two orthogonal polarization states of a reflected light pulse, that is, converting each orthogonal polarization state of an incident light pulse into a polarization state orthogonal thereto when reflecting the light pulse. For example, assuming that the two orthogonal polarization states are an x-polarization state and a y-polarization state, respectively, the x-polarization state transmitted to one polarization orthogonal rotation reflection device along the optical path is transformed into a polarization state orthogonal thereto, i.e., a y-polarization state, after polarization orthogonal rotation reflection at the reflection device, and the y-polarization state transmitted to the reflection device along the optical path is transformed into a polarization state orthogonal thereto, i.e., an x-polarization state, after polarization orthogonal rotation reflection at the reflection device.

Here, the polarization quadrature rotating reflection apparatus including the phase modulator may be referred to as "polarization quadrature rotating reflection apparatus having a phase modulation function".

The beam splitter 101 is used to split an incoming light pulse of any polarization into two light pulses for transmission along the two arms, respectively.

The two arms are used for transmitting the two light pulses respectively.

Each phase modulator is configured to phase modulate the light pulses passing therethrough in accordance with a quantum key distribution protocol. The phase modulation by the phase modulator is determined by the quantum key distribution protocol, depending on the particular application. For example, in one possible application, the phase modulator may randomly modulate a 0 degree phase or a 90 degree phase. In the case where each of the two reflecting means 102 and 103 comprises a phase modulator, the difference in the phases modulated by the two phase modulators is determined by the quantum key distribution protocol, depending on the particular application.

The phase modulator may be a single polarization phase modulator or a birefringent phase modulator. The single polarization phase modulator applies phase modulation to one polarization state and cuts off to the other polarization state. The birefringent phase modulator is adapted to apply different tunable phase modulations to the two orthogonal polarization states passing therethrough. 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 modulation experienced by each of the two orthogonal polarization states passing through the lithium niobate phase modulator may be controlled and adjusted.

The reflecting means 102 and 103 are respectively used for reflecting the two light pulses transmitted via the two arms from the beam splitter 101 back to the beam splitter 101 to be combined by the beam splitter 101 for output.

Since both reflecting means 102 and 103 are polarization quadrature rotating reflecting means, for each of the two light pulses: the two orthogonal polarization states of the light pulse are subjected to polarization orthogonal rotation reflection when the light pulse is reflected by the corresponding reflection device in the two reflection devices, so that each orthogonal polarization state of the light pulse is converted into a polarization state orthogonal to the light pulse after the light pulse is reflected by the corresponding reflection device. Thus, for the phase codec of fig. 1, the phase difference transmitted by the two arms during beam splitting to beam combining of the beam splitter for the x-polarization state of the input light pulse is exactly equal to the phase difference transmitted by the two arms during beam splitting to beam combining of the beam splitter for the y-polarization state of the light pulse using polarization quadrature rotation reflection at the polarization quadrature rotation reflection device.

The present invention proposes four inventive polarization orthogonal rotating reflecting device configurations, namely configuration 1, configuration 2, configuration 3 and configuration 4 described below.

According to configuration 1, the polarization orthogonal rotation reflecting 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, a half-wave plate being provided on the transmission optical path, the polarization direction of an optical pulse input to the half-wave plate having an angle of 45 degrees with the fast axis or the slow axis of the half-wave plate. The polarization quadrature rotating reflection means having configuration 1, when used in the phase codec of the present invention, can be coupled to one arm of the phase codec by coupling the input port of its polarizing beam splitter to the arm.

According to configuration 2, the polarization orthogonal rotation reflecting device includes 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 one transmission optical path formed by a polarization maintaining fiber whose slow axis and fast axis respectively maintain stable transmission of two orthogonal polarization states of an optical pulse inputted to the polarization maintaining fiber, that is, polarization states are unchanged, and the two output ports of the polarization beam splitter and the polarization maintaining fiber are configured such that the optical pulse outputted by the two output ports of the polarization beam splitter are both coupled to the slow axis of the polarization maintaining fiber for transmission or both coupled to the fast axis of the polarization maintaining fiber for transmission. Here, the optical pulses output by the two output ports of the polarization beam splitter are both coupled to the slow axis of the polarization maintaining fiber for transmission or both coupled to the fast axis of the polarization maintaining fiber for transmission may be implemented by twisting the polarization maintaining fiber by 90 degrees or by twisting the polarization maintaining fiber by (90+n×180 degrees), where n is an integer. The light pulse input from the slow axis of the polarization maintaining fiber is always transmitted along the slow axis (stable transmission along the slow axis) regardless of whether the polarization maintaining fiber is twisted or not, and the light pulse input from the fast axis of the polarization maintaining fiber is always transmitted along the fast axis (stable transmission along the fast axis). Polarization quadrature rotating reflecting means with configuration 2 when used in the phase codec of the present invention, the reflecting means can be coupled to one arm of the phase codec by coupling its input port of the polarizing beam splitter to said arm.

According to configuration 3, the polarization orthogonal rotation reflecting device includes 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 formed by polarization maintaining fibers having an odd number of 90 degree fusion points, each 90 degree fusion point being formed by fusion welding a slow axis of the polarization maintaining fiber with a fast axis of the polarization maintaining fiber. Polarization quadrature rotating reflecting means with configuration 3 when used in the phase codec of the present invention, the reflecting means can be coupled to one arm of the phase codec by coupling its input port of the polarizing beam splitter to said arm.

According to configuration 4, the polarization orthogonal rotation reflecting device includes: a polarizing beamsplitter having a first port, a second port, a third port, and a fourth port; a reflective device optically coupled to the third port of the polarizing beamsplitter; and the second port and the fourth port of the polarization beam splitter are optically coupled through a first transmission light path, the third port of the polarization beam splitter is coupled to the reflecting device through a second transmission light path, the phase modulator is arranged on the first transmission light path, and the reflecting device is a quarter wave plate reflecting mirror. 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, and an included angle between the polarization direction of the light pulse input into the quarter wave plate and the fast axis or the slow axis of the quarter wave plate is 45 degrees. The quarter wave plate reflector can be realized by plating a reflector on the surface of the quarter wave plate crystal, and can also be realized by plating a reflector on the end face of the polarization maintaining optical fiber with the phase difference of 90 degrees in the fast and slow axes. The first transmission optical path may be a polarization maintaining optical fiber optical path; the second transmission optical path may be a polarization maintaining optical fiber optical path, and an included angle between a slow axis of the polarization maintaining optical fiber forming the second transmission optical path and a slow axis or a fast axis of the quarter wave plate is 45 degrees. Polarization quadrature rotating reflecting means with configuration 4 when used in the phase codec of the present invention, the reflecting means may be coupled to one arm of the phase codec by coupling its first port of the polarizing beam splitter to said arm.

With the polarization quadrature rotating reflecting device of any one of the above configurations 1,2 and 3, a phase modulator may be interposed on the transmission optical path between the two output ports of the polarization beam splitter in the polarization quadrature rotating reflecting device.

Returning to the phase codec of fig. 1, at least one of the reflecting means 102 and 103 may be a polarization quadrature rotating reflecting means employing one of the above-described configurations 1, 2, 3 and 4. When one of the reflection devices 102 and 103 is a polarization orthogonal rotation reflection device using one of the above-described configurations 1, 2, 3, and 4, the other reflection device may be a polarization orthogonal rotation reflection device using one of the above-described configurations 1, 2, 3, and 4, or may be a polarization orthogonal rotation reflection device of another configuration. The polarization quadrature rotating reflecting means of the other configuration may be, for example, a quarter wave plate mirror.

One or both of the reflecting means 102 and 103 may comprise a phase modulator. For example, when only one of the reflecting means 102 and 103 is a polarization orthogonal rotation reflecting means employing the above configuration 4, the other reflecting means of the reflecting means 102 and 103 may be a polarization orthogonal rotation reflecting means employing one of the above configurations 1,2 and 3, and the other reflecting means may or may not include a phase modulator, or the other reflecting means of the reflecting means 102 and 103 may be a polarization orthogonal rotation reflecting means employing other configurations such as a quarter wave plate mirror. For another example, both of the reflecting means 102 and 103 may be polarization orthogonal rotation reflecting means adopting the above configuration 4. As another example, where two of the reflecting means 102 and 103 each employ the polarization quadrature rotating reflecting means of any one of configurations 1,2 and 3 described above, one or both of the reflecting means may comprise a phase modulator.

For the phase codec of fig. 1, the relative delay of the two light pulses can be achieved by adjusting the length of the two arms and/or adjusting the transmission light path in one or both of the two reflecting means 102 and 103 employing a configuration selected from the group consisting of configuration 1, configuration 2, configuration 3 and configuration 4.

In the case where the reflection means adopts a configuration selected from the group consisting of configuration 1, configuration 2, configuration 3, and configuration 4, two arms of the phase codec may be configured as polarization maintaining optical paths, and the optical devices on the two arms may be configured as polarization maintaining optical devices and/or non-birefringent optical devices. Thus, for each of the two optical pulses obtained by beam splitting: the two orthogonal polarization states of the light pulses of the path may be maintained unchanged during beam splitting by the beam splitter to the respective reflecting means and during beam combining by the respective reflecting means. In general, the polarization maintaining optical path may be a free space optical path or a polarization maintaining fiber optical path. Herein, "non-birefringent light device" refers to a light device having the same refractive index for different polarization states (e.g., two orthogonal polarization states). In addition, the polarization maintaining optical device may also be referred to as a polarization maintaining optical device.

Furthermore, the beam splitter 101 of the phase codec may be a polarization maintaining beam splitter.

Fig. 2 is a schematic diagram showing the constitution of a polarization quadrature rotation reflecting device having a phase modulation function which can be used for the phase codec of the present invention.

The polarization quadrature rotating reflection apparatus with phase modulation function shown in fig. 2 includes the following components: a polarizing beam splitter 202, a quarter wave plate mirror 203, and a phase modulator 204.

Polarizing beamsplitter 202 includes ports a, B, C, and D, which may be referred to as first, second, third, and fourth ports, respectively. The ports 201 on one side of the polarizing beam splitter 202, port a, are the input and output of the device. Port B of the polarization beam splitter 202 is connected to port E on one side of the phase modulator 204, and port D of the polarization beam splitter 202 is connected to port F on the other side of the phase modulator 204; that is, the phase modulator 204 is located on the transmission optical path connecting port B and port D of the polarization beam splitter 202. Port C of polarizing beam splitter 202 is connected to quarter wave plate mirror 203. The quarter wave plate reflector 203 can be realized by plating a reflector on the surface of the quarter wave plate crystal, or can be realized by plating a reflector on the end face of a polarization maintaining fiber with a phase difference of 90 degrees in the fast and slow axes. The phase modulator 204 is a single polarization phase modulator or a birefringent phase modulator. The transmission path connecting port B of the polarization beam splitter 202 with port E of the phase modulator 204, the transmission path connecting port D of the polarization beam splitter 202 with port F of the phase modulator 204, and the transmission path connecting port C of the polarization beam splitter 202 with the quarter wave plate mirror 203 are polarization maintaining optical paths such as polarization maintaining optical fibers, maintaining the polarization state of the light pulses transmitted therethrough (consistent with the two orthogonal intrinsic polarization states of the polarization beam splitter) unchanged.

In operation, an input optical pulse is input through port a of the polarizing beam splitter 202 via port 201, and is polarized and split by the polarizing beam splitter 202 into a first optical pulse and a second optical pulse with mutually orthogonal polarization states. The first optical pulse is output from the port B of the polarization beam splitter 202, input to the phase modulator 204 through the port E of the phase modulator 204, and output from the port F of the phase modulator 204 after being phase modulated. The first optical pulse output from port F of the phase modulator 204 is input to the polarizing beam splitter 202 through port D of the polarizing beam splitter 202 and output to the quarter wave plate mirror 203 through port C of the polarizing beam splitter 202. The quarter wave plate mirror 203 reflects the first light pulse input thereto back to the polarizing beam splitter 202 and out port a of the polarizing beam splitter 202. The second optical pulse is output from port C of the polarizing beam splitter 202 to the quarter wave plate mirror 203, and the quarter wave plate mirror 203 reflects the second optical pulse input thereto back to the polarizing beam splitter 202 and output from port D of the polarizing beam splitter 202. The second optical pulse output from the port D of the polarization beam splitter 202 is input to the phase modulator 204 through the port F of the phase modulator 204, phase-modulated, and then output from the port E of the phase modulator 204. The second path of light pulses output from port E of phase modulator 204 is input to polarizing beamsplitter 202 by port B of polarizing beamsplitter 202 and output by port a of polarizing beamsplitter 202. Each of the first and second optical pulses is polarized to be converted into a polarization state orthogonal thereto after being reflected by the quarter wave plate mirror 203. The phase modulator 204 performs the same phase modulation on the first optical pulse and the second optical pulse. Advantageously, the first light pulse resulting from polarization splitting by the polarization beam splitter 202 is transmitted from the polarization beam splitter 202 to the phase modulator 204 at the same time as the second light pulse resulting from polarization splitting by the polarization beam splitter 202 is transmitted from the polarization beam splitter 202 to the phase modulator 204, i.e. the first light pulse and the second light pulse resulting from polarization splitting by the polarization beam splitter 202 arrive at the phase modulator 204 at the same time—in this case, the phase modulator 204 is able to perform the same phase modulation on the first light pulse and the second light pulse once. It is also possible that the first and second light pulses resulting from polarization splitting by the polarization beam splitter 202 do not arrive at the phase modulator 204 at the same time, but in this case the phase modulator 204 needs to modulate the first and second light pulses, respectively, and the phases of their modulations need to remain the same.

Fig. 3 is a schematic diagram showing the constitution of another polarization quadrature rotation reflecting means having a phase modulation function which can be used for the phase codec of the present invention.

The polarization quadrature rotating reflection apparatus with phase modulation function shown in fig. 3 includes the following components: a polarizing beam splitter 302, a polarization maintaining fiber 303, and a phase modulator 304.

Polarizing beamsplitter 302 includes three ports, port a, port B, and port C. The ports a, B, C may be referred to as an input port, a first output port, a second output port, respectively. One port 301 of the polarizing beamsplitter 302, port a, serves as both the input port and the output port of the device. Port B and port C of the polarization beam splitter 302 are connected by a polarization maintaining fiber 303. The light pulses output by port B and port C of polarization beam splitter 302 are both coupled to the slow axis transmission of polarization maintaining fiber 303 or both coupled to the fast axis transmission of the polarization maintaining fiber. The phase modulator 304 is inserted in the optical path of the polarization maintaining fiber 303 connecting the port B and the port C of the polarization beam splitter 302.

In operation, an input optical pulse is input to polarizing beamsplitter 302 via port 301, port a of polarizing beamsplitter 302. The input light pulse may be considered to consist of two orthogonal polarization states, which may be denoted as x-polarization state and y-polarization state, respectively. The polarizing beam splitter 302 polarizes and splits the input light pulse into a first light pulse of the x-polarization state and a second light pulse of the y-polarization state for output by ports B and C of the polarizing beam splitter 302, respectively. The first optical pulse in the x-polarization state output by port B of the polarization beam splitter 302 is coupled to the slow axis transmission of the polarization maintaining fiber 303 and is input to the phase modulator 304 by port D of the phase modulator 304 for phase modulation. The first path of optical pulse after phase modulation is output by a port E of a phase modulator 304 and is transmitted to a port C of a polarization beam splitter 302 along the slow axis of a polarization maintaining optical fiber 303, the first path of optical pulse at the port C 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 path of optical pulse coupled to the port C of the polarization beam splitter 302 is y-polarization state; the first pulse of light in the y polarization state is output by port a of polarizing beam splitter 302. I.e. to achieve that the x-polarization component of the input light pulse input by port a is transformed into the y-polarization state when output by port a after reflection by the reflection means. The second optical pulse of y-polarization output by port C of polarization beam splitter 302 is coupled to the slow-axis transmission of polarization maintaining fiber 303 and phase modulated by port E of phase modulator 304. The second path of optical pulse after phase modulation is output by a port D of the phase modulator 304 and transmitted to a port B of the polarization beam splitter 302 along the slow axis of the polarization maintaining fiber 303, the second path of optical pulse at the port B 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 path of optical pulse coupled to the port B of the polarization beam splitter 302 is x-polarization state; the second pulse of light in the x-polarization state is output by port a of polarizing beam splitter 302. I.e. to achieve that the y-polarization component of the input light pulse input by port a is transformed into x-polarization when output by port a after reflection by the reflection means. The first optical pulse input to the phase modulator 304 from port D and the second optical pulse input to the phase modulator 304 from port E are input to the phase modulator 304 in the same polarization state and subjected to the same phase modulation, achieving polarization independent phase modulation. And each of the two orthogonal polarization states of the input light pulse is transformed into a polarization state orthogonal thereto when the output is reflected by the reflection means. The polarization of the two orthogonal polarization states is orthogonally rotated by the polarization maintaining fiber 303, 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.

The phase modulator 304 may be a birefringent phase modulator or a single polarization phase modulator.

Port B and port C of polarizing beamsplitter 302 may both be coupled to the fast axis of polarization maintaining fiber 303, with the results unaffected.

Fig. 4 is a schematic diagram showing the constitution of another polarization quadrature rotation reflecting means having a phase modulation function which can be used for the phase codec of the present invention.

The polarization quadrature rotating reflection apparatus with phase modulation function shown in fig. 4 includes the following components: a polarizing beam splitter 402, a polarization maintaining fiber 403, a phase modulator 404, and a 90 degree fusion splice 405.

Polarizing beamsplitter 402 includes three ports, port a, port B, and port C. The ports a, B, C may be referred to as an input port, a first output port, a second output port, respectively. One port 401 of the polarizing beamsplitter 402, port a, serves as both the input port and the output port of the device. Port B and port C of polarization beam splitter 402 are connected by polarization maintaining fiber 403. The light pulse output by port B of polarization beam splitter 402 is coupled to the slow axis of polarization maintaining fiber 403 and the light pulse output by port C of polarization beam splitter 402 is coupled to the fast axis of polarization maintaining fiber 403, or the light pulse output by port B of polarization beam splitter 402 is coupled to the fast axis of polarization maintaining fiber 403 and the light pulse output by port C of polarization beam splitter 402 is coupled to the slow axis of polarization maintaining fiber 403. The polarization maintaining fiber 403 includes a 90 degree fusion point 405, and the 90 degree fusion point 405 is formed by aligning and fusing the slow axis of the polarization maintaining fiber and the fast axis of the polarization maintaining fiber. The phase modulator 404 is inserted in the optical path of the polarization maintaining fiber 403 connecting port B and port C of the polarization beam splitter 402.

In operation, an input optical pulse is input to polarizing beamsplitter 402 via port 401, port a of polarizing beamsplitter 402. The input light pulse may be considered to consist of two orthogonal polarization states, which may be denoted as x-polarization state and y-polarization state, respectively. The polarizing beam splitter 402 polarizes and splits the input light pulse into a first light pulse of the x-polarization state and a second light pulse of the y-polarization state for output by ports B and C of the polarizing beam splitter 402, respectively. The first optical pulse in the x-polarization state output by port B of polarization beam splitter 402 is coupled to the slow axis transmission of polarization maintaining fiber 403 and phase modulated by port D of phase modulator 404 input to phase modulator 404. The first path of optical pulse after phase modulation is output by a port E of a phase modulator 404 and transmitted to a 90-degree fusion point 405 along the slow axis of a polarization maintaining fiber 403, and is transmitted to a port C of a polarization beam splitter 402 along the fast axis of the polarization maintaining fiber 403 after passing through the 90-degree fusion point 405, and the first path of optical pulse is coupled to the polarization beam splitter 402 by the fast axis of the polarization maintaining fiber 403 at the port C; the polarization state of the first light pulse coupled to port C of the polarization beam splitter 402 is the y polarization state, and the first light pulse of the y polarization state is output from port a of the polarization beam splitter 402. I.e. to achieve that the x-polarization component of the input light pulse input by port a is transformed into the y-polarization when output by port a after reflection by the device. The second optical pulse in the y polarization state output by the port C of the polarization beam splitter 402 is coupled to the fast axis of the polarization maintaining fiber 403, and is transmitted to the 90 degree fusion point 405 along the fast axis of the polarization maintaining fiber 403, is transmitted to the port E of the phase modulator 404 along the slow axis of the polarization maintaining fiber 403 after passing through the 90 degree fusion point 405, and is input to the phase modulator 404 by the port E of the phase modulator 404 for phase modulation. The second path of optical pulse after phase modulation is output by the port D of the phase modulator 404 and transmitted to the port B of the polarization beam splitter 402 along the slow axis of the polarization maintaining fiber 403, where the second path of 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 x-polarization, and the second optical pulse of x-polarization is output from port a of the polarization beam splitter 402. I.e. to achieve that the y-polarization component of the input light pulse input by port a is transformed into the x-polarization state when output by port a after reflection by the device.

The first optical pulse input to the phase modulator 404 from port D and the second optical pulse input to the phase modulator 404 from port E are input to the phase modulator 404 in the same polarization state and subjected to the same phase modulation, achieving polarization independent phase modulation. The two orthogonal polarization states of the input light pulse are each transformed into a polarization state orthogonal thereto when the output is reflected by the reflecting means.

Although only one 90 degree splice point 405 is shown in fig. 4, this is merely exemplary and the polarization maintaining fiber 403 may contain any odd number of 90 degree splice points. Each 90-degree fusion joint is formed by aligning and fusing the slow axis of the polarization maintaining optical fiber and the fast axis of the polarization maintaining optical fiber. In the case where the polarization maintaining fiber 403 includes an odd number of 90-degree fusion points greater than 1, the above result is not affected, except that the first path of light pulse and the second path of light pulse output by the ports B and C of the polarization beam splitter 402 are each converted more times between transmission along the slow axis of the polarization maintaining fiber and transmission along the fast axis of the polarization maintaining fiber while being transmitted along the polarization maintaining fiber 403, the number of times being equal to the number of 90-degree fusion points.

The polarization maintaining fiber 403 including the odd number of 90 degree fusion points is adopted 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 is kept the same as the phase between the y polarization state and the x polarization state of the output optical pulse.

The phase modulator 404 may be a birefringent phase modulator or a single polarization phase modulator.

This result is not affected when port B of polarization beam splitter 402 is coupled to the fast axis of polarization maintaining fiber 403 and port C of polarization beam splitter 402 is coupled to the slow axis of polarization maintaining fiber 403.

The positions and connection order of the phase modulator 404 and the 90 degree fusion point 405 are changed, and the above result is not affected.

Fig. 5 is a schematic diagram showing the constitution of another polarization quadrature rotation reflecting means having a phase modulation function which can be used for the phase codec of the present invention.

The polarization quadrature rotating reflection apparatus with phase modulation function shown in fig. 5 includes the following components: a polarizing beam splitter 502, a phase modulator 503, a half wave plate 504.

Polarizing beamsplitter 502 includes three ports, port a, port B, and port C. The ports a, B, C may be referred to as an input port, a first output port, a second output port, respectively. One port 501 of the polarizing beamsplitter 502, port a, serves as both the input port and the output port of the device. Port B and port C of polarizing beamsplitter 502 are connected by a transmission optical path; more specifically, port B of polarizing beamsplitter 502 is coupled to port D of phase modulator 503 via a transmission optical path, port E of phase modulator 503 is coupled to half-wave plate 504 via a transmission optical path, and half-wave plate 504 is coupled to port C of polarizing beamsplitter 502 via a transmission optical path. The transmission path between port B of the polarization beam splitter 502 and port D of the phase modulator 503, the transmission path between port E of the phase modulator 503 and the half-wave plate 504, and the transmission path between the half-wave plate 504 and port C of the polarization beam splitter 502 are all 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 504 from the ports on both sides of the half-wave plate 504 forms an angle of 45 degrees with the slow axis or the fast axis of the half-wave plate 504.

In operation, an input optical pulse is input to polarizing beamsplitter 502 through port 501, port a of polarizing beamsplitter 502. The input light pulse may be considered to consist of two orthogonal polarization states, which may be denoted as x-polarization state and y-polarization state, respectively. The polarizing beam splitter 502 polarizes and splits the input light pulse into a first light pulse of the x-polarization state and a second light pulse of the y-polarization state for output by ports B and C of the polarizing beam splitter 502, respectively. The first optical pulse in the x-polarization state output by port B of the polarization beam splitter 502 is transmitted to the phase modulator 503, input to the phase modulator 503 by port D of the phase modulator 503, and subjected to phase modulation. The phase-modulated first optical pulse is output from port E of the phase modulator 503 to the half-wave plate 504. The polarization of the first light pulse is orthogonally rotated by the half-wave plate 504, and then the polarization state of the first light pulse is converted from the x polarization state to the y polarization state. The first light pulse of y polarization output by half-wave plate 504 is transmitted to port C of polarizing beam splitter 502, input to polarizing beam splitter 502 from port C of polarizing beam splitter 502, and output from port a of polarizing beam splitter 502. In this way, it is achieved that the x-polarization component of the input light pulse input by port a is transformed into the y-polarization when output by port a after reflection by the device. The second optical pulse with y polarization state output by the port C of the polarization beam splitter 502 is transmitted to the half-wave plate 504, and the polarization state of the second optical pulse after polarization orthogonal rotation by the half-wave plate 504 is converted into x polarization state. The second optical pulse of the x-polarization state output by the half-wave plate 504 is transmitted to the port E of the phase modulator 503, input to the phase modulator 503 by the port E of the phase modulator 503 and subjected to phase modulation. The phase modulated second optical pulse is output from port D of the phase modulator 503 to port B of the polarization beam splitter 502, input to the polarization beam splitter 502 from port B of the polarization beam splitter 502, and output from port a of the polarization beam splitter 502. In this way it is achieved that the y-polarization component of the input light pulse input by port a is transformed into the x-polarization state when output by port a after reflection by the device. The two orthogonal polarization states are polarization-orthogonally rotated using half-wave plate 504 such 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.

The first optical pulse input to the phase modulator 503 through the port D and the second optical pulse input to the phase modulator 503 through the port E are input to the phase modulator 503 in the same polarization state and subjected to the same phase modulation, thereby realizing polarization independent phase modulation. The two orthogonal polarization states of the input light pulse are each transformed into a polarization state orthogonal thereto when the output is reflected by the reflecting means.

The phase modulator 503 may be a birefringent phase modulator or a single polarization phase modulator.

The positions and connection order of the phase modulator 503 and the half-wave plate 504 are changed, and the above result is not affected.

The phase codec of the present invention, such as the phase codec described above and the phase codec described later with reference to fig. 9, can be used as a component of a direct current 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 direct current modulation quantum key distribution time bit-phase codec device.

A DC modulation quantum key distribution phase encoding and decoding device based on polarization quadrature rotation reflection using the phase encoder and decoder of the present invention is shown in FIG. 6, and comprises the following components: front beam splitter 603, optical circulators 604 and 610, polarization maintaining beam splitters 605 and 611, and polarization orthogonal rotating reflecting devices 606, 607, 612 and 613 (hereinafter also referred to as reflecting devices 606, 607, 612 and 613, respectively).

The polarization maintaining beam splitter 605, the two reflecting means 606 and 607 and the two arms between the polarization maintaining beam splitter 605 and the two reflecting means constitute a first polarization maintaining unequal arm michelson interferometer, i.e. a first phase codec according to the invention. The two arms of the first phase encoder/decoder are polarization maintaining fiber light paths. At least one of the reflecting means 606 and 607 comprises a direct current phase modulator.

Similarly, the polarization maintaining beam splitter 611, the two reflecting devices 612 and 613, and the two arms between the polarization maintaining beam splitter 611 and the two reflecting devices constitute a second polarization maintaining unequal arm michelson interferometer, i.e. a second phase codec according to the invention. The two arms of the second phase codec are polarization maintaining fiber optic paths. At least one of the reflecting means 612 and 613 comprises a direct current phase modulator.

Next, an example will be described taking the codec device of fig. 6 as an example for decoding.

One of the two ports 601 and 602 on the side of the front beamsplitter 603 serves as the input port for the device. The first port a and the second port B of the optical circulator 604 are connected to one output port of the front beam splitter 603 and one input port of the polarization maintaining splitter 605, respectively. The optical pulses input to the first phase codec are decoded and output by one output port 608 of the polarization maintaining splitter 605, or transmitted to the second port B of the optical circulator 604 and output from the third port C of the optical circulator 604 via another output port of the polarization maintaining splitter 605 (i.e., the one input port of the polarization maintaining splitter 605). The first port a and the second port B of the optical circulator 610 are connected to the other output port of the front beam splitter 603 and one input port of the polarization maintaining splitter 611, respectively. The optical pulses input to the second phase codec are decoded and output by one output port 614 of the polarization maintaining beam splitter 611, or transmitted to the second port B of the optical circulator 610 and output from the third port C of the optical circulator 610 via another output port of the polarization maintaining beam splitter 611 (i.e., the one input port of the polarization maintaining beam splitter 611).

In operation, an optical pulse enters the beam splitter 603 through the port 601 or 602 of the beam splitter 603 and is split by the beam splitter 603 into a first optical pulse and a second optical pulse. The first optical pulse is input through a first port a of the optical circulator 604 and output from a second port B of the optical circulator 604 to a polarization maintaining beam splitter 605. The polarization maintaining beam splitter 605 splits the input first optical pulse into two first sub-optical pulses. One first sub-light pulse is transmitted to the reflecting device 606 through the polarization maintaining optical fiber and is reflected by the reflecting device 606, and the other first sub-light pulse is transmitted to the reflecting device 607 through the polarization maintaining optical fiber and is reflected by the reflecting device 607, wherein the direct current phase modulator in the reflecting device 606 and/or 607 carries out direct current phase modulation according to the quantum key distribution protocol. The two reflected first sub-light pulses with relative delay are output by the port 608 after being combined by the polarization maintaining beam splitter 605, or output to the second port B of the optical circulator 604 and transmitted to the third port C of the optical circulator 604 to be output by the port 609. The second optical pulse is input through the first port a of the optical circulator 610 and output to the polarization maintaining beam splitter 611 through the second port B of the optical circulator 610. The polarization maintaining beam splitter 611 splits the input second optical pulse into two second sub-optical pulses. One path of second sub-light pulse is transmitted to the reflecting device 612 through the polarization maintaining optical fiber and is reflected by the reflecting device 612, and the other path of second sub-light pulse is transmitted to the reflecting device 613 through the polarization maintaining optical fiber and is reflected by the reflecting device 613, wherein the direct current phase modulator in the reflecting device 612 and/or 613 carries out direct current phase modulation according to the quantum key distribution protocol. The two reflected second sub-light pulses with relative delay are output by the port 614 after being combined by the polarization maintaining beam splitter 611, or output to the second port B of the optical circulator 610 and transmitted to the third port C of the optical circulator 610 to be output by the port 615. Wherein the dc phase modulator in reflection means 606 and/or 607 performs dc phase modulation according to the quantum key distribution protocol and/or the dc phase modulator in reflection means 612 and/or 613 performs dc phase modulation according to the quantum key distribution protocol, resulting in a dc phase modulation by one of the first unequal arm michelson interferometer and the second unequal arm michelson interferometer differing by 90 degrees from a dc phase modulation by the other.

Next, an example will be described taking the codec of fig. 6 as an example for encoding.

One port 608 of the polarization maintaining splitter 605, the third port C of the optical circulator 604, one port 614 of the polarization maintaining splitter 611, and the third port C of the optical circulator 610 serve as input ports for the device. The first port a and the second port B of the optical circulator 604 are connected to one port of the front beam splitter 603 and the other port of the polarization maintaining beam splitter 605, respectively. 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 608 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 then 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 front beam splitter 603. The first port a and the second port B of the optical circulator 610 are connected to the other port of the front beam splitter 603 and the other port of the polarization maintaining beam splitter 611, respectively. The optical pulse input from the third port C of the optical circulator 610 is input to the second phase codec through the second port B of the optical circulator 610. The optical pulse input from the one port 614 of the polarization maintaining beam splitter 611 and the third port C of the optical circulator 610 is encoded by the second phase codec, output from the polarization maintaining beam splitter 611 to the second port B of the optical circulator 610, and transmitted from the first port a of the optical circulator 610 to the front beam splitter 603. One of the two ports 601 and 602 on one side of the front beamsplitter 603 (left side in fig. 6) serves as the output port of the device. The optical pulses input by the one port 608 of the polarization maintaining beam splitter 605, the third port C of the optical circulator 604, the one port 614 of the polarization maintaining beam splitter 611, and the third port C of the optical circulator 610 are encoded to respectively implement four phase encodings, and the encoded optical pulses are output by the port 601 or 602 after being combined by the beam splitter 603.

A quantum key distribution time bit-phase codec device based on polarization quadrature rotation reflection using the phase codec of the present invention is shown in fig. 7, and includes the following components: beam splitters 703 and 704, polarization maintaining beam splitter 707, and polarization orthogonal rotation reflecting means 708 and 709 (hereinafter also referred to as reflecting means 708 and reflecting means 709, respectively).

The polarization maintaining beam splitter 707, the two reflecting means 708 and 709 and the two arms between the polarization maintaining beam splitter 707 and the two reflecting means constitute a polarization maintaining unequal arm michelson interferometer, i.e. a phase codec according to the invention. The two arms are polarization maintaining fiber light paths. At least one of the reflecting means 708 and 709 comprises a phase modulator.

Next, an example will be described taking the codec device of fig. 7 as an example for decoding.

The beam splitter 703 acts as a front beam splitter with one of the two ports 701 and 702 on one side acting as the input port to the device. Beam splitter 704 splits a pulse of light from beam splitter 703 before outputting it through port 705 or 706. The light pulses input to the polarization maintaining unequal arm michelson interferometer are decoded and output from port 710.

In 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 by beam splitter 703 for transmission. One optical pulse from beam splitter 703 is input to beam splitter 704 and split by beam splitter 704 and output via port 705 or 706 to effect time bit decoding. The other path of light pulse from the beam splitter 703 is input to the polarization maintaining beam splitter 707, and split into two sub-light pulses by the polarization maintaining beam splitter 707. One sub-optical pulse is transmitted to the reflecting means 708 via the polarization maintaining optical fiber and reflected back by the reflecting means 708, and the other sub-optical pulse is transmitted to the reflecting means 709 via the polarization maintaining optical fiber and reflected back by the reflecting means 709, during which the phase modulators in the reflecting means 708 and/or 709 are phase modulated according to the quantum key distribution protocol. The two reflected sub-optical pulses with relative delay are output by the port 710 after being combined by the polarization-preserving beam splitter 707.

Here, it should be noted that the beam splitter 704 is optional. It is possible that the one optical pulse is directly output by the front beam splitter 703 for temporal bit decoding.

Next, an example will be described taking the codec of fig. 7 as an example for encoding.

Ports 705 and 706 of beam splitter 704 and port 710 of polarization maintaining beam splitter 707 serve as input ports for the device. The optical pulses input from ports 705 and 706 are combined by beam splitter 704 and output to front beam splitter 703 for time bit encoding. The light pulses input from port 710 are encoded by a polarization maintaining unequal arm michelson interferometer and output by polarization maintaining beam splitter 707 to pre-splitter 703, where both phase encodings are achieved by modulating the phase modulators in reflecting devices 708 and/or 709. One of ports 701 and 702 of the front beamsplitter 703 serves as the output port of the device. The beam splitter 703 combines the light pulse output from the beam splitter 704 and the light pulse output from the polarization maintaining beam splitter 707, and outputs the combined light pulse through the port 701 or 702.

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

A DC modulation quantum key distribution time bit-phase encoding and decoding device based on polarization quadrature rotation reflection using the phase encoder-decoder of the present invention is shown in FIG. 8, and comprises the following components: beam splitters 803 and 804, optical circulator 807, polarization maintaining beam splitter 808, and polarization quadrature rotating reflecting means 809 and polarization quadrature rotating reflecting means 810 (hereinafter also referred to as reflecting means 809 and reflecting means 810, respectively).

The polarization maintaining beam splitter 808, the two reflecting means 809 and 810 and the two arms between the polarization maintaining beam splitter 808 and the two reflecting means constitute a polarization maintaining unequal arm michelson interferometer, i.e. a phase codec according to the invention. The two arms are polarization maintaining fiber light paths. At least one of the reflecting means 809 and 810 comprises a direct current phase modulator.

Next, an example will be described taking the codec device of fig. 8 as an example for decoding.

Beam splitter 803 acts as a front-end beam splitter with one of the two ports 801 and 802 on one side acting as the input port for the device. Beam splitter 804 splits a pulse of light from beam splitter 803 before outputting it through ports 805 or 806. The light 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 light 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 light pulse input to the polarization maintaining unequal arm michelson interferometer is output by port 811 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 output by port 812.

In operation, an input optical pulse enters beamsplitter 803 through port 801 or 802 of beamsplitter 803 and is split by beamsplitter 803 into two optical pulses for transmission. One optical pulse from beam splitter 803 is input to beam splitter 804 and split by beam splitter 804 and output by port 805 or 806 for temporal bit decoding. Another pulse of light from beamsplitter 803 is input through a first port a of optical circulator 807 and output from a second port B of optical circulator 807 to polarization maintaining beamsplitter 808. The polarization maintaining beam splitter 808 splits the other optical pulse into two sub-optical pulses. One sub-optical pulse is transmitted to the reflecting means 809 via the polarization maintaining optical fiber and reflected back by the reflecting means 809, and the other sub-optical pulse is transmitted to the reflecting means 810 via the polarization maintaining optical fiber and reflected back by the reflecting means 810, during which the direct current phase modulators in the reflecting means 809 and/or 810 perform direct current phase modulation according to the quantum key distribution protocol. The reflected two relatively delayed sub-optical pulses are output by port 811 after being combined by polarization maintaining beam splitter 808, or transmitted to second port B of optical circulator 807 and output by third port C of optical circulator 807 and output by port 812.

Here, it should be noted that the beam splitter 804 is optional. It is possible that the one optical pulse is directly output by the pre-splitter 803 for temporal bit decoding.

Next, an example will be described taking the codec of fig. 8 as an example for encoding.

Ports 805 and 806 of beam splitter 804, port 811 of polarization maintaining splitter 808, and third port C of optical circulator 807 serve as input ports for the device. The light 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 light 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 beam splitter 804 and output to front beam splitter 803 to implement time bit encoding. The optical pulse input from the port 811 and the optical pulse 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, output from the polarization maintaining beam splitter 808 to the second port B of the optical circulator 807 and transmitted to the front beam splitter 803 through the first port a of the optical circulator 807. The optical pulses input through the port 811 of the polarization maintaining beam splitter 808 and the third port C of the optical circulator 807 are encoded to achieve two kinds of phase encoding, respectively. One of ports 801 and 802 of beamsplitter 803 serves as the output port of the device. The beam splitter 803 combines the light pulse output from the beam splitter 804 and the light pulse output from the first port a of the optical circulator 807 and outputs the combined light pulse from the port 801 or 802.

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

A phase codec according to a preferred embodiment of the present invention is shown in fig. 9, and includes the following components: polarization maintaining beam splitter 903, polarization orthogonal rotation reflecting means 904, and polarization orthogonal rotation reflecting means 905 (hereinafter also referred to as reflecting means 904 and reflecting means 905, respectively).

One of the two ports 901 and 902 on the side of the polarization maintaining splitter 903 serves as an input port for the phase codec. Polarization maintaining beam splitter 903 and reflecting devices 904 and 905 form an unequal arm Michelson interferometer, and two arms between the two are polarization maintaining fiber optical paths. At least one of the reflecting means 904 and the reflecting means 905 includes a phase modulator, and may be, for example, a polarization quadrature rotating reflecting means employing the above configuration 4. The port 901 or 902 of the polarization maintaining beam splitter 903 may be the output port of the phase codec.

In operation, an optical pulse enters the polarization maintaining beam splitter 903 through a port 901 or 902 of the polarization maintaining beam splitter 903 and is split into two optical pulses by the polarization maintaining beam splitter 903. One of the two optical pulses is transmitted to the reflecting device 904 through the polarization maintaining optical fiber and reflected by the reflecting device 904, and the other optical pulse is transmitted to the reflecting device 905 through the polarization maintaining optical fiber and reflected by the reflecting device 905, and phase modulators in the reflecting device 904 and/or 905 perform phase modulation according to a quantum key distribution protocol. The two reflected light pulses with relative delay are output from the port 901 or 902 after being combined by the polarization maintaining beam splitter 903.

In the case where one of the input port and the output port of the polarization maintaining splitter 903 is the same port, the phase codec may further include an optical circulator. The light circulator may be located at the front end of a polarization maintaining beam splitter 903. An input optical pulse of any polarization state of incidence can be input from a first port of the optical circulator and output from a second port of the optical circulator to the polarization maintaining beam splitter 903, and a combined output from the polarization maintaining beam splitter 903 is input to the second port of the optical circulator and output from a third port of the optical circulator.

The terms "beam splitter" and "beam combiner" are used interchangeably herein, and a beam splitter may also be referred to as and function as a beam combiner, and vice versa. As used herein, the term "polarization maintaining fiber optical path" refers to an optical path for transmitting an optical pulse using a polarization maintaining fiber or an optical path formed by connecting polarization maintaining fibers.

The phase codec or corresponding codec device based on polarization quadrature rotation reflection of the present invention as described above may be configured at the receiving end of the quantum key distribution system for decoding. In addition, the phase codec or the corresponding codec device based on polarization quadrature rotation reflection according to the present invention can be configured at the transmitting end of the quantum key distribution system for encoding.

While the invention has been described in connection with specific embodiments thereof, it is to be understood that these drawings are included in the spirit and scope of the invention, it is not to be limited thereto.

Claims

2. The phase codec of claim 1, wherein the two polarization quadrature rotated reflecting means are polarization quadrature rotated reflecting means of the same construction or polarization quadrature rotated reflecting means of different construction.

3. The phase codec of claim 1, wherein the first transmission optical path and/or the second transmission optical path is a polarization maintaining optical path; or alternatively

The first transmission optical path and/or the second transmission optical path are polarization maintaining optical fiber optical paths; and an included angle between the slow axis of the polarization maintaining optical fiber of the second transmission optical path and the slow axis or the fast axis of the quarter wave plate is 45 degrees.

6. A direct current modulated quantum key distribution phase codec device comprising a pre-splitter and two phase codecs according to any of claims 1-5, both of said phase codecs being optically coupled to said pre-splitter via two sub-optical paths, respectively, wherein one of the ports of the splitter of each of said phase codecs not coupled to said two arms of the phase codec is optically coupled to a respective one of said two sub-optical paths, each of said sub-optical paths being provided with an optical circulator, wherein said phase modulator is a direct current phase modulator.

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

8. The codec of claim 7, further comprising a beam splitter coupled to the front beam splitter via another sub-optical path.

9. A direct current modulated quantum key distribution time bit-phase codec device comprising a pre-splitter and a phase codec according to any one of claims 1-5, said phase codec being optically coupled to said pre-splitter via a sub-optical path, wherein one of the ports of the splitter of said phase codec not coupled to said two arms is optically coupled to said one sub-optical path, wherein said one sub-optical path is provided with an optical circulator, said phase modulator being a direct current phase modulator.

10. The codec of claim 9, further comprising a beam splitter coupled to the front beam splitter via another sub-optical 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, provided at a 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 codec device according to any one of claims 6 to 10, which is provided at a transmitting end of the quantum key distribution system for encoding.