CN109039625B

CN109039625B - Quantum key distribution time bit-phase decoding method, device and system

Info

Publication number: CN109039625B
Application number: CN201811267189.5A
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: 2018-10-29
Filing date: 2018-10-29
Publication date: 2023-05-26
Anticipated expiration: 2038-10-29
Also published as: CN109039625A

Abstract

A quantum key distribution time bit-phase decoding method and device based on polarization orthogonal rotation and a corresponding system. The method comprises the following steps: splitting an input optical pulse into a first path of optical pulse and a second path of optical pulse; and performing phase decoding on the first path of optical pulse, and performing time bit decoding on the second path of optical pulse. The phase decoding of the first optical pulse includes: splitting the first path of optical pulse into two sub-optical pulses transmitted on two sub-optical paths; and outputting the combined beam after the relative delay, wherein at least one sub-optical path comprises at least one polarization orthogonal rotation device, controlling the two orthogonal polarization states of the first path of optical pulse to respectively differ by an integral multiple of 2 pi in phase difference transmitted by the two sub-optical paths in the beam splitting to beam combining process, and carrying out phase modulation on the first path of optical pulse before beam splitting or carrying out phase modulation on one of the two sub-optical pulses in the beam splitting to beam combining process. The invention can realize the time bit-phase coding quantum key distribution scheme of the environment interference immunity.

Description

Quantum key distribution time bit-phase decoding method, device and system

Technical Field

The present invention relates to the field of optical transmission secret communication technologies, and in particular, to a method and an apparatus for decoding a quantum key distribution time bit-phase based on polarization orthogonal rotation, and a quantum key distribution system including the apparatus.

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 safely share the key 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.

At present, the ground quantum key distribution is mainly based on optical fiber channel transmission, and in the optical fiber quantum channel transmission process, the optical fiber is subjected to non-ideal conditions such as non-circular symmetry in 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 a random birefringence effect is generated. The quantum key distribution time-phase protocol employs a set of time bases encoded using time patterns of two different time positions and a set of phase bases encoded using two phase differences of the front and rear light pulses. The polarization state of the light pulse is randomly changed when the light pulse reaches a receiving end after the light pulse is transmitted by a long-distance optical fiber under the influence of the random birefringence of the optical fiber. The time base decoding in the time-phase coding is not influenced by the change of the polarization state, however, when the phase base is in interference decoding, the polarization induced fading problem exists due to the double refraction influence of the transmission optical fiber and the optical fiber of the coding-decoding interferometer, so that the decoding interference is unstable, the error rate is increased, if correction equipment is added, the system complexity and the cost are increased, and the method is difficult to stably apply to the strong interference conditions of an aerial optical cable, a road bridge optical cable and the like. How to solve the problem of unstable phase decoding interference caused by polarization induced fading when phase base decoding in the time bit-phase coding quantum key distribution application to realize stable and efficient phase interference decoding is a hotspot and a difficult problem of quantum secret communication application based on the existing optical cable infrastructure.

Disclosure of Invention

To solve at least one of the above problems, the present invention proposes a method and apparatus for quantum key distribution time bit-phase decoding based on polarization quadrature rotation.

The invention provides at least the following technical scheme:

1. a quantum key distribution time bit-phase decoding method based on polarization quadrature rotation, the method comprising:

splitting an incident input light pulse with any polarization state into a first light pulse and a second light pulse; and

according to the quantum key distribution protocol, the first path of light pulse is subjected to phase decoding and the second path of light pulse is subjected to time bit decoding,

wherein phase decoding the first optical pulse includes:

splitting the first path of light pulse into two sub-light pulses; and

transmitting the two sub-optical pulses on two sub-optical paths respectively, carrying out relative delay on the two sub-optical pulses, and then combining and outputting the two sub-optical pulses,

wherein at least one polarization orthogonal rotation device is included in the at least one of the two sub-optical paths, the polarization orthogonal rotation device being configured to respectively polarization-orthogonally rotate two orthogonal polarization states of a sub-optical pulse transmitted therethrough, such that after passing through the polarization orthogonal rotation device, each of the two orthogonal polarization states of the sub-optical pulse is respectively transformed into a polarization state orthogonal thereto, and

Wherein the phase difference between one of two orthogonal polarization states of the first optical pulse transmitted through the two sub-optical paths in the beam splitting to beam combining process and the phase difference between the other polarization state transmitted through the two sub-optical paths are controlled such that the two phase differences differ by an integer multiple of 2 pi, and

before the first path of light pulse beam splitting, the first path of light pulse is subjected to phase modulation according to a quantum key distribution protocol, or at least one of the two sub-light pulses transmitted on the two sub-light paths is subjected to phase modulation according to the quantum key distribution protocol in the process of splitting the first path of light pulse to beam combining.

2. The quantum key distribution time bit-phase decoding method based on polarization quadrature rotation according to claim 1, wherein the two sub-optical paths include optical paths having birefringence for two orthogonal polarization states of the first optical pulse, and/or the two sub-optical paths have optical devices having birefringence for two orthogonal polarization states of the first optical pulse thereon, wherein the controlling of a phase difference of one polarization state of the two orthogonal polarization states of the first optical pulse transmitted through the two sub-optical paths during splitting to combining with a phase difference of the other polarization state transmitted through the two sub-optical paths such that the two phase differences differ by an integer multiple of 2Ω includes:

Each polarization state of the two orthogonal polarization states is kept unchanged when the two sub-optical paths are transmitted in the beam splitting to beam combining process and/or the corresponding orthogonal polarization state is kept unchanged after the polarization orthogonal rotation device performs the polarization orthogonal rotation; and

the length of the optical path with birefringence and/or the birefringence of the optical device with birefringence are/is adjusted so that the phase difference transmitted by one of the two orthogonal polarization states through the two sub-optical paths during the beam splitting to the beam combining process is different from the phase difference transmitted by the other polarization state through the two sub-optical paths by an integer multiple of 2 pi.

3. The quantum key distribution time bit-phase decoding method based on polarization quadrature rotation according to scheme 1 or 2, characterized in that,

the two sub-optical paths are configured as polarization maintaining fiber optical paths, the controlling the phase difference between one polarization state of two orthogonal polarization states of the first path optical pulse transmitted through the two sub-optical paths and the phase difference between the other polarization state transmitted through the two sub-optical paths in the beam splitting to beam combining process to make the two phase differences differ by an integer multiple of 2 pi, includes:

controlling a first distance difference between a distance transmitted in the case of one intrinsic polarization state of the polarization maintaining optical fiber when transmitted on one of the two sub-optical paths and a distance transmitted in the case of an orthogonal polarization state converted to the intrinsic polarization state, and a second distance difference between a distance transmitted in the case of the intrinsic polarization state and a distance transmitted in the case of an orthogonal polarization state converted to the intrinsic polarization state when transmitted on the other of the two sub-optical paths such that the first distance difference and the second distance difference differ by an integer multiple of a beat length of the polarization maintaining optical fiber.

4. The quantum key distribution time bit-phase decoding method according to claim 1 or 3, wherein the controlling the phase difference between the phase difference transmitted through the two sub-optical paths during the beam splitting to the beam combining of one of two orthogonal polarization states of the first optical pulse and the phase difference transmitted through the two sub-optical paths of the other polarization state such that the two phase differences differ by an integer multiple of 2 pi comprises:

the two sub-optical paths each comprise a polarization orthogonal rotation device, and each polarization orthogonal rotation device is positioned at the midpoint of the sub-optical path.

5. The quantum key distribution time bit-phase decoding method based on polarization orthogonal rotation according to claim 1, wherein the polarization orthogonal rotation means is a 90 degree faraday rotator or a half wave plate.

6. The quantum key distribution time bit-phase decoding method based on polarization orthogonal rotation according to claim 1, wherein a polarization maintaining fiber stretcher and/or a birefringent phase modulator is arranged on at least one of the two sub-optical paths, wherein a difference between a phase difference transmitted through the two sub-optical paths and a phase difference transmitted through the two sub-optical paths in a beam splitting to beam combining process of one polarization state of the two orthogonal polarization states of the first path light pulse is adjusted by the polarization maintaining fiber stretcher and/or the birefringent phase modulator.

7. The quantum key distribution time bit-phase decoding method based on polarization quadrature rotation according to claim 1, wherein time bit decoding the second optical pulse comprises:

directly outputting the second path of light pulse for detection; or alternatively

And splitting the second path of light pulse and outputting the split light pulse for detection.

8. A quantum key distribution time bit-phase decoding device based on polarization quadrature rotation, the time bit-phase decoding device comprising:

the front beam splitter is configured to split an incident input light pulse with any polarization state into a first light pulse and a second light pulse; the method comprises the steps of,

a phase decoder optically coupled to the pre-splitter, configured to phase decode the first optical pulse,

the phase decoder comprises a first beam splitter, a first beam combiner and two sub-optical paths optically coupled with the first beam splitter and the first beam combiner, wherein

The first beam splitter is configured to split the first path of optical pulses into two sub-optical pulses;

the two sub-optical paths are used for respectively transmitting the two sub-optical pulses and realizing the relative delay of the two sub-optical pulses;

The first combiner is configured to combine the two sub-optical pulses after a relative delay to output,

wherein in the phase decoder, the two sub-optical paths and the optical devices thereon are configured to control the phase difference between one of two orthogonal polarization states of the first optical pulse transmitted through the two sub-optical paths and the phase difference between the other polarization state transmitted through the two sub-optical paths in the beam splitting to beam combining process such that the two phase differences differ by an integer multiple of 2 pi,

wherein the phase decoder has a phase modulator located at the front end of the first beam splitter or on either of the two sub-optical paths, the phase modulator being configured to phase modulate the light pulses passing therethrough in accordance with a quantum key distribution protocol,

Wherein the pre-splitter outputs the second optical pulse for temporal bit decoding.

9. The polarization orthogonal rotation based quantum key distribution time bit-phase decoding apparatus according to claim 8, wherein the two sub-optical paths are configured as polarization maintaining optical fiber optical paths, and the two sub-optical paths and the optical device thereon are further configured to control a first distance difference between a distance transmitted in the case of one intrinsic polarization state and a distance transmitted in the case of orthogonal polarization state converted to the intrinsic polarization state when one intrinsic polarization state is transmitted on one of the two sub-optical paths, and a second distance difference between a distance transmitted in the case of the intrinsic polarization state and a distance transmitted in the case of orthogonal polarization state converted to the intrinsic polarization state when the intrinsic polarization state is transmitted on the other of the two sub-optical paths such that the first distance difference and the second distance difference differ by an integer multiple of a polarization maintaining optical fiber length.

10. The quantum key distribution time bit-phase decoding apparatus based on polarization quadrature rotation according to claim 8 or 9, characterized in that,

11. The quantum key distribution time bit-phase decoding apparatus based on polarization orthogonal rotation according to claim 8, wherein the polarization orthogonal rotation means is a 90 degree faraday rotator or a half wave plate.

12. The quantum key distribution time bit-phase decoding apparatus based on polarization quadrature rotation according to claim 8, wherein the phase decoder further comprises:

a polarization maintaining fiber stretcher positioned on any one of the two sub-optical paths, the polarization maintaining fiber stretcher being configured to adjust a polarization maintaining fiber length of an optical path in which the polarization maintaining fiber stretcher is positioned; and/or

A birefringent phase modulator on either of the two sub-optical paths, the birefringent phase modulator being configured to apply different adjustable phase modulations to two orthogonal polarization states of an optical pulse passing therethrough.

13. The quantum key distribution time bit-phase decoding apparatus based on polarization quadrature rotation according to claim 8, wherein the phase modulator is a polarization independent phase modulator; the phase modulator is configured to randomly perform 0-degree phase modulation or 180-degree phase modulation on the light pulse passing therethrough.

14. The quantum key distribution time bit-phase decoding device based on polarization quadrature rotation according to claim 8, wherein the phase decoder adopts an optical path structure of an unequal arm mach-zehnder interferometer; or,

the phase decoder adopts the light path structure of the unequal arm Michelson interferometer, wherein the first beam combiner and the first beam splitter are the same device, and the phase decoder further comprises:

two mirrors respectively located on the two sub-optical paths and respectively configured to reflect the two sub-optical pulses transmitted via the two sub-optical paths from the first beam splitter back to the first beam splitter,

wherein the first beam splitter is configured to combine the reflected two sub-optical pulses for output.

15. The quantum key distribution time bit-phase decoding apparatus based on polarization quadrature rotation according to claim 8, wherein the first beam splitter and the first beam combiner and the optical device on the optical path between the first beam splitter and the first beam combiner are polarization maintaining optical devices or non-birefringent optical devices.

16. The quantum key distribution time bit-phase decoding device based on polarization quadrature rotation of claim 8, further comprising a second beam splitter optically coupled to the pre-splitter and configured to receive the second optical pulse and split the second optical pulse for output for time bit decoding.

17. A quantum key distribution system, comprising:

the quantum key distribution time bit-phase decoding device based on polarization orthogonal rotation according to any one of the schemes 8 to 16, which is arranged at a receiving end of the quantum key distribution system and is used for time bit-phase decoding; and/or

The quantum key distribution time bit-phase decoding device based on polarization quadrature rotation according to any one of the schemes 8 to 16, which is disposed at a transmitting end of the quantum key distribution system and is used for time bit-phase encoding.

With the solution of the invention, several advantages are achieved. For example, for a time bit-phase encoded quantum key distribution application, the present invention facilitates control of the difference between the phase differences transmitted in the two arms of the unequal arm interferometer by two orthogonal polarization states of the light pulses in the phase-based decoding by employing polarization orthogonal rotation means in the two arms of the interferometer, enabling both orthogonal polarization states to be simultaneously and effectively interfered for output at the output port, thereby enabling a phase-based decoding function of the ambient interference immunity, enabling a stable time bit-phase encoded quantum key distribution solution of the ambient interference immunity. The quantum key distribution decoding scheme of the invention can resist polarization induced fading and simultaneously avoid the need of complex deviation rectifying equipment.

Drawings

FIG. 1 is a flow chart of a quantum key distribution time bit-phase decoding method based on polarization quadrature rotation according to a preferred embodiment of the present invention;

fig. 2 is a schematic diagram showing the composition and structure of a quantum key distribution time bit-phase decoding apparatus based on polarization quadrature rotation according to a preferred embodiment of the present invention;

fig. 3 is a schematic diagram showing the composition structure of a quantum key distribution time bit-phase decoding apparatus based on polarization quadrature rotation according to another preferred embodiment of the present invention;

fig. 4 is a schematic diagram showing the composition structure of a quantum key distribution time bit-phase decoding apparatus based on polarization quadrature rotation according to another preferred embodiment of the present invention;

fig. 5 is a schematic diagram showing the composition structure of a quantum key distribution time bit-phase decoding apparatus based on polarization quadrature rotation according to another preferred embodiment of the present invention;

fig. 6 is a schematic diagram showing the composition of a quantum key distribution time bit-phase decoding apparatus based on polarization quadrature rotation according to another preferred embodiment of the present invention.

Detailed Description

Preferred embodiments of the present invention are described in detail below with reference to the attached drawing figures, which form a part of the present application and, together with the embodiments of the present invention, serve to explain the principles of the invention. 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 invention.

A quantum key distribution time bit-phase decoding method based on polarization orthogonal rotation in a preferred embodiment of the invention is shown in fig. 1, and specifically comprises the following steps:

step S101: and splitting an incident input light pulse with any polarization state into a first light pulse and a second light pulse.

Specifically, the input light pulse is in any polarization state, and can be linear polarized, circular polarized or elliptical polarized completely polarized light, or can be partial polarized light or unpolarized light.

Step S102: and according to a quantum key distribution protocol, performing phase decoding on the first path of optical pulse and performing time bit decoding on the second path of optical pulse.

As will be appreciated by those skilled in the art, each light pulse may be considered to consist of two orthogonal polarization states (e.g., orthogonal x-polarization and y-polarization states). Likewise, two sub-optical pulses resulting from the splitting of a first optical pulse may also be seen as consisting of the same two orthogonal polarization states as the first optical pulse.

Step S103: phase decoding the first optical pulse may include:

splitting the first path of light pulse into two sub-light pulses; and

and controlling the phase difference of one polarization state of two orthogonal polarization states of the first path of light pulse transmitted through the two sub-light paths in the beam splitting to beam combining process and the phase difference of the other polarization state transmitted through the two sub-light paths to enable the two phase differences to be different by an integral multiple of 2 pi.

In addition, for the method of fig. 1, the phase modulation is performed during phase decoding of the first optical pulse according to the quantum key distribution protocol as follows: before splitting the first path of light pulse, carrying out phase modulation on the first path of light pulse according to a quantum key distribution protocol; or in the process of splitting the first path of optical pulse into the combined beam, carrying out phase modulation on at least one of the two sub-optical pulses transmitted on the two sub-optical paths according to a quantum key distribution protocol. The phase modulation of the first optical pulse according to the quantum key distribution protocol prior to splitting of the first optical pulse may be achieved by phase modulating one of two adjacent front and rear input optical pulses in the first optical pulse.

The relative delay and phase modulation are performed as required and specified by the quantum key distribution protocol and are not described in detail herein.

Regarding controlling the phase difference between the phase difference transmitted by the two sub-optical paths during the beam splitting to the beam combining of one of the two orthogonal polarization states of the first optical pulse and the phase difference transmitted by the two sub-optical paths of the other polarization state such that the two phase differences differ by an integer multiple of 2 pi, for example, assuming that the two orthogonal polarization states are respectively an x-polarization state and a y-polarization state, the phase difference transmitted by the two sub-optical paths during the beam splitting to the beam combining of the x-polarization state is denoted as Δx, the phase difference transmitted by the two sub-optical paths during the beam splitting to the beam combining of the y-polarization state is denoted as Δy, the phase difference transmitted by the two sub-optical paths during the beam splitting to the beam combining of one of the two orthogonal polarization states of the first optical pulse and the phase difference transmitted by the two sub-optical paths of the other polarization state differ by an integer multiple of 2 pi, or the phase difference between the two sub-optical paths of the two orthogonal polarization states of the first optical pulse may be expressed as an integer multiple of 2 pi:

Δx – Δy = 2π* m，

Where m is an integer and may be a positive integer, a negative integer or zero.

In one possible embodiment, the two sub-optical paths for transmitting the two sub-optical pulses resulting from the splitting of the first optical pulse comprise optical paths having birefringence for the two orthogonal polarization states of the first optical pulse and/or optical devices having birefringence for the two orthogonal polarization states of the first optical pulse on the two sub-optical paths. In this case, said controlling the phase difference between the transmission of one of the two orthogonal polarization states of the first optical pulse through the two sub-optical paths and the transmission of the other polarization state through the two sub-optical paths in the beam splitting to beam combining process to make the two phase differences differ by an integer multiple of 2Ω includes: each polarization state of the two orthogonal polarization states is kept unchanged when the two sub-optical paths are transmitted in the beam splitting to beam combining process and/or the corresponding orthogonal polarization state is kept unchanged after the polarization orthogonal rotation device performs the polarization orthogonal rotation; and adjusting the length of the optical path with birefringence and/or the birefringence of the optical device with birefringence so that the phase difference of one polarization state transmitted by the two sub-optical paths during the beam splitting to the beam combining process is different from the phase difference of the other polarization state transmitted by the two sub-optical paths by an integral multiple of 2 pi, in other words, the phase difference of the two orthogonal polarization states transmitted by the two sub-optical paths during the beam splitting to the beam combining process is different from the integral multiple of 2 pi. Alternatively, this may be achieved by either: i) The two sub-optical paths are configured as polarization maintaining optical fiber optical paths, and optical devices on the polarization maintaining optical fiber optical paths are configured as non-birefringent optical devices and/or polarization maintaining optical devices; ii) one of the two sub-optical paths is configured as a free space optical path and the optical devices on the two sub-optical paths are configured as polarization maintaining optical devices. 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. "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.

In one possible implementation, the controlling the phase difference between the two orthogonal polarization states of the first optical pulse transmitted through the two sub-optical paths and the phase difference between the other polarization state transmitted through the two sub-optical paths in the beam splitting to the beam combining process includes:

the two sub-optical paths are configured as polarization maintaining optical fiber optical paths, and a first distance difference between a distance transmitted in the case of one intrinsic polarization state and a distance transmitted in the case of the orthogonal polarization state converted into the intrinsic polarization state when one intrinsic polarization state is transmitted on one of the two sub-optical paths is controlled, and a second distance difference between a distance transmitted in the case of the intrinsic polarization state and a distance transmitted in the case of the orthogonal polarization state converted into the intrinsic polarization state when the intrinsic polarization state is transmitted on the other of the two sub-optical paths is controlled, such that the first distance difference and the second distance difference differ by an integer multiple of a beat length of the polarization maintaining optical fiber, thereby further causing a phase difference between one polarization state of the first optical pulse transmitted via the two sub-optical paths and a phase difference between the other polarization state transmitted via the two sub-optical paths to be different by an integer multiple of 2 pi in a beam splitting to a beam combining process, such that the two orthogonal polarization states of the first optical pulse each differ by an integer multiple of 2 pi in a phase difference between the two sub-optical paths in a beam splitting to a beam combining process.

In one possible implementation, the two sub-optical paths may be configured as free-space optical paths, and the optical devices on the two sub-optical paths may be configured as non-birefringent optical devices.

In one possible embodiment, a polarization maintaining fiber stretcher and/or a birefringent phase modulator is arranged on at least one of the two sub-optical paths for transmitting the two sub-optical pulses obtained by splitting the first optical pulse. The polarization maintaining fiber stretcher is suitable for adjusting the length of the polarization maintaining fiber of the light path where the polarization maintaining fiber stretcher is positioned. The birefringent phase modulator is adapted to apply different adjustable phase modulations to the two orthogonal polarization states passing therethrough, whereby the polarization maintaining fiber stretcher and/or the birefringent phase modulator may be arranged to adjust the difference between the phase difference transmitted by one of the two orthogonal polarization states of the first optical pulse through the two sub-optical paths and the phase difference transmitted by the other polarization state through the two sub-optical paths during splitting to combining of the beams. 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. Thus, the birefringent phase modulator may be used to influence and adjust the difference in phase difference between two orthogonal polarization states of the first optical pulse transmitted through the two sub-optical paths during splitting to combining, respectively.

The phase modulation of an optical pulse may be achieved by a polarization independent phase modulator. The polarization independent phase modulator is adapted to perform identical phase modulation of two orthogonal polarization states of the optical pulse and is therefore referred to as polarization independent. For example, the polarization independent phase modulator may be implemented by two birefringent phase modulators in series or in parallel. Depending on the case, the phase modulation may be achieved by a number of specific means. For example, these means may include: the length of the free space optical path is modulated, or the length of the optical fiber is modulated, or a series or parallel optical waveguide phase modulator or the like is utilized. For example, the desired phase modulation may be achieved by varying the length of the free-space optical path with a motor. For another example, the length of the optical fiber may be modulated by a fiber stretcher using a piezoelectric effect, thereby achieving phase modulation. In addition, the phase modulator may be of other types suitable for voltage control, and the desired phase modulation may be achieved by applying a suitable voltage to the polarization independent phase modulator to perform the same phase modulation on the two orthogonal polarization states of the light pulse.

In a preferred embodiment, phase modulating the first optical pulse according to the quantum key distribution protocol comprises: the first path of light pulse is randomly subjected to 0-degree phase modulation or 180-degree phase modulation. In a preferred embodiment, phase modulating at least one of the two sub-optical pulses transmitted on the two sub-optical paths according to a quantum key distribution protocol comprises: one of the two sub-optical pulses transmitted on the two sub-optical paths is randomly subjected to 0-degree phase modulation or 180-degree phase modulation. Here, randomly performing 0-degree phase modulation or 180-degree phase modulation means randomly performing phase modulation selected from both 0-degree phase modulation and 180-degree phase modulation.

According to one possible implementation, the time bit decoding of the second optical pulse comprises: directly outputting the second path of light pulse for detection; or the second path of light pulse is output for detection after being split.

A quantum key distribution time bit-phase decoding device based on polarization orthogonal rotation according to a preferred embodiment of the present invention is shown in fig. 2, and includes the following components: a front beam splitter 201, a beam splitter 202 (which may also be referred to as a "second beam splitter"), a beam splitter 203 (which may also be referred to as a "first beam splitter"), a phase modulator 204, a beam combiner 205 (which may also be referred to as a "first beam combiner"), polarization quadrature rotating means 206 and 207. The beam splitter 203, the beam combiner 205, and the two sub-optical paths therebetween may be collectively referred to as a phase decoder. The polarization orthogonal rotation means 206 or 207 is configured to polarization-orthogonally rotate the two orthogonal polarization states of a sub-optical pulse transmitted therethrough, respectively, such that each of the two orthogonal polarization states of the sub-optical pulse is transformed into a polarization state orthogonal thereto, respectively, after passing through the polarization orthogonal rotation means.

Preferably, the polarization quadrature rotation means 206 or 207 may be a 90 degree faraday rotator or a half wave plate. The 90-degree Faraday rotator can rotate the sub-light pulse transmitted along the slow axis of the polarization maintaining optical fiber to the fast axis of the polarization maintaining optical fiber, and/or rotate the sub-light pulse transmitted along the fast axis of the polarization maintaining optical fiber to the slow axis of the polarization maintaining optical fiber, so that the polarization orthogonal rotation of two orthogonal polarization states of the sub-light pulse is realized. For the half-wave plate, when the polarization direction of one of the two orthogonal polarization states of the light pulse is set to be 45 degrees with the fast axis or the slow axis of the half-wave plate, the half-wave plate can respectively convert each polarization state of the two orthogonal polarization states of the sub-light pulse into the polarization state orthogonal to the half-wave plate, so that the polarization orthogonal rotation of the sub-light pulse is realized.

The front beam splitter 201 is configured to split an incident input optical pulse with any polarization into two optical pulses.

The phase decoder is optically coupled to the pre-splitter 201 for receiving and phase decoding one of the two optical pulses. For convenience, the one optical pulse received by the phase decoder is hereinafter also referred to as the first optical pulse.

The beam splitter 202 is optically coupled to the front beam splitter 201, and is configured to receive the other optical pulse (also referred to as "second optical pulse") of the two optical pulses, split the other optical pulse, and output the split optical pulse for time bit decoding. Here, it should be noted that the beam splitter 202 is optional. It is also possible that the further optical pulse is directly output by the front beam splitter 201 for time bit decoding.

The beam splitter 203 is configured to split the first optical pulse from the front beam splitter 201 into two sub-optical pulses, so as to transmit the two sub-optical paths respectively, and combine the two sub-optical paths by the beam combiner 205 for outputting after relatively delaying the two sub-optical paths. The phase modulator 204 is configured to phase modulate the sub-optical pulses transmitted through one of the two sub-optical paths in which it resides according to a quantum key distribution protocol. In particular, two sub-optical paths are used for transmitting the two sub-optical pulses, respectively, and for achieving a relative delay of the two sub-optical pulses. The relative delay of the two sub-optical pulses can be achieved by adjusting the optical path physical length of either of the two sub-optical paths between the beam splitter 203 and the beam combiner 205. The beam combiner 205 is configured to combine the two sub-optical pulses transmitted via the two sub-optical paths to output.

Preferably, the two sub-optical paths are configured as polarization maintaining optical fibers, the two sub-optical paths and the optical devices thereon being further configured to control a distance transmitted in the case of one eigenpolarization state of the polarization maintaining optical fibers and a first distance difference in the case of the orthogonal polarization state converted to the eigenpolarization state transmitted in the case of the other eigenpolarization state transmitted in the case of the eigenpolarization state transmitted in the other sub-optical path of the two sub-optical paths and a second distance difference in the case of the orthogonal polarization state converted to the eigenpolarization state such that the first distance difference and the second distance difference differ by an integer multiple of a polarization maintaining optical fiber beat length, thereby causing a phase difference in one polarization state of a first input optical pulse transmitted in the case of the two orthogonal polarization states to be a phase difference in the case of the other polarization state transmitted in the case of the eigenpolarization state transmitted in the case of the intrinsic polarization state transmitted in the case of the two sub-optical paths to be a phase difference in the integer multiple of pi of the two sub-optical paths transmitted in the case of the beam splitting to the beam.

Preferably, the phase modulator 204 is configured to randomly perform either 0-degree phase modulation or 180-degree phase modulation on the sub-optical pulses passing therethrough. As is known in the art, such a phase modulation scheme is sometimes referred to as "alternating current phase modulation".

According to the present invention, in the phase decoder, the two sub-optical paths and the optical device thereon are configured to control the phase difference between the phase difference transmitted through the two sub-optical paths and the phase difference transmitted through the two sub-optical paths in the beam splitting to beam combining process of one polarization state of the two orthogonal polarization states of the first path optical pulse such that the two phase differences differ by an integer multiple of 2 pi, in other words, such that the phase difference transmitted through the two sub-optical paths in the beam splitting to beam combining process of the two orthogonal polarization states of the first path optical pulse each differ by an integer multiple of 2 pi.

In this regard, one optical path may or may not have birefringence for two orthogonal polarization states, depending on the type of optical path. For example, free-space optical paths do not have birefringence for two orthogonal polarization states of an input optical pulse, while polarization-maintaining fiber optical paths generally have birefringence that differs significantly from each other for two orthogonal polarization states of an input optical pulse. In addition, one optical device on the optical path may or may not have birefringence for two orthogonal polarization states, depending on the type of optical device. For example, one non-birefringent optical device does not have birefringence for two orthogonal polarization states of one input optical pulse, while one polarization maintaining optical device typically has birefringence for two orthogonal polarization states of one input optical pulse that differ significantly from each other.

For the phase decoder, there may optionally be the following settings:

● The two sub-optical paths between the beam splitter and the beam combiner in the phase decoder may be free space optical paths, the optical devices in the two sub-optical paths including the phase modulator, if any, being non-birefringent and/or polarization maintaining optical devices. With this arrangement, with polarization maintaining optics, the polarization maintaining optics themselves result in two orthogonal polarization states of a first path of light pulse input to the phase decoder each being separated by an integer multiple of 2 pi in phase difference transmitted through the two sub-optical paths during beam splitting to beam combining.

● The two sub-optical paths between the beam splitter and the beam combiner in the phase decoder are polarization maintaining optical fiber optical paths, wherein at least one polarization orthogonal rotation device (for example, a 90-degree faraday rotator or a half-wave plate) is included in the at least one sub-optical path in the two sub-optical paths, and the polarization orthogonal rotation device is configured to respectively perform polarization orthogonal rotation on two orthogonal polarization states of one sub-optical pulse transmitted through the polarization orthogonal rotation device, so that each polarization state in the two orthogonal polarization states of the one sub-optical pulse is respectively transformed into a polarization state orthogonal to the one sub-optical pulse after passing through the polarization orthogonal rotation device. In addition, the optics in the two sub-paths, including the phase modulator, if any, are polarization maintaining optics and/or non-birefringent optics.

● The phase decoder further comprises a fiber stretcher and/or a birefringent phase modulator. The optical fiber stretcher can be positioned on any one of two sub-optical paths between the beam splitter and the beam combiner of the phase decoder and can be used for adjusting the length of polarization maintaining optical fibers of the sub-optical path where the optical fiber stretcher is positioned. By adjusting the length of the polarization maintaining fiber by means of the fiber stretcher, it is advantageously easy to achieve that the two orthogonal polarization states of the first optical pulse input to the phase decoder are each transmitted via two sub-optical paths with a phase difference of an integer multiple of 2 pi during splitting to combining. In addition, the fiber stretcher may also be used as a phase modulator. A birefringent phase modulator may be located on either of the two sub-optical paths, and may be used to apply different phase modulations to the two orthogonal polarization states of the sub-optical pulses passing therethrough. By controlling the birefringent phase modulator, the difference in phase modulation experienced by each of the two orthogonal polarization states of the sub-optical pulses passing therethrough is adjustable. In this way, by using a birefringent phase modulator, the difference between the phase differences transmitted by the two sub-optical paths during the splitting to combining of the two orthogonal polarization states of the first optical pulse input to the phase decoder can be conveniently influenced and adjusted, and the difference is easily realized as an integer multiple of 2pi. The birefringent phase modulator may be a lithium niobate phase modulator as described hereinbefore.

● The phase decoder adopts the structure of an unequal arm Mach-Zehnder interferometer, the optical paths of two arms of the interferometer (namely, two sub-optical paths between a beam splitter and a beam combiner of the phase decoder) adopt polarization maintaining optical fibers, and the two arms of the interferometer are assumed to respectively comprise a polarization quadrature rotating device 206 and a polarization quadrature rotating device 207. Assuming that the distance from the beam splitter to the polarization quadrature rotating means 206 in one arm is L1, the distance from the polarization quadrature rotating means 206 in the one arm to the combiner is L2, the distance from the beam splitter to the polarization quadrature rotating means 207 in the other arm is L3, and the distance from the polarization quadrature rotating means 207 in the other arm to the combiner is L4, the length relationship may preferably satisfy (L1-L2) - (L3-L4) =nβ, where n is a positive integer, a negative integer, or zero, and β is the polarization maintaining fiber beat length. In this case, the other optical devices in the two sub-optical paths cause the two orthogonal polarization states of the first optical pulse input to the phase decoder to each differ by an integer multiple of 2π in the phase difference transmitted through the two sub-optical paths during beam splitting to beam combining. In a preferred embodiment, the two polarization orthogonal rotation means may be located at the midpoints of the two arms, respectively, i.e. l1=l2 and l3=l4, the length relationship satisfying (L1-L2) - (L3-L4) =0.

● The phase decoder adopts the structure of an unequal arm Michelson interferometer. At this time, the combiner of the phase decoder and the beam splitter are the same device. In this case, the phase decoder further includes two mirrors respectively located on two sub-optical paths for transmitting the two sub-optical pulses obtained by splitting the beam splitter of the phase decoder, for reflecting the two sub-optical pulses transmitted via the two sub-optical paths from the beam splitter of the phase decoder back to be combined by a beam combiner of the same device as the beam splitter of the phase decoder. Furthermore, in one embodiment, the phase decoder may also include an optical circulator (not shown). The optical circulator may be located at a beam splitter front end of the phase decoder. A corresponding one of the light pulses from the pre-splitter 201 may be input from a first port of the optical circulator and output from a second port of the optical circulator to a splitter of the phase decoder, and a combined output from a combiner of the phase decoder (the same device as the splitter of the phase decoder) may be input to the second port of the optical circulator and output from a third port of the optical circulator. However, in another embodiment of the present invention, another port of the beamsplitter (e.g., port 514 of beamsplitter 507 in fig. 5, or port 612 of beamsplitter 605 in fig. 6) may also be utilized as the output port of the michelson interferometer. Since only one output port is needed for the phase decoder according to the present invention, a corresponding optical circulator needs to be added when the output port and the input port of the phase decoder are the same port; when the output port and the input port of the phase decoder are different, an optical circulator is not required. Preferably, the optical paths of the two arms of the interferometer formed by the first beam splitter and the two reflecting mirrors (i.e. the two sub optical paths between the beam splitter and the two reflecting mirrors of the phase decoder) employ polarization maintaining optical fibers, the two arms of the interferometer may respectively include one polarization orthogonal rotation device, the length relationship preferably satisfies 2 (L1-L2) -2 (L3-L4) =nβ, where n is a positive integer, a negative integer or zero, and β is a polarization maintaining optical fiber length, considering that the sub optical pulses are transmitted back and forth along the two arms, the distance transmitted through the slow axis or the fast axis of the polarization maintaining optical fiber is 2 times the corresponding polarization maintaining optical fiber length. In this case, the other optical devices in the two sub-optical paths cause the two orthogonal polarization states of the first optical pulse input to the phase decoder to each differ by an integer multiple of 2π in the phase difference transmitted through the two sub-optical paths during beam splitting to beam combining. In a preferred embodiment, the two polarization orthogonal rotation means may be located at the midpoints of the two arms, respectively, i.e. l1=l2 and l3=l4, the length relationship satisfying 2 (L1-L2) -2 (L3-L4) =0.

"polarization maintaining fiber beat length" is a concept known in the art and refers to the length of a polarization maintaining fiber corresponding to the phase difference of 2 pi produced by the transmission of two intrinsic polarization states of the polarization maintaining fiber along the polarization maintaining fiber.

Although fig. 2 shows that a phase modulator is arranged between the beam splitter 203 and the beam combiner 205, i.e. one of the two sub-optical pulses obtained by splitting is phase modulated according to the quantum key distribution protocol in the splitting to combining process, it is also possible that a phase modulator is arranged at the front end of the beam splitter 203, i.e. the first optical pulse is phase modulated according to the quantum key distribution protocol before splitting. Furthermore, it is also possible to provide a phase modulator, i.e. to phase modulate the incoming one input light pulse, before the front beam splitter 201.

In addition, although the phase decoder is shown in fig. 2 as having only one phase modulator, it is also possible to provide one phase modulator on each of the two sub-optical paths between the beam splitter 203 and the beam combiner 205. In the case where two phase modulators are provided, the difference in the phases modulated by the two phase modulators is determined by the quantum key distribution protocol.

For the embodiment of fig. 2, the beam splitter 203 and beam combiner 205 are preferably polarization maintaining optics. Alternatively, the beam splitter 203 and the beam combiner 205 may be a polarization maintaining beam splitter and a polarization maintaining beam combiner, respectively. With respect to polarization maintaining optical devices, which exist in two orthogonal intrinsic polarization states, the polarization state is maintained for an incident pulse of light of the intrinsic polarization state, as known to those skilled in the art.

Fig. 3 shows a quantum key distribution time bit-phase decoding apparatus based on polarization quadrature rotation according to another preferred embodiment of the present invention. As shown in fig. 3, the phase decoder therein adopts the structure of an unequal arm mach-zehnder interferometer. Specifically, the quantum key distribution time bit-phase decoding device comprises the following components: a front beam splitter 303, a beam splitter 304, a polarization maintaining beam splitter 307, a polarization maintaining fiber stretcher 309, a phase modulator 311, a polarization maintaining beam combiner 312, a polarization quadrature rotating device 308, and a polarization quadrature rotating device 310.

One of the two

ports

301 and 302 on one side of the front splitter 303 serves as an input to the quantum key distribution time bit-phase decoding device. The beam splitter 304 receives one path of the input optical pulse split by the front beam splitter 303 and splits it into two sub-optical pulses. The polarization maintaining beam splitter 307 and the polarization maintaining beam combiner 312 form part of an unequal arm mach-zehnder interferometer, and the two sub-optical paths between the polarization maintaining beam splitter 307 and the polarization maintaining beam combiner 312 (i.e., the two arms of the unequal arm mach-zehnder interferometer) may be polarization maintaining fiber optical paths, and the polarization maintaining fiber stretcher 309 and the phase modulator 311 may be inserted into the same arm of the unequal arm mach-zehnder interferometer or into the two arms of the unequal arm mach-zehnder interferometer, respectively. The two arms of the unequal arm mach-zehnder interferometer comprise at least one polarization quadrature rotating means, which may comprise, for example, one polarization quadrature rotating means 308 and one polarization quadrature rotating means 310, respectively. The optical pulse input to the polarization maintaining beam splitter 307 is decoded by the unequal arm mach-zehnder interferometer and output from the port 313.

In operation, an incident light pulse enters the front beam splitter 303 through the

port

301 or 302 of the front beam splitter 303 to be split into two light pulses (a first light pulse and a second light pulse) for transmission, wherein the first light pulse is input into the polarization maintaining beam splitter 307 to be split into two sub light pulses, one sub light pulse of the two sub light pulses is transmitted through the polarization orthogonal rotation device 308 and modulated by the polarization maintaining fiber stretcher 309 (wherein the setting sequence of the polarization orthogonal rotation device 308 and the polarization maintaining fiber stretcher 309 can be changed or simply referred to as "sequence independent"), the other sub light pulse is transmitted through the polarization orthogonal rotation device 310 and randomly modulated by the phase modulator 311 for 0 degree or 180 degree phase (sequence independent), and the two sub light pulses are output through the port 313 after being combined through the polarization maintaining beam combiner 312 after being relatively delayed. The second optical pulse output from the front splitter 303 is split into two sub-optical pulses by the splitter 304 and output via

ports

305 or 306 for time bit decoding.

Preferably, assuming that the length between polarization maintaining beam splitter 307 and polarization orthogonal rotation device 308 is L1, the length between polarization orthogonal rotation device 308 and polarization maintaining combiner 312 is L2, the length between polarization maintaining beam splitter 307 and polarization orthogonal rotation device 310 is L3, and the length between polarization orthogonal rotation device 310 and polarization maintaining combiner 312 is L4, polarization maintaining fiber stretcher 309 is modulated such that the length relationship satisfies:

(L1-L3) - (L2-L4) =nβ, or

（L1－L2）－（L3－L4）=nβ，

Wherein beta is the beat length of the polarization maintaining optical fiber, and n is an integer; thus, the difference of the phase difference transmitted by the two orthogonal polarization states of the first path light pulse at the two arms of the unequal arm Mach-Zehnder interferometer is an integer multiple of 2 pi.

The phase modulator 311 is a polarization independent optical device. If the phase modulator 311 is not connected, the polarization maintaining fiber stretcher 309 realizes the phase modulation function of the phase modulator 311, and the above result is not affected.

Fig. 4 shows a quantum key distribution time bit-phase decoding apparatus based on polarization quadrature rotation according to another preferred embodiment of the present invention, as shown in fig. 4, in which a phase decoder adopts a structure of an unequal arm mach-zehnder interferometer. The quantum key distribution time bit-phase decoding device comprises the following components: a front beam splitter 403, a polarization maintaining beam splitter 405, polarization quadrature rotating means 406 and 408, a polarization maintaining fiber stretcher 407, a phase modulator 409, and a polarization maintaining beam combiner 410.

One of the two

ports

401 and 402 on one side of the pre-splitter 403 serves as an input to the time bit-phase decoding means for receiving the incoming optical pulses. Polarization maintaining beam splitter 405 and polarization maintaining beam combiner 410 comprise an unequal arm mach-zehnder interferometer. The polarization maintaining fiber stretcher 407 and the phase modulator 409 may be inserted into the same arm of the unequal arm mach-zehnder interferometer or into two arms of the unequal arm mach-zehnder interferometer, respectively. At least one of the two arms of the unequal arm mach-zehnder interferometer comprises at least one polarization quadrature rotating device, e.g., the two arms may comprise one polarization quadrature rotating device 406 and one polarization quadrature rotating device 408, respectively, and the optical pulses input to the polarization maintaining beam splitter 405 are decoded by the unequal arm mach-zehnder interferometer and output through port 411.

In operation, an optical pulse enters the front beam splitter 403 through the

port

401 or 402 of the front beam splitter 403 and is split into two paths of optical pulses for transmission, and one path of optical pulse is directly output by the port 404; the other path of light pulse is input into a polarization-preserving beam splitter 405 to split into two sub-light pulses, wherein one sub-light pulse is transmitted by a polarization orthogonal rotation device 406 and modulated by a polarization-preserving fiber stretcher 407 (sequence is irrelevant), the other sub-light pulse is transmitted by the polarization orthogonal rotation device 408 and modulated by a phase modulator 409 (sequence is irrelevant), and the two sub-light pulses are output by a port 411 after being combined by a polarization-preserving beam combiner 410 after relatively delayed.

Preferably, assuming that the length between polarization maintaining beam splitter 405 and polarization orthogonal rotation device 406 is L1', the length between polarization orthogonal rotation device 406 and polarization maintaining combiner 410 is L2', the length between polarization maintaining beam splitter 405 and polarization orthogonal rotation device 408 is L3', and the length between polarization orthogonal rotation device 408 and polarization maintaining combiner 410 is L4', polarization maintaining fiber stretcher 407 is modulated such that the length relationship satisfies:

(L1 '-L3') - (L2 '-L4') =nβ, or

（L1’－L2’）－（L3’－L4’）=nβ，

Wherein beta is the beat length of the polarization maintaining optical fiber, and n is an integer; so that the difference between the phase differences transmitted by the two orthogonal polarization states of the input light pulse at the two arms of the unequal arm Mach-Zehnder interferometer is an integer multiple of 2 pi.

The phase modulator 409 is a polarization independent optical device. If the phase modulator 409 is not connected, the polarization maintaining fiber stretcher 407 realizes the phase modulation function of the phase modulator 409, and the above result is not affected.

Fig. 5 shows a quantum key distribution time bit-phase decoding apparatus based on polarization quadrature rotation according to another preferred embodiment of the present invention, as shown in fig. 5, in which the phase decoder adopts the structure of an unequal arm michelson interferometer. Specifically, the quantum key distribution time bit-phase decoding device comprises the following components: a front beam splitter 503, a beam splitter 504, a polarization maintaining beam splitter 507, polarization quadrature rotating means 508 and 511, a polarization maintaining fiber stretcher 509, a phase modulator 512, and mirrors 510 and 513.

One of the two

ports

501 and 502 on one side of the front beam splitter 503 serves as an input to the time bit-phase decoding device, and the beam splitter 504 receives one input optical pulse split by the front beam splitter 503 and splits it into two sub-optical pulses. Polarization maintaining beam splitter 507 and mirrors 510, 513 form an unequal arm michelson interferometer, and polarization maintaining fiber stretcher 509 and phase modulator 512 may be inserted into the same arm of the unequal arm michelson interferometer or into both arms of the unequal arm michelson interferometer, respectively. At least one of the two arms of the unequal arm michelson interferometer comprises at least one polarization quadrature rotation means, e.g. the two arms may comprise one polarization quadrature rotation means 508 and one polarization quadrature rotation means 511, respectively. The light pulse input to the polarization maintaining beam splitter 507 is decoded by the unequal arm michelson interferometer and output by a port 514 of the polarization maintaining beam splitter 507. In this example, polarization maintaining beam splitter 507 functions as both a polarization maintaining beam splitter and a polarization maintaining beam combiner.

In operation, an optical pulse enters the front beam splitter 503 through the

port

501 or 502 of the front beam splitter 503 and is split into two optical pulses for transmission, namely, a first optical pulse and a second optical pulse. The second optical pulse is input to the beam splitter 504, and then split into two sub-optical pulses, and output via the port 505 or the port 506 for time bit decoding. The first path of light pulse is input to the polarization-preserving beam splitter 507 and then split into two sub-light pulses, wherein one sub-light pulse is transmitted by the polarization orthogonal rotation device 508 and is reflected by the reflecting mirror 510 after being modulated (sequence-independent) by the polarization-preserving fiber stretcher 509, the other sub-light pulse is transmitted by the polarization orthogonal rotation device 511 and is reflected by the reflecting mirror 513 after being modulated (sequence-independent) by the phase modulator 512, and the two reflected relatively delayed sub-light pulses are output by the port 514 of the polarization-preserving beam splitter 507 after being combined by the polarization-preserving beam splitter 507, or can be output through the port of the optical circulator under the condition that the optical circulator is installed.

Preferably, assuming that the length between the polarization maintaining beam splitter 507 and the polarization orthogonal rotation device 508 is L1", the length between the polarization orthogonal rotation device 508 and the mirror 510 is L2", the length between the polarization maintaining beam splitter 507 and the polarization orthogonal rotation device 511 is L3", and the length between the polarization orthogonal rotation device 511 and the mirror 513 is L4", the polarization maintaining fiber stretcher 509 is modulated such that the length relationship satisfies:

2 (L1 "-L3") -2 (L2 "-L4") =nβ, or

2（L1”－L2”）－2（L3”－L4”）=nβ

Wherein beta is the beat length of the polarization maintaining optical fiber, and n is an integer; so that the difference between the phase differences transmitted by the two orthogonal polarization states of the first light pulse at the two arms of the unequal arm Michelson interferometer is an integer multiple of 2 pi.

The phase modulator 512 is a polarization independent optical device. If the phase modulator 512 is not connected, the polarization maintaining fiber stretcher 509 realizes the phase modulation function of the phase modulator 512, and the above result is not affected.

Fig. 6 shows a quantum key distribution time-phase decoding apparatus based on polarization quadrature rotation according to another embodiment of the present invention. As shown in fig. 6, the phase decoder therein adopts the structure of an unequal arm michelson interferometer. Specifically, the quantum key distribution time bit-phase decoding device comprises the following components: a front beam splitter 603, a polarization maintaining beam splitter 605, polarization quadrature rotating means 606 and 609, a polarization maintaining fiber stretcher 607, a phase modulator 610, and mirrors 608 and 611.

One of the two

ports

601 and 602 on one side of the front beam splitter 603 serves as an input to the time bit-phase decoding means. Polarization maintaining beam splitter 605 and mirrors 608, 611 make up an unequal arm michelson interferometer. The polarization maintaining fiber stretcher 607 and the phase modulator 610 may be inserted into the same arm of the unequal arm michelson interferometer or into both arms of the unequal arm michelson interferometer, respectively. At least one of the two arms of the unequal arm michelson interferometer comprises at least one polarization quadrature rotation means, e.g. the two arms may comprise one polarization quadrature rotation means 606 and one polarization quadrature rotation means 609, respectively. The optical pulse input to the beam splitter polarization maintaining 605 is decoded by the unequal arm michelson interferometer and output through the port 612 of the polarization maintaining beam splitter 605, or in the case of an optical circulator, may be output through the port of the optical circulator.

In operation, an optical pulse enters the front beam splitter 603 via the

port

601 or 602 of the front beam splitter 603 and is split into two optical pulses for transmission, namely a first optical pulse and a second optical pulse. The second optical pulse is directly output by port 604 for time bit decoding; the first path of light pulse is input into the polarization-preserving beam splitter 605 and split into two sub-light pulses, one sub-light pulse is transmitted by the polarization orthogonal rotation device 606 and is reflected by the reflecting mirror 608 after being modulated (sequence-independent) by the polarization-preserving fiber stretcher 607, the other sub-light pulse is transmitted by the polarization orthogonal rotation device 609 and is reflected by the reflecting mirror 611 after being modulated (sequence-independent) by the phase modulator 610, and the reflected two relatively delayed sub-light pulses are output by the port 612 of the polarization-preserving beam splitter 605 after being combined by the polarization-preserving beam splitter 605. In this example, polarization maintaining beam splitter 605 functions as both a polarization maintaining beam splitter and a polarization maintaining beam combiner.

Preferably, assuming a length L1 '"between the polarization maintaining beam splitter 605 and the polarization orthogonal rotation device 606, a length L2'" between the polarization orthogonal rotation device 606 and the mirror 608, a length L3 '"between the polarization maintaining beam splitter 605 and the polarization orthogonal rotation device 609, and a length L4'" between the polarization orthogonal rotation device 609 and the mirror 611, the polarization maintaining fiber stretcher 607 is modulated such that the length relationship satisfies:

2 (L1 '"-L3'") -2 (L2 '"-L4'") =nβ, or

2（L1”’－L2”’）－2（L3”’－L4”’）=nβ，

Wherein beta is the beat length of the polarization maintaining optical fiber, and n is an integer; so that the difference between the phase differences transmitted by the two orthogonal polarization states of the first optical pulse in the two arms of the unequal arm michelson interferometer is an integer multiple of 2 pi.

The phase modulator 610 is a polarization independent optical device. If the phase modulator 610 is not connected, the polarization maintaining fiber stretcher 607 realizes the phase modulation function of the phase modulator 610, and the above result is not affected.

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.

In still another aspect, the present invention provides a quantum key distribution system, where the quantum key distribution time bit-phase decoding device based on polarization quadrature rotation of the present invention may be configured at a receiving end of the quantum key distribution system, for time bit-phase decoding. In addition, the quantum key distribution time bit-phase decoding device based on polarization orthogonal rotation can be configured at the transmitting end of the quantum key distribution system and used for time bit-phase coding.

The invention facilitates control of the difference in phase difference between two orthogonal polarization states of an optical pulse transmitted in each of two arms of an unequal arm interferometer in phase-based decoding by employing polarization quadrature rotating means in the two arms of the interferometer. In addition, the invention can realize the effective interference output of two orthogonal polarization states of the light pulse in the phase-based decoding at the output port, which is equivalent to the polarization diversity processing of the two orthogonal polarization states, can effectively solve the problem of unstable interference decoding caused by polarization induced fading, realizes stable phase decoding of environmental interference immunity, does not need to use a polarization beam splitter and two interferometers to decode the two polarization states respectively, and eliminates the need of deviation correction.

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.

Although the exemplary embodiments have been described in detail, the foregoing description is illustrative and not restrictive in all aspects. It should be understood that numerous other modifications and variations could be devised without departing from the scope of the exemplary embodiments, which fall within the scope of the invention. Accordingly, the scope of the invention should be assessed as that of the appended claims.

Claims

wherein phase decoding the first optical pulse includes:

splitting the first path of light pulse into two sub-light pulses; and

wherein at least one polarization orthogonal rotation device is included in at least one of the two sub-optical paths, the polarization orthogonal rotation device is configured to respectively perform polarization orthogonal rotation on two orthogonal polarization states of a sub-optical pulse transmitted through the polarization orthogonal rotation device, so that after passing through the polarization orthogonal rotation device, each polarization state of the two orthogonal polarization states of the sub-optical pulse is respectively transformed into a polarization state orthogonal to the sub-optical pulse, and

wherein before the first path of light pulse beam splitting, the first path of light pulse is subjected to phase modulation according to a quantum key distribution protocol, or in the process of splitting the first path of light pulse to beam combining, at least one of the two sub-light pulses transmitted on the two sub-light paths is subjected to phase modulation according to the quantum key distribution protocol,

Wherein the two sub-optical paths include optical paths having birefringence for two orthogonal polarization states of the first optical pulse, and/or the two sub-optical paths have optical devices having birefringence for two orthogonal polarization states of the first optical pulse, wherein the controlling of a phase difference of one polarization state of the two orthogonal polarization states of the first optical pulse transmitted through the two sub-optical paths and a phase difference of the other polarization state transmitted through the two sub-optical paths in a beam splitting to beam combining process causes the two phase differences to differ by an integer multiple of 2 pi, includes:

2. The quantum key distribution time bit-phase decoding method based on polarization quadrature rotation according to claim 1, wherein,

3. The method of claim 1 or 2, wherein the controlling the phase difference between the phase difference transmitted through the two sub-optical paths during splitting to combining of one of two orthogonal polarization states of the first optical pulse and the phase difference transmitted through the two sub-optical paths of the other polarization state such that the two phase differences differ by an integer multiple of 2Ω comprises:

4. The quantum key distribution time bit-phase decoding method based on polarization orthogonal rotation according to claim 1, wherein the polarization orthogonal rotation means is a 90 degree faraday rotator or a half wave plate.

5. The quantum key distribution time bit-phase decoding method based on polarization orthogonal rotation according to claim 1, wherein a polarization maintaining fiber stretcher and/or a birefringent phase modulator is arranged on at least one of the two sub-optical paths, wherein a difference between a phase difference transmitted through the two sub-optical paths and a phase difference transmitted through the two sub-optical paths by one of two orthogonal polarization states of the first optical pulse is adjusted by the polarization maintaining fiber stretcher and/or the birefringent phase modulator in a beam splitting to beam combining process.

6. The quantum key distribution time bit-phase decoding method based on polarization quadrature rotation of claim 1, wherein time bit decoding the second optical pulse comprises:

7. A quantum key distribution time bit-phase decoding device based on polarization quadrature rotation, the time bit-phase decoding device comprising:

wherein at least one polarization orthogonal rotation device is included in at least one of the two sub-optical paths, the polarization orthogonal rotation device is configured to respectively perform polarization orthogonal rotation on two orthogonal polarization states of one sub-optical pulse transmitted by the polarization orthogonal rotation device, so that after the polarization orthogonal rotation device is used, each polarization state of the two orthogonal polarization states of the one sub-optical pulse is respectively transformed into a polarization state orthogonal to the two orthogonal polarization states of the one sub-optical pulse, wherein the two sub-optical paths comprise optical paths with double refraction on the two orthogonal polarization states of the first sub-optical pulse, and/or optical devices with double refraction on the two sub-optical paths on the two orthogonal polarization states of the first sub-optical pulse, and

Wherein in the phase decoder, the two sub-optical paths and the optical devices thereon are configured to control a phase difference between a phase difference transmitted by the two sub-optical paths and a phase difference transmitted by the other polarization state during beam splitting to beam combining such that the two phase differences differ by an integer multiple of 2 pi, wherein the controlling a phase difference transmitted by the two sub-optical paths and a phase difference transmitted by the other polarization state during beam splitting to beam combining such that the two phase differences differ by an integer multiple of 2 pi, comprises:

the length of the optical path with birefringence and/or the birefringence of the optical device with birefringence are adjusted so that the phase difference of one polarization state of the two orthogonal polarization states transmitted by the two sub-optical paths and the phase difference of the other polarization state transmitted by the two sub-optical paths in the process of splitting the beam to combining the beam are different by an integral multiple of 2 pi,

8. The polarization orthogonal rotation based quantum key distribution time bit-phase decoding device of claim 7, wherein the two sub-optical paths are configured as polarization maintaining optical fiber optical paths, the two sub-optical paths and the optical devices thereon are further configured to control a first distance difference between a distance transmitted in the case of one intrinsic polarization state and a distance transmitted in the case of orthogonal polarization state converted to the intrinsic polarization state when one of the two sub-optical paths is transmitted, and a second distance difference between a distance transmitted in the case of the intrinsic polarization state and a distance transmitted in the case of orthogonal polarization state converted to the intrinsic polarization state when the intrinsic polarization state is transmitted on the other of the two sub-optical paths such that the first distance difference and the second distance difference differ by an integer multiple of a polarization maintaining fiber length.

9. The quantum key distribution time bit-phase decoding apparatus based on polarization quadrature rotation according to claim 7 or 8, wherein,

10. The polarization quadrature rotation based quantum key distribution time bit-phase decoding apparatus of claim 7, wherein the polarization quadrature rotation means is a 90 degree faraday rotator or a half wave plate.

11. The quantum key distribution time bit-phase decoding apparatus based on polarization quadrature rotation of claim 7, wherein the phase decoder further comprises:

12. The quantum key distribution time bit-phase decoding apparatus based on polarization quadrature rotation of claim 7, wherein the phase modulator is a polarization independent phase modulator; the phase modulator is configured to randomly perform 0-degree phase modulation or 180-degree phase modulation on the light pulse passing therethrough.

13. The quantum key distribution time bit-phase decoding apparatus based on polarization quadrature rotation according to claim 7, wherein,

the phase decoder adopts an optical path structure of the unequal arm Mach-Zehnder interferometer; or,

14. The quantum key distribution time bit-phase decoding apparatus based on polarization quadrature rotation of claim 7, wherein the first beam splitter and the first beam combiner and the optical device on the optical path between the first beam splitter and the first beam combiner are polarization maintaining optical devices or non-birefringent optical devices.

15. The polarization quadrature rotation based quantum key distribution time bit-phase decoding apparatus of claim 7, further comprising a second beam splitter optically coupled to the pre-splitter and configured to receive the second optical pulse and split the second optical pulse for output for time bit decoding.

16. A quantum key distribution system, comprising:

the quantum key distribution time bit-phase decoding device based on polarization orthogonal rotation according to any one of claims 7 to 15, which is disposed at a receiving end of the quantum key distribution system and is used for time bit-phase decoding; and/or

The quantum key distribution time bit-phase decoding device based on polarization orthogonal rotation according to any one of claims 7 to 15, which is disposed at a transmitting end of the quantum key distribution system and is used for time bit-phase encoding.