CN109039625A

CN109039625A - Quantum key distribution time bit-phase decoding method, apparatus and system based on polarized orthogonal rotation

Info

Publication number: CN109039625A
Application number: CN201811267189.5A
Authority: CN
Inventors: 许华醒
Original assignee: China Electronics Technology Group Corp CETC
Current assignee: China Electronics Technology Group Corp CETC
Priority date: 2018-10-29
Filing date: 2018-10-29
Publication date: 2018-12-18
Anticipated expiration: 2038-10-29
Also published as: CN109039625B

Abstract

A kind of quantum key distribution time bit-phase decoding method and apparatus and corresponding system based on polarized orthogonal rotation.This method comprises: being the first via and the second tunnel light pulse by input optical pulse beam splitting；Phase decoding is carried out to first via light pulse, the decoding of time bit is carried out to the second tunnel light pulse.Carrying out phase decoding to first via light pulse include: by first via light pulse beam splitting is the two-way sub-light pulse transmitted in two strip optical paths；Beam output will be closed after its relative time delay, at least one sub-light road includes at least one polarized orthogonal rotating device, the phase difference through two strip optic paths differs the integral multiple of 2 π during controlling each comfortable beam splitting of two orthogonal polarisation states extremely conjunction beam of first via light pulse, and carries out phase-modulation or during beam splitting to conjunction beam to one of two-way sub-light pulse progress phase-modulation to the first via light pulse before beam splitting.The present invention is able to achieve the immune time bit-phase code quantum key distribution scheme of environmental disturbances.

Description

Quantum key distribution time bit-phase decoding method, device and system based on polarization orthogonal rotation

Technical Field

The invention relates to the technical field of optical transmission secret communication, in particular to a quantum key distribution time bit-phase decoding method and device based on polarization orthogonal rotation and a quantum key distribution system comprising the device.

Background

The quantum secret communication technology is a leading-edge hotspot field combining quantum physics and information science. Based on quantum key distribution technology and one-time pad cipher principle, quantum secret communication can realize the safe transmission of information in public channel. The quantum key distribution is based on the physical principles of quantum mechanics Heisebauer uncertain relation, quantum unclonable theorem and the like, the key can be safely shared among users, potential eavesdropping behavior can be detected, and the quantum key distribution method can be applied to the fields of high-safety information transmission requirements such as national defense, government affairs, finance, electric power and the like.

At present, ground quantum key distribution is mainly based on optical fiber channel transmission, and optical pulses generate random birefringence effect in the process of optical fiber quantum channel transmission due to the non-ideal conditions of non-circular symmetry of cross section, nonuniform distribution of refractive index of fiber core along the radial direction and the like in the process of optical fiber quantum channel transmission, and the influence of temperature, strain, bending and the like on the optical fiber in the actual environment. The quantum key distribution time-phase protocol adopts a group of time bases and a group of phase bases for coding, the time bases are coded by adopting time modes of two different time positions, and the phase bases are coded by adopting two phase differences of front and back light pulses. Under the influence of random birefringence of the optical fiber, when the optical pulse reaches a receiving end after being transmitted by the long-distance optical fiber, the polarization state of the optical pulse is randomly changed. 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 subjected to interference decoding, due to the double refraction influence of the transmission optical fiber and the coding and decoding interferometer optical fiber, the problem of polarization induced fading exists, the decoding interference is unstable, the error rate is increased, if a deviation correcting device is added, the complexity and the cost of a system are increased, and the stable application to the strong interference conditions of an overhead optical cable, a road and bridge optical cable and the like is difficult. For a quantum key distribution time-phase encoding scheme, how to solve the problem that phase decoding interference is unstable due to polarization-induced fading during phase-based decoding in time bit-phase encoding quantum key distribution application so as to realize stable and efficient phase interference decoding is a hotspot and a difficult problem in quantum secret communication application based on the existing optical cable infrastructure.

Disclosure of Invention

In order to solve at least one of the above problems, the present invention provides a quantum key distribution time bit-phase decoding method and apparatus based on polarization orthogonal rotation.

The invention provides at least the following technical scheme:

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

splitting an incident path of input light pulse in any polarization state into a first path of light pulse and a second path of light pulse; 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,

wherein performing phase decoding on the first optical pulse comprises:

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

the two paths of sub-optical pulses are respectively transmitted on two sub-optical paths, and are combined and output after relative time delay,

wherein at least one polarization orthogonal rotation device is included in at least one of the two sub-optical paths, and the polarization orthogonal rotation device is configured to perform polarization orthogonal rotation on two orthogonal polarization states of a path of sub-optical pulses 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 path of sub-optical pulses is transformed into a polarization state orthogonal to the polarization state, and

wherein one polarization state of the two orthogonal polarization states of the first path of light pulse is controlled to have a phase difference transmitted through the two sub-optical paths in the process of splitting the beam into a combined beam and a phase difference transmitted through the two sub-optical paths in the other polarization state, so that the two phase differences are different by integral multiples of 2 pi, and

before the first path of optical pulse is split, the first path of optical pulse is subjected to phase modulation according to a quantum key distribution protocol, or in the process that the first path of optical pulse is split into combined beams, at least one of the two paths of optical pulses transmitted on the two sub-optical paths is subjected to phase modulation according to the quantum key distribution protocol.

2. The method for quantum key distribution time bit-phase decoding based on polarization orthogonal rotation according to scheme 1, wherein the two sub optical paths include an optical path having birefringence for two orthogonal polarization states of the first optical pulse, and/or an optical device having birefringence for two orthogonal polarization states of the first optical pulse on the two sub optical paths, and wherein the controlling of the phase difference transmitted by one polarization state of the first optical pulse through the two sub optical paths in the process of splitting into a combined beam and the phase difference transmitted by the other polarization state through the two sub optical paths is such that the two phase differences differ by an integer multiple of 2 pi includes:

respectively keeping each polarization state in the two orthogonal polarization states unchanged when being transmitted through the two sub-optical paths in the process of splitting the beams into combined beams and/or keeping the corresponding orthogonal polarization state unchanged after the orthogonal polarization rotation is carried out by the orthogonal polarization rotation device; and

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 transmitted by one polarization state of the two orthogonal polarization states through the two sub-optical paths in the process of splitting into combined beams and the phase difference transmitted by the other polarization state through the two sub-optical paths are different by integral multiples of 2 pi.

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

the two sub-optical paths are configured as a polarization-maintaining optical fiber optical path, and the controlling of the phase difference transmitted by one polarization state of the two orthogonal polarization states of the first path of optical pulse through the two sub-optical paths and the phase difference transmitted by the other polarization state through the two sub-optical paths in the process of splitting into combined beams makes the difference between the two phase differences be an integral multiple of 2 pi includes:

a first distance difference between a distance traveled in the case of one intrinsic polarization state of the polarization maintaining fiber when traveling on one of the two sub optical paths and a distance traveled in the case of an orthogonal polarization state converted into the intrinsic polarization state, and a second distance difference between a distance traveled in the case of the intrinsic polarization state and a distance traveled in the case of the orthogonal polarization state converted into the intrinsic polarization state when traveling on the other of the two sub optical paths are 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 fiber.

4. The method for quantum key distribution time bit-phase decoding based on polarization orthogonal rotation according to scheme 1 or 3, wherein the controlling a phase difference transmitted by one polarization state of two orthogonal polarization states of the first path of optical pulse through the two sub optical paths in the process of splitting into combined beams and a phase difference transmitted by the other polarization state through the two sub optical paths so that the two phase differences are different by an integer multiple of 2 pi comprises:

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

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

6. The method for quantum key distribution time bit-phase decoding based on polarization orthogonal rotation according to scheme 1, wherein a polarization-maintaining fiber stretcher and/or a birefringent phase modulator are/is disposed on at least one of the two sub-optical paths, wherein the difference between the phase difference transmitted by one of the two sub-optical paths in the process of splitting into a combined beam and the phase difference transmitted by the other polarization state through the two sub-optical paths is adjusted by the polarization-maintaining fiber stretcher and/or the birefringent phase modulator.

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

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

And the second path of light pulse is output after beam splitting for detection.

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

the front beam splitter is configured to split an incident one path of input light pulse in any polarization state into a first path of light pulse and a second path of light pulse; and the number of the first and second groups,

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

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 two sub-optical paths are optically coupled with the first beam splitter

The first beam splitter is configured to split the first light pulse into two sub-light pulses;

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

the first beam combiner is configured to combine and output the two paths of sub-optical pulses after relative delay,

wherein in the phase decoder, the two sub-optical paths and the optical devices thereon are configured to control the phase difference transmitted by one of the two sub-optical paths in the process of splitting the light pulse into a combined light pulse and the phase difference transmitted by the other sub-optical path in the process of splitting the light pulse into the combined light pulse so that the two phase differences are different by integral multiples 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 optical pulses passing therethrough according to a quantum key distribution protocol,

wherein the pre-splitter outputs the second light pulse for time bit decoding.

9. The quantum key distribution time bit-phase decoding apparatus based on polarization orthogonal rotation according to scheme 8, wherein the two sub-optical paths are configured as a polarization-maintaining fiber optical path, the two sub-optical paths and the optical devices thereon are further configured to control a first distance difference between a distance traveled in the case of an intrinsic polarization state and a distance traveled in the case of an orthogonal polarization state converted to the intrinsic polarization state when an intrinsic polarization state of the polarization-maintaining fiber is traveled on one of the two sub-optical paths, and a second distance difference between a distance traveled in the case of the intrinsic polarization state and a distance traveled in the case of the orthogonal polarization state converted to the intrinsic polarization state when the intrinsic polarization state is traveled 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 fiber.

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

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

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

the polarization-maintaining optical fiber stretcher is positioned on any one of the two sub-optical paths and is configured for adjusting the length of the polarization-maintaining optical fiber of the optical path where the polarization-maintaining optical fiber stretcher is positioned; and/or

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

13. The apparatus for quantum key distribution time bit-phase decoding based on polarization orthogonal rotation according to claim 8, wherein the phase modulator is a polarization-independent phase modulator; the phase modulator is configured to randomly phase modulate the optical pulses passing therethrough by 0 degrees or 180 degrees.

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

the phase decoder adopts an optical path structure of an 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 through 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 two reflected sub-optical pulses and output the combined light.

15. The apparatus according to claim 8, wherein the optical devices on the optical path between the first beam splitter and the first beam combiner and between the first beam splitter and the first beam combiner are polarization-maintaining optical devices or non-birefringent optical devices.

16. The apparatus according to claim 8, wherein the apparatus further comprises a second beam splitter optically coupled to the front beam 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 schemes 8 to 16, which is arranged at a receiving end of the quantum key distribution system and 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 schemes 8 to 16, which is arranged at a transmitting end of the quantum key distribution system and is used for time bit-phase encoding.

With the solution of the invention, a number of advantages can be achieved. For example, for the time bit-phase encoding quantum key distribution application, the polarization orthogonal rotation device is adopted in the two arms of the interferometer, so that the difference between phase differences transmitted by the two orthogonal polarization states of the optical pulse in phase-based decoding in the two arms of the unequal-arm interferometer is easily controlled, the two orthogonal polarization states are simultaneously and effectively interfered and output at the output port, the phase-based decoding function of environmental interference immunity is realized, and the time bit-phase encoding quantum key distribution solution of stable environmental interference immunity can be realized. The quantum key distribution decoding scheme of the invention can resist polarization-induced fading, and simultaneously avoids the need for complex deviation rectifying equipment.

Drawings

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

fig. 2 is a schematic structural diagram of a component structure of a quantum key distribution time bit-phase decoding device based on polarization orthogonal rotation according to a preferred embodiment of the present invention;

fig. 3 is a schematic structural diagram of a component structure of a quantum key distribution time bit-phase decoding device based on polarization orthogonal rotation according to another preferred embodiment of the present invention;

fig. 4 is a schematic structural diagram of a component structure of a quantum key distribution time bit-phase decoding device based on polarization orthogonal rotation according to another preferred embodiment of the present invention;

fig. 5 is a schematic structural diagram of a component structure of a quantum key distribution time bit-phase decoding device based on polarization orthogonal rotation according to another preferred embodiment of the present invention;

fig. 6 is a schematic structural diagram of a component structure of a quantum key distribution time bit-phase decoding device based on polarization orthogonal rotation according to another preferred embodiment of the present invention.

Detailed Description

The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which form a part hereof, and which together with the embodiments of the invention serve to explain the principles of the invention. For the purpose of clarity and simplicity, a detailed description of known functions and configurations of devices described 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 according to a preferred embodiment of the present invention is shown in fig. 1, and specifically includes the following steps:

step S101: splitting one path of incident input light pulse in any polarization state into a first path of light pulse and a second path of light pulse.

Specifically, the incident input light pulse is in any polarization state, and may be a fully polarized light with linear polarization, circular polarization, or elliptical polarization, or may be a partially polarized light or an unpolarized light.

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

As will be appreciated by those skilled in the art, each optical pulse can be viewed as being composed of two orthogonal polarization states (e.g., orthogonal x-polarization state and y-polarization state). Similarly, the two sub-optical pulses obtained by splitting the first optical pulse can also be regarded as being composed of two orthogonal polarization states which are the same as those of the first optical pulse.

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

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

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

In addition, for the method in fig. 1, in the process of performing phase decoding on the first optical pulse according to the quantum key distribution protocol, phase modulation is performed 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 from splitting to combining of the first path of optical pulse, performing phase modulation on at least one of the two paths of 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 before the splitting of the first optical pulse may be implemented by performing phase modulation on one of two adjacent front and back input optical pulses in the first optical pulse.

Here, the relative delay and phase modulation are performed according to the requirements and specifications of the quantum key distribution protocol, and are not described in detail herein.

Regarding to control the phase difference transmitted by the two sub optical paths in the process of splitting into a combined beam in one of the two orthogonal polarization states of the first path of light pulse and the phase difference transmitted by the two sub optical paths in the other polarization state of the first path of light pulse so that the two phase differences are different by an integral multiple of 2 pi, for example, if the two orthogonal polarization states are an x polarization state and a y polarization state respectively, the phase difference transmitted by the two sub optical paths in the process of splitting into a combined beam in the x polarization state is represented as Δ x, and the phase difference transmitted by the two sub optical paths in the process of splitting into a combined beam in the y polarization state is represented as Δ y, the phase difference transmitted by the two sub optical paths in one of the two orthogonal polarization states of the first path of light pulse and the phase difference transmitted by the two sub optical paths in the other polarization state are different by an integral multiple of 2 pi in the process of splitting into a, or the phase difference between two orthogonal polarization states of the first path of light pulse transmitted through the two sub-optical paths in the process of splitting into a combined beam is an integral multiple of 2 pi, which can be expressed as:

Δx–Δy＝2π*m，

wherein m is an integer and can be a positive integer, a negative integer or zero.

In a possible embodiment, the two sub-optical paths for transmitting the two sub-optical pulses split by the first optical pulse include an optical path having birefringence for two orthogonal polarization states of the first optical pulse, and/or an optical device having birefringence on the two sub-optical paths for two orthogonal polarization states of the first optical pulse. In this case, the controlling of the phase difference transmitted by one polarization state of the two orthogonal polarization states of the first optical pulse through the two sub optical paths during the splitting into the combined beam and the phase difference transmitted by the other polarization state through the two sub optical paths so that the two phase differences are different by an integer multiple of 2 pi includes: respectively keeping each polarization state in the two orthogonal polarization states unchanged when being transmitted through the two sub-optical paths in the process of splitting the beams into combined beams and/or keeping the corresponding orthogonal polarization state unchanged after the orthogonal polarization rotation is carried out by the orthogonal polarization rotation device; 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 transmitted by one polarization state in the two orthogonal polarization states through the two sub-optical paths in the process of splitting into a combined beam is different from the phase difference transmitted by the other polarization state through the two sub-optical paths by an integral multiple of 2 pi, in other words, the phase difference transmitted by the two sub-optical paths in the process of splitting into a combined beam is different from the phase difference transmitted by the two orthogonal polarization states by an integral multiple of 2 pi. Optionally, this may be achieved by either: i) configuring the two sub-optical paths into a polarization-maintaining optical fiber optical path, and configuring an optical device on the polarization-maintaining optical fiber optical path into a non-birefringent optical device and/or a polarization-maintaining optical device; ii) configuring one of the two sub-optical paths as a free space optical path, and configuring the optical devices on the two sub-optical paths as polarization maintaining optical devices. Herein, the term "polarization maintaining fiber optical path" refers to an optical path formed by connecting polarization maintaining fibers or an optical path formed by transmitting optical pulses by using 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 a possible embodiment, the controlling a phase difference between one of two orthogonal polarization states of the first optical pulse transmitted through the two sub optical paths during the splitting into the combined beam and a phase difference between the other polarization state transmitted through the two sub optical paths such that the two phase differences are different by an integer multiple of 2 pi includes:

the two sub-optical paths are configured as a polarization-maintaining optical fiber optical path, a first distance difference between a distance transmitted in the intrinsic polarization state and a distance transmitted in the orthogonal polarization state converted into the intrinsic polarization state when an intrinsic polarization state of the polarization-maintaining optical fiber is transmitted on one of the two sub-optical paths, and a second distance difference between a distance transmitted in the intrinsic polarization state and a distance transmitted in 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 are controlled, so that the first distance difference and the second distance difference are different by an integral multiple of a beat length of the polarization-maintaining optical fiber, and further that a phase difference transmitted by one of the two orthogonal polarization states of the first light pulse through the two sub-optical paths during splitting into a combined beam is different by an integral multiple of 2 pi from a phase difference transmitted by the other polarization state through the two sub-optical paths, in other words, the phase difference between the two orthogonal polarization states of the first path of optical pulse transmitted through the two sub-optical paths in the process of splitting into a combined beam is different by an integral multiple of 2 pi.

In one possible embodiment, 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 disposed on at least one of two sub-optical paths for transmitting two sub-optical pulses obtained by splitting the first optical pulse. The polarization-maintaining optical fiber stretcher is suitable for adjusting the length of the polarization-maintaining optical fiber of the optical path where the polarization-maintaining optical fiber stretcher is located. The birefringent phase modulator is adapted to apply different adjustable phase modulations to the two orthogonal polarization states passing therethrough, and the polarization maintaining fiber stretcher and/or the birefringent phase modulator may be arranged to adjust a difference between a phase difference of one of the two orthogonal polarization states of the first light pulse transmitted through the two sub-optical paths during splitting into a combined beam and a phase difference of the other polarization state transmitted through the two sub-optical paths. 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. Therefore, the birefringent phase modulator can be used for influencing and adjusting the difference between the phase differences of the two orthogonal polarization states of the first optical pulse transmitted through the two sub-optical paths in the process of splitting into a combined beam.

Phase modulation of an optical pulse may be achieved by a polarization independent phase modulator. Polarization independent phase modulators are adapted to phase modulate two orthogonal polarization states of an optical pulse identically and are 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 situation, phase modulation may be achieved by a number of specific means. For example, these means may include: modulating the length of a free-space optical path, modulating the length of an optical fiber, or using serial or parallel optical waveguide phase modulators, etc. For example, the desired phase modulation can be achieved by varying the length of the free-space optical path with a motor. As another example, the phase modulation may be achieved by modulating the length of the optical fiber by an optical fiber stretcher using the piezoelectric effect. In addition, the phase modulator may be of another type suitable for voltage control, and the desired phase modulation may be achieved by applying a suitable voltage to the polarization independent phase modulator to phase modulate the same two orthogonal polarization states of the light pulse.

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

According to one possible embodiment, the time-bit decoding of the second light pulse comprises: directly outputting the second path of light pulse for detection; or the second path of light pulse is output after beam splitting for detection.

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 pre-splitter 201, a splitter 202 (also referred to as "second splitter"), a splitter 203 (also referred to as "first splitter"), a phase modulator 204, a combiner 205 (also referred to as "first combiner"), polarization quadrature rotation devices 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 device 206 or 207 is configured to perform polarization orthogonal rotation on two orthogonal polarization states of a path of sub-optical pulses transmitted therethrough, respectively, so that after passing through the polarization orthogonal rotation device, each of the two orthogonal polarization states of the path of sub-optical pulses is transformed into a polarization state orthogonal thereto, respectively.

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

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

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

The beam splitter 202 is optically coupled to the pre-beam splitter 201, and is configured to receive another optical pulse (also referred to as "second optical pulse") of the two optical pulses, and to split and output the other 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 pre-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, which are transmitted through two sub-optical paths respectively, and are relatively delayed by the two sub-optical paths, and then are combined and output by the beam combiner 205. The phase modulator 204 is configured to perform phase modulation on the sub optical pulse transmitted through one of the two sub optical paths where the sub optical pulse is located according to a quantum key distribution protocol. Specifically, the two sub-optical paths are used for respectively transmitting the two sub-optical pulses and for realizing the relative delay of the two sub-optical pulses. The relative delay of the two sub-optical pulses can be realized by adjusting the optical path physical length of any one of the two sub-optical paths between the beam splitter 203 and the beam combiner 205. The beam combiner 205 is configured to combine and output the two sub-optical pulses transmitted through the two sub-optical paths.

Preferably, the two sub-optical paths are configured as a polarization-maintaining optical fiber, and 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 an intrinsic polarization state and a distance transmitted in the case of an orthogonal polarization state converted into the intrinsic polarization state when an intrinsic polarization state of the polarization-maintaining optical fiber 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 an 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, 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, such that a phase difference transmitted by one of two orthogonal polarization states of the first input optical pulse through the two sub-optical paths and the other polarization state are split into a combined beam during splitting into the beam splitting into the combined beam The phase difference transmitted via the two sub-optical paths in the process of (2) differs by an integer multiple of 2 pi.

Preferably, the phase modulator 204 is used to randomly phase modulate the sub-optical pulses passing therethrough by 0 degrees or 180 degrees. As is known in the art, this phase modulation scheme is sometimes also referred to as "alternating current phase modulation".

According to the present invention, in the phase decoder, the two sub-optical paths and the optical devices thereon are configured to control the phase difference transmitted by one of the two sub-optical paths in the process of splitting the light pulse into the combined beam and the phase difference transmitted by the other sub-optical path in the process of splitting the light pulse into the combined beam so that the two phase differences differ by an integral multiple of 2 pi, in other words, the phase differences transmitted by the two sub-optical paths in the process of splitting the light pulse into the combined beam in each of the two orthogonal polarization states of the light pulse of the first path differ by an integral multiple of 2 pi.

In this regard, an optical path may or may not be birefringent for two orthogonal polarization states, depending on the type of optical path. For example, a free-space optical path does not have birefringence for two orthogonal polarization states of an input optical pulse, while a polarization maintaining fiber optical path typically has birefringence that differs significantly from each other for two orthogonal polarization states of an input optical pulse. In addition, an optical device in the optical path may or may not have birefringence for two orthogonal polarization states, depending on the type of optical device. For example, a non-birefringent optical device has no birefringence for two orthogonal polarization states of an input optical pulse, while a polarization maintaining optical device typically has birefringence that differs significantly from each other for two orthogonal polarization states of an input optical pulse.

For the phase decoder, there may optionally be a setting as follows:

the two sub-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-paths, including the phase modulators, if any, being non-birefringent optical devices and/or polarization-preserving optical devices. With this arrangement, in the case of the polarization maintaining optical device, the polarization maintaining optical device itself causes the phase difference between two orthogonal polarization states of the first optical pulse input to the phase decoder, which are transmitted through the two sub optical paths during the splitting to the combining, to differ by an integer multiple of 2 pi.

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

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 the polarization-maintaining optical fiber 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 realize that the phase difference of two orthogonal polarization states of the first path of light pulse input to the phase decoder, which are transmitted through the two sub-optical paths in the process of splitting into combined beams, differs by an integral multiple of 2 pi. In addition, the optical fiber stretcher can 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 arranged to apply different phase modulations to the two orthogonal states of polarisation of the sub-optical pulses passing therethrough. By controlling the birefringent phase modulator, the difference between the phase modulations experienced by each of the two orthogonal polarization states of the sub-optical pulses passing therethrough is adjustable. Thus, by using the birefringent phase modulator, the difference between the phase differences transmitted by the two sub-optical paths in the process of splitting into a combined beam of the two orthogonal polarization states of the first path of optical pulse input to the phase decoder can be conveniently influenced and adjusted, and the difference is easy to be an integral multiple of 2 pi. The birefringent phase modulator may be a lithium niobate phase modulator as described above.

the phase decoder is configured as an unequal arm mach-zehnder interferometer, the optical paths of the two arms of the interferometer (i.e., the two sub-optical paths between the beam splitter and the beam combiner of the phase decoder) are polarization-maintaining optical fibers, and the two arms of the interferometer are assumed to respectively include one polarization orthogonal rotation device 206 and one polarization orthogonal rotation device 207. assuming that the distance from the beam splitter to the polarization orthogonal rotation device 206 in one arm is L1, the distance from the polarization orthogonal rotation device 206 in the one arm to the beam combiner is L2, the distance from the beam splitter to the polarization orthogonal rotation device 207 in the other arm is L3, and the distance from the polarization orthogonal rotation device 207 in the other arm to the beam combiner is L4, the length relationship may preferably satisfy (L1-L2) - (L3-L4) ═ n β, where n is a positive integer, a negative integer or zero, β is polarization-maintaining optical fiber beat length, where the other optical devices in the two sub-optical paths cause the first optical pulse input to the phase decoder to be in the first optical path, and the first optical pulse path is transmitted through the polarization optical path in the second polarization state, and the second polarization rotation device is L1, and the second polarization rotation device is preferably equal to the same as L1 (L6342).

the phase decoder may further comprise two mirrors, which are respectively located on two sub-optical paths for transmitting two sub-optical pulses split by the beam splitter of the phase decoder, and are respectively used for reflecting the two sub-optical pulses transmitted through the two sub-optical paths from the beam splitter of the phase decoder back to be output by the beam combiner of the same phase decoder as the beam splitter, further, in one embodiment, the phase decoder may also comprise an optical circulator (not shown) which may be located at a front end of the beam splitter of the phase decoder, a corresponding optical pulse 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 the phase decoder, and may be output from the beam combiner of the phase decoder, which may be input from a first port of the optical circulator and may be output from a second port of the optical circulator to a corresponding optical pulse splitter to a second port of the phase decoder, and may be output from a second port of the optical circulator to a corresponding optical fiber splitter via a polarization beam splitter, or may be added to a corresponding optical path length of an optical fiber rotating polarization beam splitter equivalent to an optical path length of an optical fiber length ranging from the same polarization beam splitter, which may be equal to an optical path length ranging from the same optical fiber length ranging from the same polarization beam splitter, when the corresponding optical fiber splitter, such as an optical fiber polarization beam splitter, or equal to a rotating polarization length ranging from the rotating polarization beam splitter, when the rotating optical path ranging from the rotating optical fiber length ranging from the rotating optical fiber decoder, such as an optical fiber length ranging from the rotating polarization beam splitter from the rotating optical path ranging from the rotating optical fiber length ranging from the rotating optical path ranging from the rotating optical fiber decoder 26-rotating polarization decoder 26-rotating optical path from the rotating polarization decoder 26 to the rotating optical fiber length ranging from the rotating optical path from the rotating optical fiber length ranging from the rotating optical path of the rotating optical fiber length ranging from the rotating optical decoder 26 to the rotating optical fiber length ranging from the rotating optical path of the rotating optical fiber length ranging from the rotating optical fiber.

"polarization maintaining fiber beat length" is a well-known concept in the art and refers to the length of the polarization maintaining fiber corresponding to the phase difference of 2 pi generated by the transmission of two eigen 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 split sub-optical pulses is phase-modulated according to the quantum key distribution protocol during the splitting to the combining, it is also possible to arrange a phase modulator at the front end of the beam splitter 203, i.e. before the splitting of the first path of optical pulses, the phase modulator is phase-modulated according to the quantum key distribution protocol. Furthermore, it is also possible to provide a phase modulator before the front beam splitter 201, that is, to perform phase modulation on one incident input optical pulse.

In addition, although fig. 2 shows that the phase decoder has 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 between the phases modulated by the two phase modulators is determined by the quantum key distribution protocol.

For the embodiment of fig. 2, beam splitter 203 and beam combiner 205 are preferably polarization maintaining optical devices. 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 eigenstates of polarization, light pulses of an incident eigenstate of polarization remain unchanged, as is known to those skilled in the art.

Fig. 3 shows a quantum key distribution time bit-phase decoding apparatus based on polarization orthogonal rotation according to another preferred embodiment of the present invention. As shown in fig. 3, the phase decoder therein adopts a structure of unequal arm mach-zehnder interferometer. Specifically, the quantum key distribution time bit-phase decoding device comprises the following components: a pre-splitter 303, a splitter 304, a polarization maintaining splitter 307, a polarization maintaining fiber stretcher 309, a phase modulator 311, a polarization maintaining 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 of the quantum key distribution time bit-phase decoding apparatus. The beam splitter 304 receives one input optical pulse split by the pre-splitter 303 and splits it into two sub optical pulses. The polarization maintaining beam splitter 307 and the polarization maintaining beam combiner 312 constitute a part of the unequal arm mach-zehnder interferometer, two sub optical paths between the polarization maintaining beam splitter 307 and the polarization maintaining beam combiner 312 (i.e., 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 may be respectively inserted into two arms of the unequal arm mach-zehnder interferometer. The two arms of the unequal arm mach-zehnder interferometer may include at least one orthogonal polarization rotation device, such as an orthogonal polarization rotation device 308 and an orthogonal polarization rotation device 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.

During operation, an incident light pulse enters the pre-beam splitter 303 through the port 301 or 302 of the pre-beam splitter 303 and is 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 and is split into two sub light pulses, one of the two sub light pulses is transmitted through the polarization orthogonal rotating device 308 and modulated by the polarization maintaining fiber stretcher 309 (the setting sequence of the polarization orthogonal rotating device 308 and the polarization maintaining fiber stretcher 309 is changeable or simply referred to as "sequence independence"), the other sub light pulse is transmitted through the polarization orthogonal rotating device 310 and randomly modulated by the phase modulator 311 at 0 degree or 180 degree (sequence independence), and the two sub light pulses are output through the port 313 after being relatively delayed and combined by the polarization maintaining beam combiner 312. The second optical pulse output from the pre-splitter 303 is input into a splitter 304 to be split into two sub-optical pulses, and the two sub-optical pulses are output through ports 305 or 306 for time bit decoding.

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

(L1-L3) - (L2-L4) ═ n β, or so-called

(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 of the phase difference of two orthogonal polarization states of the first path of light pulse transmitted by two arms of the unequal arm Mach-Zehnder interferometer is an integral multiple of 2 pi.

The phase modulator 311 is a polarization-independent optical device. If the phase modulator 311 is not connected, but the polarization maintaining fiber stretcher 309 is used to realize the phase modulation function of the phase modulator 311, 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 employs a structure of an unequal arm mach-zehnder interferometer. The quantum key distribution time bit-phase decoding device comprises the following components: a pre-splitter 403, a polarization maintaining splitter 405, polarization quadrature rotating devices 406 and 408, a polarization maintaining fiber stretcher 407, a phase modulator 409, and a polarization maintaining combiner 410.

One of the two ports 401 and 402 on one side of the pre-splitter 403 serves as an input of the time bit-phase decoding means for receiving the incoming optical pulses. Polarization maintaining beam splitter 405 and polarization maintaining beam combiner 410 form 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 may be inserted into both arms of the unequal arm mach-zehnder interferometer. At least one of the two arms of the unequal arm mach-zehnder interferometer includes at least one polarization quadrature rotator, for example, each of the two arms may include one polarization quadrature rotator 406 and one polarization quadrature rotator 408, and the optical pulse input to the polarization maintaining beam splitter 405 is decoded by the unequal arm mach-zehnder interferometer and output through the port 411.

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

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

(L1 '-L3') - (L2 '-L4') ═ n β, or stated otherwise

(L1’－L2’)－(L3’－L4’)＝nβ，

wherein beta is the beat length of the polarization maintaining fiber, and n is an integer, so that the difference of the phase difference of two orthogonal polarization states of the input optical pulse transmitted on two arms of the unequal arm Mach-Zehnder interferometer is integral multiple of 2 pi.

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

Fig. 5 shows a quantum key distribution time bit-phase decoding apparatus based on polarization orthogonal rotation according to another preferred embodiment of the present invention, as shown in fig. 5, wherein 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 orthogonal rotation devices 508 and 511, a polarization maintaining fiber stretcher 509, a phase modulator 512, and mirrors 510 and 513.

One of two ports 501 and 502 on one side of the front beam splitter 503 serves as an input end of the time bit-phase decoding apparatus, 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. The polarization-maintaining beam splitter 507 and the mirrors 510 and 513 constitute an unequal-arm michelson interferometer, and the polarization-maintaining fiber stretcher 509 and the phase modulator 512 can be inserted into the same arm of the unequal-arm michelson interferometer or into two arms of the unequal-arm michelson interferometer respectively. At least one of the arms of the unequal-arm michelson interferometer includes at least one orthogonal polarization rotation device, e.g., each arm may include one orthogonal polarization rotation device 508 and one orthogonal polarization rotation device 511. The optical pulse input to the polarization maintaining beam splitter 507 is decoded by the unequal arm michelson interferometer and then output from the 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.

During operation, light pulses enter the pre-splitter 503 through the port 501 or 502 of the pre-splitter 503 and are split into two paths of light pulses for transmission, that is, a first path of light pulses and a second path of light pulses. After the second optical pulse is input to the beam splitter 504, the second optical pulse is 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 into the polarization-maintaining beam splitter 507 and split into two paths of sub-light pulses, wherein one path of sub-light pulse is transmitted by the polarization orthogonal rotation device 508 and modulated (sequence-independent) by the polarization-maintaining optical fiber stretcher 509 and then reflected by the reflector 510, the other path of sub-light pulse is transmitted by the polarization orthogonal rotation device 511 and modulated (sequence-independent) by the phase modulator 512 and then reflected by the reflector 513, and the two paths of reflected relatively delayed sub-light pulses are combined by the polarization-maintaining beam splitter 507 and then output by the port 514 of the polarization-maintaining 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 polarization maintaining beam splitter 507 and polarization orthogonal rotation device 508 is L1 ", the length between polarization orthogonal rotation device 508 and mirror 510 is L2", the length between polarization maintaining beam splitter 507 and polarization orthogonal rotation device 511 is L3 ", and the length between polarization orthogonal rotation device 511 and mirror 513 is L4", polarization maintaining fiber stretcher 509 is modulated such that the length relationship satisfies:

2(L1 "-L3") -2 (L2 "-L4") ═ n β, or

2(L1”－L2”)－2(L3”－L4”)＝nβ

wherein beta is the beat length of the polarization maintaining fiber, and n is an integer, so that the difference of the phase difference of two orthogonal polarization states of the first path of light pulse transmitted by two arms of the unequal-arm Michelson interferometer is an integral multiple of 2 pi.

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

Fig. 6 shows a quantum key distribution time-phase decoding apparatus based on polarization orthogonal 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 pre-beam splitter 603, a polarization maintaining beam splitter 605, polarization quadrature rotating devices 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 of one side of the front beam splitter 603 serves as an input of the time bit-phase decoding apparatus. Polarization maintaining beam splitter 605 and mirrors 608, 611 form an unequal-arm michelson interferometer. Polarization maintaining fiber stretcher 607 and 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 arms of the unequal-arm michelson interferometer includes at least one orthogonal polarization rotation device, e.g., each arm may include one orthogonal polarization rotation device 606 and one orthogonal polarization rotation device 609. The optical pulse input to the beam splitter polarization-maintaining 605 is decoded by the unequal arm michelson interferometer and then output through the port 612 of the polarization-maintaining beam splitter 605, or may be output through the port of an optical circulator when the optical circulator is mounted.

During operation, light pulses enter the pre-splitter 603 through the port 601 or 602 of the pre-splitter 603 and are split into two paths of light pulses for transmission, that is, a first path of light pulses and a second path of light pulses. The second optical pulse is directly output by the port 604 for time bit decoding; the first path of light pulse is input into the polarization maintaining beam splitter 605 and split into two paths of sub light pulses, one path of the sub light pulse is transmitted by the polarization orthogonal rotating device 606 and modulated (sequence-independent) by the polarization maintaining fiber stretcher 607 and then reflected back by the reflecting mirror 608, the other path of the sub light pulse is transmitted by the polarization orthogonal rotating device 609 and modulated (sequence-independent) by the phase modulator 610 and then reflected back by the reflecting mirror 611, and the two paths of the reflected and relatively delayed sub light pulses are combined by the polarization maintaining beam splitter 605 and then output by the port 612 of the polarization maintaining 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 that the length between the polarization maintaining beam splitter 605 and the polarization orthogonal rotation device 606 is L1 '″, the length between the polarization orthogonal rotation device 606 and the mirror 608 is L2' ″, the length between the polarization maintaining beam splitter 605 and the polarization orthogonal rotation device 609 is L3 '″, the length between the polarization orthogonal rotation device 609 and the mirror 611 is L4' ″, the polarization maintaining fiber stretcher 607 is modulated such that the length relationship is satisfied:

2(L1 "' -L3" ') -2 (L2 "' -L4" ') -n β, or L4 "')

2(L1”’－L2”’)－2(L3”’－L4”’)＝nβ，

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

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

Herein, the terms "beam splitter" and "beam combiner" are used interchangeably, and a beam splitter may also be referred to and used as a beam combiner, and vice versa.

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

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

While the invention has been described in connection with specific embodiments thereof, it is to be understood that it is intended by the appended drawings that all such modifications as fall within the true spirit and scope of the invention are intended to be included within the scope of the invention.

While the exemplary embodiments have been described in detail, the foregoing description is in all aspects illustrative and not restrictive. It should be understood that numerous other modifications and variations can be devised without departing from the scope of the exemplary embodiments, which fall within the scope of the invention. Therefore, the protection scope of the present invention should be subject to the appended claims.

Claims

wherein performing phase decoding on the first optical pulse comprises:

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

2. The method for quantum key distribution time-phase decoding based on polarization orthogonal rotation according to claim 1, wherein the two sub-optical paths include an optical path having birefringence for two orthogonal polarization states of the first optical pulse, and/or an optical device having birefringence for two orthogonal polarization states of the first optical pulse on the two sub-optical paths, wherein the controlling a phase difference transmitted by one polarization state of the first optical pulse through the two sub-optical paths during splitting into a combined beam and a phase difference transmitted by the other polarization state through the two sub-optical paths is different from each other by an integer multiple of 2 pi comprises:

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

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

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

6. The method according to claim 1, wherein a polarization-maintaining fiber stretcher and/or a birefringent phase modulator are disposed on at least one of the two sub-optical paths, and wherein the difference between the phase difference transmitted through the two sub-optical paths in the process of splitting into combined beams and the phase difference transmitted through the two sub-optical paths in the other polarization state is adjusted by the polarization-maintaining fiber stretcher and/or the birefringent phase modulator in one of the two orthogonal polarization states of the first optical pulse.

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

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

wherein the pre-splitter outputs the second light pulse for time bit decoding.

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

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

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

13. The quantum key distribution time bit-phase decoding apparatus based on polarization orthogonal rotation according to claim 8, wherein the phase modulator is a polarization independent phase modulator; the phase modulator is configured to randomly phase modulate the optical pulses passing therethrough by 0 degrees or 180 degrees.

14. The quantum key distribution time-phase decoding apparatus based on polarization orthogonal rotation according to claim 8,

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

15. The apparatus of claim 8, wherein the first beam splitter and the first beam combiner and the optical devices 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 polarization quadrature rotation based quantum key distribution time bit-phase decoding apparatus of claim 8, wherein the time bit-phase decoding apparatus further comprises a second beam splitter optically coupled to the front beam splitter and configured to receive the second optical pulse and to beamdivide the second optical pulse into beams for output for time bit decoding.

17. A quantum key distribution system, comprising:

the quantum key distribution time bit-phase decoding device based on the polarization orthogonal rotation as claimed in any one of claims 8 to 16, which is arranged at the receiving end of the quantum key distribution system 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 8 to 16, which is arranged at the transmitting end of the quantum key distribution system and is used for time bit-phase encoding.