CN109150523B - Quantum key distribution time bit-phase decoding method and device and corresponding system - Google Patents

Quantum key distribution time bit-phase decoding method and device and corresponding system Download PDF

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CN109150523B
CN109150523B CN201811264198.9A CN201811264198A CN109150523B CN 109150523 B CN109150523 B CN 109150523B CN 201811264198 A CN201811264198 A CN 201811264198A CN 109150523 B CN109150523 B CN 109150523B
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CN109150523A (en
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许华醒
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China Academy of Electronic and Information Technology of CETC
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L9/00—Cryptographic mechanisms or cryptographic arrangements for secret or secure communications; Network security protocols
    • H04L9/08—Key distribution or management, e.g. generation, sharing or updating, of cryptographic keys or passwords
    • H04L9/0816—Key establishment, i.e. cryptographic processes or cryptographic protocols whereby a shared secret becomes available to two or more parties, for subsequent use
    • H04L9/0852—Quantum cryptography
    • H04L9/0858—Details about key distillation or coding, e.g. reconciliation, error correction, privacy amplification, polarisation coding or phase coding
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/70—Photonic quantum communication
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/50—Transmitters
    • H04B10/516—Details of coding or modulation
    • H04B10/524—Pulse modulation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/50—Transmitters
    • H04B10/516—Details of coding or modulation
    • H04B10/532—Polarisation modulation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/50—Transmitters
    • H04B10/516—Details of coding or modulation
    • H04B10/548—Phase or frequency modulation
    • H04B10/556—Digital modulation, e.g. differential phase shift keying [DPSK] or frequency shift keying [FSK]
    • H04B10/5561—Digital phase modulation

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  • Theoretical Computer Science (AREA)
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  • Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)

Abstract

The invention provides a quantum key distribution time bit-phase decoding method and device controlled by polarization phase difference and a corresponding system. The method comprises the following steps: splitting an input optical pulse into first and second optical pulses; according to the quantum key distribution protocol, the first path of optical pulse is subjected to phase decoding and the second path of optical pulse is subjected to time bit decoding. The phase decoding of the first optical pulse includes: it is incident on an interferometer comprising a beam splitter and a beam combiner to split it by the beam splitter into first and second sub-optical pulses, which are transmitted along a first and a second arm of the interferometer, respectively, and which are beam-combined by the beam combiner for output after a relative delay, wherein two orthogonal polarization states of a first optical pulse are controlled to each differ in the interferometer by an integer multiple of 2 pi in phase difference transmitted via the first and the second arm, either the first optical pulse before splitting or one of the first and the second sub-optical pulse. The scheme of the invention can resist polarization induced fading.

Description

Quantum key distribution time bit-phase decoding method and device and corresponding system
Technical Field
The present invention relates to the field of optical transmission secret communication technology, and in particular, to a method and an apparatus for decoding a quantum key distribution time-phase by controlling a polarization phase difference, 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 realize safe sharing of keys among users, can detect potential eavesdropping behaviors, and can be applied to the fields of national defense, government affairs, finance, electric power and other high-safety information transmission requirements.
The time bit-phase encoded quantum key distribution 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 ground quantum key distribution is mainly based on fiber channel transmission, but the optical fiber manufacturing has non-ideal conditions of non-circular symmetry in section, non-uniform distribution of refractive index of fiber cores along radial directions and the like, and the optical fiber is influenced by temperature, strain, bending and the like in an actual environment, so that random birefringence effect can be generated. 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 bit-phase coding is not influenced by the change of the polarization state, however, when the phase base is in interference decoding, due to the influence of double refraction of a transmission optical fiber and an optical fiber of a coding-decoding interferometer, the problem of polarization induced fading exists, decoding interference is unstable, the error rate is increased, correction equipment is required to be added, the complexity and the cost of a system are increased, and stable application is difficult to realize under the condition of strong interference such as an overhead optical cable, a road bridge optical cable and the like. For a quantum key distribution time bit-phase encoding scheme, how to stably and efficiently perform phase interference decoding is a hotspot and a difficulty in quantum secret communication application based on the existing optical cable infrastructure.
Disclosure of Invention
The invention mainly aims to provide a quantum key distribution time bit-phase decoding method and device for polarization phase difference control, which are used for solving the problem of unstable phase decoding interference caused by polarization induced fading in phase base decoding in time bit-phase coding quantum key distribution application.
The invention provides at least the following technical scheme:
1. a quantum key distribution time bit-phase decoding method for split polarization phase difference control, 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:
the first path of light pulse is incident to an interferometer comprising a beam splitter and a beam combiner, so that the beam splitter splits the first path of light pulse into a first path of sub-light pulse and a second path of sub-light pulse;
transmitting the first path of sub-optical pulse and the second path of sub-optical pulse along a first arm and a second arm of the interferometer respectively, carrying out relative delay on the first path of sub-optical pulse and the second path of sub-optical pulse, then outputting the combined beam by the beam combiner,
Wherein for the first sub-optical pulse transmitted at least along the first arm: the first sub-light pulse is polarized and split into two polarized sub-light pulses with mutually orthogonal polarization states, the two polarized sub-light pulses are transmitted along two sub-light paths, then the two polarized sub-light pulses are combined into the first sub-light pulse, the first sub-light pulse is transmitted to the beam combiner along the first arm,
wherein two orthogonal polarization states of the first path of light pulse are controlled to be respectively different from each other by an integer multiple of 2 pi in phase difference transmitted by the first arm and the second arm in the interferometer,
the input light pulse before splitting or the first path of light pulse before splitting or at least one of the first path of sub-light pulse and the second path of sub-light pulse is subjected to phase modulation according to a quantum key distribution protocol in the process of splitting the beam by the beam splitter to the beam combiner.
2. The quantum key distribution time bit-phase decoding method of split-polarization phase difference control according to claim 1, wherein the first and second arms include optical paths having birefringence for the two orthogonal polarization states, and/or the first and second arms have thereon optical devices having birefringence for the two orthogonal polarization states, wherein the controlling the two orthogonal polarization states of the first optical pulse each differs by an integer multiple of 2 pi in the interferometer by a phase difference transmitted through the first and second arms includes:
Respectively maintaining the polarization states of the two orthogonal polarization states unchanged when the two orthogonal polarization states are transmitted along the first arm and the second arm in the interferometer; and
the length of the optical path in which the birefringence is present and/or the magnitude of the birefringence of the optical device in which the birefringence is present are adjusted such that the two orthogonal polarization states each differ in the interferometer by an integer multiple of 2 pi in phase difference transmitted through the first and second arms.
3. The quantum key distribution time bit-phase decoding method for split-polarization phase difference control according to claim 1 or 2, wherein the first arm and the second arm are configured as polarization maintaining fiber optical paths, and the optical devices on the first arm and the second arm are configured as non-birefringent optical devices and/or polarization maintaining optical devices.
4. The quantum key distribution time bit-phase decoding method of split-polarization phase difference control according to claim 2, wherein a polarization maintaining fiber stretcher and/or a birefringent phase modulator is provided on at least one of the first arm and the second arm, wherein a difference between phase differences transmitted in the interferometer via the first arm and the second arm by two orthogonal polarization states of the first optical pulse is adjusted by the polarization maintaining fiber stretcher and/or the birefringent phase modulator.
5. The quantum key distribution time bit-phase decoding method of split polarization phase difference control according to claim 1, wherein at least one sub-optical pulse of the first and second sub-optical pulses is phase-modulated according to a quantum key distribution protocol in the process of splitting the beam by the beam splitter to the beam combiner, wherein
The at least one sub-light pulse comprises the first sub-light pulse, and the phase modulating the at least one sub-light pulse of the first sub-light pulse and the second sub-light pulse according to a quantum key distribution protocol in the process of splitting the beam by the beam splitter to the beam combiner comprises: the first path of polarized sub-light pulse is subjected to phase modulation before polarization beam splitting or after beam combination is carried out on the two paths of polarized sub-light pulses, or the two paths of polarized sub-light pulses are subjected to the same phase modulation in the process from polarization beam splitting to beam combination is carried out on the two paths of polarized sub-light pulses; and/or
The at least one sub-light pulse comprises the second sub-light pulse, and the phase modulating the at least one sub-light pulse of the first sub-light pulse and the second sub-light pulse according to a quantum key distribution protocol in the process of splitting the beam by the beam splitter to the beam combiner comprises: and carrying out phase modulation on the second sub-optical pulse in the process of splitting the beam by the beam splitter to the beam combiner.
6. The quantum key distribution time bit-phase decoding method of split-polarization phase difference control of claim 1, wherein at least one of the two polarized sub-optical pulses is phase-controlled during transmission of the two polarized sub-optical pulses along the two sub-optical paths.
7. The quantum key distribution time bit-phase decoding method of split-polarization phase difference control according to claim 6, wherein performing phase control on at least one of the two polarized sub-optical pulses comprises:
and adjusting the phase of one polarized oscillator optical pulse in the two polarized oscillator optical pulses.
8. A quantum key distribution time bit-phase decoding device for split polarization phase difference control, characterized in that the decoding device comprises a front beam splitter and an interferometer, the interferometer comprises a first beam splitter, a first beam combiner, and a first arm and a second arm optically coupled with the first beam splitter and optically coupled with the first beam combiner, wherein
The front beam splitter is used for splitting one path of input light pulse with any incident polarization state into a first path of light pulse and a second path of light pulse;
the interferometer is optically coupled to the front beam splitter for phase decoding the first optical pulse, wherein
The first beam splitter is used for splitting the first path of light pulse into a first path of sub-light pulse and a second path of sub-light pulse;
the first and second arms are for transmitting the first and second sub-optical pulses, respectively, and for achieving a relative delay of the first and second sub-optical pulses;
the first beam combiner is used for combining and outputting the first path of sub-optical pulse and the second path of sub-optical pulse which are relatively delayed,
wherein at least the first arm is provided with a polarization phase difference control device, the polarization phase difference control device comprises a polarization beam splitter, a second beam combiner and two sub-light paths which are optically coupled with the polarization beam splitter and the second beam combiner, wherein the polarization beam splitter is used for splitting the polarization beam, the second beam combiner is used for splitting the polarization beam, and the first beam combiner is used for splitting the polarization beam
The polarization beam splitter is used for polarization splitting of the first path of sub-light pulses into two paths of polarized oscillator light pulses with mutually orthogonal polarization states;
the two sub-optical paths are used for respectively transmitting the two polarized oscillator optical pulses;
the second beam combiner is used for combining the two polarized light pulses transmitted by the two sub-light paths into the first sub-light pulse and transmitting the first sub-light pulse to the first beam combiner along the first arm,
Wherein the first and second arms and the optics thereon are configured such that the two orthogonal polarization states of the first optical pulse each differ in the interferometer by an integer multiple of 2 pi in phase difference transmitted through the first and second arms,
the decoding device is further provided with a phase modulator, and the phase modulator is used for carrying out phase modulation on at least one sub-optical pulse of the first sub-optical pulse and the second sub-optical pulse according to a quantum key distribution protocol in the process of splitting the input optical pulse before splitting or the first path optical pulse before splitting or splitting the first beam splitter to the first beam combiner.
9. The quantum key distribution time bit-phase decoding apparatus for split-polarization phase difference control according to claim 8, wherein the first arm and the second arm are polarization maintaining fiber optical paths, and the optical devices on the first arm and the second arm are polarization maintaining optical devices and/or non-birefringent optical devices.
10. The quantum key distribution time bit-phase decoding apparatus of claim 8, wherein the decoding apparatus further comprises:
the polarization maintaining optical fiber stretcher is positioned on any one of the first arm and the second arm and is used for adjusting the length of the polarization maintaining optical fiber of the arm where the polarization maintaining optical fiber stretcher is positioned; and/or
A birefringent phase modulator located on either of the first and second arms for applying different adjustable phase modulations to two orthogonal polarization states of the light pulses passing therethrough.
11. The quantum key distribution time bit-phase decoding apparatus of claim 8, wherein the phase modulator comprises:
the phase modulator is positioned at the front end of the interferometer and is used for carrying out phase modulation on the first path of optical pulse before splitting; or (b)
A phase modulator on the second arm for phase modulating the second sub-optical pulse during beam splitting by the first beam splitter to beam combining by the first beam combiner, wherein the at least one sub-optical pulse comprises the second sub-optical pulse; or (b)
A phase modulator arranged on the first arm before the polarization beam splitter and used for carrying out phase modulation on the first path of polarized sub-light pulse before polarization beam splitting, or a phase modulator arranged on the first arm after the second beam combiner and used for carrying out phase modulation on the first path of sub-light pulse after beam splitting on the two paths of polarized sub-light pulses, or two phase modulators respectively arranged on the two paths of sub-light and used for carrying out the same phase modulation on the two paths of polarized sub-light pulses in the process of polarization beam splitting to beam combining on the two paths of polarized sub-light pulses, wherein the at least one sub-light pulse comprises the first sub-light pulse.
12. The quantum key distribution time bit-phase decoding device for split polarization phase difference control according to claim 8, wherein an optical fiber phase shifter or a phase modulator is disposed on at least one of the two sub-optical paths, and the optical fiber phase shifter or the phase modulator is used for adjusting the phase of a polarized sub-optical pulse transmitted through the sub-optical path where the optical fiber phase shifter or the phase modulator is located.
13. The quantum key distribution time bit-phase decoding apparatus of the split-polarization phase difference control according to claim 8, characterized in that,
the interferometer adopts a structure of an unequal arm Mach-Zehnder interferometer; or alternatively
The interferometer adopts the structure of an unequal arm Michelson interferometer, the first beam combiner and the first beam splitter are the same device, and the interferometer further comprises:
a first mirror on the first arm for reflecting the first sub-light pulse transmitted through the first arm from the first beam splitter back to the first beam combiner;
and a second mirror on the second arm for reflecting the second sub-optical pulse transmitted from the first beam splitter via the second arm back to the first beam combiner.
14. The quantum key distribution time bit-phase decoding apparatus of the split-polarization phase difference control according to claim 8 or 13, characterized in that,
The polarization splitting phase difference control device adopts a Mach-Zehnder optical path structure; or alternatively
The polarization splitting phase difference control device adopts a Michelson optical path structure, the polarization beam splitter and the second beam combiner are the same device, and the polarization splitting phase difference control device further comprises two reflecting mirrors, wherein one of the two reflecting mirrors is positioned on one of the two sub-optical paths and is used for reflecting polarized sub-optical pulses transmitted by the one sub-optical path from the polarization beam splitter back to the second beam combiner; the other of the two reflectors is positioned on the other of the two sub-light paths and is used for reflecting polarized sub-light pulses transmitted by the other sub-light path from the polarization beam splitter back to the second beam combiner, wherein the interferometer adopts the structure of an unequal-arm Michelson interferometer, and one of the two reflectors is the first reflector.
15. The quantum key distribution time bit-phase decoding apparatus according to any one of schemes 8 to 13, wherein the second beam combiner is a polarization maintaining coupler or a polarization beam combiner.
16. The quantum key distribution time bit-phase decoding apparatus according to claim 8, wherein the decoding apparatus further comprises a second beam splitter optically coupled to the front beam splitter for receiving the second optical pulse and splitting the second optical pulse and outputting the second optical pulse for time bit decoding.
17. A quantum key distribution system comprising:
the quantum key distribution time bit-phase decoding device for sub-polarization phase difference control according to any one of the schemes 8 to 16, which is provided at a receiving end of the quantum key distribution system, for time bit-phase decoding; and/or
The quantum key distribution time bit-phase decoding device for sub-polarization phase difference control according to any one of the schemes 8 to 16, which is provided at a transmitting end of the quantum key distribution system, is used for time bit-phase encoding.
With the solution of the invention, several advantages are achieved. For the application of time bit-phase coding quantum key distribution, the invention realizes the effective interference output of two orthogonal polarization states at the output port by controlling the difference of phase differences transmitted by two orthogonal polarization states of an input light pulse in two arms of an unequal arm interferometer in phase-based decoding, thereby realizing the phase-based decoding function of environment interference immunity and enabling the realization of a stable time bit-phase coding quantum key distribution solution of environment interference immunity. In addition, by performing polarization diversity processing on the light pulses transmitted along at least one arm of the interferometer, it is possible to independently phase control the two orthogonal polarization states of the light pulses, thereby making it easier to achieve that the difference in phase differences transmitted in the two arms of the unequal arm interferometer, respectively, of the two orthogonal polarization states of the input light pulses meets the requirements (i.e., is an integer multiple of 2pi). The invention provides a convenient and feasible time bit-phase coding quantum key distribution solution for resisting polarization induced fading, and simultaneously avoids the need for complex deviation rectifying equipment. In addition, the invention has no constraint on the type of the interferometer adopted by the decoding device, and can use the most commonly used unequal arm Mach-Zehnder type interferometer, so that the optical pulse only needs to pass through the phase modulator once during decoding, thereby being beneficial to reducing the insertion loss of a receiving end and improving the system efficiency.
Drawings
FIG. 1 is a flow chart of a quantum key distribution time bit-phase decoding method for split-polarization phase difference control according to a preferred embodiment of the present invention;
FIG. 2 is a schematic diagram showing the structure of a quantum key distribution time bit-phase decoding apparatus for sub-polarization phase difference control according to a preferred embodiment of the present invention;
FIG. 3 is a schematic diagram showing the constitution of a quantum key distribution time bit-phase decoding apparatus for split-polarization phase difference control according to another preferred embodiment of the present invention;
FIG. 4 is a schematic diagram showing the constitution of a quantum key distribution time bit-phase decoding apparatus for split-polarization phase difference control according to another preferred embodiment of the present invention;
FIG. 5 is a schematic diagram showing the constitution of a quantum key distribution time bit-phase decoding apparatus for split-polarization phase difference control according to another preferred embodiment of the present invention;
FIG. 6 is a schematic diagram showing the constitution of a quantum key distribution time bit-phase decoding apparatus for split-polarization phase difference control according to another preferred embodiment of the present invention;
FIG. 7 is a schematic diagram showing the constitution of a quantum key distribution time bit-phase decoding apparatus for split-polarization phase difference control according to another preferred embodiment of the present invention;
fig. 8 is a schematic diagram showing the composition of a quantum key distribution time bit-phase decoding apparatus for split-polarization phase difference control 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 for split-polarization phase difference control according to a preferred embodiment of the present invention is shown in fig. 1, and includes 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.
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 seen as consisting of two orthogonal polarization states. Naturally, the two sub-optical pulses resulting from the splitting of the first optical pulse can also be seen as consisting of the same two orthogonal polarization states as the optical pulse of this path.
Phase decoding the first optical pulse may include: the first path of light pulse is incident to an interferometer comprising a beam splitter and a beam combiner, so that the beam splitter splits the first path of light pulse into a first path of sub-light pulse and a second path of sub-light pulse; transmitting the first path of sub-optical pulse and the second path of sub-optical pulse along a first arm and a second arm of the interferometer respectively, and carrying out relative delay on the first path of sub-optical pulse and the second path of sub-optical pulse and then outputting the combined beam by the beam combiner.
In the method, the input light pulse before splitting is subjected to phase modulation according to a quantum key distribution protocol, or the first path of light pulse before splitting is subjected to phase modulation according to the quantum key distribution protocol, or at least one sub light pulse in the first path of sub light pulse and the second path of sub light pulse is subjected to phase modulation according to the quantum key distribution protocol in the process of splitting the beam by the beam splitter to the beam combiner.
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.
According to the method of the invention, any one of the first sub-light pulse and the second sub-light pulse which are obtained by beam splitting of the first light pulse and respectively transmitted along the first arm and the second arm can be subjected to polarization diversity processing, or both the first sub-light pulse and the second sub-light pulse can be respectively subjected to polarization diversity processing.
Taking as an example the polarization diversity processing of a first sub-optical pulse transmitted along said first arm, for this first sub-optical pulse: the first sub-optical pulse is polarized and split into two polarized sub-optical pulses with mutually orthogonal polarization states, the two polarized sub-optical pulses are transmitted along two sub-optical paths, and then the two polarized sub-optical pulses are combined into the first sub-optical pulse, and the first sub-optical pulse is transmitted to the beam combiner along the first arm.
According to the method of the invention, two orthogonal polarization states of the first light pulse are controlled to each differ in the interferometer by an integer multiple of 2 pi in phase difference transmitted through the first and second arms.
For example, assuming that the two orthogonal polarization states are an x-polarization state and a y-polarization state, respectively, a phase difference of x-polarization state transmitted through the first arm and the second arm during beam splitting by the beam splitter into the beam combiner is denoted as Δx, and a phase difference of y-polarization state transmitted through the first arm and the second arm during beam splitting by the beam splitter into the beam combiner is denoted as Δy, then each of the two orthogonal polarization states of the first optical pulse during beam splitting by the beam splitter into the beam combiner in the interferometer may be expressed as an integer multiple of a phase difference of 2pi:
Δx–Δy＝2π.m，
Where m is an integer and may be a positive integer, a negative integer or zero.
To achieve that the two orthogonal polarization states of the first optical pulse each differ in the interferometer by an integer multiple of 2 pi in phase difference transmitted through the first and second arms, any one or any combination of the following means may be employed:
● And performing phase control on at least one of two polarized sub-optical pulses obtained by polarization beam splitting of one sub-optical pulse subjected to polarization diversity treatment in the first sub-optical pulse and the second sub-optical pulse. Taking as an example the polarization diversity processing of the first sub-optical pulse transmitted along said first arm, in this case: at least one of the two polarized sub-optical pulses may be phase controlled during transmission of the two polarized sub-optical pulses resulting from polarization splitting of the first sub-optical pulse along the two sub-optical paths. For example, phase controlling at least one of the two polarized sub-pulses of light may include: the phase of either of the two polarized light pulses or the phase of each of the two polarized light pulses is adjusted. For example, an optical fiber phase shifter or a phase modulator may be disposed on a sub-optical path transmitting one of the two polarized sub-optical pulses, or on each sub-optical path transmitting each of the two polarized sub-optical pulses, as needed, so as to adjust the transmission phase of the corresponding polarized optical pulse by the optical fiber phase shifter or the phase modulator. The optical fiber phase shifter is suitable for adjusting the length of the optical path where the optical fiber phase shifter is located and adjusting the transmission phase of the optical pulse transmitted by the optical path where the optical fiber phase shifter is located accordingly, and is particularly suitable for adjusting the length of the polarization maintaining optical fiber optical path.
● The first and second arms include optical paths that are birefringent for the two orthogonal polarization states, and/or the first and second arms have optical devices thereon that are birefringent for the two orthogonal polarization states, in which case the difference in phase differences between the two orthogonal polarization states of the first optical pulse transmitted through the first and second arms in the interferometer are each controlled as follows: respectively maintaining the polarization states of the two orthogonal polarization states unchanged when the two orthogonal polarization states are transmitted along the first arm and the second arm in the interferometer; and adjusting the length of the optical path in which birefringence exists and/or the magnitude of the birefringence of the optical device in which birefringence exists such that the two orthogonal polarization states each differ in the interferometer by an integer multiple of 2π in phase difference transmitted through the first and second arms. Alternatively, this may be achieved by either: i) Configuring the first arm and the second arm as polarization maintaining fiber light paths, and configuring optical devices on the first arm and the second arm as non-birefringent optical devices and/or polarization maintaining optical devices; ii) configuring the first and second arms as free space optical paths, and configuring the optics on the first and second arms as polarization maintaining optics. In the case of i), a polarization maintaining fiber stretcher and/or a birefringent phase modulator may be provided on at least one of the first arm and the second arm. 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 and may thus be arranged to influence and adjust the difference in phase difference between the two orthogonal polarization states of the first optical pulse transmitted in the interferometer via said first and second arms, respectively. 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 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 a first optical pulse transmitted in the interferometer via the first and second arms, respectively.
● The first and second arms are configured as free space optical paths and the optical devices on the first and second arms are configured as non-birefringent optical devices. In this case, the two orthogonal polarization states of the first light pulse each do not change in polarization state when transmitted along the first and second arms in the interferometer, and the phase difference of the two orthogonal polarization states each transmitted along the first and second arms in the interferometer may be the same.
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.
As described above, at least one of the first and second sub-optical pulses may optionally be phase modulated according to a quantum key distribution protocol during beam splitting by a beam splitter of the interferometer to beam combining by a beam combiner of the interferometer. In addition, polarization diversity processing may be performed on either or both of the first and second sub-optical pulses transmitted along the first and second arms of the interferometer, respectively. Phase modulating any one of the sub-optical pulses subjected to polarization diversity processing, for example, the first sub-optical pulse, may be achieved by any one of: the first sub-optical pulse is phase modulated before polarization beam splitting, or the first sub-optical pulse is phase modulated after beam combination of the two corresponding polarization sub-optical pulses, or the same phase modulation is performed on the two polarization sub-optical pulses in the process of polarization beam splitting to beam combination of the two corresponding polarization sub-optical pulses. The phase modulating of the sub-optical pulses, if any, e.g. the second sub-optical pulses, which have not undergone polarization diversity processing may comprise: the second sub-optical pulse is phase modulated during beam splitting by a beam splitter of the interferometer to beam combining by a beam combiner of the interferometer. In either case, phase modulating the corresponding light pulse may include randomly 0-degree phase modulating or 180-degree phase modulating the light pulse. 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.
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.
Decoding the time bits of the second optical pulse may include: 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 apparatus for split-polarization phase difference control according to a preferred embodiment of the present invention is shown in fig. 2, and includes the following components: front beam splitter 201, beam splitters 202 and 203, polarizing beam splitter 204, polarizing beam combiner 205, phase modulator 206, and beam combiner 207.
The beam splitter 203, the beam combiner 207, and both arms therebetween constitute an interferometer. The polarizing beam splitter 204, the polarizing beam combiner 205, and the two sub-optical paths therebetween may be collectively referred to as a split-polarization phase difference control device. The split-polarization phase difference control device and the phase modulator 206 are inserted into the two arms of the interferometer, respectively. For convenience, the arm of the interferometer into which the split-polarization phase difference control device is inserted is hereinafter also referred to as a first arm, and the arm of the interferometer into which the phase modulator 206 is inserted is hereinafter also referred to as a second arm.
The front beam splitter 201 is configured to split an incident input optical pulse with any polarization into two optical pulses.
The interferometer 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 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 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 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 path of light pulses from the front beam splitter 201 into a first path of sub-light pulses and a second path of sub-light pulses.
The first arm and the second arm are for transmitting the first sub-light pulse and the second sub-light pulse, respectively, and for achieving a relative delay of the first sub-light pulse and the second sub-light pulse.
The beam combiner 207 is configured to combine the relatively delayed first sub-optical pulse and the second sub-optical pulse to output.
The phase modulator 206 is used to phase modulate the sub-optical pulses transmitted via the arm in which it resides according to the quantum key distribution protocol.
The polarization beam splitter 204 is configured to split the first polarized sub-light pulse into two polarized sub-light pulses with mutually orthogonal polarization states.
The two sub-optical paths are used for respectively transmitting the two polarized sub-optical pulses.
The polarization beam combiner 205 is configured to polarization-combine the two polarized sub-optical pulses transmitted via the two sub-optical paths into the first sub-optical pulse, and transmit the first sub-optical pulse to the beam combiner 207 along the first arm.
For the decoding apparatus of fig. 2, the first and second arms of the interferometer and the optics thereon are configured such that the two orthogonal polarization states of a first path of light pulse incident to the interferometer each differ in the interferometer by an integer multiple of 2 pi in phase difference transmitted through the first and second arms.
The relative delay of the two sub-optical pulses can be achieved by adjusting the optical path physical length of either of the first and second arms between the beam splitter 203 and the beam combiner 207.
The phase modulator 206 may randomly perform 0-degree phase modulation or 180-degree phase modulation on the sub-optical pulses passing therethrough.
The phase modulator 206 may be a polarization independent phase modulator, including a birefringent device with birefringence compensation (e.g., implemented by two birefringent phase modulators in series or parallel), or other polarization independent phase modulators as previously mentioned.
Although it is shown in fig. 2 that the split-polarization phase difference control means is provided on only the first arm of the interferometer, it is also possible that the split-polarization phase difference control means is provided on only the second arm of the interferometer or that the split-polarization phase difference control means is provided on both the first and second arms of the interferometer.
Although the polarization splitting phase difference control device in fig. 2 uses the polarization beam combiner 205, it is possible to combine two polarized light pulses by using a polarization maintaining coupler instead of the polarization beam combiner 205.
Although the interferometer in FIG. 2 is a structure of an unequal arm Mach-Zehnder interferometer, the interferometer may be a structure of an unequal arm Michelson interferometer.
In addition, although the split-polarization phase difference control device in fig. 2 has a structure of a mach-zehnder optical path, it is possible to adopt a structure of a michelson optical path.
Although fig. 2 shows the phase modulator being provided on only the second arm of the interferometer, it is also possible to provide the phase modulator on only the first arm of the interferometer or one phase modulator on each of the first and second arms of the interferometer. In the case where one phase modulator is provided on each of the first and second arms of the interferometer, the difference in the phases modulated by the two phase modulators is determined by the quantum key distribution protocol. In addition, instead of providing a phase modulator on one or both of the first and second arms of the interferometer, a phase modulator may be provided at the front end of the beam splitter 203, i.e. the first optical pulse before splitting is phase modulated according to the quantum key distribution protocol. Furthermore, it is also possible to provide a phase modulator, i.e. to phase modulate the incoming input light pulses, before the front beam splitter 201.
For the decoding device of fig. 2, the interferometer may optionally have any one or any combination of the following settings:
● The first arm and the second arm of the interferometer are polarization maintaining fiber light paths, and the optical devices on the first arm and the second arm are polarization maintaining optical devices and/or non-birefringent optical devices.
● The interferometer further comprises: the polarization maintaining optical fiber stretcher is positioned on any one of the first arm and the second arm and is used for adjusting the length of the polarization maintaining optical fiber of the arm where the polarization maintaining optical fiber stretcher is positioned; and/or a birefringent phase modulator located on either of the first and second arms for applying different tunable phase modulations to the two orthogonal polarization states of the sub-optical pulses passing therethrough.
● At least one of the two sub-optical paths of the inserted polarization splitting phase difference control device is provided with an optical fiber phase shifter or a phase modulator, and the optical fiber phase shifter or the phase modulator is used for adjusting the transmission phase of the polarized sub-optical pulse transmitted by the sub-optical path where the optical fiber phase shifter or the phase modulator is positioned.
● The interferometer adopts the structure of an unequal arm Michelson interferometer, a beam splitter and a beam combiner of the interferometer are the same device, and the interferometer further comprises: a first mirror on the first arm for reflecting the first sub-pulses transmitted via the first arm from a beam splitter of the interferometer back to a beam combiner of the interferometer; a second mirror on the second arm for reflecting the second sub-optical pulse transmitted via the second arm from the beam splitter of the interferometer back to a beam combiner of the interferometer.
● The inserted polarization splitting phase difference control device adopts a Michelson optical path structure, the polarization beam splitter and the polarization beam combiner are the same device, and the polarization splitting phase difference control device further comprises two reflecting mirrors, wherein one of the two reflecting mirrors is positioned on one of the two sub-optical paths and is used for reflecting polarized sub-optical pulses transmitted by the one sub-optical path from the polarization beam splitter back to the polarization beam combiner; the other of the two reflectors is positioned on the other of the two sub-light paths and is used for reflecting polarized sub-light pulses transmitted by the other sub-light path from the polarization beam splitter back to the polarization beam combiner, wherein the interferometer adopts the structure of an unequal-arm Michelson interferometer, and the first reflector is one of the two reflectors of the polarization splitting phase difference control device.
● The interferometer adopts the structure of the unequal arm Michelson interferometer, the input port and the output port of the interferometer are the same port, the interferometer further comprises an optical circulator, the optical circulator is positioned at the front end of the beam splitter of the interferometer, the first path of light pulse from the front beam splitter is input from the first port of the optical circulator and output from the second port of the optical circulator to the beam splitter of the interferometer, and the beam combining output from the beam combiner of the interferometer is input to the second port of the optical circulator and output from the third port of the optical circulator.
In the case where polarization maintaining fiber stretchers are provided on the first arm and/or the second arm of the interferometer, the polarization maintaining fiber stretchers may optionally be used as phase modulators for phase modulating the light pulses transmitted via the arm in which they are located.
For an arm of an interferometer into which a split-polarization phase difference control device is inserted, a fiber stretcher, such as a polarization maintaining fiber stretcher, may be provided on the arm before or after the split-polarization phase difference control device.
In case that the two sub-optical paths of the polarization splitting phase difference control device are each provided with an optical fiber phase shifter, the optical fiber phase shifter may be optionally used as a phase modulator for performing the same phase modulation on the two polarized sub-optical pulses.
A quantum key distribution time bit-phase decoding apparatus for split-polarization phase difference control according to another preferred embodiment of the present invention is shown in fig. 3, and includes the following components: beam splitters 303 and 304, polarization maintaining beam splitter 307, polarization beam splitter 308, polarization maintaining fiber phase shifter 309, polarization combiner 310, phase modulator 311, polarization maintaining combiner 312.
The splitter 303 acts as a front splitter with one of the two ports 301 and 302 on one side acting as the input to the device. The polarization maintaining beam splitter 307, the polarization maintaining beam combiner 312 and two arms therebetween constitute one polarization maintaining mach-zehnder interferometer. The polarizing beam splitter 308, the polarizing beam combiner 310, and the two sub-optical paths therebetween may be collectively referred to as a split-polarization phase difference control device. The split-polarization phase difference control device and the phase modulator 311 are inserted into the two arms of the mach-zehnder interferometer, respectively. The polarization maintaining fiber phase shifter 309 is inserted into either one of the two sub-optical paths of the split-polarization phase difference control device. For convenience, the arm of the mach-zehnder interferometer into which the split-polarization phase difference control device is inserted is hereinafter also referred to as a first arm, and the arm of the mach-zehnder interferometer into which the phase modulator 311 is inserted is hereinafter also referred to as a second arm.
In operation, an incident input optical pulse enters the beam splitter 303 through the port 301 or 302 of the front beam splitter 303, and is split into two optical pulses by the beam splitter 303 for transmission.
One optical pulse from the front splitter 303 is input to the splitter 304 and split by the splitter 304 and output via port 305 or port 306 for time bit decoding.
The other path of light pulse from the front beam splitter 303 is input to the polarization maintaining beam splitter 307, and is split into a first path of sub-light pulse and a second path of sub-light pulse by the polarization maintaining beam splitter 307. The first sub-optical pulse is polarized and split into two polarized oscillator optical pulses by the polarization beam splitter 308; the two polarized sub-light pulses are transmitted to the polarization beam combiner 310 through two first sub-light paths respectively, and are polarized and combined by the polarization beam combiner 310 into a first sub-light pulse, and are transmitted to the polarization maintaining beam combiner 312 along the first arm. The second sub-optical pulse is transmitted to the polarization maintaining beam combiner 312 after being randomly modulated by the phase modulator 311 with a 0 degree phase or 180 degree phase. The first sub-optical pulse and the second sub-optical pulse, which are transmitted to the polarization maintaining beam combiner 312 and are relatively delayed, are combined by the polarization maintaining beam combiner 312 and then output from the port 313. During the period from polarization beam splitting to beam combining of the first sub-optical pulse, the phase of the polarized sub-optical pulse transmitted through the sub-optical path where the polarization maintaining optical phase shifter 309 is located may be adjusted by the polarization maintaining optical phase shifter 309.
The phase modulator 311 is a polarization independent device including a birefringent device with birefringence compensation (e.g., implemented by two birefringent phase modulators in series or parallel), or other polarization independent phase modulators as previously mentioned.
The two sub-optical paths of the polarization splitting phase difference control device can be respectively inserted into an optical fiber phase shifter. In this case, the same phase modulation can be performed on the two polarized sub-optical pulses by the two optical fiber phase shifters on the two sub-optical paths, thereby realizing the phase modulation function of the phase modulator 311; that is, the phase modulator 311 may be omitted.
In addition, the split-polarization phase difference control device and the phase modulator 311 may be inserted into the same arm of the mach-zehnder interferometer, without the above-described operation being affected.
A quantum key distribution time bit-phase decoding apparatus for split-polarization phase difference control according to another preferred embodiment of the present invention is shown in fig. 4, and includes the following components: beam splitter 403, polarization maintaining beam splitter 405, polarization maintaining fiber phase shifter 407, polarization combiner 408, phase modulator 409, polarization maintaining combiner 410.
Beam splitter 403 acts as a front-end beam splitter with one of the two ports 401 and 402 on one side acting as the input to the device. Polarization maintaining beam splitter 405, polarization maintaining beam combiner 410, and two arms therebetween form a polarization maintaining mach-zehnder interferometer. The polarizing beam splitter 406, polarizing beam combiner 408, and the two sub-optical paths therebetween may be collectively referred to as a split polarization phase difference control device. The split-polarization phase difference control device and the phase modulator 409 are inserted into the two arms of the mach-zehnder interferometer, respectively. The polarization maintaining fiber phase shifter 407 is inserted into either one of the two sub-optical paths of the polarization splitting phase difference control device. For convenience, the arm of the mach-zehnder interferometer into which the split-polarization phase difference control device is inserted is hereinafter also referred to as a first arm, and the arm of the mach-zehnder interferometer into which the phase modulator 409 is inserted is hereinafter also referred to as a second arm.
In operation, an incoming optical pulse enters beam splitter 403 through port 401 or 402 of beam splitter 403 and is split into two optical pulses by beam splitter 403 for transmission.
One optical pulse from pre-splitter 403 is directly output for temporal bit decoding.
The other light pulse from the pre-splitter 403 is input to the polarization maintaining splitter 405 and split into a first sub-light pulse and a second sub-light pulse by the polarization maintaining splitter 405. The first sub-optical pulse is polarized and split into two polarized oscillator optical pulses by the polarization beam splitter 406; the two polarized sub-light pulses are transmitted to the polarization beam combiner 408 through two first sub-light paths respectively, and are polarized and combined into a first sub-light pulse by the polarization beam combiner 408 and transmitted to the polarization-preserving beam combiner 410 along the first arm. The second sub-optical pulse is transmitted to the polarization maintaining beam combiner 410 after being randomly modulated by the phase modulator 409 in a phase of 0 degrees or 180 degrees. The first sub-optical pulse and the second sub-optical pulse transmitted to the polarization maintaining beam combiner 410 after being relatively delayed are combined by the polarization maintaining beam combiner 410 and then output by the port 411. During the period from polarization beam splitting to beam combination of the first path of sub-optical pulse, the phase of the polarized sub-optical pulse transmitted through the sub-optical path where the polarization maintaining optical phase shifter 407 is located can be adjusted by the polarization maintaining optical phase shifter 407.
The phase modulator 409 is a polarization independent device including a birefringent device with birefringence compensation (e.g., implemented by two birefringent phase modulators in series or parallel), or other polarization independent phase modulators as previously mentioned.
The two sub-optical paths of the polarization splitting phase difference control device can be respectively inserted into an optical fiber phase shifter. In this case, the same phase modulation can be performed on the two polarized sub-optical pulses by the two optical fiber phase shifters on the two sub-optical paths, thereby realizing the phase modulation function of the phase modulator 409; that is, the phase modulator 409 may be omitted.
In addition, the split-polarization phase difference control device and the phase modulator 409 may be inserted into the same arm of the mach-zehnder interferometer without the above-described operation being affected.
A quantum key distribution time bit-phase decoding apparatus for split-polarization phase difference control according to another preferred embodiment of the present invention is shown in fig. 5, and includes the following components: beam splitters 503 and 504, polarization maintaining beam splitter 507, polarization beam splitter 508, polarization maintaining fiber phase shifter 509, polarization combiner 510, phase modulator 512, and mirrors 511 and 513.
The beam splitter 503 acts as a front-end beam splitter with one of the two ports 501 and 502 on one side as the input to the device. Polarization maintaining beam splitter 507, two mirrors 511 and 513, and two arms between polarization maintaining beam splitter 507 and the two mirrors constitute a polarization maintaining michelson interferometer. The polarizing beam splitter 508, polarizing beam combiner 510, and the two sub-optical paths therebetween may be collectively referred to as a split-polarization phase difference control device. The split-polarization phase difference control device and the phase modulator 512 are inserted into the two arms of the michelson interferometer, respectively. The polarization maintaining fiber phase shifter 509 is inserted into either one of the two sub-optical paths of the split-polarization phase difference control device. For convenience, the arm of the michelson interferometer with the split-polarization phase difference control device inserted is referred to below as the first arm, and the arm of the michelson interferometer with the phase modulator 512 inserted is referred to below as the second arm.
In operation, an incident input optical pulse enters the beam splitter 503 through the port 501 or 502 of the front beam splitter 503, and is split into two optical pulses by the beam splitter 503 for transmission.
One optical pulse from the front splitter 503 is input to the splitter 504 and split by the splitter 504 and output via port 505 or port 506 for temporal bit decoding.
The other path of light pulse from the front beam splitter 503 is input to the polarization maintaining beam splitter 507, and is split into a first path of sub-light pulse and a second path of sub-light pulse by the polarization maintaining beam splitter 507. The first sub-optical pulse is polarized and split into two polarized oscillator optical pulses by the polarization beam splitter 508; the two polarized sub-light pulses are transmitted to the polarization beam combiner 510 through two first sub-light paths respectively, and are polarized and combined into a first sub-light pulse by the polarization beam combiner 510, transmitted to the mirror 511 along the first arm, and reflected back by the mirror 511. The second sub-optical pulse is randomly modulated by the phase modulator 512 with a 0-degree phase or a 180-degree phase, and then transmitted to the mirror 513 and reflected back by the mirror 513. The reflected first sub-optical pulse and the reflected second sub-optical pulse with relative delay are combined by the polarization maintaining beam splitter 507 and then output by the port 514. During the period from polarization beam splitting to beam combination of the first sub-optical pulse, the phase of the polarized sub-optical pulse transmitted through the sub-optical path where the polarization maintaining optical phase shifter 509 is located can be adjusted by the polarization maintaining optical phase shifter 509.
The phase modulator 512 is a polarization independent device including a birefringent device with birefringence compensation (e.g., implemented by two birefringent phase modulators in series or parallel), or other polarization independent phase modulators as previously mentioned.
The two sub-optical paths of the polarization splitting phase difference control device can be respectively inserted into an optical fiber phase shifter. In this case, the same phase modulation can be performed on the two polarized sub-optical pulses by the two optical fiber phase shifters on the two sub-optical paths, thereby implementing the phase modulation function of the phase modulator 512; that is, the phase modulator 512 may be omitted.
Alternatively, the split-polarization phase difference control device and phase modulator 512 may be inserted into the same arm of the Michelson interferometer without the above-described operation being affected.
A quantum key distribution time bit-phase decoding apparatus for split-polarization phase difference control according to another preferred embodiment of the present invention is shown in fig. 6, and includes the following components: beam splitters 603 and 604, polarization maintaining beam splitter 607, polarization beam splitter 608, polarization maintaining fiber phase shifter 609, phase modulator 612, and mirrors 610, 611, and 613.
The beam splitter 603 acts as a front beam splitter with one of the two ports 601 and 602 on one side as the input to the device. Polarization maintaining beam splitter 607, one arm between polarization maintaining beam splitter 607 and two mirrors 610 and 611, the other arm between polarization maintaining beam splitter 607 and mirror 613, and mirrors 610, 611, 613 comprise a polarization maintaining michelson interferometer. The polarizing beam splitter 608, the two sub-optical paths between the polarizing beam splitter 608 and the two mirrors 610 and 611 may be collectively referred to as a split polarization phase difference control device. The split-polarization phase difference control device and the phase modulator 612 are inserted into the two arms of the michelson interferometer, respectively. The polarization maintaining fiber phase shifter 609 is inserted into either one of the two sub-optical paths of the split polarization phase difference control device. For convenience, the arm of the michelson interferometer with the split-polarization phase difference control device inserted is referred to below as the first arm, and the arm of the michelson interferometer with the phase modulator 612 inserted is referred to below as the second arm.
In operation, an incident input optical pulse enters the beam splitter 603 through the port 601 or 602 of the front beam splitter 603, and is split into two optical pulses by the beam splitter 603 for transmission.
One optical pulse from the front splitter 603 is input to the splitter 604 and split by the splitter 604 and output via port 605 or port 606 for temporal bit decoding.
The other path of light pulse from the front beam splitter 603 is input to the polarization maintaining beam splitter 607, and is split into a first path of sub-light pulse and a second path of sub-light pulse by the polarization maintaining beam splitter 607. The first sub-optical pulse is polarized and split into two polarized oscillator optical pulses by the polarization beam splitter 608; the two polarized sub-light pulses are respectively transmitted to the mirrors 610 and 611 through two first sub-light paths, respectively, and are respectively reflected by the mirrors 610 and 611 back to the polarization beam splitter 608, and are polarized by the polarization beam splitter 608 to be combined into first sub-light pulses, and are transmitted to the polarization beam splitter 607 along the first arm. The second sub-optical pulse is transmitted to the mirror 613 after being randomly modulated with a 0-degree phase or a 180-degree phase by the phase modulator 612 and is reflected back by the mirror 613. The reflected first sub-optical pulse and the second sub-optical pulse with relative delay are output by the port 614 after being combined by the polarization maintaining beam splitter 607. During the period from polarization beam splitting to beam combining of the first sub-optical pulse, the phase of the polarized sub-optical pulse transmitted through the sub-optical path where the polarization maintaining optical phase shifter 609 is located can be adjusted by the polarization maintaining optical phase shifter 609.
The phase modulator 612 is a polarization independent device including a birefringent device that is birefringence compensated (e.g., by two birefringent phase modulators in series or parallel), or other polarization independent phase modulators as previously mentioned.
The two sub-optical paths of the polarization splitting phase difference control device can be respectively inserted into an optical fiber phase shifter. In this case, the same phase modulation can be performed on the two polarized sub-optical pulses by the two optical fiber phase shifters on the two sub-optical paths, thereby implementing the phase modulation function of the phase modulator 612; that is, the phase modulator 612 may be omitted.
Alternatively, the split-polarization phase difference control device and phase modulator 612 may be inserted into the same arm of the Michelson interferometer without the above-described operation being affected.
A quantum key distribution time bit-phase decoding apparatus for split-polarization phase difference control according to another preferred embodiment of the present invention is shown in fig. 7, and includes the following components: beam splitter 703, polarization maintaining beam splitter 705, polarization beam splitter 706, polarization maintaining fiber phase shifter 707, polarization combiner 708, phase modulator 710, and mirrors 709 and 711.
The beam splitter 703 acts as a front beam splitter with one of the two ports 701 and 702 on one side acting as the input to the device. Polarization maintaining beam splitter 705, two mirrors 709 and 711, and two arms between polarization maintaining beam splitter 705 and the two mirrors constitute a polarization maintaining michelson interferometer. Polarizing beam splitter 706, polarizing beam combiner 708, and the two sub-optical paths therebetween may be collectively referred to as a split polarization phase difference control device. The split-polarization phase difference control device and the phase modulator 710 are inserted into the two arms of the michelson interferometer, respectively. The polarization maintaining fiber phase shifter 707 is interposed between either of the two sub-optical paths of the polarization splitting phase difference control apparatus. For convenience, the arm of the michelson interferometer with the split-polarization phase difference control device inserted is referred to below as the first arm, and the arm of the michelson interferometer with the phase modulator 710 inserted is referred to below as the second arm.
In operation, an incoming optical pulse enters beam splitter 703 through port 701 or 702 of front beam splitter 703 and is split into two optical pulses by beam splitter 703 for transmission.
One optical pulse from the pre-splitter 703 is directly output for temporal bit decoding.
The other path of light pulse from the front beam splitter 703 is input to the polarization maintaining beam splitter 705, and is split into a first path of sub-light pulse and a second path of sub-light pulse by the polarization maintaining beam splitter 705. The first sub-optical pulse is polarized and split into two polarized oscillator optical pulses by the polarization beam splitter 706; the two polarized sub-optical pulses are transmitted to the polarization beam combiner 708 via two first sub-optical paths, respectively, and the first sub-optical pulses are polarized and combined by the polarization beam combiner 708 into a first sub-optical pulse, which is transmitted along the first arm to the mirror 709 and reflected back by the mirror 709. The second sub-optical pulse is randomly modulated by the phase modulator 710 in a phase of 0 degree or 180 degrees, transmitted to the mirror 711, and reflected by the mirror 711. The reflected first sub-optical pulse and the second sub-optical pulse with relative delay are output by the port 712 after being combined by the polarization maintaining beam splitter 705. During the period from polarization splitting to beam combining of the first sub-optical pulse, the phase of the polarized sub-optical pulse transmitted through the sub-optical path where the polarization maintaining optical phase shifter 707 is located may be adjusted by the polarization maintaining optical phase shifter 707.
The phase modulator 710 is a polarization independent device including a birefringent device with birefringence compensation (e.g., implemented by two birefringent phase modulators in series or parallel), or other polarization independent phase modulators as previously mentioned.
The two sub-optical paths of the polarization splitting phase difference control device can be respectively inserted into an optical fiber phase shifter. In this case, the same phase modulation can be performed on the two polarized sub-optical pulses by the two optical fiber phase shifters on the two sub-optical paths, thereby implementing the phase modulation function of the phase modulator 710; that is, the phase modulator 710 may be omitted.
In addition, the split-polarization phase difference control device and the phase modulator 710 may be inserted into the same arm of the michelson interferometer without the above-described operation being affected.
A quantum key distribution time bit-phase decoding apparatus for split-polarization phase difference control according to another preferred embodiment of the present invention is shown in fig. 8, and includes the following components: beam splitter 803, polarization maintaining beam splitter 805, polarizing beam splitter 806, polarization maintaining fiber phase shifter 807, phase modulator 810, and mirrors 808, 809, and 811.
Beam splitter 803 acts as a front-end beam splitter with one of the two ports 801 and 802 on one side acting as the input to the device. Polarization maintaining beam splitter 805, one arm between polarization maintaining beam splitter 805 and two mirrors 808 and 809, the other arm between polarization maintaining beam splitter 805 and mirror 811, mirrors 808, 809, 811 constitute a polarization maintaining michelson interferometer. The polarizing beam splitter 806, the two mirrors 808 and 809 and the two sub-optical paths between the polarizing beam splitter 806 and the two mirrors may be collectively referred to as a split polarization phase difference control device. The split-polarization phase difference control device and the phase modulator 810 are inserted into the two arms of the michelson interferometer, respectively. The polarization maintaining fiber phase shifter 807 is interposed in either of the two sub-optical paths of the split-polarization phase difference control device. For convenience, the arm of the michelson interferometer with the split-polarization phase difference control device inserted is referred to below as the first arm, and the arm of the michelson interferometer with the phase modulator 810 inserted is referred to below as the second arm.
In operation, an incoming input optical pulse enters beamsplitter 803 through port 801 or 802 of front beamsplitter 803 and is split into two optical pulses by beamsplitter 803 for transmission.
One optical pulse from the pre-splitter 803 is directly output for temporal bit decoding.
The other path of light pulse from the front beam splitter 803 is input to the polarization maintaining beam splitter 805, and split into a first path of sub-light pulse and a second path of sub-light pulse by the polarization maintaining beam splitter 805. The first sub-optical pulse is polarized and split into two polarized oscillator optical pulses by the polarization beam splitter 806; the two polarized sub-light pulses are respectively transmitted to the reflectors 808 and 809 through two first sub-light paths, respectively, and are respectively reflected by the reflectors 808 and 809 back to the polarization beam splitter 806, and are polarized by the polarization beam splitter 806 to be combined into first sub-light pulses, and transmitted to the polarization beam splitter 805 along the first arm. The second sub-optical pulse is randomly modulated by the phase modulator 810 in phase of 0 degrees or 180 degrees, and then transmitted to the mirror 811 and reflected back by the mirror 811. The reflected first sub-optical pulse and the second sub-optical pulse with relative delay are output by the port 812 after being combined by the polarization maintaining beam splitter 805. During the period from polarization splitting to beam combining of the first sub-optical pulse, the phase of the polarized sub-optical pulse transmitted through the sub-optical path where the polarization maintaining optical phase shifter 807 is located may be adjusted by the polarization maintaining optical phase shifter 807.
The phase modulator 810 is a polarization independent device including a birefringent device (e.g., implemented by two birefringent phase modulators in series or parallel) that is birefringent compensated, or other polarization independent phase modulators as previously mentioned.
The two sub-optical paths of the polarization splitting phase difference control device can be respectively inserted into an optical fiber phase shifter. In this case, the same phase modulation can be performed on the two polarized sub-optical pulses by the two optical fiber phase shifters on the two sub-optical paths, thereby implementing the phase modulation function of the phase modulator 810; that is, the phase modulator 810 may be omitted.
Alternatively, the split-polarization phase difference control device and phase modulator 810 may be inserted into the same arm of the Michelson interferometer without the above-described operation being affected.
The decoding device of the present invention, such as the decoding device shown in any one of fig. 3 to 8, has two arms of the interferometer and the optical devices on the two arms configured such that two orthogonal polarization states of a first path light pulse incident to the interferometer each differ in the interferometer by an integer multiple of 2 pi in phase difference transmitted through the two arms. In addition, the optical pulses transmitted on at least one of the two arms are subjected to polarization diversity processing, whereby the transmission phases of the two orthogonal polarization states of the first path optical pulse are controlled by the polarization, so that the difference in the above-described phase differences is easily achieved.
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. The terms "polarizing beam splitter" and "polarizing beam combiner" are used interchangeably, and a polarizing beam splitter may also be referred to as and function as a polarizing beam combiner, and vice versa
The quantum key distribution time bit-phase decoding device of the split polarization phase difference control of the present invention can be configured at the receiving end of the quantum key distribution system for time bit-phase decoding. In addition, the quantum key distribution time bit-phase decoding device for the polarization splitting phase difference control can be configured at the transmitting end of the quantum key distribution system and used for time bit-phase encoding.
While the invention has been described in connection with specific embodiments thereof, it is to be understood that these drawings are included in the spirit and scope of the invention, it is not to be limited thereto.

Claims (17)

1. A quantum key distribution time bit-phase decoding method for split polarization phase difference control, 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:
the first path of light pulse is incident to an interferometer comprising a beam splitter and a beam combiner, so that the beam splitter splits the first path of light pulse into a first path of sub-light pulse and a second path of sub-light pulse;
transmitting the first path of sub-optical pulse and the second path of sub-optical pulse along a first arm and a second arm of the interferometer respectively, carrying out relative delay on the first path of sub-optical pulse and the second path of sub-optical pulse, then outputting the combined beam by the beam combiner,
wherein for the first sub-optical pulse transmitted at least along the first arm: the first sub-light pulse is polarized and split into two polarized sub-light pulses with mutually orthogonal polarization states, the two polarized sub-light pulses are transmitted along two sub-light paths, then the two polarized sub-light pulses are combined into the first sub-light pulse, the first sub-light pulse is transmitted to the beam combiner along the first arm,
wherein two orthogonal polarization states of the first path of light pulse are controlled to be respectively different from each other by an integer multiple of 2 pi in phase difference transmitted by the first arm and the second arm in the interferometer,
The input light pulse before splitting or the first path of light pulse before splitting or at least one of the first path of sub-light pulse and the second path of sub-light pulse is subjected to phase modulation according to a quantum key distribution protocol in the process of splitting the beam by the beam splitter to the beam combiner.
2. The quantum key distribution time bit-phase decoding method of split-polarization phase difference control of claim 1, wherein the first and second arms comprise optical paths having birefringence for the two orthogonal polarization states and/or the first and second arms have optical devices thereon having birefringence for the two orthogonal polarization states, wherein the controlling the two orthogonal polarization states of the first optical pulse to each differ by an integer multiple of 2 pi in phase difference transmitted in the interferometer via the first and second arms comprises:
respectively maintaining the polarization states of the two orthogonal polarization states unchanged when the two orthogonal polarization states are transmitted along the first arm and the second arm in the interferometer; and
the length of the optical path in which the birefringence is present and/or the magnitude of the birefringence of the optical device in which the birefringence is present are adjusted such that the two orthogonal polarization states each differ in the interferometer by an integer multiple of 2 pi in phase difference transmitted through the first and second arms.
3. The quantum key distribution time bit-phase decoding method of split-polarization phase difference control according to claim 1 or 2, wherein the first and second arms are configured as polarization maintaining fiber optical paths, and the optical devices on the first and second arms are configured as non-birefringent optical devices and/or polarization maintaining optical devices.
4. The quantum key distribution time bit-phase decoding method of split-polarization phase difference control according to claim 2, wherein a polarization maintaining fiber stretcher and/or a birefringent phase modulator is arranged on at least one of the first arm and the second arm, wherein a difference between phase differences transmitted in the interferometer via the first arm and the second arm by two orthogonal polarization states of the first optical pulse are adjusted by the polarization maintaining fiber stretcher and/or the birefringent phase modulator, respectively.
5. The quantum key distribution time bit-phase decoding method of split polarization phase difference control according to claim 1, wherein at least one sub-optical pulse of the first and second sub-optical pulses is phase-modulated according to a quantum key distribution protocol in the process of splitting the beam by the beam splitter to the beam combiner, wherein
The at least one sub-light pulse comprises the first sub-light pulse, and the phase modulating the at least one sub-light pulse of the first sub-light pulse and the second sub-light pulse according to a quantum key distribution protocol in the process of splitting the beam by the beam splitter to the beam combiner comprises: the first path of polarized sub-light pulse is subjected to phase modulation before polarization beam splitting or after beam combination is carried out on the two paths of polarized sub-light pulses, or the two paths of polarized sub-light pulses are subjected to the same phase modulation in the process from polarization beam splitting to beam combination is carried out on the two paths of polarized sub-light pulses; and/or
The at least one sub-light pulse comprises the second sub-light pulse, and the phase modulating the at least one sub-light pulse of the first sub-light pulse and the second sub-light pulse according to a quantum key distribution protocol in the process of splitting the beam by the beam splitter to the beam combiner comprises: and carrying out phase modulation on the second sub-optical pulse in the process of splitting the beam by the beam splitter to the beam combiner.
6. The split-polarization phase-difference controlled quantum key distribution time bit-phase decoding method of claim 1, wherein at least one of the two polarized sub-optical pulses is phase-controlled during transmission of the two polarized sub-optical pulses along the two sub-optical paths.
7. The quantum key distribution time bit-phase decoding method of split-polarization phase difference control of claim 6, wherein phase controlling at least one of the two polarized sub-pulses of light comprises:
and adjusting the phase of one polarized oscillator optical pulse in the two polarized oscillator optical pulses.
8. A quantum key distribution time bit-phase decoding device for split polarization phase difference control, characterized in that the decoding device comprises a front beam splitter and an interferometer, the interferometer comprises a first beam splitter, a first beam combiner, and a first arm and a second arm optically coupled with the first beam splitter and optically coupled with the first beam combiner, wherein
The front beam splitter is used for splitting one path of input light pulse with any incident polarization state into a first path of light pulse and a second path of light pulse;
the interferometer is optically coupled to the front beam splitter for phase decoding the first optical pulse, wherein
The first beam splitter is used for splitting the first path of light pulse into a first path of sub-light pulse and a second path of sub-light pulse;
the first and second arms are for transmitting the first and second sub-optical pulses, respectively, and for achieving a relative delay of the first and second sub-optical pulses;
The first beam combiner is used for combining and outputting the first path of sub-optical pulse and the second path of sub-optical pulse which are relatively delayed,
wherein at least the first arm is provided with a polarization phase difference control device, the polarization phase difference control device comprises a polarization beam splitter, a second beam combiner and two sub-light paths which are optically coupled with the polarization beam splitter and the second beam combiner, wherein the polarization beam splitter is used for splitting the polarization beam, the second beam combiner is used for splitting the polarization beam, and the first beam combiner is used for splitting the polarization beam
The polarization beam splitter is used for polarization splitting of the first path of sub-light pulses into two paths of polarized oscillator light pulses with mutually orthogonal polarization states;
the two sub-optical paths are used for respectively transmitting the two polarized oscillator optical pulses;
the second beam combiner is used for combining the two polarized light pulses transmitted by the two sub-light paths into the first sub-light pulse and transmitting the first sub-light pulse to the first beam combiner along the first arm,
wherein the first and second arms and the optics thereon are configured such that the two orthogonal polarization states of the first optical pulse each differ in the interferometer by an integer multiple of 2 pi in phase difference transmitted through the first and second arms,
the decoding device is further provided with a phase modulator, and the phase modulator is used for carrying out phase modulation on at least one sub-optical pulse of the first sub-optical pulse and the second sub-optical pulse according to a quantum key distribution protocol in the process of splitting the input optical pulse before splitting or the first path optical pulse before splitting or splitting the first beam splitter to the first beam combiner.
9. The quantum key distribution time bit-phase decoding apparatus for split-polarization phase difference control according to claim 8, wherein the first arm and the second arm are polarization maintaining fiber optical paths, and the optical devices on the first arm and the second arm are polarization maintaining optical devices and/or non-birefringent optical devices.
10. The quantum key distribution time bit-phase decoding apparatus of split-polarization phase difference control of claim 8, wherein the decoding apparatus further comprises:
the polarization maintaining optical fiber stretcher is positioned on any one of the first arm and the second arm and is used for adjusting the length of the polarization maintaining optical fiber of the arm where the polarization maintaining optical fiber stretcher is positioned; and/or
A birefringent phase modulator located on either of the first and second arms for applying different adjustable phase modulations to two orthogonal polarization states of the light pulses passing therethrough.
11. The quantum key distribution time bit-phase decoding apparatus of claim 8, wherein the phase modulator comprises:
the phase modulator is positioned at the front end of the interferometer and is used for carrying out phase modulation on the first path of optical pulse before splitting; or (b)
A phase modulator on the second arm for phase modulating the second sub-optical pulse during beam splitting by the first beam splitter to beam combining by the first beam combiner, wherein the at least one sub-optical pulse comprises the second sub-optical pulse; or (b)
A phase modulator arranged on the first arm before the polarization beam splitter and used for carrying out phase modulation on the first path of polarized sub-light pulse before polarization beam splitting, or a phase modulator arranged on the first arm after the second beam combiner and used for carrying out phase modulation on the first path of sub-light pulse after beam splitting on the two paths of polarized sub-light pulses, or two phase modulators respectively arranged on the two paths of sub-light and used for carrying out the same phase modulation on the two paths of polarized sub-light pulses in the process of polarization beam splitting to beam combining on the two paths of polarized sub-light pulses, wherein the at least one sub-light pulse comprises the first sub-light pulse.
12. The quantum key distribution time bit-phase decoding device for split-polarization phase difference control according to claim 8, wherein an optical fiber phase shifter or a phase modulator is disposed on at least one of the two sub-optical paths, and is used for adjusting the phase of the polarized sub-optical pulse transmitted through the sub-optical path where the optical fiber phase shifter or the phase modulator is located.
13. The split-polarization phase-difference controlled quantum key distribution time bit-phase decoding device according to claim 8, wherein,
the interferometer adopts a structure of an unequal arm Mach-Zehnder interferometer; or alternatively
The interferometer adopts the structure of an unequal arm Michelson interferometer, the first beam combiner and the first beam splitter are the same device, and the interferometer further comprises:
a first mirror on the first arm for reflecting the first sub-light pulse transmitted through the first arm from the first beam splitter back to the first beam combiner;
and a second mirror on the second arm for reflecting the second sub-optical pulse transmitted from the first beam splitter via the second arm back to the first beam combiner.
14. The quantum key distribution time bit-phase decoding apparatus for split-polarization phase difference control according to claim 8 or 13, wherein,
the polarization splitting phase difference control device adopts a Mach-Zehnder optical path structure; or alternatively
The polarization splitting phase difference control device adopts a Michelson optical path structure, the polarization beam splitter and the second beam combiner are the same device, and the polarization splitting phase difference control device further comprises two reflecting mirrors, wherein one of the two reflecting mirrors is positioned on one of the two sub-optical paths and is used for reflecting polarized sub-optical pulses transmitted by the one sub-optical path from the polarization beam splitter back to the second beam combiner; the other of the two reflectors is positioned on the other of the two sub-light paths and is used for reflecting polarized sub-light pulses transmitted by the other sub-light path from the polarization beam splitter back to the second beam combiner, wherein the interferometer adopts the structure of an unequal-arm Michelson interferometer, and one of the two reflectors is a first reflector.
15. The quantum key distribution time bit-phase decoding apparatus for split-polarization phase difference control according to any one of claims 8 to 13, wherein the second beam combiner is a polarization maintaining coupler or a polarization beam combiner.
16. The quantum key distribution time bit-phase decoding apparatus of claim 8, further comprising a second beam splitter optically coupled to the pre-splitter for receiving the second optical pulse and splitting 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 for split polarization phase difference control according to any one of claims 8 to 16, which is provided at a receiving end of the quantum key distribution system, for time bit-phase decoding; and/or
A quantum key distribution time bit-phase decoding apparatus for split polarization phase difference control according to any one of claims 8 to 16, which is provided at a transmitting end of the quantum key distribution system for time bit-phase encoding.
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