CN109150524B - Phase decoding method and device and quantum key distribution system - Google Patents
Phase decoding method and device and quantum key distribution system Download PDFInfo
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- 238000000034 method Methods 0.000 title claims abstract description 49
- 238000009826 distribution Methods 0.000 title claims abstract description 47
- 230000003287 optical effect Effects 0.000 claims abstract description 265
- 230000010287 polarization Effects 0.000 claims abstract description 174
- 239000000835 fiber Substances 0.000 claims description 61
- 239000013307 optical fiber Substances 0.000 claims description 16
- 238000001514 detection method Methods 0.000 claims description 3
- 238000003780 insertion Methods 0.000 abstract description 5
- 230000037431 insertion Effects 0.000 abstract description 5
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- 230000005540 biological transmission Effects 0.000 description 9
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- 238000004891 communication Methods 0.000 description 3
- GQYHUHYESMUTHG-UHFFFAOYSA-N lithium niobate Chemical compound [Li+].[O-][Nb](=O)=O GQYHUHYESMUTHG-UHFFFAOYSA-N 0.000 description 3
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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
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- 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
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- 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
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- 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
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- 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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- Optics & Photonics (AREA)
- Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)
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Abstract
The invention provides a phase decoding method, a phase decoding device and a quantum key distribution system. The method comprises the following steps: splitting an input light pulse with any polarization state into two light pulses to be transmitted on two light paths respectively, and carrying out relative delay on the two light pulses to be output in a beam combining way, wherein the light pulses before splitting or at least one of the two light pulses obtained by splitting in the beam combining process are subjected to phase modulation according to a quantum key distribution protocol, and the two orthogonal polarization states of the input light pulse are controlled to be respectively different by an integral multiple of 2 pi in phase difference transmitted through the two light paths in the beam combining process. The invention can effectively solve the influence of the random change of the polarization state of the light pulse on the system stability, and realize stable phase decoding resistant to environmental interference. In addition, the invention can adopt the unequal arm Mach-Zehnder interferometer, and the optical pulse only needs to pass through the phase modulator once when decoding, thereby reducing the insertion loss of the receiving end and improving the system efficiency.
Description
Technical Field
The present invention relates to the field of optical transmission secret communication technology, and in particular, to a phase decoding method, a device and a quantum key distribution system.
Background
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 and gradually goes to application. For a phase coding quantum key distribution system based on an unequal arm interferometer, in the process of transmitting optical pulses through an optical fiber quantum channel, the optical fiber is made to have non-ideal conditions such as non-circular symmetry in section, non-uniform distribution of refractive index of a fiber core along radial direction and the like, and the optical fiber is influenced by temperature, strain, bending and the like in an actual environment to generate a double refraction effect, the polarization state of the optical pulses when reaching a receiving end can be randomly changed, so that an output result of the phase decoding interferometer is unstable, and the phenomenon is obviously deteriorated along with the increase of the distance of the optical fiber.
In the prior art, an unequal arm faraday-michelson interferometer is proposed that allows light pulses to remain stably output of the interference result when subjected to the random birefringence of the fibre channel and the resulting polarization state changes. However, such interferometers are large in loss, and the insertion loss of the phase modulator is one of the main factors causing the large loss. Specifically, when the phase modulator is placed on an arm of the interferometer, the light pulse passes through the phase modulator twice due to back and forth transmission, which results in a larger loss of the interferometer and lower system efficiency.
Disclosure of Invention
The invention mainly aims to provide a phase decoding method, a phase decoding device and a quantum key distribution system, which are used for solving the problem that the interference output result of a receiving end is unstable due to the polarization state change in phase coding quantum key distribution application.
The invention provides at least the following technical scheme:
1. a method of phase decoding, the method comprising:
Splitting one path of input light pulse with any polarization state into two paths of light pulses;
transmitting the two paths of light pulses on two light paths respectively, carrying out relative delay on the two paths of light pulses, then combining and outputting,
Wherein the input light pulse before splitting or at least one of the two light pulses is phase modulated according to a quantum key distribution protocol in the process of splitting to combining, and
Wherein, two orthogonal polarization states of the input light pulse are controlled to respectively have an integral multiple of 2 pi phase difference in the process of splitting and combining the light beams.
2. The phase decoding method according to claim 1, wherein the two optical paths include an optical path having birefringence for the two orthogonal polarization states, and/or the two optical paths have an optical device having birefringence for the two orthogonal polarization states thereon, wherein the controlling of the two orthogonal polarization states of the input optical pulse to each have a phase difference of 2 pi between integral multiples of a phase difference transmitted through the two optical paths during beam splitting to beam combining includes:
Respectively keeping the polarization states of the two orthogonal polarization states unchanged when the two orthogonal polarization states are transmitted on the two light paths in the beam splitting to beam combining process; and
The length of the optical path in which the birefringence exists and/or the birefringence of the optical device in which the birefringence exists are adjusted so that the two orthogonal polarization states each differ by an integer multiple of 2 pi in phase difference transmitted through the two optical paths in the process of splitting the beam to combining the beam.
3. The phase decoding method according to claim 1 or 2, characterized in that the two optical paths are configured as free space optical paths, and the optical devices on the two optical paths are configured as non-birefringent optical devices and/or polarization maintaining optical devices.
4. The phase decoding method according to claim 1 or 2, characterized in that the two optical paths are configured as polarization maintaining fiber optical paths, and the optical devices on the two optical paths are configured as non-birefringent optical devices and/or polarization maintaining optical devices.
5. The phase decoding method according to any one of claims 2 to 4, characterized in that a polarization maintaining fiber stretcher and/or a birefringent phase modulator is arranged on at least one of the two optical paths, wherein a difference between phase differences transmitted through the two optical paths in the beam splitting to beam combining process of two orthogonal polarization states of the input optical pulse are adjusted by the polarization maintaining fiber stretcher and/or the birefringent phase modulator.
6. A phase decoding device, characterized in that the phase decoding device comprises: a beam splitter; a beam combiner; and two optical paths optically coupled to the beam splitter and to the beam combiner, wherein at least one of the two optical paths or the front end of the beam splitter has a phase modulator,
The beam splitter is used for splitting one path of input light pulse with any incident polarization state into two paths of light pulses;
The two light paths are used for respectively transmitting the two light pulses and realizing the relative delay of the two light pulses;
The phase modulator is used for carrying out phase modulation on one path of input optical pulse or at least one of the two paths of optical pulses in any polarization state before beam splitting transmitted by the optical path where the phase modulator is positioned according to a quantum key distribution protocol;
The beam combiner is used for combining the two paths of light pulses to output,
Wherein the two optical paths and the optical devices thereon are configured such that the two orthogonal polarization states of the input optical pulse each differ by an integer multiple of 2 pi in phase difference transmitted through the two optical paths during beam splitting to beam combining.
7. The phase decoding apparatus according to claim 6, wherein the two optical paths are free space optical paths, and the optical devices on the two optical paths are non-birefringent optical devices and/or polarization maintaining optical devices.
8. The phase decoding apparatus according to claim 6, wherein the two optical paths are polarization maintaining fiber optical paths, and the optical devices on the two optical paths are polarization maintaining optical devices and/or non-birefringent optical devices.
9. The phase decoding apparatus according to claim 7 or 8, characterized in that the phase decoding apparatus further comprises:
the polarization maintaining optical fiber stretcher is positioned on any one of the two optical paths and used for adjusting the length of the polarization maintaining optical fiber of the optical path where the polarization maintaining optical fiber stretcher is positioned; and/or
A birefringent phase modulator located on either of the two optical paths for applying different adjustable phase modulations to two orthogonal polarization states of the optical pulses passing therethrough.
10. The phase decoding apparatus according to claim 6, wherein the phase decoding apparatus is configured by an unequal arm mach-zehnder interferometer or an unequal arm michelson interferometer.
11. The phase decoding device according to claim 6, 8 or 10, wherein the phase decoding device adopts a structure of an unequal arm mach-zehnder interferometer, and the two optical paths are polarization maintaining fiber optical paths, wherein a difference between lengths of polarization maintaining fibers of the two optical paths is an integer multiple of a beat length of the polarization maintaining fiber.
12. The phase decoding device according to claim 6, 8 or 10, wherein the phase decoding device adopts a structure of an unequal arm michelson interferometer, and the two optical paths are polarization maintaining fiber optical paths, wherein a difference between lengths of polarization maintaining fibers of the two optical paths is an integer multiple of half of a beat length of the polarization maintaining fiber.
13. The phase decoding apparatus according to claim 6 or 10, wherein the phase decoding apparatus adopts a structure of an unequal arm michelson interferometer, the beam combiner and the beam splitter are the same device, and the phase decoding apparatus further comprises:
two reflectors respectively positioned on the two light paths for reflecting the two light pulses transmitted from the beam splitter via the two light paths back to the beam combiner,
Wherein the input port and the output port of the unequal arm michelson interferometer are the same port, the phase decoding device further comprises:
The optical circulator is positioned at the front end of the beam splitter, one input optical pulse with any incident polarization state is input from the first port of the optical circulator and output from the second port of the optical circulator to the beam splitter, and the combined beam output from the beam combiner is input to the second port of the optical circulator and output from the third port of the optical circulator.
14. A quantum key distribution system, comprising: a single photon source, a phase encoder, a quantum channel, a single photon detector, and a phase decoding device according to any one of schemes 6 to 13,
The single photon source is used for generating single photon light pulses;
the phase encoder is used for carrying out phase encoding on the single photon light pulse generated by the single photon source according to a quantum key distribution protocol;
the quantum channel is used for transmitting single photon light pulses;
the phase decoding device is used for performing phase decoding on the single photon light pulse transmitted by the quantum channel according to a quantum key distribution protocol, wherein the single photon light pulse transmitted by the quantum channel is used as one path of input light pulse of any incident polarization state;
The single photon detector is used for detecting the single photon light pulse output by the phase decoding device and carrying out quantum key distribution according to a detection result and a quantum key distribution protocol, wherein the single photon light pulse output by the phase decoding device is combined beam output from the beam combiner.
15. The quantum key distribution system according to claim 14, wherein the phase encoder employs the phase decoding apparatus according to any one of claims 6 to 13.
As mentioned above, when an optical pulse is transmitted through an optical fiber quantum channel, the polarization state of the optical pulse transmitted to the receiving end is randomly changed due to environmental influence, which affects the working stability of the quantum secret communication system. The invention can effectively solve the influence of the random change of the polarization state of the input light pulse on the system stability, and realize stable phase decoding of the interference immunity of the transmission optical fiber environment. In addition, the invention has no restriction on the type of the interferometer adopted by the phase decoding device, and can use the most commonly used unequal arm Mach-Zehnder interferometer, so that the optical pulse only needs to pass through the phase modulator once when being decoded, thereby reducing the insertion loss of a receiving end and remarkably improving the system efficiency. The invention can provide a technical scheme of a stable and efficient quantum key distribution system with low insertion loss. In addition, the scheme of the invention is easy to realize.
Drawings
FIG. 1 is a flow chart of a phase decoding method according to a preferred embodiment of the present invention;
fig. 2 is a schematic diagram showing the construction of a phase decoding apparatus according to a preferred embodiment of the present invention;
fig. 3 is a schematic diagram showing the composition and structure of a phase decoding apparatus according to another preferred embodiment of the present invention;
fig. 4 is a schematic diagram showing the composition and structure of a phase decoding apparatus according to another preferred embodiment of the present invention;
Fig. 5 is a schematic diagram showing the composition and structure of a quantum key distribution system according to a preferred embodiment of the present invention.
Detailed Description
Preferred embodiments of the present application are described in detail below with reference to the attached drawing figures, which form a part of the present application and are used in conjunction with the embodiments of the present application to illustrate the principles of the present application. For the purposes of clarity and simplicity, detailed descriptions of known functions and configurations incorporated herein will be omitted when it may obscure the subject matter of the present application.
A phase decoding method according to a preferred embodiment of the present invention is shown in FIG. 1, and specifically comprises the following steps:
step S101: and splitting one path of input light pulse with any incident polarization state into two paths of light pulses.
In particular, the polarization state of an incoming input light pulse may be any polarization state, and may be considered to consist of two orthogonal polarization states. Naturally, the two optical pulses resulting from the splitting can also be seen as consisting of the same two orthogonal polarization states as the incoming optical pulse.
Step S102: and transmitting the two paths of light pulses obtained by beam splitting on the two light paths respectively, and carrying out relative delay on the two paths of light pulses and then combining and outputting the two paths of light pulses.
In the method, the input optical pulse before splitting can be subjected to phase modulation according to a quantum key distribution protocol before splitting, or at least one of the two optical pulses can be subjected to phase modulation according to the quantum key distribution protocol in the process of splitting to combining.
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.
In the method, two orthogonal polarization states of an input optical pulse are controlled to be respectively different by an integral multiple of 2 pi in phase difference transmitted through the two optical paths in the beam splitting to beam combining process.
For example, assuming that the two orthogonal polarization states are an x-polarization state and a y-polarization state, respectively, a phase difference of the x-polarization state transmitted through the two optical paths in the beam splitting to the beam combining process is denoted as Δx, a phase difference of the y-polarization state transmitted through the two optical paths in the beam splitting to the beam combining process is denoted as Δy, and an integer multiple of a phase difference of the two optical paths transmitted through the two optical paths in the beam splitting to the beam combining process of each of the two orthogonal polarization states of the light pulse may be expressed as:
Δx–Δy＝2π.m，
where m is an integer and may be a positive integer, a negative integer or zero.
In one possible embodiment, the two optical paths between splitting and combining comprise optical paths having birefringence for two orthogonal polarization states of the input optical pulse, and/or optical devices having birefringence for two orthogonal polarization states on the two optical paths. In this case, controlling the two orthogonal polarization states of the input optical pulse to each differ by an integer multiple of 2Ω in phase difference transmitted through the two optical paths in the beam splitting to beam combining process includes: respectively keeping the polarization states of the two orthogonal polarization states unchanged when the two orthogonal polarization states are transmitted on the two light paths in the beam splitting to beam combining process; and adjusting the length of the optical path with birefringence and/or the birefringence of the optical device with birefringence so that the phase difference of two orthogonal polarization states transmitted by the two optical paths in the process of splitting to combining is different by an integral multiple of 2 pi. Alternatively, this may be achieved by either: i) Configuring the two light paths as polarization maintaining fiber light paths, and configuring optical devices on the polarization maintaining fiber light paths as non-birefringent optical devices and/or polarization maintaining optical devices; ii) configuring the two light paths as free space light paths, and configuring the optics on the two light paths as polarization maintaining optics. As used herein, the term "polarization maintaining fiber optical path" refers to an optical path for transmitting an optical pulse using a polarization maintaining fiber or an optical path formed by connecting polarization maintaining fibers. "non-birefringent light device" refers to a light device having the same refractive index for different polarization states (e.g., two orthogonal polarization states). In addition, the polarization maintaining optical device may also be referred to as a polarization maintaining optical device.
In one possible implementation, the two optical paths may be configured as free space optical paths, and the optical devices on the two optical paths may be configured as non-birefringent optical devices. In this case, the polarization states of the two orthogonal polarization states each remain unchanged when transmitted on the two optical paths during the beam splitting to the beam combining, and the phase differences of the two orthogonal polarization states each transmitted through the two optical paths during the beam splitting to the beam combining may be the same.
In one possible implementation, a polarization maintaining fiber stretcher and/or a birefringent phase modulator is configured on at least one of the two optical paths. 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 input light pulses transmitted through the two optical paths during splitting to combining 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 an input optical pulse transmitted through the two optical paths during splitting to combining, respectively.
A phase decoding device according to a preferred embodiment of the present invention is shown in fig. 2, and includes the following components: beam splitter 201, phase modulator 202, beam combiner 203. The beam splitter 201 and the beam combiner 203 are optically coupled by two optical paths, and the phase modulator 202 is located on one of the optical paths.
The beam splitter 201 is configured to split an incident input optical pulse of any polarization into two optical pulses.
The two light paths between the beam splitter 201 and the beam combiner 203 are used for respectively transmitting two light pulses obtained by beam splitting, and realizing the relative delay of the two light pulses.
Specifically, by adjusting the physical transmission lengths of the two optical paths between the beam splitter 201 and the beam combiner 203, the relative delay of the two optical pulses is achieved.
The phase modulator 202 is configured to phase modulate one of the two optical pulses transmitted via the optical path in which it is located according to a quantum key distribution protocol.
Although fig. 2 shows that a phase modulator is arranged between the beam splitter 201 and the beam combiner 203, that is, one of two optical pulses obtained by splitting is subjected to phase modulation according to a quantum key distribution protocol in the process of splitting to combining, it is also possible that a phase modulator is arranged at the front end of the beam splitter 201, that is, one input optical pulse of any polarization state of incidence before splitting is subjected to phase modulation according to the quantum key distribution protocol.
In addition, although only one phase modulator is shown in fig. 2, it is also possible to provide one phase modulator on each of the two optical paths between the beam splitter 201 and the beam combiner 203. In the case where two phase modulators are provided, the difference in the phases modulated by the two phase modulators is determined by the quantum key distribution protocol.
The beam combiner 203 is configured to combine the two paths of light pulses obtained by relatively delayed beam splitting.
According to the invention, the two light paths and the optical devices thereon are configured such that the two orthogonal polarization states of the input light pulse each differ by an integer multiple of 2 pi in the phase difference transmitted through the two light paths during beam splitting to beam combining.
In this regard, one optical path may or may not have birefringence for two orthogonal polarization states, depending on the type of optical path. For example, free-space optical paths do not have birefringence for two orthogonal polarization states of an input optical pulse, while polarization-maintaining fiber optical paths generally have birefringence that differs significantly from each other for two orthogonal polarization states of an input optical pulse. In addition, one optical device on the optical path may or may not have birefringence for two orthogonal polarization states, depending on the type of optical device. For example, one non-birefringent optical device does not have birefringence for two orthogonal polarization states of one input optical pulse, while one polarization maintaining optical device typically has birefringence for two orthogonal polarization states of one input optical pulse that differ significantly from each other.
According to the invention, the type and/or length of the two light paths and the type and/or birefringence magnitude of the optical devices thereon result in that the two orthogonal polarization states of the input light pulse are each transmitted via the two light paths with a phase difference of an integer multiple of 2 pi during the beam splitting to beam combining process.
In one possible embodiment, the optical path between beam splitter 201 and beam combiner 203 is a free space optical path, and phase modulator 202 and other optical devices in the optical path are non-birefringent and/or polarization maintaining optical devices. For this embodiment, with polarization maintaining optics, the polarization maintaining optics itself causes two orthogonal polarization states of the light pulses to differ by an integer multiple of 2 pi in phase difference between the beam splitter 201 and the beam combiner 203 transmitted via the two optical paths.
In one possible implementation, the two optical paths between beam splitter 201 and beam combiner 203 are polarization maintaining fiber optical paths, and phase modulator 202 and other optical devices in the optical paths are polarization maintaining optical devices and/or non-birefringent optical devices.
In one possible embodiment, the phase decoding device may further comprise a fiber stretcher and/or a birefringent phase modulator.
The optical fiber stretcher can be positioned on any one of two optical paths between the beam splitter 201 and the beam combiner 203, and can be used for adjusting the length of polarization maintaining optical fibers of the optical path where the optical fiber stretcher is positioned. By adjusting the polarization maintaining fiber length by means of the fiber stretcher, it is advantageously easy to achieve that the two orthogonal polarization states of the light pulses are transmitted between the beam splitter 201 and the beam combiner 203 via two optical paths with a phase difference of an integer multiple of 2 pi.
The birefringent phase modulator may be located in either of the two optical paths and may be used to apply different phase modulations to the two orthogonal polarization states of the optical pulses passing therethrough. By controlling the birefringent phase modulator, the difference in phase modulation experienced by each of the two orthogonal polarization states of the light pulses passing through it is adjustable. Thus, by using the birefringent phase modulator, the difference between the phase differences transmitted through the two optical paths in the beam splitting to beam combining process of the two orthogonal polarization states of the optical pulse can be conveniently influenced and adjusted, and the difference is easily realized to be an integer multiple of 2pi. The birefringent phase modulator may be a lithium niobate phase modulator as described hereinbefore.
Alternatively, the phase decoding device may adopt a structure of an unequal arm mach-zehnder interferometer or an unequal arm michelson interferometer.
In one possible implementation, the phase decoding device adopts a structure of an unequal arm mach-zehnder interferometer, and optical paths of two arms of the interferometer (i.e., two optical paths between a beam splitter and a beam combiner) adopt polarization maintaining fibers, wherein the difference between lengths of the polarization maintaining fibers of the two optical paths is an integer multiple of the beat length of the polarization maintaining fibers. In this case, the optical devices in the two optical paths cause the two orthogonal polarization states of the optical pulses to each differ by an integer multiple of 2π in phase difference between the beam splitter 201 and the beam combiner 203 transmitted via the two optical paths.
In one possible implementation, the phase decoding device adopts a michelson interferometer structure with unequal arms, and optical paths of two arms of the interferometer (i.e., two optical paths, which are optically coupled to a beam splitter and a beam combiner of the same device and are respectively used for transmitting two optical pulses obtained by beam splitting) adopt polarization maintaining fibers, and the difference between lengths of the polarization maintaining fibers of the two optical paths is an integer multiple of half of the beat length of the polarization maintaining fibers. In this case, the other optical devices in the two optical paths cause the two orthogonal polarization states of the optical pulses to each differ by an integer multiple of 2π in the phase difference transmitted through the two optical paths during beam splitting to beam combining.
"Polarization maintaining fiber beat length" is a concept known in the art and refers to the length of a polarization maintaining fiber corresponding to the phase difference of 2 pi produced by the transmission of two intrinsic polarization states of the polarization maintaining fiber along the polarization maintaining fiber.
For the embodiment of fig. 2, the beam splitter 201 is preferably a polarization maintaining beam splitter, and the beam combiner 203 is preferably a polarization maintaining beam combiner.
In one possible embodiment, the phase decoding device adopts the structure of an unequal arm michelson interferometer. At this time, the beam combiner and the beam splitter are the same device. In this case, the phase decoding apparatus further includes two reflecting mirrors respectively disposed on the aforementioned two optical paths for transmitting the two optical pulses obtained by splitting the beam, and respectively configured to reflect the two optical pulses transmitted through the two optical paths from the beam splitter back for beam combination output by a beam combiner that is the same device as the beam splitter. Furthermore, the input port and the output port of the unequal arm michelson interferometer may be the same port, and the phase decoding apparatus further comprises an optical circulator.
The optical circulator may be located at a front end of the beam splitter. The input optical pulse may be input from a first port of the optical circulator and output from a second port of the optical circulator to the beam splitter. The combined output from the beam combiner (which is the same device as the beam splitter) may be input to and output from the second port of the optical circulator.
As shown in FIG. 3, a phase decoding device according to another preferred embodiment of the present invention adopts an unequal arm Mach-Zehnder interferometer structure, comprising the following components: beam splitter 303, phase modulator 304, beam combiner 305.
One of the two ports 301 and 302 on one side of the beam splitter 303 serves as an input of the phase decoding device, one of the two ports 306 and 307 on one side of the beam combiner 305 serves as an output of the phase decoding device, and the phase modulator 304 is inserted into either one of the two arms of the mach-zehnder interferometer. In operation, an incident light pulse of any polarization state enters the beam splitter 303 through the port 301 or 302 of the beam splitter 303 and is divided into two light pulses for transmission, wherein one light pulse is directly transmitted to the beam combiner 305, the other path of light pulse is transmitted to the beam combiner 305 after being subjected to phase modulation by the phase modulator 304, and the two paths of light pulses are combined by the beam combiner 305 after being subjected to relative delay and output by the port 306 or 307 after being combined. The beam splitter 303 and the beam combiner 305 may employ polarization maintaining fiber couplers, and the phase modulator 304 may be a polarization independent device. The lengths of the polarization maintaining fibers of the two optical paths between the beam splitter 303 and the beam combiner 305 are adjusted so that the two orthogonal polarization states of the light pulses are equal in phase difference or are different by an integer multiple of 2 pi between the transmission of the two optical paths between the beam splitter 303 and the beam combiner 305. If the optical device between the beam splitter 303 and the beam combiner 305 is a non-birefringent optical device, or if there is an optical device between the beam splitter 303 and the beam combiner 305 that has birefringence for the two orthogonal polarization states, the difference between the phase differences of the two orthogonal polarization states transmitted between the beam splitter 303 and the beam combiner 305 through the two optical paths is an integer multiple of 2pi, and the difference between the polarization maintaining fiber lengths of the two optical paths between the beam splitter 303 and the beam combiner 305 is adjusted to be an integer multiple of the beat length of the polarization maintaining fiber. The length of the polarization maintaining fiber can be adjusted by precisely cutting the fiber or arranging a polarization maintaining fiber stretcher on any arm of the Mach-Zehnder interferometer.
The phase decoding device according to another preferred embodiment of the present invention, as shown in fig. 4, adopts an unequal arm michelson interferometer structure, comprising the following components: beam splitter 403, phase modulator 405, two mirrors 404 and 406.
Two ports 401 and 402 on one side of beam splitter 403 serve as input and output, respectively, of the phase decoding means; one of the two ports on the other side of beam splitter 403 is directly coupled to mirror 404 and the other port on the same side is sequentially coupled to phase modulator 405 and mirror 406. In operation, an input light pulse with any polarization state enters beam splitter 403 through port 401 of beam splitter 403, is split into two light pulses for transmission, one light pulse is transmitted to mirror 404 and reflected back through mirror 404, the other light pulse is transmitted to mirror 406 after being phase modulated by phase modulator 405 and reflected back by mirror 406, and the two light pulses are combined through beam splitter 403 after relative delay and output through port 402 after being combined. The result is the same when an optical pulse is input from port 402, output from port 401, and input and output from either port 401 or 402 simultaneously. When port 401 or 402 is used as both input and output of the unequal arm michelson interferometer, the port (port 401 or 402 of beam splitter 403) which is used as both input and output is connected to the optical circulator; the input optical pulse is input through a first port of an optical circulator and output from a second port of the optical circulator to beam splitter 403, and then split into two optical pulses by beam splitter 403; the two optical pulses are combined by beam splitter 403 after a relative delay and the output is input to and output from the second port of the optical circulator. Beam splitter 403 may employ a polarization maintaining fiber coupler and phase modulator 405 may be a polarization independent device. The length of the polarization maintaining fiber between beam splitter 403 and the two mirrors 404 and 406 is adjusted such that the two orthogonal polarization states of the light pulses are transmitted between beam splitter 403 and the two mirrors 404 and 406 over two optical paths with equal phase differences or with an integer multiple of 2 pi. If the optical device between beam splitter 403 and two mirrors 404 and 406 is a non-birefringent optical device, or if there is an optical device between beam splitter 403 and two mirrors 404 and 406 that is birefringent for these two orthogonal polarization states and the resulting difference in phase difference between the two optical paths transmitted through beam splitter 403 and two mirrors 404 and 406 during beam splitting to combining is an integer multiple of 2 pi, the difference in polarization maintaining fiber lengths of the two optical paths between beam splitter 403 and two mirrors 404 and 406 is adjusted to be an integer multiple of half the beat length of the polarization maintaining fiber. The length of the polarization maintaining fiber can be adjusted by precisely cutting the fiber or arranging a polarization maintaining fiber stretcher on any one arm of the unequal arm Michelson interferometer.
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 phase decoding device of the present invention can be configured at the receiving end of the quantum key distribution system for phase decoding. In addition, the phase decoding device of the present invention may be disposed at the transmitting end of the quantum key distribution system for phase encoding.
Although the embodiments of fig. 3 and 4 phase modulate one of the two optical pulses split during the splitting to combining process according to the quantum key distribution protocol, it is also possible that: and in the process of splitting the beam to combining the beam, the two paths of light pulses obtained by splitting the beam are respectively subjected to phase modulation according to a quantum key distribution protocol, or one path of input light pulse in any incident polarization state before splitting the beam is subjected to phase modulation according to the quantum key distribution protocol.
A quantum key distribution system according to a preferred embodiment of the present invention is shown in fig. 5, and comprises the following components: a single photon source 501, a phase encoder 502, a quantum channel 503, two single photon detectors 505 and 506, and a phase decoding device 504 as described above.
The single photon source 501 is used to generate single photon light pulses.
The phase encoder 502 is used to phase encode the single photon light pulses generated by the single photon source 501 in accordance with a quantum key distribution protocol.
Quantum channel 503 is used to transmit single photon light pulses. In particular, quantum channel 503 transmits phase encoded single photon light pulses to phase decoding device 504.
The phase decoding means 504 is arranged to phase decode the single photon light pulses transmitted via the quantum channel 503 in accordance with a quantum key distribution protocol.
The single photon detectors 505 and 506 are configured to detect the single photon light pulse output from the phase decoding device 504, and perform quantum key distribution according to the detection result and the quantum key distribution protocol.
The single photon source 501 emits a single photon light pulse into the phase encoder 502, the phase encoder 502 performs phase encoding on the single photon light pulse, the light pulse after phase encoding is transmitted to the phase decoding device 504 through the quantum channel 503, the phase decoding device 504 performs phase decoding on the incident single photon pulse, and the light pulse output by the phase decoding device 504 is sent to the single photon detector 505 or the single photon detector 506. The phase encoder 502 and the phase decoding means 504 phase encode and phase decode the light pulses, respectively, according to a quantum key distribution protocol, and perform key distribution according to the quantum key distribution protocol.
Specifically, the phase encoder 502 employs any one of the following: unequal arm mach-zehnder interferometer, unequal arm michelson interferometer, unequal arm faraday-michelson interferometer, phase decoding apparatus described above.
The quantum channel 503 may be an optical waveguide, an optical fiber, a free space, a discrete optical element, a planar waveguide optical element, a fiber optical element, or a light propagation channel combining any two or more of the foregoing.
By splitting an input light pulse with any incident polarization state into two paths at the receiving end, respectively transmitting the two paths through two light paths and controlling the relationship between the phase differences of the two orthogonal polarization states of the input light pulse, which are respectively transmitted through the two light paths in the process of splitting the light beam to combining the light beam, the invention can realize stable interference output for the input light pulse with any polarization state.
In addition, with the present invention, the interferometer used by the phase decoding device at the receiving end may be of various types including an unequal arm mach-zehnder interferometer, and is not limited to an unequal arm michelson interferometer. Therefore, by selecting a proper interferometer, the problem of system stability is solved, and meanwhile, the lower insertion loss of the receiving end can be realized.
From the foregoing description, it will be appreciated that specific details and functions of the invention have been set forth in order to achieve the desired objects, but that the drawings are merely for purposes of illustration and description, and are not intended to be limiting.
Claims (15)
1. A method of phase decoding, the method comprising:
Splitting one path of input light pulse with any polarization state into two paths of light pulses;
transmitting the two paths of light pulses on two light paths respectively, carrying out relative delay on the two paths of light pulses, then combining and outputting,
Wherein the input light pulse before splitting or at least one of the two light pulses is phase modulated according to a quantum key distribution protocol in the process of splitting to combining, and
Wherein, two orthogonal polarization states of the input light pulse are controlled to respectively have an integral multiple of 2 pi phase difference in the process of splitting and combining the light beams.
2. The phase decoding method of claim 1, wherein the two optical paths include optical paths having birefringence for the two orthogonal polarization states, and/or the two optical paths have optical devices having birefringence for the two orthogonal polarization states thereon, wherein the controlling the phase difference of the two orthogonal polarization states of the input optical pulse transmitted through the two optical paths during splitting to combining each includes an integer multiple of 2 pi:
Respectively keeping the polarization states of the two orthogonal polarization states unchanged when the two orthogonal polarization states are transmitted on the two light paths in the beam splitting to beam combining process; and
The length of the optical path in which the birefringence exists and/or the birefringence of the optical device in which the birefringence exists are adjusted so that the two orthogonal polarization states each differ by an integer multiple of 2 pi in phase difference transmitted through the two optical paths in the process of splitting the beam to combining the beam.
3. A phase decoding method according to claim 1 or 2, characterized in that the two optical paths are configured as free space optical paths, and the optical devices on the two optical paths are configured as non-birefringent optical devices and/or polarization maintaining optical devices.
4. The phase decoding method according to claim 1 or 2, characterized in that the two optical paths are configured as polarization maintaining fiber optical paths, and the optical devices on the two optical paths are configured as non-birefringent optical devices and/or polarization maintaining optical devices.
5. The phase decoding method according to claim 2, wherein a polarization maintaining fiber stretcher and/or a birefringent phase modulator is arranged on at least one of the two optical paths, wherein a difference between phase differences transmitted through the two optical paths in the splitting-to-combining process of the two orthogonal polarization states of the input optical pulse are adjusted by the polarization maintaining fiber stretcher and/or the birefringent phase modulator.
6. A phase decoding device, characterized in that the phase decoding device comprises: a beam splitter; a beam combiner; and two optical paths optically coupled to the beam splitter and to the beam combiner, wherein at least one of the two optical paths or the front end of the beam splitter has a phase modulator,
The beam splitter is used for splitting one path of input light pulse with any incident polarization state into two paths of light pulses;
The two light paths are used for respectively transmitting the two light pulses and realizing the relative delay of the two light pulses;
The phase modulator is used for carrying out phase modulation on one path of input optical pulse or at least one of the two paths of optical pulses in any polarization state before beam splitting transmitted by the optical path where the phase modulator is positioned according to a quantum key distribution protocol;
The beam combiner is used for combining the two paths of light pulses to output,
Wherein the two optical paths and the optical devices thereon are configured such that the two orthogonal polarization states of the input optical pulse each differ by an integer multiple of 2 pi in phase difference transmitted through the two optical paths during beam splitting to beam combining.
7. The phase decoding apparatus according to claim 6, wherein the two optical paths are free space optical paths, and the optical devices on the two optical paths are non-birefringent optical devices and/or polarization maintaining optical devices.
8. The phase decoding apparatus according to claim 6, wherein the two optical paths are polarization maintaining fiber optical paths, and the optical devices on the two optical paths are polarization maintaining optical devices and/or non-birefringent optical devices.
9. The phase decoding apparatus according to claim 7 or 8, characterized in that the phase decoding apparatus further comprises:
the polarization maintaining optical fiber stretcher is positioned on any one of the two optical paths and used for adjusting the length of the polarization maintaining optical fiber of the optical path where the polarization maintaining optical fiber stretcher is positioned; and/or
A birefringent phase modulator located on either of the two optical paths for applying different adjustable phase modulations to two orthogonal polarization states of the optical pulses passing therethrough.
10. The phase decoding device according to claim 6, wherein the phase decoding device has a structure of an unequal arm mach-zehnder interferometer or an unequal arm michelson interferometer.
11. The phase decoding apparatus according to claim 6, 8 or 10, wherein the phase decoding apparatus adopts a structure of an unequal arm mach-zehnder interferometer, and the two optical paths are polarization maintaining fiber optical paths, wherein a difference between polarization maintaining fiber lengths of the two optical paths is an integer multiple of a beat length of the polarization maintaining fiber.
12. The phase decoding device according to claim 6, 8 or 10, wherein the phase decoding device adopts a structure of an unequal arm michelson interferometer, and the two optical paths are polarization maintaining fiber optical paths, wherein a difference between polarization maintaining fiber lengths of the two optical paths is an integer multiple of half of a beat length of the polarization maintaining fiber.
13. The phase decoding apparatus according to claim 6 or 10, wherein the phase decoding apparatus adopts a structure of an unequal arm michelson interferometer, the beam combiner and the beam splitter are the same device, and the phase decoding apparatus further comprises:
two reflectors respectively positioned on the two light paths for reflecting the two light pulses transmitted from the beam splitter via the two light paths back to the beam combiner,
Wherein the input port and the output port of the unequal arm michelson interferometer are the same port, the phase decoding device further comprises:
The optical circulator is positioned at the front end of the beam splitter, one input optical pulse with any incident polarization state is input from the first port of the optical circulator and output from the second port of the optical circulator to the beam splitter, and the combined beam output from the beam combiner is input to the second port of the optical circulator and output from the third port of the optical circulator.
14. A quantum key distribution system, comprising: a single photon source, a phase encoder, a quantum channel, a single photon detector and a phase decoding device according to any one of claims 6 to 13,
The single photon source is used for generating single photon light pulses;
the phase encoder is used for carrying out phase encoding on the single photon light pulse generated by the single photon source according to a quantum key distribution protocol;
the quantum channel is used for transmitting single photon light pulses;
the phase decoding device is used for performing phase decoding on the single photon light pulse transmitted by the quantum channel according to a quantum key distribution protocol, wherein the single photon light pulse transmitted by the quantum channel is used as one path of input light pulse of any incident polarization state;
The single photon detector is used for detecting the single photon light pulse output by the phase decoding device and carrying out quantum key distribution according to a detection result and a quantum key distribution protocol, wherein the single photon light pulse output by the phase decoding device is combined beam output from the beam combiner.
15. The quantum key distribution system of claim 14, wherein the phase encoder employs the phase decoding apparatus of any one of claims 6 to 13.
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| CN110572263A (en) * | 2019-09-23 | 2019-12-13 | 中国电子科技集团公司电子科学研究院 | Continuous variable quantum key distribution decoding method, device and system |
| CN110601768B (en) * | 2019-10-18 | 2024-07-16 | 中国电子科技集团公司电子科学研究院 | Integrated waveguide decoding device and quantum key distribution system |
| CN111896126B (en) * | 2020-08-03 | 2021-11-09 | 四川九洲电器集团有限责任公司 | Optical path delay coding method and coding system |
| CN113972982B (en) * | 2021-12-22 | 2022-03-29 | 杭州慧明量子通信技术有限公司 | Phase encoding device for quantum key distribution system stabilization |
| CN114285566B (en) * | 2021-12-30 | 2024-01-30 | 武汉光谷信息光电子创新中心有限公司 | Signal processing device and method, electronic device, and storage medium |
| CN114401089B (en) * | 2022-01-26 | 2024-04-26 | 杭州慧明量子通信技术有限公司 | Adjustable time delay interferometer and quantum key distribution system |
| CN114785493A (en) * | 2022-03-17 | 2022-07-22 | 银川产业技术研究院 | Quantum key distribution phase encoding method and device based on IQ (in-phase Quadrature) optical modulator |
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