CN106161010B - High-bit-rate point-to-point QKD system, transmitting end, receiving end and QKD method - Google Patents
High-bit-rate point-to-point QKD system, transmitting end, receiving end and QKD method Download PDFInfo
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- 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
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Abstract
The invention discloses a high-bitrate point-to-point QKD system, a transmitting end, a receiving end and a QKD method, wherein the transmitting end sequentially comprises a light source module, an encoder, a wavelength division multiplexer and an optical fiber interface along the transmission direction of optical signals, and a main control logic module for sending control signals to the encoder is further arranged, the light source module comprises a plurality of light sources and respectively has different transmitting wavelengths, the output end of each light source is correspondingly connected with a beam splitter, and the beam splitter divides the output of the corresponding light source into multiple paths of optical signals to be output to the encoder for encoding respectively; the optical fiber interface is a multi-core optical fiber interface and is used for switching the optical signals from the wavelength division multiplexer to the multi-core optical fiber so as to transmit the optical signals to the receiving end. When the QKD system is provided with n light sources with different wavelengths and m-core multi-core optical fibers, n x m quantum key distribution lines can be accommodated, and the code rate is n x m times that of the original single-set QKD system.
Description
Technical Field
The invention relates to a point-to-point QKD technology, in particular to a point-to-point QKD system with high bit rate, a transmitting end, a receiving end and a QKD method.
Background
The quantum communication is a new technology combining quantum mechanics and cryptoanalysis, the safety of the quantum communication is ensured by the basic principles of quantum mechanics, namely the Hessenberg measurement inaccuracy principle and the quantum unclonable theorem, and the safety of key distribution and transmission can be ensured, so that the quantum communication is highly valued by the government and military departments of various countries, and many research institutions develop the research of many quantum communication. Quantum communication research includes directions of Quantum Key Distribution (QKD), quantum storage and relay, quantum digital signature, quantum encoding, quantum secret sharing, quantum security computing protocol and the like, wherein QKD is core research content and is the earliest research field of practical application of quantum secret communication.
The QKD can realize that both communication parties generate the same secret key, the key generation rate, namely the bit rate, is an important index for measuring the system performance, and more data can be encrypted by a high bit rate to form a more complex encryption system, and the improvement of the bit rate is always the direction of improvement and perfection of the QKD system since the first QKD system in the world is produced in 1991. However, the reported QKD system has low code rate, and the document "Room temperature single-photon detectors for high bit rate quantum key distribution" in 2014 (APPLIED PHYSICS LETTERS) reports that the best QKD system has 1.2Mbps at 50km of optical fiber, but the 100km of code rate is 1.2kbps, and the lower code rate limits the large-scale commercial use of the QKD system, such as distributed storage network, large data disaster recovery backup transfer between banks, access of more users and other requirements, so that one-time one-cipher unconditional security encryption needs to be carried out on a large amount of transmitted data, and therefore, a QKD system with a higher code rate is urgently needed.
At present, a plurality of methods for improving the code rate are available, and the literature Room temperature single-photon detectors for high bit rate quantum key distribution in APPLIED PHYSICS LETTERS in 2014 adopts a method for improving the performance of a single photon detector, so that the single photon detector can obtain higher code rate even when working at room temperature; the Chinese patent application of application number 200610170557.5 discloses a QKD system for phase-coded polarization detection, overcomes the defect of low code rate caused by random selection of a single photon path in the prior QKD system, and improves the anti-interference capability and the single photon utilization efficiency of the system; the Chinese patent application of application number 201020112228.7 discloses a two-way key generation receiving end, which realizes two-way distribution of keys by adding an optical path selector on a QKD optical path, and realizes a QKD system with higher code rate and higher safety; the chinese patent application of application number 201310434795.2 discloses a synchronization device of QKD system, which can reduce the influence of synchronous light on signal light when the frequency of the transmitted signal is far greater than that of the synchronous signal, thereby being beneficial to improving the working frequency of the system and effectively improving the code rate of the QKD system. In summary, the main methods for improving the code rate of the QKD system are to improve the performance of the single photon detector, improve the QKD optical path system, and improve the working frequency of the QKD system, which are all aimed at improving the single QKD system, and the working frequency and the working efficiency of the laser, the single photon detector and other devices in the single QKD system are limited, so the improvement of the code rate is limited.
There are many reports about QKD systems using wavelength division multiplexers for multi-user access, for example, chinese patent application No. 201410337054.7 discloses a multi-user wavelength division multiplexing quantum key distribution network system and a method for distributing and sharing keys thereof, so as to implement key distribution between 1 Alice and a plurality of bobs. Chinese patent application No. 200680022303.7 discloses a system and method for achieving quantum key distribution between a transmitter and a receiver using a wavelength division multiplexed link, enabling key distribution for a plurality of quantum transmitting units and a plurality of quantum receiving units. The quantum key distribution lines of the two patents all use a plug-and-play QKD system scheme based on phase coding, and the signal light interference contrast on a decoding module is not high enough due to the limitation of the modulation precision of two phase modulators at a transmitting end and a receiving end, so that the measurement result of a detector is influenced, and the code rate of the system is reduced.
Problems of the prior art:
the laser, single photon detector, etc. devices in a single QKD system have limitations on their operating frequency and efficiency, and therefore limited improvement in the rate of formation.
The existing plug-and-play system scheme based on phase coding, which is used by the one-to-many and many-to-many QKD multiplexing network, has the bit rate limited by the modulation precision of the phase modulators at the transmitting and receiving ends.
Disclosure of Invention
The invention provides a QKD system, which comprises a transmitting end and a receiving end which are matched with each other, the QKD system integrates a beam splitter technology, a wavelength division multiplexing technology and a multi-core optical fiber technology, the code rate of the point-to-point QKD system is improved by expanding the number of quantum key distribution lines, when time-phase unbalanced basis vector coding is adopted, the insertion loss of the lines is reduced, the detection result is not limited by the device modulation precision, high-contrast coding is realized, a two-way communication QKD system can be constructed by utilizing an optical path selection device according to requirements, and the code rate is further improved. The transmitting end of the high-bit-rate point-to-point QKD system sequentially comprises a light source module, an encoder, a wavelength division multiplexer and an optical fiber interface along the transmission direction of optical signals, and is also provided with a main control logic module for transmitting control signals to the encoder, wherein the light source module comprises a plurality of light sources and respectively has different transmitting wavelengths, the output end of each light source is correspondingly connected with a beam splitter, and the beam splitter divides the output of the corresponding light source into multiple paths of optical signals to be output to the encoder for encoding respectively; the optical fiber interface is a multi-core optical fiber interface and is used for switching the optical signals from the wavelength division multiplexer to the multi-core optical fiber so as to transmit the optical signals to the receiving end.
The emission end of the QKD system can greatly improve the number of quantum key distribution lines by using a beam splitter, for example, n light sources (lasers) with different wavelengths are arranged, all the light sources can be integrated on one laser plate, the beam splitter corresponding to each light source divides emergent light into m quantum key distribution lines, at the moment, the system can accommodate n x m quantum key distribution lines, and the code rate is n x m times that of the original single QKD system. The rate of the bit can be increased by a factor of about 2n x m when the system communicates bi-directionally.
Because the optical wave band selected for quantum communication has a certain limit, how to fully utilize the optional wave band is one of the problems existing in the prior art, and the number of light sources with different emission wavelengths in a limited wave band range is correspondingly limited.
In the invention, because the number of the constructed quantum key distribution lines is large, in order to improve the integration level of equipment, hardware resources are fully utilized, and as the optimization, the encoder is provided with the coding modules corresponding to all paths of optical signals one by one, the main control logic module is provided with corresponding signal modulators for each path of coding module, and all the signal modulators share the same random number generator in the main control logic module.
Although the optical signals of each quantum key distribution line need to be respectively encoded, each encoding module needs a signal modulator, the random numbers utilized by each signal modulator can be uniformly distributed by adopting the existing random number generators in the main control logic module (different random numbers are respectively distributed), the random number generators do not need to be respectively configured for each signal modulator, and the hardware cost is reduced.
In the encoding mode, the existing protocol can be adopted, and in the invention, the encoder is a signal optical encoding module based on time-phase encoding, namely, a time-phase encoding scheme is preferably adopted.
The signal light coding module is provided with coding modules corresponding to all paths of signal lights. For example, when the light source module has n light sources, each beam splitter is divided into m paths, there are n×m paths of signal light, and n×m coding modules are arranged in the corresponding signal light coding module.
When time-phase encoding is adopted, each encoding module is a time-phase encoding module, and specifically comprises a time encoding sub-module and a phase encoding sub-module.
The time coding submodule carries out a time coding process, modulates the intensities of two light pulses in a front and a rear in a line (namely a quantum key distribution line) through an intensity modulator, randomly modulates the light intensity of one light pulse into a vacuum state close to zero, and generates a double pulse sequence with only a first light pulse or only a second light pulse to realize coding.
The phase coding sub-module is an X-base vector phase coding sub-module or a Y-base vector phase coding sub-module. In phase encoding, the effective phase difference between two optical pulses on a line is 0 or pi by modulating one of the two optical pulses by a phase modulator PM (i.e. X-basis vector phase encoding), or pi/2 or 3 pi/2 by modulating the other two optical pulses (i.e. Y-basis vector phase encoding). The phase decoding process does not need a phase modulator, so that the insertion loss of a line is reduced, the detection result is not limited by the precision of a modulation device, high-contrast coding can be realized, and the system code rate is improved in the two aspects. The selection proportion of the two groups of basis vectors of time and phase is controlled, and the unbalanced basis vector is used for encoding and decoding the signal light, so that the code rate of the system can be further improved.
For a discussion of the impact of unbalanced-basis-vector schemes on QKD-rate, reference is made to document "Efficient quantum key distribution scheme and a proof of its unconditional security" of Journal of Cryptology, 2005.
Preferably, the number of the beam splitters is multiple, the light sources are in one-to-one correspondence, the number of the wavelength division multiplexers is multiple, the input end of each wavelength division multiplexer is connected with one path of output of each beam splitter, and the output end of each wavelength division multiplexer is connected with the multi-core optical fiber interface and corresponds to one fiber core.
Because the input requirement of the wavelength division multiplexer is different optical signals of wavelength, only one path of output of each beam splitter is received, one of the key points of the invention is that a multi-core optical fiber is adopted, different fiber cores of the multi-core optical fiber are separated in space, parallel transmission of a plurality of paths of identical optical signals can be realized, the number of common transmission channels between a transmitting end and a receiving end is expanded, the number of quantum key distribution lines which can be accommodated by a system is increased, the improvement of the system code rate is realized, the output of each wavelength division multiplexer further improves the integration level through a multi-core optical fiber interface, and the multi-core optical fiber interface can adopt an optical fiber fan-shaped output device, so that signal transmission can be carried out between the receiving end and the transmitting end of a QKD system through the multi-core optical fiber.
Preferably, the number of the beam splitters is the same and equal to the number of the wavelength division multiplexers. This can improve the standardization of the device.
The transmitting end of the invention is also provided with a synchronous light source and a corresponding synchronous light transmitting line, wherein the synchronous light source is connected with the input end of any wavelength division multiplexer.
In order to further improve the code rate, the transmitting end of the invention can realize two-way communication by adding the optical path selecting device, and preferably, each optical signal transmission path between the encoder and the wavelength division multiplexer is respectively provided with the optical path selecting device, and the transmitting end is also provided with a signal receiving module which is coupled into the corresponding optical signal transmission path through each optical path selecting device.
The optical path selection device may employ a fiber optic circulator.
The receiving module is matched with the transmitting part and comprises a decoder and a detector for receiving each path of optical signals and performing corresponding processing.
The invention also provides a receiving end of the high-bit-rate point-to-point QKD system, which sequentially comprises an optical fiber interface, a wavelength division multiplexer, a decoder and a detector along the transmission direction of optical signals, wherein the optical fiber interface is a multi-core optical fiber interface for receiving signals of a transmitting end, and each fiber core of the multi-core optical fiber corresponds to one input of the wavelength division multiplexer.
And the same as the transmitting end, n x m quantum key distribution lines can be obtained through demultiplexing the wavelength division multiplexing signals by a wavelength division multiplexer, and the code rate is n x m times of the original single QKD system. The rate of the bit can be increased by a factor of about 2n x m when the system communicates bi-directionally.
The receiving end is adapted to the transmitting end in decoding mode, and preferably, the decoder is a signal light decoding module based on time-phase decoding.
In order to realize two-way communication, preferably, an optical path selection device is respectively arranged on each optical signal transmission path between the wavelength division multiplexer and the decoder, and the receiving end is further provided with a signal transmitting module which is coupled into the corresponding optical signal transmission path through each optical path selection device.
The optical path selection device may employ a fiber optic circulator.
The configuration of the transmitting module is matched with the receiving part, and the corresponding part in the transmitting end of the present invention can be adopted, and will not be described herein.
The invention also provides a high-bit-rate point-to-point QKD system, which comprises a transmitting end and a receiving end which are mutually matched, wherein the transmitting end is the transmitting end of the high-bit-rate point-to-point QKD system, and the transmitting end and the receiving end are communicated through a multi-core optical fiber connected between optical fiber interfaces.
Preferably, the receiving end is the receiving end of the high-bit-rate point-to-point QKD system.
Based on the high-bit-rate point-to-point QKD system of the invention, the invention also provides a high-bit-rate point-to-point QKD method, which comprises the steps of generating multiple paths of optical signals with different wavelengths at a transmitting end, splitting each path of optical signals, respectively encoding, performing wavelength division multiplexing processing after encoding, and transmitting through a multi-core optical fiber; the signal is received at the receiving end and is subjected to demultiplexing, decoding, detection and post-processing in sequence.
The beneficial effects of the invention are as follows:
1. when the system is provided with n light sources with different wavelengths and m-core multi-core optical fibers, n x m quantum key distribution lines can be accommodated, and the code rate is n x m times that of the original single QKD system.
2. The QKD system scheme based on time-phase unbalanced basis vector coding reduces the insertion loss of a circuit, the detection result is not limited by the precision of a modulation device, and high-contrast coding is realized, so that the code rate of the system is improved; the selection proportion of the two groups of base vectors is controlled, and the unbalanced base vectors are used for encoding and decoding the signal light, so that the code rate of the system can be further improved.
3. Improved two-way communication QKD systems using fiber optic circulators as optical path selection devices allow Alice and Bob to transmit and receive quantum information simultaneously, with system bit rates up to about 2n x m times.
Drawings
FIG. 1 is a schematic diagram of a high-bitrate point-to-point QKD system of the present invention;
fig. 2 is a schematic diagram of time-phase based codec operation on a Shan Zhiliang subkey distribution line;
fig. 3 is a schematic diagram of a QKD system of fig. 1 with four-way power division multiplexing for each light source;
fig. 4 is a schematic diagram of two-way communication of the high-bitrate point-to-point QKD system of the present invention.
Fig. 5 is a schematic diagram of a QKD system of fig. 4 with four-way power division multiplexing for each light source.
Detailed Description
Example 1 unidirectional communication
As shown in fig. 1, a high-bitrate point-to-point QKD system of the present invention includes a transmitting end (Alice) and a receiving end (Bob) that are matched to each other, and the transmitting end and the receiving end communicate with each other through a multicore fiber 104.
The transmitting end includes a light source module (specifically, a laser board), a plurality of beam splitters (for example, beam splitter 101), an encoder (signal light encoding module), a plurality of wavelength division multiplexers (for example, wavelength division multiplexer 102), and a fiber fan-shaped output 103 serving as a multi-core fiber interface.
In addition, a main control logic module, namely Alice's electronic equipment in fig. 1, is also provided for controlling the laser board and the synchronous light source to emit light and generating modulation signals for the signal light coding module, etc.
A logic control unit in Alice's electronic equipment controls a random number generator to generate a random signal and send the random signal to a signal generator and a signal modulator, the signal generator controls a laser plate to emit signal light corresponding to the random signal, simultaneously controls a synchronous light source to emit synchronous light corresponding to the signal light, the signal modulator receives the random signal and then generates a corresponding modulation signal, and a signal light coding module is controlled to code the optical signal.
The receiving end includes a fiber fan-shaped output 105 as a multi-core fiber interface, a plurality of wavelength division multiplexers (e.g., wavelength division multiplexer 106), a decoder (signal light decoding module), and a detector (signal light detecting module).
In addition, a data recording unit and an auxiliary circuit, namely, electronic equipment of Bob in fig. 1, are also provided for collecting the measurement result of the receiving end. The synchronous light is received and processed by the synchronous light detector and then becomes a synchronous light signal, the synchronous light signal is processed in a delay way by the delay control unit to be used as a trigger signal of the detector module to start detection of the signal light, the data recording unit records response of the detector module, arrival time of the signal light and arrival time of the synchronous light, and the position information of the signal light is obtained according to the time difference of the signal light and the synchronous light, and the response of the detector module and the position information of the signal light are used as measurement results of the receiving end.
In fig. 1, all quantum key distribution lines are synchronously controlled by a synchronous optical signal, wherein a synchronous optical source is located at a transmitting end, and a synchronous optical detector is located at a receiving end. In fig. 1, alice's electronic device and Bob's electronic device may both adopt the prior art, and in this embodiment, each quantum key distribution circuit is a circuit with the same working principle and structure, all circuits work synchronously at the same time, and all circuits are driven and controlled by a common electronic device.
The laser plate is provided with n light sources with different wavelengths, the wavelengths of the light sources 1-n correspond to lambda 1-lambda n respectively, each light source corresponds to one beam splitter, the light sources can be multiplexed, the 1*m beam splitters can divide the emergent light power of each light source to m quantum key distribution lines, if the system is provided with n light sources with different wavelengths, n x m quantum key distribution lines can be expanded, and the utilization rate of the light sources of the system is improved, and meanwhile the code rate of the system is improved.
Wavelength division multiplexers (e.g., wavelength division multiplexer 102 or wavelength division multiplexer 106) may integrate the output of the encoder at different wavelengths of light or the input of the decoder at different wavelengths into one fiber, as shown in fig. 1, as one input to fiber fan-out 103 or one output to fiber fan-out 105. The use of wavelength division multiplexers reduces the need for multiplexing systems to branch the fiber optic sector output while reducing the size of the system (the number of existing fiber optic sector output branches is limited, typically 4 branches and 7 branches). The number of branches of the wavelength division multiplexer is the same as the number of light sources of different wavelengths of light of the system, and when the system has n light sources of different wavelengths, the wavelength division multiplexer has n branches, each branch being connected to a different light source. The number of wavelength division multiplexers is determined by the number of quantum key distribution lines that each light source splits through a beam splitter, when m-way multiplexing is performed on each light source by using the beam splitter, m wavelength division multiplexers are needed at the transmitting end to multiplex m groups of mixed light inputs with different light wavelengths to one optical fiber, and m identical wavelength division multiplexers are needed at the receiving end of the system to perform demultiplexing of mixed light, as described above, at this time, the system needs 2m wavelength division multiplexers of 1*n branches in total.
The optical fiber fanout 103 and the optical fiber fanout 105 are further integrated on the basis of wavelength division multiplexer (e.g. wavelength division multiplexer 102 and wavelength division multiplexer 106) to the optical path of the system, and the optical fiber fanout 103 and the optical fiber fanout 105 are symmetrically identical devices in the system. The optical fiber fan-shaped output device is a component device of a plurality of single-mode fibers and a multi-core optical fiber, the fiber cores of the single-mode fibers are aligned with different fiber cores of the multi-core optical fiber, and light transmitted in each single-mode fiber continuously and independently transmits in parallel along the corresponding fiber core of the multi-core optical fiber, so that the spatial multiplexing or demultiplexing of the light is realized. When (when)When m-core multi-core light is used, m independent parallel light paths are arranged in the optical fiber fan-shaped output device, and m single-mode optical fibers are correspondingly arranged for input or output. At Alice's end, n wavelength inputs integrated by each wavelength division multiplexer form a mixed optical group (lambda 1 、λ 2 、...λ n ) Outputs and transmits the mixed optical group into one input of the fiber fan-out 103. The m wavelength division multiplexers form m identical mixed optical groups (lambda 1 、λ 2 、...λ n ) In order to avoid crosstalk between optical signals of the same wavelength, m mixed optical groups (lambda 1 、λ 2 、...λ n ) Independently in parallel in different optical paths of the fiber optic fanout 103. The wavelength division multiplexer (for example, wavelength division multiplexer 102 and wavelength division multiplexer 106) symmetrical at two ends of the system, the symmetrical optical fiber fan-shaped output device (optical fiber fan-shaped output device 103 and optical fiber fan-shaped output device 105) and the multi-core optical fiber 104 form an intensive optical path together, so as to transmit optical information between the encoder and the decoder.
The synchronous light source uses a different wavelength lambda from the signal light n+1 The optical fiber transmission line is integrated with the quantum key distribution line of Alice end through the wavelength division multiplexer 107 at the left end of the system and the optical fiber fan-shaped output device 103, and finally demultiplexed by the wavelength division multiplexer 108 symmetrical to the Bob end, separated from the signal light, and input into the synchronous optical detector to generate a synchronous signal.
In summary, when n different light sources are respectively subjected to m-channel power division multiplexing through the beam splitter in the point-to-point QKD system, an intensive optical path composed of the wavelength division multiplexer, the optical fiber fan-shaped output device and the m-core multi-core optical fiber can be used for expanding n×m quantum key distribution lines, so that the code rate of the point-to-point QKD system is improved by n×m times.
Fig. 2 is a schematic diagram of the operation of the single-arm quantum key distribution circuit of the present embodiment, which performs a time-phase based encoding and decoding process on signal light on a dual unequal arm MZ interferometer structure. The Shan Zhiliang subkey distribution circuit mainly comprises a transmitting end (Alice), a single-mode optical fiber transmission channel 206 and a receiving end (Bob). The transmitting end (Alice) is composed of a light source 1, a coding module (corresponding to a part of a signal light coding module) and a main control logic module (Alice electronic device), wherein the polarization maintaining beam splitter 201, the phase modulator PM, the optical fiber delay coil 202, the polarization maintaining beam splitter 203, the intensity modulator 204 and the intensity attenuator 205 form the coding module. The receiving end (Bob) is composed of a decoding module (corresponding to a part of the signal light decoding module), a detection module (corresponding to a part of the detection module) and a data recording unit, wherein the beam splitter 207, the 90-degree polarization rotator 208, the polarization maintaining polarization splitter 209, the optical fiber delay coil 210 and the polarization maintaining beam splitter 211 form the decoding module. The detector module consists of three single photon detectors, wherein the detector D1 and the detector D2 are used for measuring single photon pulses output by phase decoding, and the detector D3 is used for measuring single photon pulses output by time decoding. The response values of the respective detectors are collected and processed by the data recording unit.
In the encoding module and the decoding module, an unequal arm MZ interferometer structure is formed by an upper optical path and a lower optical path between the polarization maintaining beam splitter 201 and the polarization maintaining beam splitter 203 and between the polarization maintaining beam splitter 209 and the polarization maintaining beam splitter 211, the upper optical path is a long arm of the MZ interferometer, the lower optical path is a short arm of the MZ interferometer, and the optical path difference between the long and the short arms of the two unequal arm MZ interferometers at two ends of the quantum key distribution line is equal. All devices and fibers at the transmitting end (Alice) and the receiving end (Bob) are polarization maintaining. Each device in the quantum key distribution circuit is a mature prior art, and in a specific embodiment, other devices with the same function can be used for substitution.
The light source 1 emits pulsed light of strong linear polarization. The phase modulator PM is a phase encoding unit of the signal light, which randomly performs 0 or pi phase modulation on the passing light pulse. The intensity modulator 204 is a time-encoding unit of signal light that intensity-modulates two light pulses output from the long and short arms of the MZ interferometer at the left end of the line, randomly attenuating one of the light pulses to a vacuum state close to 0.
The elements of Alice's electronics in fig. 2 perform the same function as in fig. 1, wherein the signal modulator randomly selects either the phase modulator PM or the intensity modulator 204 based on the random signal generated by the random number generator, and controls the random encoding process of the signal light by either the phase modulator PM or the intensity modulator 204.
In Alice's electronic device in fig. 1, a separate signal modulator is connected to each encoding module of the signal light encoding module, and performs the same function as in fig. 2, with all signal modulators being controlled by a common random number generator.
The fiber delay coil 202 and the fiber delay coil 210 are used to increase the MZ interferometer long arm optical path to create an optical path difference between the MZ interferometer long and short arms.
The polarization-maintaining beam splitter 201 achieves beam splitting of the optical wave power on the premise of maintaining the original polarization state of the input light.
The polarization-maintaining beam splitter 211 performs beam combination and beam splitting of the optical wave power on the premise of maintaining the original polarization state of the input light.
Polarization maintaining polarization beam splitter 203 and polarization maintaining polarization beam splitter 209 respectively realize functions of polarization beam combining and polarization beam splitting in the line, and realize 90 ° change of the polarization direction of the fixed branch light.
The intensity attenuator 205 is used to attenuate the input light into light pulses of single photon level.
The beam splitter 207 is a normal 1*2 optical power beam splitter, one of the two outputs of which is connected to a signal light phase decoding unit (90 ° polarization rotator 208 and unequal arm MZ interferometer structure at the right end of the line), and the other is connected to a detector unit D3 for time decoding of the signal light. The beam splitter 207 is not actively modulated by the electronic device in the line, i.e. is a passive modulation device (the passive modulation device has lower cost compared with the active modulation device controlled by the electronic device), when the beam splitting ratio is 50: values other than 50, a time-phase imbalance base vector based codec scheme is formed. The system selects the phase basis vector or the time basis vector at different rates for the decoding and detection of the signal light. The 90 polarization rotator 208 is used to rotate the polarization direction of incident light by 90, such as changing vertically polarized light (V) to horizontally polarized light (H) or vice versa.
In the phase encoding and decoding process, as shown in fig. 2, at Alice transmitting end, the light source 1 emits strong linear polarized light pulse (H), and after being split by the polarization-preserving beam splitter 201, the light pulse is shaped in the short arm and the long arm of the MZ interferometer at the left end of the lineIn two identical horizontal polarization beams (H a 、H b ) A, b represent the two outputs of the polarization maintaining beam splitter 201, respectively, and also represent the short and long arms of the line left MZ interferometer, respectively.
Short arm light pulse (H) a ) Is transmitted through the polarization maintaining fiber and directly input into the polarization maintaining polarization beam splitter 203, and the light pulse (H b ) The phase modulator PM randomly carries out 0 or pi phase modulation, and the phase modulation is transmitted through the delay of the optical fiber delay coil 202 and then is input into the polarization maintaining polarization beam splitter 203. The two light pulses pass through the polarization maintaining polarization splitter 203 before and after, and the polarization maintaining polarization splitter 203 selectively changes the light pulse (H) according to the different input ports b ) Rotated by 90 degrees to become an optical pulse (V b ) The method comprises the steps of carrying out a first treatment on the surface of the Light pulse (H) a ) The polarization direction of (2) is unchanged. Polarization maintaining polarization beam splitter 203 finally outputs light pulse (H) with orthogonal front and rear polarization directions a 、V b ) And two are light pulse (H) a 、V b ) Is attenuated to a single photon level after passing through an intensity attenuator.
The two light pulses reach the Bob's receiving end via a single-mode fiber transmission channel 206. At Bob's receiving end, the light enters a 90-degree polarization rotator 208 after passing through a beam splitter 207, and the 90-degree polarization rotator 208 changes the front and rear light pulses (H a 、V b ) Is of the polarization direction of two light pulses (V a 、H b ) Polarization maintaining polarization beam splitter 209 splits the light pulse (V a ) The polarization direction is changed by 90 ° and is input into the long arm of the line right MZ interferometer, the light pulse (H b ) Directly into the interferometer short arm. To this end, the light pulses (H a 、H b ) After passing through the same single-mode optical fiber transmission channel 206, cross-transmission is performed in the short and long arms of the line-right MZ interferometer, i.e. the short and long arms of the line-right MZ interferometer transmit optical pulses (H b 、H a )。
Because the optical path difference between the long and short arms of the MZ interferometer at both ends is the same, both optical pulses pass through the same fiber transmission channel 206, and thus the optical paths of both optical pulses are the same, i.e., both optical pulses arrive at the polarization-maintaining beam splitter 211 at the same time. Because of two light pulses (H a 、H b ) For light of the same frequency, with the same polarization direction, with a stable phase difference 0 or pi modulated by the phase modulator PM, i.e. satisfying the light interference condition, two light pulses (H a 、H b ) Interference on the polarization maintaining beam splitter 211. When the phase modulator PM randomly selects the 0-phase modulation, the detector D1 responds; the detector D2 responds when the phase modulator PM randomly selects pi phase modulation.
In the time encoding and decoding process, as shown in fig. 2, as in the phase encoding and decoding process, the strong linear polarized light pulse (H) emitted from the light source 1 passes through the MZ interferometer at the left end of the line, and then passes through the optical pulse (H a 、V b ) And outputting in a form. At this time, the phase modulator 204 randomly modulates one of the light pulses into a vacuum state, and the intensity attenuator 205 attenuates the two light pulses (H a 、V b ) Attenuated to the single photon level and transmitted through single mode fiber transmission channel 206 to Bob's receiving end. Two light pulses (H a 、V b ) After passing through the beam splitter 207, the light enters the detector unit D3, and the detector D3 performs time resolution capturing on two light pulses through synchronous operation with an Alice transmitting end, measures light intensity values of the front pulse and the rear pulse, and finally obtains a time decoded quantum state.
The quantum state obtained by the phase encoding and decoding and the quantum state obtained by the time encoding and decoding together form a quantum state sequence of the Bob receiving end. After Alice and Bob basic vectors are compared, the original secret key is obtained by both parties, and then the same security secret key is finally obtained by both parties through data post-processing operations such as error correction, privacy amplification and the like. For the process of post-processing of the basis vector ratio peer-to-peer data, reference is made to Christian Kollmitzer et al, appliced Quantum Cryptography, for a description of quantum key distribution in the third chapter.
Fig. 3 is a schematic diagram of the structure of the present invention when 4-way power division multiplexing is performed on a system light source. Only the quantum key distribution circuit of the point-to-point QKD system is shown in the figure, and the synchronous optical transceiver circuit is the same as that of fig. 1, i.e. all the quantum key distribution circuits use a synchronous optical signal together, and the synchronous optical path is transmitted by a wavelength division multiplexer incorporated into a corresponding optical group at the transmitting end, and is transmitted in the same fiber core of the optical fiber fan-shaped output device with the optical group, and finally demultiplexed by the wavelength division multiplexer symmetrical to the receiving end, and the synchronous optical signal is detected and processed by a synchronous optical detector to generate the synchronous optical signal, which acts on the detection module of the receiving end.
In this embodiment, the laser plate integrates 5 pulse light sources with different wavelengths, and the external electronic device controls the 5 pulse light sources to operate synchronously. 4 paths of power division multiplexing are carried out on the emergent light of each pulse light source by adopting a 1*4 beam splitter, and 4 quantum key distribution lines are respectively led out. At this time, the system forms an intensive optical path by using 8 1*5 wavelength division multiplexers, 2 optical fiber fan-shaped exporters with 1*4 ports and four-core optical fibers, and performs optical information transmission between the signal optical coding module and the signal optical decoding module. In the embodiment, 4 paths of power division multiplexing are respectively carried out on 5 light sources with different wavelengths through the beam splitter, and expansion of 5*4 quantum key distribution lines can be realized by means of the intensive light path, so that the code rate of a point-to-point QKD system is improved by 5*4 times.
In fig. 3, taking 4 quantum key distribution lines led out of the light source 1 as an example, the system is illustrated to expand the 5*4 quantum key distribution lines and encode and decode and detect the signal light. The light source 1 introduces 4 quantum key distribution lines through a 1*4 beam splitter 301. On the 1 st quantum key distribution line, a polarization maintaining beam splitter 302, a phase modulator PM, an optical fiber delay coil 303, a polarization maintaining beam splitter 304, an intensity modulator 305, and an intensity attenuator 306 form a coding module, and the coding modules of the other 3 quantum key distribution lines form the same coding module. The signal lights on the 4 quantum key distribution lines are encoded by the encoding modules and then respectively enter 4 identical 1*5 wavelength division multiplexers (such as the wavelength division multiplexer 307), and the signal light of the 1 st quantum key distribution line enters one input end of the wavelength division multiplexer 307. When the system enables the other 4 light sources (light sources 2 to 5) to expand the quantum key distribution circuit, the other 4 light sources (light sources 2 to 5) respectively introduce 4 new quantum key distribution circuits in the same way, and the 4 quantum key distribution circuits introduced by the same light source respectively enter the 4 1*5 wavelength division multiplexers (such as the wavelength division multiplexer 307) after being encoded into the module. The external electronic device controls the phase encoding units (phase modulators PM) or time encoding units (intensity modulators) of all quantum key distribution lines simultaneously, namely, each signal modulator in Alice's electronic device controls the random modulation process of each phase modulator PM or each intensity modulator on signal light simultaneously. Five input light wavelengths are combined in a 1*5 wavelength division multiplexer (e.g., wavelength division multiplexer 307) and then transmitted in a mixed optical group in one optical fiber, and 4 1*5 wavelength division multiplexers (e.g., wavelength division multiplexer 307) of Alice output 4 optical groups. The 4 optical groups are respectively and independently transmitted in different fiber cores of the four-core optical fiber 309 through the coupling action of the optical fiber fan-shaped output device 308, and enter 4 different single-mode optical fibers for continuous transmission after being coupled out by the symmetrical optical fiber fan-shaped output device 310. The 4 optical groups are transmitted into Bob's 4 1*5 wavelength division multiplexers (e.g., wavelength division multiplexer 311), respectively, where a five-wavelength beam splitting process is to be performed. On the 1 st quantum key distribution line led in by the light source 1, the beam splitter 312, the 90 degree polarization rotator 313, the polarization maintaining polarization splitter 314, the optical fiber delay coil 315 and the polarization maintaining beam splitter 316 form a decoding module, the detector D1, the detector D2 and the detector D3 form a detection module, and other quantum key distribution line related modules in the system form the same module. Each output end of the 4 1*5 wavelength division multiplexers (e.g., wavelength division multiplexer 311) of Bob is connected to a decoding module and a detection module of the signal light, and the quantum key distribution circuit will complete the decoding and detection process of the signal light.
Example 2 bidirectional communication
Fig. 4 is a high-bitrate point-to-point QKD system that can communicate bi-directionally. The method is a modification of the embodiment 1, and devices symmetrical to the original multiplexing system are respectively added at the Alice transmitting end and the Bob receiving end of the embodiment 1 by using optical fiber circulators (for example, the optical fiber circulators 401 and 407): the signal light decoding module 2, the signal light detecting module 2, the laser plate 2 and the signal light encoding module 2 make quantum communication in the forward and backward directions share an intensive optical path composed of a wavelength division multiplexer (for example, the wavelength division multiplexer 402 and the wavelength division multiplexer 406), an optical fiber fan-shaped output device (the optical fiber fan-shaped output device 403 and the optical fiber fan-shaped output device 405) and a multi-core optical fiber 404, and simultaneously add a synchronous light transmitting and receiving line (a synchronous light source 2 to a synchronous light detector 2) from a Bob end to an Alice end in the system. Optical signals 1 of the optical fiber circulators (for example, the optical fiber circulators 401 and 407) as path selection devices for optical output are incident from the port 1, exit from the port 2 through path selection of the circulators, and the optical signals 2 are incident from the port 2, exit from the port 3 through path selection of the circulators.
The quantum information transmitting and receiving processes of the Alice end and the Bob end can be simultaneously driven and controlled by adding a main control logic module, a data recording unit and an auxiliary circuit which are symmetrical to the original multiplexing system, and the synchronous light source 1 and the synchronous light detector 1 form a synchronous light transmitting and receiving line 1 which is used for controlling the synchronization of the transmitting end Alice and the receiving end Bob during forward communication; the synchronous light source 2 and the synchronous light detector 2 form a synchronous light transmitting and receiving line 2, which is used for controlling the synchronization of the transmitting end Bob and the receiving end Alice during reverse communication. As described above, alice and Bob can simultaneously transmit and receive quantum information, and thus the system bitrate is improved by about 2n x m times.
The Shan Zhiliang subkey distribution line of the two-way communication system still uses the QKD structure of embodiment 1 (in fig. 2) based on a time-phase encoding scheme, which is not repeated here. The communication in two directions of the system is provided with devices such as independent encoding and decoding and detection, and the two devices only multiplex the integrated optical circuit.
Fig. 5 is a schematic diagram of the structure of the present invention when 4-way power division multiplexing is performed on the system light source. The signal lights emitted by the light sources at two ends are encoded by the encoding modules at two ends, the encoded signal lights are subjected to path selection at each end of the optical fiber circulator (such as the optical fiber circulator 501), for the transmitting end, the signal lights are input by the port 1 and output by the port 2, the signal lights emitted by all the light sources (such as the light sources 1 to 5) enter the wavelength division multiplexer (such as the wavelength division multiplexer 502 and the wavelength division multiplexer 507) in the form of the same light groups (lambda 1, lambda 2 and lambda 5) at the wavelength division multiplexer (such as the wavelength division multiplexer 502 and the wavelength division multiplexer 503), the optical fiber fanout (such as the optical fiber fanout 504 and the optical fiber fanout 506) and the multi-core optical fiber 505, for the transmitting end, the signal lights are input by the port 2 and output by the port 3, and the output signal lights are decoded by the decoding modules and then detected by the corresponding detectors at the other end of the optical fiber circulator (such as the optical fiber circulator 508). Alice and Bob can send quantum information at the same time, and through simultaneous encoding and decoding, the quantum information is received at both ends at the same time, and the system has a code rate of 2 x 4 x 5 times. The components and functions of the optical fiber beam splitter, the coding module, the decoding module and the detection module in the system are the same as those in fig. 3 of the first embodiment, and will not be described one by one.
Claims (10)
1. A high-bit-rate point-to-point QKD system comprises a transmitting end and a receiving end which are matched with each other, and is characterized in that,
the transmitting end sequentially comprises a light source module, a beam splitter, an encoder, a wavelength division multiplexer and an optical fiber fan-shaped output device serving as a multi-core optical fiber interface along the transmission direction of the optical signals, and is further provided with a main control logic module used for transmitting control signals to the encoder, wherein the light source module comprises a plurality of light sources and respectively has different transmission wavelengths, the output end of each light source is correspondingly connected with one beam splitter, the beam splitter divides the output of the corresponding light source into a plurality of paths of optical signals to be output to the encoder for encoding respectively, the wavelength division multiplexer comprises a plurality of wavelength division multiplexers, the output ends of different optical wavelengths of the encoder are assembled into an optical fiber, the input ends of different optical wavelengths of the wavelength division multiplexer are integrated form a mixed optical group, the mixed optical group is transmitted into one input end port of the optical fiber fan-shaped output device, the optical fiber fan-shaped output device is a plurality of single-mode optical fibers and a component device of the multi-core optical fibers, the optical cores of the single-mode optical fibers are aligned, the light transmitted in each single-mode optical fiber continues to be independently and in parallel along the corresponding fiber core to form a plurality of independent parallel optical paths, and the optical paths are used for switching the optical signals from the wavelength division multiplexer to the receiving end of the multi-core optical fibers for transmitting signals to the multi-core optical fibers;
the receiving end sequentially comprises an optical fiber fan-shaped output device serving as a multi-core optical fiber interface, a wavelength division multiplexer, a decoder and a detector along the optical signal transmission direction, wherein each fiber core of the multi-core optical fiber corresponds to a plurality of independent parallel light paths in the optical fiber fan-shaped output device, the optical fiber fan-shaped output device integrally sends optical signals from each fiber core of the multi-core optical fiber to the wavelength division multiplexer, and the optical signals correspond to one input path of the wavelength division multiplexer;
the transmitting end and the receiving end are communicated through a multi-core optical fiber connected between the optical fiber interfaces.
2. The high-bitrate point-to-point QKD system of claim 1, wherein the encoders have encoding modules therein that are in one-to-one correspondence with each optical signal, and wherein the master logic module is configured with a corresponding signal modulator for each encoding module, all signal modulators sharing the same random number generator in the master logic module.
3. The high-bitrate point-to-point QKD system of claim 1, wherein the encoder is a time-phase encoding based signal optical encoding module.
4. The high-bitrate point-to-point QKD system of claim 1, wherein the plurality of splitters is one-to-one with each light source, and wherein the output of each wavelength division multiplexer is connected to the multi-core fiber interface and corresponds to one of the cores.
5. The high-bitrate point-to-point QKD system of claim 4, wherein the number of splitters is the same and equal to the number of wavelength division multiplexers.
6. The high-bitrate point-to-point QKD system of claim 1, further comprising a synchronous optical source and corresponding synchronous optical transmit lines, wherein the synchronous optical source is coupled to an input of any one of the wavelength division multiplexers.
7. The high-bitrate point-to-point QKD system of any one of claims 1-6, wherein each optical signal transmission path between the encoder and the wavelength division multiplexer is provided with an optical path selection device, and the transmitting end is further provided with a signal receiving module coupled into the corresponding optical signal transmission path via each optical path selection device;
and each optical signal transmission path between the encoder and the wavelength division multiplexer is respectively provided with an optical path selection device, and the transmitting end is also provided with a signal receiving module which is coupled into the corresponding optical signal transmission path through each optical path selection device.
8. The high-bitrate point-to-point QKD system of claim 1, wherein the decoder is a time-phase decoding based signal light decoding module.
9. A high-bitrate point-to-point QKD system according to claim 1 or 8, wherein each optical signal transmission path between a wavelength division multiplexer and a decoder is provided with an optical path selection device, and the receiving end is further provided with a signal transmission module coupled into the respective optical signal transmission path via each optical path selection device.
10. The high-bit-rate point-to-point QKD method of claim 1, which is characterized in that the method is implemented in a high-bit-rate point-to-point QKD system, multiple optical signals with different wavelengths are generated at the transmitting end, each optical signal is split and encoded respectively, and after encoding, the wavelength division multiplexing is performed and then the multi-core optical fiber is used for transmitting; the signal is received at the receiving end and is subjected to demultiplexing, decoding, detection and post-processing in sequence.
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