CN112702162B - One-dimensional continuous variable quantum key distribution system based on discrete state and its realization method - Google Patents
One-dimensional continuous variable quantum key distribution system based on discrete state and its realization method Download PDFInfo
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Abstract
The invention discloses a one-dimensional continuous variable quantum key distribution system based on discrete state and a realization method thereof, wherein the system comprises a sending end, a transmission channel and a receiving end, the sending end couples signal light subjected to one-dimensional discrete modulation with a local oscillator and then transmits the coupled signal light to the receiving end through the quantum channel, and the receiving end processes and detects the coupled signal light and the local oscillator and then performs error correction negotiation with the sending end through a classical channel to obtain a final quantum key; the invention simplifies the modulation scheme through one-dimensional discrete modulation, reduces the realization cost and leads the quantum key distribution to be more widely applied in short-distance key transmission.
Description
Technical Field
The invention belongs to the technical field of quantum key distribution, and particularly relates to a one-dimensional continuous variable quantum key distribution system based on discrete states and an implementation method thereof.
Background
Quantum Key Distribution (QKD) technology is one of the most promising and feasible technologies at present, and has a wide application prospect in quantum physics, and Discrete Variable Quantum Key Distribution (DVQKD) and Continuous Variable Quantum Key Distribution (CVQKD) are two main implementation modes of quantum key distribution technology, and some remarkable achievements are achieved.
The discrete variable quantum key distribution technology is started earlier and has the characteristic of longer safe transmission distance, but the discrete variable quantum key distribution technology adopts single photons as carriers to transmit information, the preparation of a single photon source is difficult, and the existing laboratory adopts pulsed light with the average photon number of 0.1/pulse as the single photon source, so that the safety of the system is reduced because the pulsed light is not real single photons; in addition, the single photon detector has high manufacturing cost, is only suitable for experimental realization and cannot be manufactured and used in large-scale commercialization.
Compared with the discrete variable quantum key distribution technology, the continuous variable quantum key distribution technology has the following advantages: 1. in the experiment, weak coherent light can be used as an information carrier by using a continuous variable quantum key distribution technology, a single photon source does not need to be prepared, a single photon detector with high manufacturing cost is also not needed, and a balanced homodyne detector or a heterodyne detector can be used; 2. the continuous variable quantum key distribution technology can adopt modulation schemes in classical optical communication such as quadrature amplitude modulation, and the experiment complexity is simplified; 3. the continuous variable quantum key distribution technology has higher key rate, so the continuous variable quantum key distribution technology has better application prospect.
There are two main states in continuous variable quantum key distribution, namely, gaussian state and discrete state, and although a higher key rate can be achieved by using gaussian state, in fact, gaussian coherent state is difficult to be completely realized in practical application; compared to the gaussian state, the discrete state has the following advantages: 1. the discrete state is characterized by a small constellation, so that the error correction process is greatly simplified; 2. discrete states can simplify the state preparation process; 3. the use of discrete states may enable high negotiation efficiency at low signal-to-noise ratios.
Disclosure of Invention
The invention aims to provide a one-dimensional continuous variable quantum key distribution system based on an discrete state, which generates a one-dimensional discrete state continuous variable through an electro-optical phase modulator, obtains a phase component detection result at a receiving end by using a homodyne detector, and has the advantages of low implementation cost, simple preparation process of the discrete state of the continuous variable and high efficiency when carrying out error correction negotiation on the discrete state of the continuous variable.
The invention also aims to provide a realization method of the one-dimensional continuous variable quantum key distribution system based on the discrete state, which greatly simplifies the preparation process and the error correction process of the quantum state, improves the negotiation efficiency of the quantum key distribution system at low signal-to-noise ratio and promotes the practicability of the continuous variable quantum key.
The technical scheme adopted by the invention is that the one-dimensional continuous variable quantum key distribution system based on the discrete state comprises a sending end, a transmission channel and a receiving end;
the transmitting end comprises:
a pulsed laser for generating pulsed coherent light;
the optical phase modulator comprises a beam splitter 1, a polarization coupler 1 and an electro-optic phase modulator 1, wherein the beam splitter 1 is used for splitting pulse coherent light into 1% signal light and 99% local oscillator light, sending the local oscillator light to the polarization coupler 1 through a delay optical fiber and sending the signal light to the electro-optic phase modulator 1;
the classical computer PC1 is used for generating a uniform random number signal and sending the random number signal to the electro-optical phase modulator 1 to control the electro-optical phase modulator to perform one-dimensional discrete modulation on signal light;
the adjustable attenuator attenuates the signal light subjected to one-dimensional discrete modulation to a quantum level and sends the signal light to the polarization coupler 1;
the polarization coupler 1 is used for coupling the signal light of the quantum level and the local oscillator light into a quantum signal;
the transmission channel comprises a quantum channel and a classical channel, the polarization coupler 1 transmits quantum signals to a receiving end through the quantum channel, and the classical computer PC1 is connected with the receiving end through the classical channel.
Further, the receiving end includes:
the polarization controller is used for receiving the quantum signals sent by the polarization coupler 1, carrying out polarization compensation on the quantum signals and then sending the quantum signals to the beam splitter 2;
the beam splitter 2 is used for splitting the quantum signals into 1% signal light and 99% local oscillation light, inputting the local oscillation light into the electro-optical phase modulator 2, and inputting the signal light into the polarization coupler 2 through a delay optical fiber;
the electro-optical phase modulator 2 is used for carrying out phase modulation on the local oscillator light to enable the phase difference between the local oscillator light and the signal light to be 0 or pi/2;
the polarization coupler 2 is used for interfering the local oscillation light and the signal light after phase modulation and inputting an interference result into a homodyne detector;
the homodyne detector is used for carrying out homodyne detection on the interference result to obtain a phase component detection result;
and the classical computer PC2 is used for controlling the electro-optical phase modulator 2 to perform phase modulation, acquiring a homodyne detection result, and negotiating with the classical computer PC1 through a classical channel to obtain a quantum key.
Further, the pulse laser adopts a Thorlabs OPG1015 picosecond optical pulse generator, the beam splitter 1 adopts a beam splitter with a port type of 1 × 2, the model of the electro-optical phase modulator 1 is MPZ-LN-10, the adjustable attenuator adopts a polarization-maintaining adjustable laser attenuator with a model of VOA780PM-FC, and the model of the polarization coupler 1 is Thorlabs PBC980 PM-FC.
Further, the beam splitter 2 is a 1 × 2 port type beam splitter, the electro-optical phase modulator 2 is MPZ-LN-10 in model, and the polarization coupler 2 is Thorlabs PBC980PM-FC in model.
The implementation method of the one-dimensional continuous variable quantum key distribution system based on the discrete state comprises the following steps:
s1, separating the pulse coherent light generated by the pulse laser into a main beam and a signal beam by using the beam splitter 1, and transmitting the signal beam to the electro-optic phase modulator 1, generating a random number signal by a classical computer PC1, and inputting the random number signal into the electro-optic phase modulator 1 to control the electro-optic phase modulator to perform one-dimensional discrete modulation on the signal beam, wherein the one-dimensional discrete modulation process is as follows:
s11, generating a uniform random number set {0,1,2, …, N-1} by an FPGA signal generating card contained in a classical computer PC1, and sending the random number set to an electro-optical phase modulator 1;
s12, the electro-optic phase modulator 1 randomly extracts the number k from the set {0,1,2, …, N-1} with the same probability, and modulates the signal light to obtain the discrete quantum state | alpha |k>＝|Aei(2k+1)π/N>The N-type discrete quantum states form a set SN，SN＝{|Aei π/N>,…,|Ae(2k+1)iπ/N>,…,|Ae(2N-1)iπ/N>H, wherein i is an imaginary number and A is an amplitude;
s2, the electro-optic phase modulator 1 will SNThe input adjustable attenuator is used for inputting the signals into the polarization coupler 1 after being attenuated to the quantum level, and the polarization coupler 1 couples the signals and the local oscillator light into quantum signals which are transmitted to the polarization controller through a quantum channel;
s3, the polarization controller carries out polarization compensation on the quantum signals, then the quantum signals are incident to the beam splitter 2 and are separated into 1% signal light and 99% local oscillator light, the local oscillator light is subjected to phase modulation through the electro-optic phase modulator 2, then the local oscillator light is input into the polarization coupler 2 to interfere with the signal light, and the interference light is sent to the homodyne detector to carry out phase component detection;
and S4, the classical computer PC2 acquires the phase component detection result, and performs parameter estimation, error correction, consistency check and security enhancement operations with the classical computer PC1 through a classical channel to obtain a final shared key.
Further, the step 4 comprises the following steps:
s41, the classical computer PC1 and the classical computer PC2 respectively select part of key bits with the same positions from the original keys for public comparison, the quantum error rate is calculated, if the quantum error rate is larger than or equal to a threshold value, the key transmitted at this time is abandoned, and if the quantum error rate is smaller than the threshold value, S42 is carried out;
s42, the classical computer PC1 obtains check bits through coding, the check bits are sent to the classical computer PC2 through a classical channel, the classical computer PC2 mixes the check bits with original key bits and carries out decoding operation to correct error code bits in the original key bits obtained by the classical computer PC 2;
s43, respectively calculating hash values of the key bits after error correction by the classic computer PC1 and the classic computer PC2, if the calculation results of the hash values are the same, successfully correcting the error, reserving the group of key bits and carrying out S44, otherwise, discarding the group of key bits;
and S44, performing security enhancement on the key bits to obtain a final shared key.
Further, the encoding in step 42 includes the following steps:
(1) setting 0 on n-k' check bits, defining a double diagonal matrix for the check matrix according to a DVB-S2 protocol to obtain a check bit address list;
(2) taking 360 information bits as a group, carrying out XOR calculation on the first group of information bit data and the check bit in the first row in the check bit address list correspondingly, carrying out XOR calculation on the second group of information bit data and the check bit in the second row in the check bit address list correspondingly, and obtaining the values of all check bits in the check bit address list in the same way;
said different isOr calculated as follows: { x + (mmod360 × q) } mod (n-k'), where m is a variable indicating the number of information bits, and x is the m +1 st information bit imThe corresponding check bit address, q is the parameter corresponding to the selected code rate;
(3) using formulas
Obtain the final parity bit piA 1 is to piAttaching to the information sequence to obtain a coded codeword, piWhere i is a variable indicating the number of check bits, i is 1,2,3, …, n-k' -1.
Further, the decoding in step S42 includes the following steps:
(1) information initialization: the initial probability likelihood ratio information received by the variable node a is L (P)a) The initial information transmitted from the variable node a to the check node b is L(0)(qab)＝L(Pa)，qabThe external probability information from the variable node a to the check node b;
(2) and processing and updating the check nodes by using the following calculation:
where u is the number of iterations, L(u)(rba) For the information, R, passed from check node b to variable node a at the u-th iterationbA is a set of variable nodes connected to check node b except variable node a, c is a set of variable nodes connected to check node b except variable node a, L(u-1)(qcb) The information transmitted to the check node b by the variable node c for the u-1 iteration;
the variable nodes are updated using the following calculation:
L(u)(qab) For the information passed from the variable node a to the check node b at the u-th iteration, CaB is a set of check nodes connected to a variable node a except for check node bD is a check node connected to the variable node a except the check node b, L(u)(rda) The information transmitted to the variable node a by the check node d in the u-th iteration is obtained;
and (3) decoding judgment:
wherein L is(u)(qa) All information collected for variable node a, CaIs the set of all check nodes connected with the variable node a if L(u)(qa) Is considered to be > 0
Otherwise
A decoded output sequence for variable node a;
and (4) iteration termination: when in use
Or stopping iteration when reaching preset iteration times to obtain a code word after decoding judgment, wherein H is a corresponding parity check matrix,
t is the transpose of the matrix formed by the decoded sequences obtained by decoding.
The invention has the beneficial effects that: the invention directly uses digital signal modulation to obtain discrete continuous variable, the preparation process is simple and easy to realize high speed, after single phase modulation, the secret information is coded on the phase regular component of quantum state, the receiving end uses homodyne detector to detect it, the invention reduces the realization cost and is easy to popularize, and the invention also carries out error correction negotiation on the key received by the receiving end, thus improving the negotiation efficiency of quantum key.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the drawings without creative efforts.
In fig. 1: (a) the distribution diagram of the discrete state in the phase space obtained by the invention, and (b) the distribution of the discrete state in the phase space obtained by the traditional method.
Fig. 2 is a system configuration diagram of the present invention.
Fig. 3 is a flow chart of key distribution according to the present invention.
Fig. 4 is a graph comparing the effects of the examples of the present invention.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
As shown in fig. 2, the system for distributing a one-dimensional continuous variable quantum key based on a discrete state includes a transmitting end, a transmission channel, and a receiving end, where the transmitting end includes a pulse laser, the pulse laser transmits generated pulse coherent light to a beam splitter 1, the beam splitter 1 splits the pulse coherent light into 1% of original signal light and 99% of original local oscillator light, and transmits the original local oscillator light to a polarization coupler 1 through an adjustable delay optical fiber, the original signal light is input to an electro-optic phase modulator 1, an FPGA signal generation card included in a classical computer PC1 transmits a generated uniform random number signal to the electro-optic phase modulator 1, and controls the electro-optic phase modulator to perform one-dimensional discrete modulation on the original signal light, the phase modulator 1 inputs a one-dimensional discrete modulation result to an adjustable attenuator, the adjustable attenuator attenuates the discrete modulation signal to a quantum level and then inputs to the polarization coupler 1, the polarization coupler 1 couples the signal light and the original local oscillator light into a quantum signal, and the quantum signal is transmitted to a receiving end through a quantum channel.
The receiving end comprises a polarization controller, the polarization controller performs polarization compensation on the received quantum signals and inputs the compensated quantum signals into a beam splitter 2, the beam splitter 2 separates the quantum signals into 1% of signal light and 99% of local oscillator light, the signal light is input into a polarization coupler 2, the local oscillator light is input into an electro-optical phase modulator 2 for phase modulation, so that the phase difference between the local oscillator light and the signal light is 0 or pi/2, the electro-optical phase modulator 2 inputs the modulated local oscillator light into the polarization coupler 2, and the polarization coupler 2 couples the phase modulated local oscillator light and the signal light and then inputs the phase modulated local oscillator light and the signal light into a homodyne detector for homodyne detection to obtain a phase component detection result; the FPGA data acquisition card contained in the classic computer PC2 acquires a phase component detection result, interacts with the classic computer PC1 through a classic channel, and verifies, corrects and enhances secrecy of the phase component detection result to obtain final quantum key data.
The pulse laser adopts a Thorlabs OPG1015 picosecond optical pulse generator, can produce laser pulse less than or equal to 3ps, the frequency is 10GHz, the beam splitter adopts the NS series port type of Agiltron company to be 1 x 2 beam splitter, the working wavelength is 780-1800 nm, the beam splitting ratio in the whole bandwidth is adjustable, the model of the electro-optic phase modulator 1 and the electro-optic phase modulator 2 is MPZ-LN-10, have the characteristics of high extinction ratio (>20dB), low loss (2.5dB), high bandwidth (10GHz), can meet the quantum key communication system of higher speed, reduce the extra loss brought by optical device; the adjustable attenuator adopts a polarization-maintaining adjustable laser attenuator with the model of VOA780 PM-FC; the polarization couplers 1 and 2 are of the type Thorlabs PBC980PM-FC, high extinction ratio (>18dB), low loss (<2 dB); the polarization controller adopts MLC-15-PQH-SM-FA, and the working wavelength is 15-1550 nm; the homodyne detector adopts a Thorlabs PDA435A balanced amplification photoelectric detector, the common mode rejection ratio is more than 20dB, and the bandwidth can reach 350 MHz; the FPGA signal generation card and the FPGA data acquisition card in the PC of the classic computer adopt Xilinx VC707, and the sampling rate can reach 5 GSa/s.
The implementation flow of one-dimensional continuous variable quantum key distribution based on discrete states in this embodiment is shown in fig. 3, and includes the following steps:
s1, preparing a discrete quantum signal;
the beam splitter 1 transmits the pulse coherent light emitted by the pulse laser according to the ratio of 99: 1 is divided into local oscillation light and signal light, the signal light is transmitted to an electro-optical phase modulator 1, an FPGA signal generation card contained in a classic computer PC1 generates a uniform random number signal, the random number signal is input into the electro-optical phase modulator 1 to control the electro-optical phase modulator to discretely modulate the signal light, and the classic computer PC1 controls the electro-optical phase modulator 1 to discretely modulate the signal light as follows:
s11, generating a uniform random number set {0,1,2, …, N-1} by an FPGA signal generating card contained in a classical computer PC1, and sending the random number set to an electro-optical phase modulator 1;
s12, the electro-optic phase modulator 1 randomly extracts a number k from the random number set {0,1,2, …, N-1} with the same probability to obtain a set S of N discrete quantum statesN＝{|Aeiπ/N>,…,|Ae(2k+1)iπ/N>,…,|Ae(2N-1)iπ/NWhere i is an imaginary number, a is an amplitude, the value of the phase component satisfies (2k +1) pi/N, and the amplitude a is unknown because single-phase modulation is employed and the amplitude component is not modulated;
the distribution of the discrete state obtained by modulation of the electro-optical phase modulator 1 in the phase space is shown in (a) in fig. 1, compared with the distribution of the discrete state in the phase space obtained by the traditional method (shown in (b) in fig. 1), the key information is loaded on the phase of the quantum state in the quantum key transmission process, and the information transmission is irrelevant to the amplitude;
s2, the electro-optical phase modulator 1 inputs the N discrete quantum states into the adjustable attenuator to be attenuated to the quantum level, then inputs the N discrete quantum states into the polarization coupler 1, the local oscillator light obtained by the separation of the beam splitter 1 is transmitted to the polarization coupler 1 through the delay optical fiber, the polarization coupler 1 couples the attenuated signal light and the local oscillator light into a light beam, and the light beam is transmitted to the polarization controller of the receiving end through the quantum channel;
s3, the polarization controller carries out polarization compensation on the received quantum light beam, then the quantum light beam enters the beam splitter 2 to be separated into 1% of signal light and 99% of local oscillator light, the local oscillator light is input into the electro-optic phase modulator 2 to be subjected to phase modulation, the modulation result is input into the polarization coupler 2, the signal light is input into the polarization coupler 2 through a delay optical fiber by the beam splitter 2, and the local oscillator light and the signal light are input into a homodyne detector to be subjected to phase component detection after being interfered by the polarization coupler 2; because one-dimensional single-phase modulation is adopted, the modulation basis and the measurement basis of the sending end and the receiving end are phase basis components, the key screening process can be omitted;
s4, collecting phase component detection result by FPGA data collection card contained in PC2, and carrying out parameter estimation, error correction, consistency check and security enhancement to obtain quantum key data.
As shown in fig. 3, the process of parameter estimation, error correction, consistency check and security enhancement is as follows:
s41, the classical computer PC1 and the classical computer PC2 jointly select part of key bits from the original key for public comparison, the quantum error rate is calculated according to the comparison result, if the quantum error rate is larger than or equal to a threshold value, all the key bits transmitted at this time are abandoned, and if the quantum error rate is smaller than the threshold value, S42 is carried out;
s42, generating check bits by the classical computer PC1, sending the check bits to the classical computer PC2, mixing the check bits with the bits of the local original key by the classical computer PC2, and correcting error code bits in the bits of the local original key by decoding operation;
s43, calculating the hash value of the key bit after error correction by the classic computer PC1 and the classic computer PC2 respectively by using a hash algorithm, if the hash values of the two are the same, successfully correcting the error, reserving the group of key bits and carrying out S44, otherwise, abandoning the group of keys;
and S44, performing information compression, namely privacy enhancement on the key bits to obtain the final shared security key.
As a preferred technical solution, in S42, the process of performing encoding operation using a check code with a code length of 16200 bits and a code rate of 1/3 is as follows:
(1) at n-k' check bits 0, i.e. p0＝pi＝…＝pn-k′-1＝0，piWherein i is a variable indicating the number of check bits, i is 1,2,3iFor the ith check digit, defining a double diagonal matrix for different check matrixes according to a DVB-S2 protocol to obtain a check digit address list;
(2) taking 360 information bits as a group, carrying out XOR calculation on the first group of information bit data and all check bits in a first row in a check bit address list correspondingly, carrying out XOR calculation on the second group of information bit data and all check bits in a second row in the check bit address list correspondingly, and obtaining the values of all check bits in the check bit address list in the same way;
the formula of the exclusive or calculation is as follows: { x + (mmod360 × q) } mod (n-k'), where m is a variable indicating the number of information bits, and x is the m +1 st information bit imThe corresponding check bit address, q is the parameter corresponding to the selected code rate;
(3) using formulas
Obtain the final parity bit piA 1 is to piAnd attaching the information sequence to obtain a coded code word.
As a preferred technical solution, the process of performing the decoding operation of error correction in step S42 is as follows:
(1) information initialization: the initial probability likelihood ratio information received by the variable node a is L (P)a) The initial information transmitted from the variable node a to the check node b is L(0)(qab)＝L(Pa)，qabThe external probability information from the variable node a to the check node b;
(2) and (3) processing and updating by the check node:
where u is the number of iterations, rbaFor external information from check node b to variable node a, L(u)(rba) For the information, R, passed from check node b to variable node a at the u-th iterationbA is a set of variable nodes connected to check node b except variable node a, c is a set of variable nodes connected to check node b except variable node a, L(u-1)(qcb) The information transmitted to the check node b by the variable node c for the u-1 iteration;
and (3) variable node updating:
L(u)(qab) For the information passed from the variable node a to the check node b at the u-th iteration, CaV.b is a set of check nodes connected to variable node a except check node b, d is a set of check nodes connected to variable node a except check node b, L(u)(rda) The information transmitted to the variable node a by the check node d in the u-th iteration is obtained;
and (3) decoding judgment:
wherein L is(u)(qa) All information collected for variable node a, CaIs the set of all check nodes connected with the variable node a if L(u)(qa) If > 0, the judgment is regarded as
Otherwise
A decoded output sequence for variable node a;
(3) and (4) iteration termination: when in use
Or stopping iteration when reaching preset iteration times to obtain a code word after decoding judgment, wherein H is a corresponding parity check matrix,
t is the transpose of the matrix formed by the decoded sequences obtained by decoding.
In the prior art, discrete modulation needs to be carried out by using an amplitude modulator and a phase modulator to modulate signal light at the same time, the discrete state based on the discrete state one-dimensional modulation technology can obtain discrete states with different phases only by using single-phase modulation, and amplitude components are not modulated, so that a modulation scheme can be simplified, the implementation cost is reduced, the application of the short-distance secret key transmission is wider, the quantum communication can be used for local area network construction when the quantum communication is mature, and the method is more suitable for popularization; the invention also applies the DVB-S2 protocol in 5G communication to the key error correction negotiation, so that the transmission efficiency of the quantum key is improved, and the accuracy is increased.
Examples
A sending end Alice and a receiving end Bob carry out communication initialization on the system, including initialization on an information source, a modem, a detector and a control circuit in the system; alice then phase modulates the weak coherent light from the pulsed laser using electro-optic phase modulator 1 to produce discrete quantum state | αk〉＝|Aei(2k+1)π/NObtain a set of discrete quantum states SNThen transmitted to a remote receiving end Bob through a phase sensitive quantum channel, and the receiving end Bob modulates the orthogonal component by using a homodyne detector
And (4) carrying out homodyne measurement, and finally obtaining a final security key by both communication parties through error correction and privacy enhancement.
Without loss of generality, the discrete states mainly include two, four and eight states, and the set S is changed according to the type of the discrete states in different phasesNThe relationship between the key rate and the transmission distance of the continuous variable quantum key distribution system in the three types of discrete states is obtained, the performance of the continuous variable quantum key distribution system in the discrete state and the Gaussian state is compared, the comparison result is shown in FIG. 4, it can be known from FIG. 4 that the key rate in the Gaussian state is higher than that in the discrete state when the transmission distance is less than 20km, and the key rate in the discrete state is higher than that in the Gaussian state when the transmission distance is between 20km and 70km, i.e. the application of the discrete state in short-distance key transmission is more extensive.
The above description is only for the preferred embodiment of the present invention, and is not intended to limit the scope of the present invention. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention shall fall within the protection scope of the present invention.
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