CN113162767B

CN113162767B - Heterodyne measurement-based four-state quantum key distribution method and system

Info

Publication number: CN113162767B
Application number: CN202110661084.3A
Authority: CN
Inventors: 尹华磊; 刘文博; 陈增兵
Original assignee: Nanjing University
Current assignee: Nanjing University
Priority date: 2020-11-16
Filing date: 2021-06-15
Publication date: 2023-03-10
Anticipated expiration: 2041-06-15
Also published as: CN113162767A; CN112311541A

Abstract

The invention provides a heterodyne measurement-based four-state quantum key distribution method and a heterodyne measurement-based four-state quantum key distribution system. And after the encoding of the transmitting and receiving parties is finished, performing classical error correction, calculating the security key rate in a privacy amplification link by adopting a key rate calculation method based on convex optimization and dual problems, performing privacy amplification based on the calculated security key rate, and finally obtaining the security key. The invention reduces the difficulty of phase precision preparation and phase feedback; on the other hand, the system has few types of sending signal light and is easy to prepare, the system is simplified, the safe transmission distance and the safe key rate of other continuous variable quantum key distribution systems can be surpassed, and the key distribution with simplicity, safety, efficiency and longer distance is realized.

Description

Heterodyne measurement-based four-state quantum key distribution method and system

Technical Field

The invention relates to the field of quantum key distribution, in particular to a heterodyne measurement-based four-state quantum key distribution method and a heterodyne measurement-based four-state quantum key distribution system.

Background

The quantum key distribution is matched with the one-time pad technology, so that the unconditional and safe secret communication of information in theory can be realized. Under the condition that the confidentiality of the traditional classical cryptosystem is threatened by the parallel operation capability of a quantum computer, quantum key distribution becomes an optimal scheme.

The distribution of the discrete variable quantum key proposed earlier needs to utilize a certain discretization degree of freedom of a single photon, so that the final safe key rate is not enough to meet the existing communication requirements under the constraint of the problems of large single photon channel loss, insufficient precision of measuring equipment and the like. The subsequent continuous variable quantum key distribution based on Gaussian modulation theoretically utilizes continuous freedom and can carry more than one bit of information; each signal light produces codeable data using interferometric measurements with intense light. Therefore, the secure key rate of continuous variable quantum key distribution has the capability of surpassing the secure key rate of discrete variable quantum key distribution in a short distance. In addition, the equipment required by the continuous variable quantum key distribution is basically the same as the equipment used by the existing optical communication system, and the distribution system is easier to combine with the existing optical communication system, so that the distribution system has more beneficial prospect and is easier to realize in a metropolitan area range than the distribution system of the discrete variable quantum key.

However, the security of the continuous variable quantum key distribution has been highly dependent on the condition that the regular component of the optical pulse must be a continuous gaussian distribution, because the continuous gaussian distribution makes the overall quantum state of the distribution system possess mathematically strong symmetry-U (n) symmetry (symmetry of an n-dimensional unitary matrix). Multiple synchronization, feedback methods for continuous variable quantum key distribution also often require that the data have a gaussian distribution to be able to be implemented. However, although the regular component can be theoretically made to conform to the continuous gaussian distribution by gaussian modulation, the continuous gaussian distribution cannot be prepared in practice, and only the gaussian distribution can be discretized. The scattered different signal states are enough, and dozens of different signal states are often needed to construct the symmetry allowed by the error. In addition, accurate measurements are needed to calculate the true covariance and thus a trustworthy key rate. These experimental techniques are still immature, and the error estimation for this security is large, which in turn results in that the key rate can only surpass the discrete variable quantum key distribution over a short distance, and quickly decays to 0 over a long distance.

Although many persons skilled in the relevant art consider that a discrete modulation continuous variable key distribution system is implemented by transmitting fewer signal states, the fewer signal states means that the U (n) symmetry caused by gaussian modulation is broken, and the distribution system loses the security protected by the symmetry. Most discrete modulation schemes either assume that the channel is a linear channel, thereby limiting the attack means of an attacker, or assume that gaussian attack is the optimal attack method of the attacker, thereby reusing relevant security proof of gaussian modulation. In the face of discrete modulation, however, this assumption is mathematically proven to approach the true safe encoding rate lower limit well only when the intensity goes towards 0. The light intensity of the signal light used in actual distribution cannot be so small, otherwise the transmission distance is obviously limited; however, if the light intensity is not small enough, the lower limit is too loose, the encoding rate is attenuated to 0 rapidly, and finally, the encoding rate and the transmission distance are both low, and even not enough to exceed the discrete variable quantum key distribution.

In practice, the existing continuous variable quantum key distribution system usually needs to jointly modulate the amplitude (amplitude) and phase of a pulse to be able to modulate a signal state to a state of realizing key information encoding, so that the operation is relatively complex. Meanwhile, a continuous variable quantum key distribution system generally uses continuous laser to cooperate with at least two intensity modulators to realize preparation of pulses, so that more equipment is provided, and much noise is easily introduced; the unstable condition of the actual voltage is not favorable for maintaining the stable continuous laser of intensity, and the chopping of the continuous light into pulses is extinction, which wastes a large amount of energy. In a continuous variable quantum key distribution system, a phase compensation value needs to be obtained through complex detection and calculation and then acts on relevant equipment of the system, so that a fixed phase difference is kept between a signal state and local oscillator light, and the real-time performance is poor for high-speed pulses; or to reduce as much as possible the environmental changes that cause phase drift, but not to essentially eliminate it.

Disclosure of Invention

The invention aims to: the invention provides a quantum Key Distribution method and a quantum Key Distribution system for realizing four-state resolution by utilizing heterodyne measurement, wherein a Key rate calculation method (Jie Lin, twoshUpdhyaya, and Norbert Lutkehaus, asymmetric secure Analysis of relationship-Modulated Continuous-variable Quantum Key Distribution, phys.Rev.X 9, 041064) for solving the problems of convex optimization and dual is introduced into a four-state protocol, and the Discrete modulation quantum Key Distribution protocol provided by the method is improved to be free from strict limitation on signal light phases (the central phases of four signal lights and the local oscillator light central phase are required to be kept fixed)

) And the phase difference between the signal light and the local oscillator light only affects the operation of data post-processing, but does not reduce the code rate. After the limitation is removed, the difficulty of accurate phase preparation is reduced, meanwhile, the continuous phase compensation calculation is not needed in the measurement process and is applied to relevant equipment of the system, and the difficulty of phase feedback is reduced; on the other hand, the system has few types of signal light for sending and is easy to prepare, the system is simplified, noise introduced by complex equipment is reduced, the system can surpass the safe transmission distance and the safe key rate of other continuous variable quantum key distribution systems, and key distribution with simplicity, safety, efficiency and longer distance is realized.

The technical scheme is as follows: in order to achieve the purpose, the invention provides the following technical scheme:

a heterodyne measurement-based four-state quantum key distribution method comprises the following steps:

(1) The method comprises the steps that local oscillator light and original signal light are prepared at a sending end, then the original signal light is prepared into one of four kinds of signal light in an equal probability and random mode, the degrees of freedom of the local oscillator light and the degrees of freedom of the signal light except for phase amplitude are differentiated, and then the local oscillator light and the signal light are subjected to phase amplitude differentiationThen the local oscillator light and the signal light are combined and then sent to a receiving end through a quantum channel; in each round of code forming process, four kinds of signal light meet the following conditions: the phase of any one of the signal lights is taken as a reference phase, and the phase differences between the phases of the other three signal lights and the reference phase are respectively 90 degrees, 180 degrees and 270 degrees; the local oscillator light satisfies: the phase of the local oscillator light forms a fixed phase difference with the reference phase

；

(2) The method comprises the steps that a sending end encodes a signal state corresponding to signal light sent in each coding process into classical bits, wherein the classical bits are initial key character strings of the sending end;

(3) The receiving end re-unifies the degrees of freedom between the local oscillator light and the signal light except for the phase amplitude, the phase of the local oscillator light is taken as a regular coordinate axis, a regular coordinate value and a corresponding regular momentum value of the signal light are obtained through heterodyne measurement, and a complex result is formed by taking the measured regular coordinate as a real part and the measured regular momentum as an imaginary part;

(4) The receiving end maps a plurality of complex results obtained in one-round code formation into signal states corresponding to four signal lights, and encodes the obtained signal states into an initial key character string of the receiving end in the same way as the sending end;

(5) The sending end and the receiving end calculate the safe key rate by adopting a key rate calculation method based on convex optimization and dual problems, then carry out classical error correction on the initial key character string held by the sending end and the receiving end, carry out privacy amplification based on the calculated safe key rate, and finally obtain the safe key.

Several alternatives are provided below for the above method, but not as an additional limitation to the above general solution, but only as a further addition or preference, each alternative being combinable individually for the above general solution or among several alternatives without technical or logical contradictions.

Optionally, in the step (3), the step of mapping the complex result into a signal state specifically includes:

1) Constructing a complex coordinate system by taking the local oscillator optical phase before heterodyne measurement as 0 phase, and calculating the phase difference between the reference phase and the 0 phase

Is a phase angle of

For the rest three phase angles, four rays corresponding to the four phase angles are led out from the origin of the complex coordinate system;

2) Four fan-shaped areas are formed by respectively taking the four rays as centers, and each fan-shaped area corresponds to one signal state;

3) And mapping each complex result to a complex coordinate system, and obtaining a signal state corresponding to each complex result according to the sector area in which the complex result falls.

Optionally, the specific step of calculating the security key rate by using the key rate calculation method based on convex optimization and dual problems includes:

constructing a key rate calculation model:

wherein R is ^∞ Representing the key rate, p _AB Representing a combined density matrix, S representing a constraint condition, H (rho | sigma) representing the information amount which is still unknown by an attacker after being attacked by means of mutual entropy, and obtaining the lower limit of the minimum value after solving by a convex optimization algorithm;

is a mapping that relates a density matrix of quanta to classical bits,

indicating that the shrinking quantum channel is to

The mapping result of (2) is projected on the classical bit, and the correlation of different classical bit results is abandoned；δ _EC Representing the bit loss, δ, caused by bit error correction of classical data _EC = 1- β H (Z) - β H (Z | X), where β is the error correction efficiency, H (Z) is the information entropy of Z, and H (Z | X) is the information entropy of Z in the case where X is known; pr (Pr) of _save Indicating the probability that data of a certain pulse is retained for generating a key.

The invention also provides a heterodyne measurement-based four-state quantum key distribution system, which is used for realizing the method and comprises a sending end and a receiving end;

the transmitting end comprises: pulse light source module, n:1, a light splitting module, a signal local oscillator dissimilarity module, a four-state preparation module, a merging output module and a sending end control module; the pulse light source module is used for preparing high-speed pulse laser; n:1, preparing high-speed pulse laser into original signal light and local oscillator light by a light splitting module; the signal local oscillator dissimilarity module selects and modulates a plurality of degrees of freedom of the original signal light and the local oscillator light except for phase amplitude, so that the original signal light and the local oscillator light can be distinguished according to the degrees of freedom after being transmitted through the same optical path; the four-state preparation module randomly prepares original signal light into the four kinds of signal light; the combining output module combines the four kinds of signal light and the local oscillator light into one path and outputs the path to a receiving end; the sending end control module controls other modules of the sending end to realize respective functions and carries out key negotiation with the receiving end control module;

the receiving end includes: the device comprises a transmission calibration module, a signal local oscillator de-diversification module, a heterodyne measurement four-state resolution module and a receiving end control module; the transmission calibration module is used for receiving the combined light and eliminating parameter drift caused by the combined light in quantum channel transmission; the signal local oscillator de-dissimilarity module separates the signal light and the local oscillator light and restores the degree of freedom of the signal light and the local oscillator light modulated by the signal local oscillator dissimilarity module to the original state; the heterodyne measurement four-state distinguishing module measures the four signal lights to obtain a regular coordinate taking a local oscillation state phase as a reference and a corresponding regular momentum, and distinguishes the four signal states according to the regular coordinate and the regular momentum; the receiving end control module controls other modules of the receiving end to realize respective functions and carries out key agreement with the sending end control module.

For the above system, several alternatives are provided below, and each alternative may be combined separately for the above overall scheme, or may be combined among multiple alternatives, without technical or logical contradictions.

Optionally, the receiving end includes an electronic polarization controller, first to third beam splitters, first to fourth detectors, first to second polarization maintaining polarization beam splitters, and first to second differential amplifiers, which are connected by a polarization maintaining fiber; wherein, the first and the second end of the pipe are connected with each other,

the electronic polarization controller receives the optical pulse sent by the sending end, corrects the polarization of the optical pulse, rotates the polarization corresponding to the local oscillation light to the fast axis of the optical fiber for transmission, and transmits the polarization corresponding to the signal light to the slow axis of the optical fiber;

the first beam splitter evenly divides the calibrated light into two beams which are respectively sent into the first polarization-maintaining polarization beam splitter and the second polarization-maintaining polarization beam splitter;

the first polarization-preserving polarization beam splitter separates signal light and local oscillator light components in the light beam according to local oscillator light reflection and signal light transmission, rotates the local oscillator light polarization by 90 degrees, keeps the local oscillator light and the signal light polarization consistent, and then respectively transmits the signal light and the local oscillator light to the second beam splitter through two sections of optical fibers with different lengths, so that the local oscillator light phase is increased

The second beam splitter conducts interference operation of local oscillation light and signal light, the two beams of interfered light are respectively sent to a first detector and a second detector for measurement, and the measurement results of the first detector and the second detector are sent to a first differential amplifier for differential amplification to obtain regular momentum p;

the second polarization-maintaining polarization beam splitter separates the signal light and the local oscillator light in the light beam according to local oscillator light reflection and signal light transmission, rotates the local oscillator light polarization by 90 degrees to keep the local oscillator light and the signal light polarization consistent, and then respectively transmits the signal light and the local oscillator light to the third beam splitter through two sections of optical fibers with different lengths to enable the local oscillator light and the signal light to simultaneously enter the third beam splitter; and the third beam splitter performs local oscillation light and signal light interference operation, the two beams of interfered light are respectively sent to a third detector and a fourth detector for measurement, and the measurement results of the third detector and the fourth detector are sent to a second differential amplifier for differential amplification to obtain a regular coordinate q.

Optionally, the transmitting end includes a pulse laser connected by a polarization maintaining fiber, n:1, a beam splitter, a phase modulator and a polarization-maintaining polarization beam splitter; wherein, the first and the second end of the pipe are connected with each other,

the pulse laser sends pulses to the n:1 a beam splitter;

n:1, a beam splitter splits a pulse into original signal light and local oscillation light, the original signal light is sent to a phase modulator, and the local oscillation light is sent to a polarization-preserving polarization beam splitter;

the phase modulator is used for carrying out phase modulation on the original signal light to obtain signal light corresponding to any one signal state, and then the signal light is sent to the polarization-preserving polarization beam splitter;

the signal light and the local oscillation light respectively pass through optical fibers with different lengths from the beam splitter to the polarization-preserving polarization beam splitter, so that the time when the current signal light reaches the polarization-preserving polarization beam splitter is positioned in the middle of the time when the current local oscillation light reaches the polarization-preserving polarization beam splitter next time; and the polarization-maintaining polarization beam splitter rotates the polarization of the signal light after phase modulation by 90 degrees and reflects the signal light, directly transmits the local oscillation light, and finally sends the beam combining light to a sending end through a single-mode optical fiber.

Optionally, the transmitting end includes a master laser, a first slave laser, a second slave laser, a beam splitter, a first circulator, a second circulator, a fixed attenuator, a phase modulator, and a polarization-preserving polarization beam splitter; wherein the content of the first and second substances,

the master laser generates seed light and divides the seed light into two parts through the beam splitter, wherein one part is injected into the first slave laser through the first circulator, and the other part is injected into the second slave laser through the second circulator;

generating original signal light by a first slave laser, transmitting the original signal light to a fixed attenuator for attenuation through a first circulator, transmitting the attenuated light beam to a phase modulator for phase modulation to form signal light corresponding to any one signal state, and transmitting the generated signal light to a polarization-preserving polarization beam splitter;

the second slave laser generates local oscillation light, and the local oscillation light is sent to the polarization-preserving polarization beam splitter through the second circulator;

and the polarization-maintaining polarization beam splitter combines the local oscillation light and the signal light and transmits the combined signal to a receiving end through a single-mode optical fiber.

Optionally, the transmitting end includes a master laser, a first slave laser, a second slave laser, a beam splitter, a first circulator, a second circulator, a fixed attenuator, and a polarization-preserving polarization beam splitter; wherein the content of the first and second substances,

under the input voltage, the master laser generates a beam of seed light in each period and divides the seed light into two parts through the beam splitter, wherein one part is injected into the first slave laser through the first circulator, and the other part is injected into the second slave laser through the second circulator;

the second slave laser realizes injection locking based on seed light input by the master laser in the first half period of each period to generate local oscillator light, and the local oscillator light is sent to the polarization-preserving polarization beam splitter through the second circulator;

the first slave laser realizes injection locking based on seed light input by the master laser in the second half period of each period to generate signal light, the signal light is sent into the fixed attenuator through the first circulator to be attenuated, and the attenuated signal light is sent into the polarization-preserving polarization beam splitter;

and the polarization-maintaining polarization beam splitter combines the local oscillation light and the signal beam and transmits the combined beam to a receiving end through a single-mode optical fiber.

The technical effects are as follows: compared with the existing continuous variable quantum key distribution scheme, the method has the following advantages:

(1) The sending end only needs to prepare four signal states with different phases, so that the coding can be carried out only by modulating the phases, and the combination of an amplitude modulation device and a phase modulation device is not needed to realize the coding, thereby greatly reducing the requirements on equipment in the system;

(2) The signal state only needs to satisfy the following condition: the phase of any one of the signal lights is taken as a reference phase, and the phase differences between the phases of the other three signal lights and the reference phase are respectively 90 degrees, 180 degrees and 270 degrees; therefore, when preparing signal light, it is not necessary to limit the phase relation between two adjacent signal lights, so that it can be separated from the limitation of selecting continuous laser whose phase must be stable, and can select continuous laser or pulse light source whose phase is irrelevant.

(3) In the invention, the local oscillator light only needs to ensure that a fixed phase difference is formed between the phase and the reference phase within the short time of one-time code forming

Namely, phase difference

The value of (A) is not specific and can be arbitrary. Therefore, the operation of adjusting the state of the system equipment in real time according to the phase feedback result can not be introduced into the channel, so that the preparation difficulty and the phase feedback difficulty of signals and local oscillators are reduced, and the system is further simplified.

Drawings

FIG. 1 is a light path diagram of the system module of the present invention;

fig. 2 is a schematic diagram of an optical path structure according to a first embodiment of the present invention;

fig. 3 is a schematic diagram of an optical path structure according to a second embodiment of the present invention;

fig. 4 is a schematic diagram of an optical path structure in a third embodiment of the present invention.

Detailed Description

The invention provides a four-state quantum key distribution method, which reduces the degree of dependence of the phase between local oscillator light and signal light, but does not reduce the code rate, but reduces the difficulty of phase feedback, and the corresponding system can be simpler, so that the code rate is improved.

The four-state quantum key distribution method is described as follows:

firstly, preparing local oscillator light and four kinds of signal light by a sending end; the local oscillator light is considered to be classical strong light, and the signal light is coherent light with a much smaller number of photons than the classical strong light. The signal light prepared in this step satisfies the following conditions: taking the phase of one signal light as a reference phase, the phase difference between the phases of the other three signal lights and the reference phase is 90 degrees, 180 degrees and 270 degrees; the local oscillator light prepared in the step meets the following requirements: in a short time enough to form a round of code formation, the phase of the local oscillator light and the reference phase have a fixed phase difference, but the specific value can be random in different rounds without control.

Secondly, combining the local oscillator light and the signal light in a one-to-one correspondence manner, and then sending the combined beams to a receiving party through a quantum channel for measurement; in the step, the local oscillator light and the signal light have differentiability in one or more degrees of freedom except for phase amplitude, so that photon leakage in transmission can be reduced, and a receiver can be conveniently separated and used.

Thirdly, the receiving end reunites the degrees of freedom between the local oscillator light and the signal light except for the phase amplitude, the phase of the local oscillator light is taken as a regular coordinate axis, and two measuring results are obtained by a heterodyne measuring method.

Fourthly, after the data is generated, the receiving end estimates the phase in the one-round code forming, so that the corresponding relation between the measuring result and the secret key can be determined, and the secret key is generated; then calculating the rate of the security key, performing classical error correction and privacy amplification; the final security key is obtained.

The invention also provides a four-state quantum key distribution system matched with the method, which consists of three parts, namely a sending end (Alice), a channel transmission part and a receiving end (Bob), wherein the three parts are separated in space and have a front-back relation in time. This is explained in more detail below in connection with fig. 1:

the sending end comprises a pulse light source module and n:1 light splitting module, signal local oscillator dissimilarity module, four state preparation module and merge output module. In addition, the control area which is used for carrying out correlation regulation and control on each module, carrying out post-processing on data and carrying out classical communication with a receiving end is not directly embodied in a light path diagram because the control area does not directly act with light pulses.

The channel transmission part comprises a quantum channel and a classical channel, the quantum channel is realized by optical fibers, free space and other paths depending on the space separation condition between the transmitting end and the receiving end, the quantum channel is used for transmitting the output pulse prepared by the transmitting end to the receiving end in time, and the classical channel is used for transmitting classical information.

And the receiving end comprises a transmission calibration module, a signal local oscillator de-diversification module and a heterodyne measurement four-state resolution module. In addition, the receiving end is also provided with a control area for performing correlation regulation and control on each module, performing post-processing on data and performing classical communication with the transmitting end, and the control area is not directly embodied in a light path diagram for simplifying the drawing.

The functions to be implemented by each module and the devices implementing the corresponding functions are explained below:

the function of the pulse light source module is to prepare high-speed pulse laser with stable light intensity, fixed time sequence interval and extremely small, and to stably input and connect to n:1 slow axis of polarization maintaining fiber of the splitting module. It is particularly emphasized that the phase between the pulsed light may be independent, and the system implementation is independent of the specific phase of the pulsed light. Devices that can accomplish this include, but are not limited to, an inter-modulation pulse laser, an electro-absorption modulation pulse laser. Preferably, a fixed attenuator can be added after the laser to remove stray light and improve the extinction ratio.

n: the function of the optical splitting module 1 is to prepare original signal light and local oscillator light which have the same phase and have a plurality of magnitude differences in photon number at this moment, and devices which can realize the function include but are not limited to one n: the beam splitter of 1 can also use a combination of a polarizing film which can deflect a small-angle deflection part of slow axis light into a fast axis and a polarization-maintaining polarization beam splitter, and finally can also realize the preparation of two lights with the same phase and a plurality of magnitude differences of photon numbers and output the two lights to two polarization-maintaining optical fibers respectively, wherein the light with the small photon number is signal light, and the light with the large photon number is local oscillation light. Preferably, a fixed attenuation can be inserted into the polarization maintaining fiber to which the signal light leads, so as to further expand the photon number difference of the two beams and achieve the most favorable distribution ratio.

The function of the signal local oscillator dissimilarity module is to realize the distinguishability of the signal light and the local oscillator light after being transmitted on the same optical path. Further, several degrees of freedom of the signal light and the local oscillator light except for the phase amplitude are selected and modulated respectively, so that the two have distinguishable characteristics in the degrees of freedom. Devices that can accomplish this include, but are not limited to, different lengths of fiber that can produce time separation, polarization maintaining polarization splitters that make the polarizations orthogonal, acousto-optic modulators that make small differences in frequency.

Since n:1, the original signal light and the local oscillator light output by the optical splitting module have changed and drifted phase when they are transmitted in the optical fiber, so after the local oscillator signal is converted into an abnormal signal, the four-state preparation module is required to randomly prepare the signal light into a signal light with a phase difference of 0,

π、

The four kinds of signal light. Suppose that the phase difference between the original signal light and the corresponding local oscillator light is

The phase difference between the prepared four signal lights and the corresponding local oscillator lights is

Can take any value between 0,2 pi), but because the phase difference of the four signal lights prepared by us is all

Multiple of (1), thus

Each time exceeding

All have a phase difference between the signal light and the local oscillator light

Therefore, it is assumed in this embodiment that the phase difference between the reference signal light and the local oscillation light selected by us is the forward minimum. In addition, the phase difference between the four kinds of signal light and the original signal light can be uniformly increased by an angle, but the angle can be equivalent to that the original signal light and the corresponding local oscillator light are differentiated by the angle and then prepared according to the angle, namely the angle is still included

The device implementing this function preferably requires only one phase modulator, but not the only device system implemented. Particularly, taking a phase modulator as the module as an example, the phase modulator may be placed between extended optical fibers through which signal states pass, thereby implementing preparation of four kinds of signal light in the process of implementing functional time separation of the local oscillation dissimilatory module.

The function of the merging output module is to merge the signal light and the local oscillator light into one path and stably input the signal light and the local oscillator light into a quantum channel provided by the channel transmission part. Devices that perform this function include, but are not limited to, a polarization maintaining polarizing beamsplitter. Particularly, the device can meet the requirement of combining the output module to synthesize the light beam, and meanwhile, the polarization-maintaining positive beam splitter rotates the polarization of the signal light by 90 degrees, so that the signal light is reflected when passing through the device, and the local oscillator light is directly transmitted out of the device, namely, the polarization of the signal light and the polarization of the local oscillator light are orthogonalized, so that the functional requirement of the signal local oscillator diversification module is met.

The function of the transmission calibration module is to eliminate parameter drift caused by quantum channel transmission with long optical path as completely as possible. The apparatus for this function includes, but is not limited to, an electronic polarization controller that re-aligns the polarization drift due to the channel and accurately inputs light with orthogonal polarizations into the slow and fast axes of the polarization maintaining fiber, respectively.

The function of the signal local oscillator de-differentiation module is the reverse operation of the signal local oscillator differentiation module, and the signal local oscillator de-differentiation module finally outputs two beams of light of the signal light and the local oscillator light respectively, and the degrees of freedom of the signal light and the local oscillator light except for the phase amplitude are regressed to be in the same state. According to the selection of the signal local oscillator dissimilarity module, carrying out optical fiber extension on the channel transmitted by the light with the selected short optical fiber, wherein the length of the extension is the same as the length of the extension shortened at the transmitting end, and the length of the selection is different from the length of the channel transmitted by the light with the selected short optical fiber; tuning back the same with polarization-preserving polarizing beam splitters with different selective polarizations, and so on.

The heterodyne measurement four-state resolution module has the function of efficiently and accurately resolving four signal states with different phases. Devices that may be used include, but are not limited to, beam splitters, photodetectors, and differential amplifiers. The heterodyne measurement simultaneously obtains the regular coordinate taking the phase of the local oscillation state as the reference and the corresponding regular momentum, and the two results are combined together to construct four different conditions respectively corresponding to the four signal states, so that the four signal states can be efficiently and accurately distinguished.

In addition, the control area located at the transmitting end performs association regulation and control on each module of the transmitting end, performs post-processing on data, and performs classical communication with the receiving end, and the control area needs to have the following capabilities: the method comprises the steps that an attacker cannot access the key, random number strings of specific preparation phases at each time are generated to serve as original keys, samples are extracted randomly, corresponding measurement results of the samples are obtained through classical communication, unconditional safe key rate is calculated through a safe key rate calculation method integrating a convex optimization algorithm according to the measurement results, and the key is extracted by combining error correction and privacy amplification of a receiving end.

The control area located at the receiving end for performing association regulation and control on each module of the receiving end, performing post-processing on data, and performing classical communication with the transmitting end needs to have the following capabilities: the method comprises the steps that an attacker cannot access the key, a number string with different phases and different numbers in one-to-one correspondence is generated according to a measurement result and serves as an original key, partial measurement results are given through classical communication according to requirements of a sending end, and the sending end is combined to correct errors and amplify privacy to extract the key.

It should be noted that all the lines in fig. 1 do not represent the distance of the optical path between any actual systems. For any of the connections of the optical paths of the system embodying the invention to the device, polarization maintaining fibers are used in both the transmitting end and the receiving end, respectively. The specific connections of the polarization maintaining fibers vary for each module and for the specific requirements of polarization between modules, and have been described in the module functions and embodiments, if necessary. If not, the slow axes of the polarization-maintaining optical fibers connected with the front end and the rear end of the equipment are aligned by default.

The present invention is further illustrated in detail below with reference to three examples.

The first embodiment is as follows:

fig. 2 shows an optical diagram of a specific apparatus for connecting a transmitting end (Alice) and a receiving end (Bob) via an unsecured single mode fiber. This embodiment provides a device and a connection case with the minimum required device in the optical path included in the system of the present invention. In this embodiment, the sending end Alice includes an inner modulation pulse laser, and one of the inner modulation pulse lasers may be preferably 999:1, a phase modulator and a polarization-maintaining polarization beam splitter, and connecting them according to an optical path diagram by using polarization-maintaining optical fibers.

The internal modulation pulse laser sends high-speed and light-intensity stable pulses to the optical fiber slow axis by stably adding direct current and alternating current voltage, and the function of the pulse light source module is realized.

The beam splitter splits the pulse in the slow axis into signal light and local oscillation light which are respectively input into the slow axis, and n:1 spectral module function.

The split weak light as signal light enters a phase modulator through an optical fiber, and the phase modulator uniformly and randomly performs on the signal light

One of the phase rotation is recorded as {1,2,3 and 4} according to the phase rotation, and the slow axis of the optical fiber is input again, so that the function of the four-state preparation module is realized.

The signal light and the local oscillator light are generated from the beam splitter and then reach the polarization-preserving polarization beam splitter, the lengths of the passing optical fibers are different, the optical fiber through which the signal light passes in the section is longer than the local oscillator light, but the signal light does not reach the polarization-preserving polarization beam splitter later than the next local oscillator light but is in the middle of the time when the local oscillator light and the next local oscillator light reach the polarization-preserving polarization beam splitter; meanwhile, the polarization-maintaining polarization beam splitter is utilized to rotate the signal light for 90 degrees in polarization and then reflect the signal light, and the local oscillation light is directly transmitted. Therefore, the function of the signal local oscillator dissimilarity module in the time degree of freedom and the polarization degree of freedom is realized, and the function of the merging output module is also completed.

And finally, inputting the beam-combining light into the single-mode optical fiber quantum channel by the sending end.

The receiving end comprises an electronic polarization controller, three beam splitters, two polarization-preserving polarization beam splitters, four detectors and two differential amplifiers, and the polarization-preserving fiber is used for connecting the electronic polarization controller, the three beam splitters, the two polarization-preserving polarization beam splitters, the four detectors and the two differential amplifiers according to an optical path diagram.

The electronic polarization controller corrects the polarization of the light pulse passing through the single-mode fiber, so that the light pulse is aligned and input into the polarization-maintaining fiber, in particular, the polarization corresponding to the local oscillation light is rotated to the fast axis for transmission, and the polarization corresponding to the corresponding signal light is transmitted in the slow axis of the fiber, thereby realizing the function required by the transmission calibration module.

The rest equipment, three beam splitters, two polarization-maintaining polarization beam splitters, four detectors and two differential amplifiers form a set of simplest heterodyne measurement four-state resolution module. The first beam splitter is used for uniformly splitting light into two beams of light for generating two outputs for heterodyne measurement, and the first polarization-preserving polarization beam splitter and the second polarization-preserving polarization beam splitter are used for respectively separating signal light and local oscillator light components in the two beams of light according to local oscillator light reflection and signal light transmission and keeping the local oscillator light polarization consistent with the signal light polarization after rotating the local oscillator light polarization by 90 degrees. From the first polarization-preserving polarization beam splitter to the second beam splitter, the optical fiber through which the local oscillation light passes is longer than the signal light, the longer part is the optical fiber length shorter than the signal light at the Alice end, and meanwhile, a short length is supplemented to increase the phase of the local oscillation light

The time delay of the local oscillation light and the signal light enter the second beam splitter in the same time window, and the local oscillation light is additionally added

For measuring the regular momentum p. The local oscillation light between the second polarization-maintaining polarization beam splitter and the third beam splitter is longer than the signal light in the passing optical fiber, the longer part is the optical fiber length shorter than the signal light at the Alice end, so that the local oscillation light is delayed in time and enters the third beam splitter together with the signal light, but the local oscillation light does not additionally change the phase and is used for measuring the regular coordinate q. The second beam splitter and the third beam splitter respectively perform local oscillation light and signal light interference operation, and output measurement results through corresponding detectors and differential amplifiers, wherein the measurement results are two real numbers, and a complex number result is formed by taking a regular coordinate result as a real part and a regular momentum result as an imaginary part. With the phase of the non-rotated local oscillation light as 0 phase (positive real axis), the phase difference between the signal light having the reference phase and the non-rotated local oscillation light is determined

Is a phase angle, and

for the phase angle, four rays are extracted from the complex spatial origin. Phase difference

Comprising a phase difference during preparation

And phase drift introduced in the channel transmission. The four rays are respectively taken as the center, and the complex number result is increased or decreased in a certain ray

Given the same key signature, to

Increase or decrease for center

Within is denoted by 1, to

Increase or decrease for center

Within is denoted by 2, to

Increase or decrease for center

Within is denoted by 3, to

Increase or decrease for center

The symbol within is 4. Preferably, estimating

The resulting phase error can be reduced by eliminating data in a small range near the boundary of different key signatures, thereby reducing the error rate. Preferably, to further reduce the error rate, the measurement results falling in the small area with radius a around the origin may not be used to generate {1,2,3,4} key string, but only to be published when randomly extracted for parameter estimation and key rate calculation. The specific value of the radius a should be optimized by the result of the key rate calculation. It can be seen that while the heterodyne measurement four-state resolution module is realized, the function of the signal local oscillator de-differentiation module is also realized together.

Through the optical path, all module functions are realized, required data are collected, and key distribution can be realized as long as the related functions of the control area described in the specific embodiment are operated. The following describes in more detail how data is processed in turn in the control area and key distribution is implemented to make the construction and use of the system of the present invention more complete.

Parameter estimation is first achieved by randomly disclosing partial data results. These parameters are then used to calculate the security key rate and error correction and privacy amplification of the key. If the key rate calculation result is too small or even negative, the data is discarded and the operation is started from the pulse light source module again. The mathematical description of the specific calculation method is as follows:

recording original signal light sent by pulse light source module as

Rotate the phase by

Respectively, is recorded as | mu ₁ >、|μ ₂ >、|μ ₃ >、|μ ₄ >And the events of transmitting the four kinds of signal light are recorded as |1 respectively>、|2>、|3>、|4>Thus, the transmitting end is equivalent to prepare an entangled state

Therein

The portion is sent to the receiving end and,

after a complete positive and trace-keeping transformation in the channel, the channel becomes B, and then a joint density matrix rho of the whole system light path part is obtained _AB . Since an attacker may exist in a channel transmission part, the conversion specific form of complete positive definition and trace-keeping is unknown, but the measurement result of the heterodyne measurement four-state resolution module ensures rho _AB Some constraints are satisfied. These constraints, denoted as S, include but are not limited to: 1. the nature of the density matrix, trace 1 and positive semidefinite; 2. fully positive and vestigial transforms require p _AB Calculating the bias trace and p for the middle pair B _AA′ The results of the bias trace calculation of the middle A' are consistent; 3. the expected value (average value) of the corresponding measurement operator acting on the density matrix is obtained from the randomly published part of the measurement results. The density matrix meeting the constraint conditions is not unique and cannot reversely deduce the specific attack of an attacker, but the key rate formula and the density matrix variable meet convex optimization, and the most favorable result for the attacker, namely the key rate lower limit, can be searched according to a convex optimization algorithm.

The progressive secret key rate formula is R ^∞ ＝Pr _save (I(X；Z)-max _ρ∈S χ (Z: E)), wherein R ^∞ Is the key rate, pr _save The probability of reserving data of a certain pulse for generating a key is represented and depends on the size of an area with a radius a deducted when the area is optimized, and I (X; Z) is classical mutual information, wherein X refers to a key string of a transmitting end and Z refers to a key string of a receiving end and indicates that the two have classical information quantity with the same value at corresponding positions; and χ (Z: E) is Holevo information and represents the degree of understanding of Z by the attacker E, which means that the inverse coordination based on the receiving end key string Z is used. ρ is ρ _AB Is a density matrix that satisfies the constraint S described above.

The formula can be further rewritten into

The former term H (ρ | | | σ) is a mutual entropy indicating the amount of information still unknown to the attacker after being attacked, and the lower limit of the minimum value is obtained by the convex optimization algorithm. Therein are

Is a mapping that relates a density matrix of quanta to classical bits,

then it is the shrinking quantum channel that will

The mapping result of (2) is projected onto the classical bit, and correlation of different classical bit results is abandoned. Delta in the latter term _EC Representing the bit loss, δ, caused by bit error correction of classical data _EC = 1- β H (Z) - β H (Z | X), where β is the error correction efficiency, typically less than 1, H (Z) is the information entropy of Z, and H (Z | X) is the information entropy of Z (i.e., the conditional entropy) given X. Phys.Rev.X 9,041064 considers that the problem of solving the minimum mutual entropy under the constraint S is a mathematical convex optimization problem, and the minimum lower limit is solved by utilizing the existing convex optimization algorithm and the dual theory.

And finally, when the key rate obtained by using the minimum lower limit is greater than 0, performing reverse coordination, performing classical error correction, error verification and privacy amplification, and finally extracting a final key from the original key.

The second embodiment:

fig. 3 shows another optical path diagram of a four-state quantum key distribution system based on heterodyne measurement. As shown in the figure, the present embodiment is different from fig. 2 only in the transmitting end from the viewpoint of the equipment and connection configuration. In particular, the structure of the pulse light source module and the relation between n: the order of implementation of the 1 splitter module is different from that of fig. 2.

Three internal modulation pulse lasers exist in a sending end, wherein one internal modulation pulse laser serves as a master laser and two slave lasers; the polarization beam splitter comprises a beam splitter, two circulators, a fixed attenuator, a phase modulator and a polarization-maintaining polarization beam splitter.

The master laser generates seed light for injecting to the slave laser, the slave first laser generates signal light, and the second slave laser generates local oscillator light. The first circulator and the second circulator respectively induce seed light to be injected from the master laser into the first slave laser and the second slave laser, and then light pulses generated by the first slave laser and the second slave laser are respectively led into different optical fibers connected with the fixed attenuator and directly connected with the polarization-maintaining polarization beam splitter. Note that the beam splitter is placed at the output end of the main laser, and the beam splitter still carries a function of separating the signal light and the local oscillator light, but it actually separates seed light for generating the signal light and the local oscillator light, thereby indirectly separating the signal light and the local oscillator light. Because what produce signal light from first laser instrument utilizes is the principle of laser injection, and the light intensity of production is almost irrelevant with the light intensity of seed light, and the light intensity of production can't produce the contrast with the local oscillator light by strong and weak, consequently need place a fixed attenuator on signal light path and cooperate and realize an n:1, beam splitting effect. That is to say, the pulse light source module includes the signal light and the local oscillator light that seed light and laser injection produced, but produce signal light and local oscillator light by belong to n:1 beam splitting module, thus showing a pulsed light source module and n:1 alternate implementation of splitter modules, it is not possible to strictly distinguish between two modules on a device connection, but it is clear for each device which module functions to implement.

In addition, this method of generating an optical pulse by driving seed light into a slave laser is called injection locking, and has a good phase locking function, that is, a stable phase relationship is provided between the light generated from the laser and the injected light. This phase relationship can be calibrated in advance. The signal light and the local oscillator light respectively keep stable phase relation with the same original seed light, so that a stable phase relation is obtained between the signal light and the local oscillator light. The phase relationship has little drift along with the change of time, and can be regarded as no change in a short time, so that the unknown phase between the local oscillator light and the signal light is explained in the method

And directly processed in phase feedback without independent consideration.

Example three:

in conjunction with fig. 4, this embodiment specifically provides an optical path diagram of a device in which a single device implements the functional requirements of multiple modules. Fig. 4 (a) in fig. 4 of this embodiment shows an apparatus and an optical path diagram at a transmitting end, which uses a transmission channel and a receiving end that are completely the same as those of fig. 3, and therefore, they are not repeated here. Fig. 4 (b) of the present embodiment corresponds to the input voltage diagrams of the first slave laser, the master laser, and the second slave laser in fig. 4 (a) on the left side thereof in order from top to bottom. Note that these three voltage curves are time-aligned, and the three voltage points passed by any vertical line are the voltage levels used by the master laser at this time, and the voltage levels used by the two slave lasers when the light generated by the master laser reaches the slave lasers at this time.

As can be seen from the figure, the master laser may be a continuous laser, to which a stable dc voltage is applied, the dotted line in the figure divides the time axis of the figure into four regions at equal intervals, the voltage is applied to the second slave laser in the first half of each time domain, the light input at the time corresponding to the master laser is injection-locked, and the first half is used to generate local oscillation light; and in the second half part of each time domain, the first slave laser is electrified with voltage, the light input at the moment corresponding to the master laser realizes injection locking, and the second half part is used for generating signal light.

An additional feature exists in the main laser's energized voltage, which places a small voltage ripple on the input voltage of the main laser during the very short middle time deltat of each time domain. The main laser as a continuous laser has a stable central frequency when stably powered on, and when the voltage changes, the central frequency has an offset Δ v, which results in that the phase evolves according to the new central frequency in the change time, and when the voltage returns to the initial stable size again, the central frequency also returns to the original position, and the phase evolution recovers again. However, the phase evolution in delta t is different from the original evolution, so that a phase difference exists between the front part and the rear part of the time domain, and the phase difference can be determined by

It is given. Therefore, when the variation time deltat is given, the frequency deviation deltav can be controlled by controlling the voltage fluctuation, and the generated phase difference can be controlled, when the light before and after injection locking is controlled, a phase difference is generated for the light before and after the injection locking in addition to the phase relation caused by the injection locking

The four different voltage fluctuations in 4 (b) of fig. 4 are qualitative representations of the relationship of voltage magnitude to be used, and do not represent the true voltage magnitude, and the true used voltage fluctuation value is calibrated for a specific laser. In a small voltage fluctuation range, the size of a fluctuation voltage signal and phase deflection are approximately in a linear relationship. Therefore, the front part and the rear part can be respectively used as seed light to generate by adjusting voltage fluctuation

The phase difference of (1). The phase difference of the seed light can be stably reflected on the local oscillation light and the signal light through injection locking, that is, four kinds of voltage fluctuation are randomly selected, and signal states of four kinds of phases can be correspondingly prepared. This is why 4 (a) in fig. 4 is compared with fig. 3, and one phase modulator is absent at the transmitting end. That is, these laser groups simultaneously realize the functions of the pulsed light source module and the four-state preparation module. Meanwhile, because the seed light of the signal light and the local oscillator light is separated in time, the function of the signal local oscillator dissimilarity module is also realized. Therefore, it should be emphasized that the time-domain separation of the signal light and the local oscillator light due to the temporal separation of the seed light also needs to be compensated by the signal local oscillator de-differentiating module at the receiving end. However, the compensation method is only to lengthen or shorten the original compensation fiber, and no equipment is added or connection mode is changed, so that the optical path diagram of the receiving end is not given at a time for simplicity.

It should be noted that the main laser does not need to be a continuous laser, and only the phase difference between the signal light and the corresponding local oscillator light affects the measurement result, so the main laser is changed to a pulse laser having a wider platform as the input voltage, and the pulse laser can be divided into front and rear regions within one platform time to prepare the seed light of the signal light and the local oscillator light, and the above functions can also be implemented.

All the embodiments described above only represent some specific embodiments of the system of the present invention, and the description is specific and detailed, but it should not be understood that the scope of the invention is limited thereby. It should be noted that various changes and modifications can be made by those skilled in the art without departing from the spirit of the invention, and these changes and modifications are all within the scope of the invention. Therefore, the protection scope of the present patent shall be subject to the appended claims.

Claims

1. A four-state quantum key distribution method based on heterodyne measurement is characterized in that: the method comprises the following steps:

(1) The method comprises the steps that a sending end prepares local oscillator light and original signal light, then the original signal light is prepared into one of four kinds of signal light at equal probability and randomly, the degrees of freedom of the local oscillator light and the signal light except for phase amplitude are differentiated, and then the local oscillator light and the signal light are combined and then sent to a receiving end through a quantum channel; in each round of code forming process, four kinds of signal light meet the following conditions: the phase of any one of the signal lights is taken as a reference phase, and the phase differences between the phases of the other three signal lights and the reference phase are respectively 90 degrees, 180 degrees and 270 degrees; the local oscillator light satisfies: the phase of the local oscillator light forms a fixed phase difference with the reference phase

(2) The method comprises the steps that a sending end encodes a signal state corresponding to signal light sent in each code forming process into classical bits, wherein the classical bits are initial key character strings of the sending end;

(5) The sending end and the receiving end calculate the safe key rate by adopting a key rate calculation method based on convex optimization and dual problems, then carry out classical error correction on the initial key character string held by the sending end and the receiving end, carry out privacy amplification based on the calculated safe key rate, and finally obtain a safe key;

the step of calculating the security key rate comprises:

constructing and solving a secret key rate calculation model:

is a mapping that relates a density matrix of quanta to classical bits,

indicating that the shrinking quantum channel is to

The mapping result of (2) is projected onto the classical bits; delta. For the preparation of a coating _EC Representing the bit loss, δ, caused by bit error correction of classical data _EC = 1- β H (Z) - β H (Z | X), where β is the error correction efficiency, H (Z) is the information entropy of Z, and H (Z | X) is the information entropy of Z in the case where X is known; pr (Pr) _save Indicating the probability that data for a certain pulse is retained for generating a key.

2. The heterodyne measurement-based four-state quantum key distribution method as recited in claim 1, wherein: in the step (4), the step of mapping the complex result into a signal state specifically includes:

1) Before heterodyne measurement is carried outConstructing a complex coordinate system by using the local oscillator optical phase as the 0 phase, and calculating the phase difference between the reference phase and the 0 phase

Is a phase angle of

3. A heterodyne measurement based four-state quantum key distribution system, configured to implement the method according to any one of claims 1 to 2, including a transmitting end and a receiving end, and characterized in that:

the transmitting end comprises: the system comprises a pulse light source module, an n:1 light splitting module, a signal local oscillator dissimilarity module, a four-state preparation module, a merging output module and a sending end control module; the pulse light source module is used for preparing high-speed pulse laser; n:1, the light splitting module prepares the high-speed pulse laser into original signal light and local oscillator light; the signal local oscillator dissimilarity module selects and modulates a plurality of degrees of freedom of original signal light and local oscillator light except phase amplitude, so that the original signal light and the local oscillator light can be distinguished according to the degrees of freedom after being transmitted through the same optical path; the four-state preparation module randomly prepares original signal light into the four kinds of signal light; the combining output module combines the four kinds of signal light and the local oscillator light into one path and outputs the path to a receiving end; the sending end control module controls other modules of the sending end to realize respective functions and carries out key agreement with the receiving end control module;

the receiving end includes: the device comprises a transmission calibration module, a signal local oscillator de-dissimilarity module, a heterodyne measurement four-state resolution module and a receiving end control module; the transmission calibration module is used for receiving the combined light and eliminating parameter drift caused by the combined light in quantum channel transmission; the signal local oscillator de-dissimilarity module separates the signal light and the local oscillator light and restores the degree of freedom of the signal light and the local oscillator light modulated by the signal local oscillator dissimilarity module to the original state; the heterodyne measurement four-state distinguishing module measures the four signal lights to obtain a regular coordinate with the local oscillation state phase as a reference and a corresponding regular momentum, and distinguishes the four signal states according to the regular coordinate and the regular momentum; the receiving end control module controls other modules of the receiving end to realize respective functions and carries out key agreement with the sending end control module.

4. The heterodyne measurement-based four-state quantum key distribution system of claim 3, wherein:

the receiving end comprises an electronic polarization controller, first to third beam splitters, first to fourth detectors, first to second polarization-maintaining polarization beam splitters and first to second differential amplifiers which are connected through polarization-maintaining optical fibers; wherein the content of the first and second substances,

the first beam splitter evenly splits the calibrated light into two beams which are respectively sent into the first polarization-preserving polarization beam splitter and the second polarization-preserving polarization beam splitter;

The second beam splitter performs local oscillation light and signal light interference operation, and sends the two beams after interference to the first beam splitter and the second beam splitter respectively,The second detector measures, and the measurement results of the first detector and the second detector are sent to the first differential amplifier for differential amplification to obtain regular momentum p;

the second polarization-maintaining polarization beam splitter separates the signal light and the local oscillator light component in the light beam according to local oscillator light reflection and signal light transmission, rotates the local oscillator light polarization by 90 degrees, keeps the local oscillator light and the signal light polarization consistent, and then respectively transmits the signal light and the local oscillator light to the third beam splitter through two sections of optical fibers with different lengths, so that the local oscillator light and the signal light enter the third beam splitter simultaneously; and the third beam splitter performs local oscillation light and signal light interference operation, the two beams of interfered light are respectively sent to a third detector and a fourth detector for measurement, and the measurement results of the third detector and the fourth detector are sent to a second differential amplifier for differential amplification to obtain a regular coordinate q.

5. The heterodyne measurement-based four-state quantum key distribution system of claim 4, wherein:

the transmitting end comprises a pulse laser, an n:1 beam splitter, a phase modulator and a polarization-maintaining polarization beam splitter which are connected through a polarization-maintaining optical fiber; wherein the content of the first and second substances,

the pulse laser sends pulses to the n:1 beam splitter through the polarization-maintaining optical fiber;

n:1, splitting the pulse into original signal light and local oscillation light by a beam splitter, sending the original signal light to a phase modulator, and sending the local oscillation light to a polarization-preserving polarization beam splitter;

the signal light and the local oscillation light respectively pass through optical fibers with different lengths from the beam splitter to the polarization-preserving polarization beam splitter, so that the time when the current signal light reaches the polarization-preserving polarization beam splitter is positioned between the time when the current local oscillation light reaches the polarization-preserving polarization beam splitter and the time when the current local oscillation light reaches the polarization-preserving polarization beam splitter next time; and the polarization-maintaining polarization beam splitter rotates the polarization of the signal light after the phase modulation by 90 degrees and then reflects the signal light, directly transmits the local oscillation light, and finally sends the combined light to a sending end through a single-mode optical fiber.

6. The heterodyne measurement-based four-state quantum key distribution system of claim 4, wherein:

the transmitting end comprises a master laser, a first slave laser, a second slave laser, a beam splitter, a first circulator, a second circulator, a fixed attenuator, a phase modulator and a polarization-preserving polarization beam splitter; wherein, the first and the second end of the pipe are connected with each other,

generating local oscillation light by the second slave laser, and sending the local oscillation light to the polarization-preserving polarization beam splitter through the second circulator;

7. The heterodyne measurement-based four-state quantum key distribution system of claim 4, wherein:

the transmitting end comprises a master laser, a first slave laser, a second slave laser, a beam splitter, a first circulator, a second circulator, a fixed attenuator and a polarization-preserving polarization beam splitter; wherein the content of the first and second substances,

the first slave laser realizes injection locking based on seed light input by the master laser in the second half period of each period to generate signal light, the signal light is sent to the fixed attenuator through the first circulator to be attenuated, and the attenuated signal light is sent to the polarization-preserving polarization beam splitter;