CN113422653B - Quantum communication system without polarization feedback and quantum secure direct communication method - Google Patents

Abstract

The invention relates to a quantum communication system without polarization feedback and a quantum secure direct communication method, wherein the quantum communication system comprises a communication receiving module, a communication sending module and a quantum channel between the two parties; a first phase modulator in the communication receiving module can prepare photons in four phases randomly for the communication sending module, and a single photon detector detects the photons returned by the communication sending module to obtain quantum bits; the communication sending module comprises an encoding light path module and a safety detection module. The invention avoids the influence of polarization drift caused by birefringence by using the consistency of the coding optical path and the stability of the safety detection optical path, realizes the self-compensation of the polarization drift of the coding optical path, and reduces the quantum bit error rate, thereby improving the communication quality.

Description

A quantum communication system without polarization feedback and a quantum-safe direct communication method

technical field

The invention relates to the technical field of quantum information, in particular to a quantum communication system without polarization feedback and a quantum safe direct communication method.

Background technique

Quantum secure direct communication is a new type of communication mode proposed by Long Guilu et al. in 2002. It breaks the traditional dual-channel model, and both parties use only one quantum channel to transmit information. Quantum secure direct communication is a combination of classical coding theory and eavesdropping channel theory. It does not require the use of quantum memory or the generation of keys to encrypt information. It realizes efficient, real-time, and secure information transmission.

Quantum secure direct communication needs to meet two requirements: 1. After the communication receiver receives the quantum state as the information carrier, it can directly read the confidential information sent by the sender without exchanging additional classical auxiliary information with the communication sender; 2. The eavesdropper has eavesdropped on the quantum channel and cannot obtain any confidential information.

Compared with traditional optical communication, quantum-secure direct communication is very different. The traditional optical communication uses strong light as the information carrier. When the strong light is modulated in phase, polarization and amplitude, the influence of the surrounding environment changes on the system is relatively limited. However, quantum secure direct communication uses single photon, and environmental changes and system noise have a great influence on single photon, which is easy to cause polarization drift of single photon, so it needs to be improved.

In the current system model, the two parties of quantum secure direct communication use single-mode optical fiber to connect, and the field environment of optical fiber installation as quantum channel is usually complex and unstable. As photons travel through the fiber channel, the inherent birefringence of single-mode fiber causes the polarization of the photons to drift, and this drift changes with environmental fluctuations such as temperature or other environmental influences. As a result, the polarization state of the photon becomes unpredictable when it reaches the receiving end, resulting in the degradation of the performance of the polarization-sensitive quantum secure direct communication system. In the existing system models, a polarization controller is usually used to manually modulate the polarization drift, which often increases the complexity of the system and the difficulty of experimental operation.

The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person of ordinary skill in the art.

SUMMARY OF THE INVENTION

In view of this, the technical problem to be solved by the present invention is how to provide a quantum communication system without polarization feedback and a quantum safe direct communication method, so as to solve the need to manually modulate and operate the polarization drift in the existing quantum safe direct communication. difficult problem.

In order to solve the above technical problems, the present invention provides a quantum communication system without polarization feedback, comprising a communication receiving module (Bob), a communication sending module (Alice) and a quantum channel (channel) between the two parties;

The communication receiving module includes a laser source, an attenuator, a first circulator, a first beam splitter, a second beam splitter, a first phase modulator, a first single photon detector and a second single photon detector; The laser source is connected to the attenuator, the attenuator is connected to the first circulator, the first circulator is connected to the first beam splitter, and the first beam splitter is respectively connected to the The first phase modulator is connected to the second beam splitter, the first single photon detector is connected to the first circulator, and the second single photon detector is connected to the first beam splitter the laser source, the attenuator, the first circulator, the first beam splitter, the second beam splitter, the first phase modulator, the first single photon detection The connection between the detector and the second single photon detector is through a polarization maintaining fiber;

The communication sending module includes a filter coupler, an encoding optical path module and a safety detection optical path module;

The encoding optical path module includes a first polarization-maintaining beam splitter, a second phase modulator and a Faraday rotator; the first polarization-maintaining beam splitter is respectively connected to the second phase modulator and the Faraday rotator, the second phase modulator is connected with the Faraday rotator; the connections between the first polarization maintaining beam splitter, the second phase modulator and the Faraday rotator are all connected through polarization maintaining fibers;

The security detection optical path module includes a second circulator, a second polarization-maintaining beam splitter, a third polarization-maintaining beam splitter, a third beam splitter, a fourth beam splitter, a third phase modulator, and a third single-phase modulator. a photon detector and a fourth single-photon detector; the second circulator is respectively connected to the second polarization-maintaining beam splitter and the third single-photon detector through a single-mode fiber; the third polarization-maintaining splitter The beam splitter is connected to the fourth single-photon detector through a single-mode fiber; the second polarization-maintaining beam splitter is connected to the third beam splitter and the fourth beam splitter, and the third beam splitter and the fourth beam splitter, the third phase modulator, and the third polarization-maintaining beam splitter are respectively connected, and the fourth beam splitter is connected to the third phase modulator, the third The polarization maintaining beam splitters are respectively connected; between the second circulator, the third beam splitter, the fourth beam splitter, the third phase modulator, and the third polarization maintaining beam splitter The connections are all connected by polarization-maintaining fibers;

The filter coupler and the first polarization maintaining beam splitter are connected by a single-mode fiber delay line, and the filter coupler and the second circulator are connected by a single-mode fiber;

The quantum channel is a single-mode fiber, and the second beam splitter in the communication receiving module and the filter coupler in the communication sending module are connected through a single-mode fiber.

Preferably, the laser source is a laser diode, and the first single-photon detector, the second single-photon detector, the third single-photon detector and the fourth single-photon detector are all superconducting Nanowire single-photon detectors.

A method for quantum-safe direct communication using the above quantum communication system, the method comprising:

Step S1, the laser source emits a light beam, the light beam passes through the attenuator to form a single-photon pulse, and the single-photon pulse is divided into two beams after reaching the first beam splitter; The arm is transmitted to the second beam splitter, set P _{1 short} ; another beam of light along the long arm is modulated in phase by the first phase modulator and then transmitted to the second beam splitter, set P _{1 long} ; The second beam splitter combines the two beams into one beam;

Step S2, the second beam splitter sends the combined beam of light to the filter coupler, the filter coupler divides the beam into two beams, and one beam is sent to the security detection optical path module, another beam of light is sent to the encoded optical path module along a single-mode fiber delay line;

In step S3, the light beam reaching the safety detection optical path module passes through the second circulator, and is then divided into two orthogonal polarization components by the second polarization maintaining beam splitter, wherein the horizontal polarization beam is set to P _2H , and the vertical beam is set to P 2H . The polarization beam is set to P _2V ;

The horizontally polarized light beam P _2H is transmitted to the third beam splitter, and is split into two beams by the third beam splitter, one beam is transmitted to the fourth beam splitter along the short arm, set P _{2H short} , and the other A beam along the long arm is modulated in phase by the third phase modulator and then transmitted to the fourth beam splitter, set as P _{2H long} ; the beam P _{2H short} and the beam P _{2H long} pass through the fourth beam splitter are respectively divided into two beams, one beam is reflected back to the second polarization-maintaining beam splitter, and the other beam is transmitted to the third polarization-maintaining beam splitter;

The vertically polarized light beam P _2V is transmitted to the fourth beam splitter, split into two beams by the fourth beam splitter, one beam is transmitted to the third beam splitter along the short arm, set P _{2V short} , and the other A beam along the long arm is modulated by the third phase modulator and then transmitted to the third beam splitter, which is set as P _{2V long} ; the beam P _{2V short} and the beam P _{2V long} pass through the third beam splitter are respectively divided into two beams, one beam is reflected back to the second polarization-maintaining beam splitter, and the other beam is transmitted to the third polarization-maintaining beam splitter;

When the light beams interfere when passing through the second polarization-maintaining beam splitter and the third polarization-maintaining beam splitter respectively, the third single-photon detector and the fourth single-photon detector can detect the amount of the beam In the case of interference with the light beam, if the detection bit error rate is higher than or equal to the preset threshold, the encoding optical path does not encode information; if the detection bit error rate is less than the preset threshold, the encoding optical path performs encoding information;

Step S4, the light beam reaching the encoding optical path module is divided into two orthogonal polarization components by the first polarization-maintaining beam splitter, wherein the horizontal polarization state light beam is set as P _1H , and the vertical polarization state light beam is set as P _1V ;

The horizontally polarized light beam P _1H passes through the second phase modulator and the Faraday rotator in sequence, and the light beam is rotated by the Faraday rotator by 90°, set as P _1HV , and then reflected back to the first polarization-maintaining beam splitter ; The vertical polarization beam P _1V first passes through the Faraday rotator and is rotated by 90°, set as P _1VH , passes through the second phase modulator, and is then reflected back to the first polarization-maintaining beam splitter; the beam P _1HV and beam P _1VH are combined into a beam of light in the first polarization maintaining beam splitter, and reflected back to the second beam splitter along the optical path, and the second beam splitter divides it into two beams, one beam along the optical path. The short arm is transmitted back to the first beam splitter, set as P _{return short} , and the other beam passes through the first phase modulator and then transmitted back to the first beam splitter, set as P _{return long} , two beams The interference result that occurs through the first beam splitter is detected by the second single photon detector.

四种态随机调制相位；光束P_1短比光束P_1长先到达所述第二分束器，所述第二分束器将光束P_1短和光束P_1长首尾相连合为一束光。In a possible implementation manner, in step S1: the single-photon pulse satisfies the Poisson distribution; the length of the light beam P _{1 is} used by the first phase modulator

The four states are randomly modulated in phase; the _shorter beam P1 than the _longer beam P1 first reaches the second beam splitter, and the second beam splitter combines the _short beam P1 and the _long beam P1 end-to-end into one beam .

In a possible implementation manner, in step S2: the light beam sent to the security detection optical path module arrives first, and the light beam sent to the encoding light path module arrives later.

两种态对光束的每一个光子进行随机调制。In a possible implementation manner, in step S3: the length of the light beam P _{2H and the length} of the light beam P _2V are both used by the third phase modulator when passing through the third phase modulator respectively.

The two states randomly modulate each photon of the beam.

则所述第三单光子探测器响 应，若相位差为0则所述第四单光子探测器响应；若探测比特错误率高于或等于预先设定 的阙值，则编码光路不编码信息，若探测比特错误率小于预先设定的阙值，则编码光路执 行编码信息。In a possible implementation manner, in step S3: the light beams interfere when passing through the second polarization-maintaining beam splitter and the third polarization-maintaining beam splitter respectively; if the phase difference between the two interfered beams is

Then the third single-photon detector responds, and if the phase difference is 0, the fourth single-photon detector responds; if the detection bit error rate is higher than or equal to the preset threshold, the encoding optical path does not encode information, If the detected bit error rate is less than a preset threshold, the encoding optical path performs encoding information.

In a possible implementation manner, in step S4:

When the light beam P _1H and the light beam P _1VH respectively pass through the second phase modulator, each photon is modulated by the second phase modulator with two states of {0, π};

The beam P _1HV and the beam P _1VH are combined into one beam at the first polarization-maintaining beam splitter, and reflected back to the second beam splitter along the optical path. The second beam splitter divides it into two beams, one beam. The beam is transmitted back to the first beam splitter along the short arm, and is set as P _{return short} , and the other beam passes through the first phase modulator and then is transmitted back to the first beam splitter, set as P _{return long} , two beams Interference occurs when passing through the first beam splitter. If the phase difference between the two beams of light is π, the first single-photon detector responds. If the phase difference between the two beams of light is 0, the second single-photon detector responds. When the detector responds, the communication receiving module can know the operations and qubits performed by the communication sending module on the light beam.

The quantum communication system of the present invention uses polarization-maintaining optical fibers to connect the internal structures of both communication parties, and adopts a symmetric structure for the security detection optical path module, so that the polarization state of the light beam is stable during internal transmission, and the sensitivity of the system to polarization is reduced; the present invention utilizes the round-trip of the light beam. The optical path consistency can avoid the influence of single-mode fiber birefringence, and the encoding optical path realizes polarization drift self-compensation, which not only improves the system performance, but also reduces the difficulty of experimental operation and the qubit error rate, thereby improving the communication quality.

Other features and aspects of the present invention will become apparent from the following detailed description of exemplary embodiments with reference to the accompanying drawings.

Description of drawings

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate exemplary embodiments, features and aspects of the invention and together with the description, serve to explain the principles of the invention.

Fig. 1 shows the structure diagram of the quantum communication system provided by the present invention;

2 shows a schematic diagram of the safety detection optical path provided by the present invention;

3 shows a schematic diagram of an encoded optical path provided by the present invention;

FIG. 4 shows a flow chart of quantum-safe direct communication provided by the present invention.

In the figure, 1-communication receiving module, 2-quantum channel, 3-communication sending module, 4-encoding optical path module, 5-safety detection optical path module, 6-laser source, 7-attenuator, 8-first circulator , 9 - the first beam splitter, 10 - the first phase modulator, 11 - the second beam splitter, 12 - the first single photon detector, 13 - the second single photon detector. 14-filter coupler, 15-first polarization maintaining beam splitter, 16-second phase modulator, 17-Faraday rotator, 18-second circulator, 19-second polarization maintaining beam splitter, 20-th Three beam splitters, 21 - third phase modulator, 22 - third polarization maintaining beam splitter, 23 - fourth beam splitter, 24 - third single photon detector, 25 - fourth single photon detector.

Detailed ways

The specific embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings, but it should be understood that the protection scope of the present invention is not limited by the specific embodiments.

In order to make the purposes, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments These are some embodiments of the present invention, but not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention. Unless expressly stated otherwise, throughout the specification and claims, the term "comprising" or its conjugations such as "comprising" or "comprising" and the like will be understood to include the stated elements or components, and Other elements or other components are not excluded.

The word "exemplary" is used exclusively herein to mean "serving as an example, embodiment, or illustration." Any embodiment described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments.

In addition, in order to better illustrate the present invention, numerous specific details are given in the following detailed description. It will be understood by those skilled in the art that the present invention may be practiced without certain specific details. In some instances, methods, means and elements well known to those skilled in the art have not been described in detail so as to highlight the subject matter of the present invention.

Example 1

FIG. 1 shows a structural diagram of a quantum communication system without polarization feedback provided by Embodiment 1 of the present invention. As shown in FIG. 1 , the quantum communication system of the present invention includes a communication receiving module 1, a communication sending module 3 and a quantum channel between the two parties. 2;

The communication receiving module 1 includes a laser source 6, an attenuator 7, a first circulator 8, a first beam splitter 9, a second beam splitter 11, a first phase modulator 10, a first single photon detector 12 and a second The single photon detector 13; the laser source 6 is connected to the attenuator 7, the attenuator 7 is connected to the first circulator 8, the first circulator 8 is connected to the first beam splitter 9, and the first beam splitter 9 is respectively connected to the first The phase modulator 10 is connected with the second beam splitter 11, the first circulator 8 is connected with a first single photon detector 12, the first beam splitter 9 is connected with a second single photon detector 13; the laser sources 6, The connection between the attenuator 7, the first circulator 8, the first beam splitter 9, the second beam splitter 11, the first phase modulator 10, the first single photon detector 12 and the second single photon detector 13 is Connected by polarization maintaining fiber;

The communication sending module 3 includes a filter coupler 14, an encoding optical path module 4 and a safety detection optical path module 5;

The encoded optical path module 4 includes a first polarization maintaining beam splitter 15, a second phase modulator 16 and a Faraday rotator 17; the first polarization maintaining beam splitter 15 is connected to the second phase modulator 16 and the Faraday rotator 17 respectively, and the first polarization maintaining beam splitter 15 is respectively connected to the second phase modulator 16 and the Faraday rotator 17 The two-phase modulator 16 is connected with the Faraday rotator 17; the connections between the first polarization-maintaining beam splitter 15, the second phase modulator 16 and the Faraday rotator 17 are all connected through polarization-maintaining fibers;

The security detection optical path module 5 includes a second circulator 18, a second polarization-maintaining beam splitter 19, a third polarization-maintaining beam splitter 22, a third beam splitter 20, a fourth beam splitter 23, and a third phase modulator 21. The third single-photon detector 24 and the fourth single-photon detector 25; the second circulator 18 is respectively connected to the second polarization-maintaining beam splitter 19 and the third single-photon detector 24 through a single-mode fiber; The polarization beam splitter 22 is connected to the fourth single-photon detector 25 through a single-mode fiber; the second polarization maintaining beam splitter 19 is connected to the third beam splitter 20 and the fourth beam splitter 23, and the third beam splitter 20 is connected to the The fourth beam splitter 23, the third phase modulator 21, and the third polarization maintaining beam splitter 22 are respectively connected, and the fourth beam splitter 23 is respectively connected with the third phase modulator 21 and the third polarization maintaining beam splitter 22; The connections between the second circulator 18, the third beam splitter 20, the fourth beam splitter 23, the third phase modulator 21, and the third polarization-maintaining beam splitter 22 are all connected through polarization-maintaining fibers;

The filter coupler 14 and the first polarization maintaining beam splitter 15 are connected through a single-mode fiber delay line, and the filter coupler 14 and the second circulator 18 are connected through a single-mode fiber;

The quantum channel 2 is a single-mode fiber, and the second beam splitter 11 in the communication receiving module 1 and the filter coupler 14 in the communication sending module 3 are connected through a single-mode fiber.

The laser source 6 is a laser diode, and the first single-photon detector 12 , the second single-photon detector 13 , the third single-photon detector 24 and the fourth single-photon detector 25 are all superconducting nanowire single-photon detectors.

A method for quantum secure direct communication using the above quantum communication system, the method comprising:

四种态随机调制相位后传输到第二分束器11，设为P_1长；光束P_1短比光束P_1长先到达第二分束器11，第二分束器 11将将光束P_1短和光束P_1长首尾相连合为一束光；In step S1, the laser source 6 emits a light beam, and the light beam passes through the attenuator 7 to form a single-photon pulse satisfying the Poisson distribution, and the single-photon pulse is divided into two beams after reaching the first beam splitter 9; one of the beams is transmitted along the short arm. to the second beam splitter 11, set P _{1 short} ; the other beam is used by the first phase modulator 10 along the long arm

The four states are randomly modulated in phase and then transmitted to the second beam splitter 11, and set as P _{1 long} ; the beam P _{1 is shorter} than the beam P _{1 and} reaches the second beam splitter 11 first, and the second beam splitter 11 will split the beam P _{1 short} and beam P _{1 long} are combined end to end to form a beam;

In step S2, the second beam splitter 11 sends the combined beam of light to the filter coupler 14, and the filter coupler 14 divides the beam into two beams, one of which is sent to the security detection optical path module 5, and the other beam is sent to the safety detection optical path module 5. along the single-mode fiber delay line and sent to the encoding optical path module 4; the light beam sent to the safety detection optical path module 5 arrives first, and the light beam sent to the encoding optical path module 4 arrives later;

In step S3, the light beam reaching the security detection optical path module 5 passes through the second circulator 18, and is then divided into two orthogonal polarization components by the second polarization maintaining beam splitter 19, wherein the horizontal polarization beam is set as P _2H , and the vertical polarization beam is set as P 2H . Set to P _2V ;

The horizontally polarized light beam P _2H is transmitted to the third beam splitter 20, and is split into two beams by the third beam splitter 20. One beam is transmitted to the fourth beam splitter 23 along the _short arm. The long arm is modulated in phase by the third phase modulator 21 and then transmitted to the fourth beam splitter 23, set as P _{2H long} ; the _short beam P _{2H and the long} beam P 2H are divided into two parts respectively when passing through the fourth beam splitter 23. beams, one beam is reflected back to the second polarization maintaining beam splitter 19, and the other beam is transmitted to the third polarization maintaining beam splitter 22;

The vertically polarized light beam P _2V is transmitted to the fourth beam splitter 23, and is divided into two beams by the fourth beam splitter 23, one beam is transmitted to the third beam splitter 20 along the short arm, set P _{2V short} , and the other beam The long arm is transmitted to the third beam splitter 20 after modulating the phase by the third phase modulator 21, and is set to be P _{2V long} ; the light beam P _{2V short} and the light beam P _{2V long} are divided into two parts respectively when passing through the third beam splitter 20. beams, one beam is reflected back to the second polarization maintaining beam splitter 19, and the other beam is transmitted to the third polarization maintaining beam splitter 22;

两种态对光束的每一个光子进行随机调制；Among them, the length of the light beam P _{2H and the length} of the light beam P _2V are both used by the third phase modulator 21 when they pass through the third phase modulator 21 respectively.

The two states randomly modulate each photon of the beam;

则第三单光子探测器24响应，若相位差为0则第四单光子探测器25响应；若探测比特错误率高于或等于预先设定的阙值，则编码光路不编码信息，若探测比特错误率小于预先设定的阙值，则编码光路执行编码信息；The light beams interfere when passing through the second polarization-maintaining beam splitter 19 and the third polarization-maintaining beam splitter 22 respectively, then the third single-photon detector 24 and the fourth single-photon detector 25 can detect the amount of the beam and the difference between the beam interference. If the phase difference between the two interfering beams is

Then the third single-photon detector 24 responds, and if the phase difference is 0, the fourth single-photon detector 25 responds; if the detected bit error rate is higher than or equal to the preset threshold, the encoded optical path does not encode information. If the bit error rate is less than the preset threshold, the encoding optical path executes encoding information;

Step S4, the light beam reaching the encoding optical path module 4 is divided into two orthogonal polarization components by the first polarization maintaining beam splitter 15, wherein the horizontal polarization state light beam is set as P _1H , and the vertical polarization state light beam is set as P _1V ;

The horizontally polarized light beam P _1H passes through the second phase modulator 16 and the Faraday rotator 17 in turn, and the light beam is rotated by the Faraday rotator 17 by 90°, set as P _1HV , and then reflected back to the first polarization maintaining beam splitter 15; the vertical polarization state The beam P _1V first passes through the Faraday rotator 17 and is rotated by 90°, set as P _1VH , then passes through the second phase modulator 16 , and is then reflected back to the first polarization-maintaining beam splitter 15 ; the beam P _1HV and the beam P _1VH are A polarization maintaining beam splitter 15 combines a beam of light, which is reflected back to the second beam splitter 11 along the optical path, the second beam splitter 11 divides it into two beams, and one beam is transmitted back to the first beam splitter 9 along the short arm , set P to be _short , the other beam first passes through the first phase modulator 10 and then transmitted back to the first beam splitter 9, and set P to be _long , two beams interfere when passing through the first beam splitter 9, if the two beams pass through the first beam splitter 9 If the phase difference between the beams of light is π, the first single-photon detector 12 responds. If the phase difference between the two beams of light is 0, the second single-photon detector 13 responds, and the communication receiving module 1 can know that the communication sending module 3 is paired The operations and qubits performed by the beam;

Wherein, when the light beam P _1H and the light beam P _1VH pass through the second phase modulator 16 respectively, each photon is modulated by the second phase modulator 16 with two states of {0, π};

The foregoing descriptions of specific exemplary embodiments of the present invention have been presented for purposes of illustration and description. These descriptions are not intended to limit the invention to the precise form disclosed, and obviously many changes and variations are possible in light of the above teachings. The exemplary embodiments were chosen and described for the purpose of explaining certain principles of the invention and its practical application, to thereby enable one skilled in the art to make and utilize various exemplary embodiments and various different aspects of the invention. Choose and change. The scope of the invention is intended to be defined by the claims and their equivalents.

The device embodiments described above are only illustrative, wherein the units described as separate components may or may not be physically separated, and the components shown as units may or may not be physical units, that is, they may be located in One place, or it can be distributed over multiple network elements. Some or all of the modules may be selected according to actual needs to achieve the purpose of the solution in this embodiment. Those of ordinary skill in the art can understand and implement it without creative effort.

Claims (8)

1. a quantum communication system without polarization feedback, is characterized in that, comprises the quantum channel between communication receiving module, communication sending module and both parties; The communication receiving module includes a laser source, an attenuator, a first circulator, a first beam splitter, a second beam splitter, a first phase modulator, a first single photon detector and a second single photon detector; The laser source is connected to the attenuator, the attenuator is connected to the first circulator, the first circulator is connected to the first beam splitter, and the first beam splitter is respectively connected to the The first phase modulator is connected to the second beam splitter, the first single photon detector is connected to the first circulator, and the second single photon detector is connected to the first beam splitter the laser source, the attenuator, the first circulator, the first beam splitter, the second beam splitter, the first phase modulator, the first single photon detection The connection between the detector and the second single photon detector is through a polarization maintaining fiber; The communication sending module includes a filter coupler, an encoding optical path module and a safety detection optical path module; The encoding optical path module includes a first polarization-maintaining beam splitter, a second phase modulator and a Faraday rotator; the first polarization-maintaining beam splitter is respectively connected to the second phase modulator and the Faraday rotator, The second phase modulator is connected to the Faraday rotator; the connections between the first polarization-maintaining beam splitter, the second phase modulator and the Faraday rotator are all connected through polarization-maintaining fibers; The security detection optical path module includes a second circulator, a second polarization-maintaining beam splitter, a third polarization-maintaining beam splitter, a third beam splitter, a fourth beam splitter, a third phase modulator, and a third single-phase modulator. a photon detector and a fourth single-photon detector; the second circulator is respectively connected to the second polarization-maintaining beam splitter and the third single-photon detector through a single-mode fiber; the third polarization-maintaining splitter The beam splitter is connected to the fourth single-photon detector through a single-mode fiber; the second polarization-maintaining beam splitter is connected to the third beam splitter and the fourth beam splitter, and the third beam splitter and the fourth beam splitter, the third phase modulator, and the third polarization-maintaining beam splitter are respectively connected, and the fourth beam splitter is connected to the third phase modulator, the third The polarization maintaining beam splitters are respectively connected; between the second circulator, the third beam splitter, the fourth beam splitter, the third phase modulator, and the third polarization maintaining beam splitter The connections are all connected by polarization-maintaining fibers; The filter coupler and the first polarization-maintaining beam splitter are connected through a single-mode fiber delay line, and the filter coupler and the second circulator are connected through a single-mode fiber; The quantum channel is a single-mode fiber, and the second beam splitter in the communication receiving module and the filter coupler in the communication sending module are connected through a single-mode fiber. 2. The quantum communication system according to claim 1, wherein the laser source is a laser diode, the first single-photon detector, the second single-photon detector, and the third single-photon detector Both the device and the fourth single-photon detector are superconducting nanowire single-photon detectors. 3. A method for quantum-safe direct communication utilizing the quantum communication system according to claim 1, wherein the method comprises: Step S1, the laser source emits a beam, the beam passes through the attenuator to form a single-photon pulse, and the single-photon pulse is divided into two beams after reaching the first beam splitter; one beam of light is transmitted along the short arm to the second beam splitter, set P _{1 short} ; another beam of light along the long arm is modulated in phase by the first phase modulator and then transmitted to the second beam splitter, set P _{1 long} ; so The second beam splitter combines the two beams into one beam; Step S2, the second beam splitter sends the combined beam of light to the filter coupler, the filter coupler divides the beam into two beams, and one beam is sent to the security detection optical path module, another beam of light is sent to the encoded optical path module along the single-mode fiber delay line; In step S3, the light beam reaching the safety detection optical path module passes through the second circulator, and is then divided into two orthogonal polarization components by the second polarization maintaining beam splitter, wherein the horizontal polarization beam is set to P _2H , and the vertical beam is set to P 2H . The polarization beam is set to P _2V ; The horizontally polarized light beam P _2H is transmitted to the third beam splitter, and is split into two beams by the third beam splitter, one beam is transmitted to the fourth beam splitter along the short arm, set P _{2H short} , and the other A beam along the long arm is modulated in phase by the third phase modulator and then transmitted to the fourth beam splitter, set as P _{2H long} ; the beam P _{2H short} and the beam P _{2H long} pass through the fourth beam splitter are respectively divided into two beams, one beam is reflected back to the second polarization-maintaining beam splitter, and the other beam is transmitted to the third polarization-maintaining beam splitter; The vertically polarized light beam P _2V is transmitted to the fourth beam splitter, split into two beams by the fourth beam splitter, one beam is transmitted to the third beam splitter along the short arm, set P _{2V short} , and the other A beam along the long arm is modulated by the third phase modulator and then transmitted to the third beam splitter, which is set as P _{2V long} ; the beam P _{2V short} and the beam P _{2V long} pass through the third beam splitter are respectively divided into two beams, one beam is reflected back to the second polarization-maintaining beam splitter, and the other beam is transmitted to the third polarization-maintaining beam splitter; When the light beams interfere when passing through the second polarization-maintaining beam splitter and the third polarization-maintaining beam splitter respectively, the third single-photon detector and the fourth single-photon detector can detect the amount of the beam In the case of interference with the light beam, if the detection bit error rate is higher than or equal to the preset threshold, the encoding optical path does not encode information; if the detection bit error rate is less than the preset threshold, the encoding optical path performs encoding information; Step S4, the light beam reaching the encoding optical path module is divided into two orthogonal polarization components by the first polarization-maintaining beam splitter, wherein the horizontal polarization state light beam is set as P _1H , and the vertical polarization state light beam is set as P _1V ; The horizontally polarized light beam P _1H passes through the second phase modulator and the Faraday rotator in sequence, and the light beam is rotated by the Faraday rotator by 90°, set as P _1HV , and then reflected back to the first polarization-maintaining beam splitter ; The vertical polarization beam P _1V first passes through the Faraday rotator and is rotated by 90°, set as P _1VH , passes through the second phase modulator, and is then reflected back to the first polarization-maintaining beam splitter; the beam P _1HV and beam P _1VH are combined into one beam at the first polarization maintaining beam splitter and reflected back to the second beam splitter along the optical path, and the second beam splitter divides it into two beams, one beam along the optical path. The short arm is transmitted back to the first beam splitter, set as P _{return short} , and the other beam passes through the first phase modulator and then transmitted back to the first beam splitter, set as P _{return long} , two beams The interference result that occurs through the first beam splitter is detected by the second single photon detector. 4. The method according to claim 3, characterized in that, in step S1: the single-photon pulse satisfies the Poisson distribution; the _length of the light beam P1 is used by the first phase modulator

The four states are randomly modulated in phase; the _shorter beam P1 than the _longer beam P1 first reaches the second beam splitter, and the second beam splitter combines the _short beam P1 and the _long beam P1 end-to-end into one beam . 5 . The method according to claim 3 , wherein, in step S2 : the light beam sent to the security detection optical path module arrives first, and the light beam sent to the encoding light path module arrives later. 6 . 6. The method according to claim 3, wherein in step S3: the length of the light beam P _{2H and the length} of the light beam P _2V are both used by the third phase modulator when passing through the third phase modulator respectively.

The two states randomly modulate each photon of the beam. 7. The method according to claim 3, wherein in step S3: the light beams interfere when passing through the second polarization-maintaining beam splitter and the third polarization-maintaining beam splitter respectively; If the phase difference of the beam is

Then the third single-photon detector responds, and if the phase difference is 0, the fourth single-photon detector responds; if the detection bit error rate is higher than or equal to the preset threshold, the encoding optical path does not encode information, If the detected bit error rate is less than a preset threshold, the encoding optical path performs encoding information. 8. according to the described method of claim 3, it is characterized in that, in step S4: When the light beam P _1H and the light beam P _1VH respectively pass through the second phase modulator, each photon is modulated by the second phase modulator with two states of {0, π}; The beam P _1HV and the beam P _1VH are combined into one beam at the first polarization-maintaining beam splitter, and reflected back to the second beam splitter along the optical path. The second beam splitter divides it into two beams, one beam. The beam is transmitted back to the first beam splitter along the short arm, and is set as P _{return short} , and the other beam passes through the first phase modulator and then is transmitted back to the first beam splitter, set as P _{return long} , two beams Interference occurs when passing through the first beam splitter. If the phase difference between the two beams of light is π, the first single-photon detector responds. If the phase difference between the two beams of light is 0, the second single-photon detector responds. When the detector responds, the communication receiving module can know the operations and qubits performed by the communication sending module on the light beam.

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