CN113422653B - Quantum communication system without polarization feedback and quantum secure direct communication method - Google Patents
Quantum communication system without polarization feedback and quantum secure direct communication method Download PDFInfo
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- CN113422653B CN113422653B CN202110676118.6A CN202110676118A CN113422653B CN 113422653 B CN113422653 B CN 113422653B CN 202110676118 A CN202110676118 A CN 202110676118A CN 113422653 B CN113422653 B CN 113422653B
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/70—Photonic quantum communication
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- H—ELECTRICITY
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- H04L9/00—Cryptographic mechanisms or cryptographic arrangements for secret or secure communications; Network security protocols
- H04L9/08—Key distribution or management, e.g. generation, sharing or updating, of cryptographic keys or passwords
- H04L9/0816—Key establishment, i.e. cryptographic processes or cryptographic protocols whereby a shared secret becomes available to two or more parties, for subsequent use
- H04L9/0852—Quantum cryptography
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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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
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 security direct communication method.
Background
The quantum secure direct communication is a novel communication mode proposed by Longgui Lu et al 2002, and breaks through the traditional dual-channel model, and two communication parties only use one quantum channel to transmit information. The quantum secure direct communication is the combination of the classical coding theory and the eavesdropping channel theory, does not need to use a quantum memory, does not need to generate key encryption information, and realizes efficient, real-time and secure information transfer.
Quantum secure direct communication needs to meet two requirements: 1. after receiving the quantum state as the information carrier, the communication receiver can directly read the confidential information sent by the sender without exchanging additional classical auxiliary information with the communication sender; an eavesdropper eavesdrops on the quantum channel and cannot obtain any confidential information.
Quantum secure direct communication is a great difference compared to conventional optical communication. In the conventional optical communication, strong light is used as an information carrier, and when the strong light is subjected to phase, polarization and amplitude modulation, the influence of the change of the surrounding environment on the system is relatively limited. The quantum secure direct communication uses single photons, and environmental changes and system noise have great influence on the single photons, so that the single photons are easy to generate polarization drift, and therefore improvement is needed.
In the existing system model, both sides of quantum secure direct communication are connected by using single-mode optical fiber, and the field environment of optical fiber installation as a quantum channel is usually complex and unstable. When photons are transmitted through a fiber channel, the inherent birefringence of a single-mode fiber shifts the polarization of the photons, which shifts as environmental fluctuations, such as temperature or other environmental influences, change. Therefore, the polarization state becomes unpredictable when photons reach the receiving end, resulting in a degradation of the performance of the polarization sensitive quantum secure direct communication system. In the existing system model, it is common to use a polarization controller to manually modulate the polarization drift, which often increases the complexity of the system and the operational difficulty of the experiment.
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 that is already known to a person skilled in the art.
Disclosure of Invention
In view of this, the technical problem to be solved by the present invention is how to provide a quantum communication system and a quantum secure direct communication method without polarization feedback, so as to solve the problems that the polarization drift needs to be manually modulated and the operation difficulty is large in the existing quantum secure direct communication.
In order to solve the technical problems, the invention provides a quantum communication system without polarization feedback, which comprises a communication receiving module (Bob), a communication sending module (Alice) and a quantum channel (channel) between the communication receiving module (Bob) and the communication sending module (Alice);
the communication receiving module comprises 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 with the attenuator, the attenuator is connected with the first circulator, the first circulator is connected with the first beam splitter, the first beam splitter is respectively connected with the first phase modulator and the second beam splitter, the first circulator is connected with the first single-photon detector, and the first beam splitter is connected with the second single-photon detector; the laser source, the attenuator, the first circulator, the first beam splitter, the second beam splitter, the first phase modulator, the first single-photon detector and the second single-photon detector are connected through polarization-maintaining optical fibers;
the communication sending module comprises a filtering coupler, an encoding light path module and a safety detection light path module;
the coding light path module comprises a first polarization-preserving beam splitter, a second phase modulator and a Faraday rotator; the first polarization-maintaining beam splitter is respectively connected with the second phase modulator and the Faraday rotator, and the second phase modulator is connected with the Faraday rotator; the first polarization-preserving beam splitter, the second phase modulator and the Faraday rotator are connected through polarization-preserving optical fibers;
the safety detection light path module comprises a second circulator, a second polarization-maintaining beam splitter, a third beam splitter, a fourth beam splitter, a third phase modulator, a third single-photon detector and a fourth single-photon detector; the second circulator is respectively connected with the second polarization-maintaining beam splitter and the third single-photon detector through single-mode fibers; the third polarization-maintaining beam splitter is connected with the fourth single-photon detector through a single-mode fiber; the second polarization-maintaining beam splitter is connected with the third beam splitter and the fourth beam splitter, the third beam splitter is respectively connected with the fourth beam splitter, the third phase modulator and the third polarization-maintaining beam splitter, and the fourth beam splitter is respectively connected with the third phase modulator and the third polarization-maintaining beam splitter; the second circulator, the third beam splitter, the fourth beam splitter, the third phase modulator and the third polarization-maintaining beam splitter are all connected through polarization-maintaining optical fibers;
the filter coupler is connected with the first polarization-preserving beam splitter through a single-mode fiber delay line, and the filter coupler is connected with the second circulator through a single-mode fiber;
the quantum channel is a single-mode fiber, and the second beam splitter in the communication receiving module is connected with the filter coupler in the communication transmitting module through the 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 superconducting nanowire single-photon detectors.
The method for quantum secure direct communication by using the quantum communication system comprises the following steps:
step S1, the laser source emits a light beam, the light beam forms a single photon pulse through the attenuator, and the single photon pulse is divided into two beams after reaching the first beam splitter; one beam of light is transmitted to the second beam splitter along the short arm and is set as P 1 short (ii) a The other beam of light is transmitted to the second beam splitter after being modulated in phase by the first phase modulator along the long arm, and the modulation is set as P 1 long (ii) a The second beam splitter combines the two beams of light into one beam;
step S2, the second beam splitter sends the light synthesized into one beam to the filter coupler, the filter coupler splits the light beam into two beams, one beam of light is sent to the safety detection optical path module, and the other beam of light is sent to the coding optical path module along a single-mode fiber delay line;
step S3, the light beam reaching the safety detection optical path module passes through the second circulator and is then split into two orthogonal polarization components by the second polarization maintaining beam splitter, wherein the horizontal polarization state light beam is set as P 2H With the vertically polarized beam set to P 2V ；
Horizontal polarized light beam P 2H Transmitted to the third beam splitter to be transmitted to the third beam splitterThe third beam splitter is divided into two beams, one beam is transmitted to the fourth beam splitter along the short arm and is set as P 2H short The other beam is transmitted to the fourth beam splitter after being modulated in phase by the third phase modulator along the long arm, and is set as P 2H long (ii) a Light beam P 2H short And a light beam P 2H long When passing through the fourth beam splitter, the light beams are respectively split 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;
vertically polarized light beam P 2V Transmitted to the fourth beam splitter and split into two beams by the fourth beam splitter, one beam is transmitted to the third beam splitter along a short arm and is set as P 2V short The other beam passes through the third phase modulator along the long arm to be transmitted to the third beam splitter after being modulated in phase, and the modulation phase is set as P 2V long (ii) a Light beam P 2V short And a light beam P 2V long When passing through the third beam splitter, the light beams are respectively split 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;
if the detection bit error rate is higher than or equal to a preset threshold value, the coded light path does not encode information, and if the detection bit error rate is smaller than the preset threshold value, the coded light path executes the coded information;
step S4, the light beam reaching the encoding light 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 With the vertically polarized beam set to P 1V ；
Horizontal polarized light beam P 1H Sequentially passes through the second phase modulator and the Faraday rotator, and the light beam is rotated by 90 degrees by the Faraday rotator to be set as P 1HV And then reflected back to the first polarizing beamsplitter; vertically polarized light beam P 1V Passes through the Faraday rotator and is rotated by 90 DEG to be set as P 1VH Then passes throughSaid second phase modulator subsequently reflected back to said first polarization-preserving beam splitter; light beam P 1HV And a light beam P 1VH The first polarization-preserving beam splitter synthesizes a beam of light which is reflected back to the second beam splitter along a light path, the second beam splitter splits the beam of light into two beams, one beam is transmitted back to the first beam splitter along a short arm and is set as P Shortening of the return The other beam is transmitted back to the first beam splitter after passing through the first phase modulator, and is set as P Back length And the interference result of the two beams of light passing through the first beam splitter is detected by the second single-photon detector.
In one possible implementation, in step S1: the single photon pulse satisfies Poisson distribution; light beam P 1 length For use by the first phase modulator
Randomly modulating the phase in four states; light beam P 1 short Specific beam P 1 length First reaches the second beam splitter, which splits the beam P 1 short And a light beam P 1 long End-to-end into a beam of light.
In one possible implementation, in step S2: the light beam sent to the safety detection light path module arrives first, and the light beam sent to the coding light path module arrives later.
In one possible implementation, in step S3: light beam P 2H long And a light beam P 2V long Are used by the third phase modulators when passing through the third phase modulators, respectively
The two states randomly modulate each photon of the beam.
In one possible implementation, 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 interfering beams is
Then theResponding by a third single-photon detector, and responding by a fourth single-photon detector if the phase difference is 0; and if the detection bit error rate is higher than or equal to a preset threshold value, the coding light path does not code information, and if the detection bit error rate is smaller than the preset threshold value, the coding light path executes coding information.
In one possible implementation, in step S4:
light beam P 1H And a light beam P 1VH Modulating each photon with {0, pi } two states by the second phase modulator while passing through the second phase modulator, respectively;
light beam P 1HV And a light beam P 1VH The first polarization-preserving beam splitter is combined into a beam of light, the light is reflected back to the second beam splitter along a light path, the second beam splitter splits the light into two beams of light, one beam of light is transmitted back to the first beam splitter along a short arm and is set as P Return to the short The other beam is transmitted back to the first beam splitter after passing through the first phase modulator, and is set as P Back length The two beams of light interfere with each other when passing through the first beam splitter, the first single-photon detector responds if the phase difference of the two beams of light is pi, the second single-photon detector responds if the phase difference of the two beams of light is 0, and the communication receiving module can know the operation and the qubit executed by the communication sending module on the beams of light.
The quantum communication system adopts the polarization maintaining optical fiber connection in the internal structures of both communication parties, and adopts the symmetrical structure in the safety detection light path module, so that the polarization state of light beams is stable during internal transmission, and the polarization sensitivity of the system is reduced; the invention avoids the influence caused by single-mode fiber birefringence by using the consistency of the round-trip optical path of the light beam, realizes polarization drift self-compensation by the coding optical path, improves the system performance, and reduces the difficulty of experimental operation and the quantum bit 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, which proceeds with reference to the accompanying drawings.
Drawings
The accompanying drawings, which are incorporated in and constitute a part of this 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 illustrates a block diagram of a quantum communication system provided by the present invention;
FIG. 2 illustrates a schematic diagram of a safety detection optical path provided by the present invention;
FIG. 3 illustrates a schematic diagram of the encoding light path provided by the present invention;
fig. 4 shows a quantum secure direct communication flow diagram provided by the present invention.
In the figure, 1-communication receiving module, 2-quantum channel, 3-communication sending module, 4-coding optical path module, 5-safety detection optical path module, 6-laser source, 7-attenuator, 8-first circulator, 9-first beam splitter, 10-first phase modulator, 11-second beam splitter, 12-first single-photon detector and 13-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-third beam splitter, 21-third phase modulator, 22-third polarization-maintaining beam splitter, 23-fourth beam splitter, 24-third single photon detector and 25-fourth single photon detector.
Detailed Description
The following detailed description of the embodiments of the present invention is provided in conjunction with the accompanying drawings, but it should be understood that the scope of the present invention is not limited to the specific embodiments.
In order to make the objects, 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 drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention. Throughout the specification and claims, unless explicitly stated otherwise, the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element or component but not the exclusion of any other element or component.
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.
Furthermore, in the following detailed description, numerous specific details are set forth in order to provide a better understanding of the present invention. It will be understood by those skilled in the art that the present invention may be practiced without some of these specific details. In some instances, methods, means, elements well known to those skilled in the art have not been described in detail so as not to obscure the present invention.
Example one
Fig. 1 is a structural diagram of a quantum communication system without polarization feedback according to an embodiment of the present invention, and fig. 1 shows that the quantum communication system of the present invention includes a communication receiving module 1, a communication sending module 3, and a quantum channel 2 between the two parties;
the communication receiving module 1 comprises 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 single-photon detector 13; the laser source 6 is connected with the attenuator 7, the attenuator 7 is connected with the first circulator 8, the first circulator 8 is connected with the first beam splitter 9, the first beam splitter 9 is respectively connected with the first phase modulator 10 and the second beam splitter 11, the first circulator 8 is connected with the first single-photon detector 12, and the first beam splitter 9 is connected with the second single-photon detector 13; the laser source 6, 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 are connected through polarization-maintaining optical fibers;
the communication sending module 3 comprises a filtering coupler 14, an encoding optical path module 4 and a safety detection optical path module 5;
the encoding optical path module 4 comprises a first polarization-preserving beam splitter 15, a second phase modulator 16 and a Faraday rotator 17; the first polarization-preserving beam splitter 15 is respectively connected with a second phase modulator 16 and a Faraday rotator 17, and the second phase modulator 16 is connected with the Faraday rotator 17; the first polarization-preserving beam splitter 15, the second phase modulator 16 and the Faraday rotator 17 are connected through polarization-preserving optical fibers;
the safety detection optical path module 5 comprises 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, a third phase modulator 21, a third single-photon detector 24 and a fourth single-photon detector 25; the second circulator 18 is respectively connected with a second polarization-maintaining beam splitter 19 and a third single-photon detector 24 through single-mode fibers; the third polarization-maintaining beam splitter 22 is connected with the fourth single-photon detector 25 through a single-mode fiber; the second polarization-maintaining beam splitter 19 is connected with the third beam splitter 20 and the fourth beam splitter 23, the third beam splitter 20 is respectively connected with the fourth beam splitter 23, the third phase modulator 21 and the third polarization-maintaining beam splitter 22, and the fourth beam splitter 23 is respectively connected with the third phase modulator 21 and the third polarization-maintaining beam splitter 22; 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 optical fibers;
the filter coupler 14 is connected with the first polarization-preserving beam splitter 15 through a single-mode fiber delay line, and the filter coupler 14 is connected with the second circulator 18 through a single-mode fiber;
the quantum channel 2 is a single-mode optical fiber, and the second beam splitter 11 in the communication receiving module 1 and the filter coupler 14 in the communication transmitting module 3 are connected by the single-mode optical 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 superconducting nanowire single-photon detectors.
A method of quantum secure direct communication using the above quantum communication system, the method comprising:
step S1, the laser source 6 emits a light beam, the light beam forms a single photon pulse satisfying Poisson distribution through the attenuator 7, and the single photon pulse is divided into two beams after reaching the first beam splitter 9(ii) a One of the beams is transmitted along the short arm to the second beam splitter 11, set as P 1 short (ii) a The other beam of light is used by the first phase modulator 10 along the long arm
The four states are transmitted to the second beam splitter 11 after randomly modulating the phases, and are set as P 1 long (ii) a Light beam P 1 short Specific beam P 1 long First reaches the second beam splitter 11, and the second beam splitter 11 will split the light beam P 1 short And a light beam P 1 long End-to-end to form a beam of light;
step S2, the second beam splitter 11 sends the light synthesized into one beam to the filter coupler 14, the filter coupler 14 divides the light beam into two beams, one beam of light is sent to the safety detection light path module 5, and the other beam of light is sent to the coding light path module 4 along the single-mode fiber delay line; the light beam sent to the safety detection light path module 5 arrives first, and the light beam sent to the coding light path module 4 arrives later;
in step S3, the light beam reaching the safety detection optical path module 5 passes through the second circulator 18 and is then split into two orthogonal polarization components by the second polarization-maintaining beam splitter 19, where the horizontally polarized light beam is set as P 2H With the vertically polarized beam set to P 2V ；
Horizontal polarized light beam P 2H Transmitted to the third beam splitter 20, is split into two beams by the third beam splitter 20, one beam is transmitted along the short arm to the fourth beam splitter 23, set as P 2H short The other beam is modulated in phase by the third phase modulator 21 along the long arm and transmitted to the fourth beam splitter 23, where P is set 2H long (ii) a Light beam P 2H short And a light beam P 2H long While passing through the fourth beam splitter 23, the light beams are respectively split into two 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;
vertically polarized light beam P 2V Transmitted to the fourth beam splitter 23 and split into two by the fourth beam splitter 23, one transmitted along the short arm to the third beam splitter 20, set to P 2V short The other beam passes through the third phase modulator 21 along the long arm and is transmitted to the third beam splitter 20 after being modulated in phase, and the modulation is set as P 2V long (ii) a Light beam P 2V short And a light beam P 2V long While passing through the third beam splitter 20, the light beams are respectively split into two 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;
wherein the light beam P 2H long And a light beam P 2V long While passing through the third phase modulators 21, are used by the third phase modulators 21
The two states randomly modulate each photon of the beam;
when the light beams interfere with each other when passing through the second polarization-maintaining beam splitter 19 and the third polarization-maintaining beam splitter 22, the third single-photon detector 24 and the fourth single-photon detector 25 can detect the amount of the light beams and the interference of the light beams, and if the phase difference between the two interfering light beams is equal to
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 a preset threshold value, the coding light path does not code information, and if the detected bit error rate is smaller than the preset threshold value, the coding light path executes coding information;
in step S4, the light beam reaching the encoding optical path module 4 is split into two orthogonal polarization components by the first polarization maintaining beam splitter 15, where the horizontally polarized light beam is set as P 1H With the vertically polarized beam set to P 1V ；
Horizontal polarized light beam P 1H Sequentially passes through the second phase modulator 16 and the Faraday rotator 17, and the light beam is rotated by 90 DEG by the Faraday rotator 17 to be P 1HV And then reflected back to the first polarizing beamsplitter 15; vertically polarized light beam P 1V Passes through Faraday rotator 17 and is rotated by 90 DEG to P 1VH And then through the second phase modulator 16 and then back to the first polarization-preserving beam splitter 15; light beam P 1HV And a light beam P 1VH The first 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 splits it into two beams,one beam is transmitted back to the first beam splitter 9 along the short arm, set as P Return to the short The other beam is transmitted back to the first beam splitter 9 through the first phase modulator 10, and is set as P Back length Two beams of light interfere when passing through the first beam splitter 9, if the phase difference of the two beams of light is pi, the first single-photon detector 12 responds, if the phase difference of the two beams of light is 0, the second single-photon detector 13 responds, and the communication receiving module 1 can know the operation and the quantum bit executed by the communication sending module 3 on the beams of light;
wherein the light beam P 1H And a light beam P 1VH Each photon is modulated by the second phase modulator 16 in two states {0, } while passing through the second phase modulator 16, respectively;
the foregoing descriptions of specific exemplary embodiments of the present invention have been presented for purposes of illustration and description. It is not intended to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. The exemplary embodiments were chosen and described in order to explain certain principles of the invention and its practical application to enable one skilled in the art to make and use various exemplary embodiments of the invention and various alternatives and modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims and their equivalents.
The above-described embodiments of the apparatus are merely illustrative, and the units described as separate parts may or may not be physically separate, and parts displayed as units may or may not be physical units, that is, may be located in one place, or may be distributed on a plurality of network units. Some or all of the modules may be selected according to actual needs to achieve the purpose of the solution of the present embodiment. One of ordinary skill in the art can understand and implement it without inventive exercise.
Claims (8)
1. A quantum communication system without polarization feedback is characterized by comprising a communication receiving module, a communication sending module and a quantum channel between the two parties;
the communication receiving module comprises 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 with the attenuator, the attenuator is connected with the first circulator, the first circulator is connected with the first beam splitter, the first beam splitter is respectively connected with the first phase modulator and the second beam splitter, the first circulator is connected with the first single-photon detector, and the first beam splitter is connected with the second single-photon detector; the laser source, the attenuator, the first circulator, the first beam splitter, the second beam splitter, the first phase modulator, the first single-photon detector and the second single-photon detector are connected through polarization-maintaining optical fibers;
the communication sending module comprises a filtering coupler, an encoding light path module and a safety detection light path module;
the coding light path module comprises a first polarization-preserving beam splitter, a second phase modulator and a Faraday rotator; the first polarization-preserving beam splitter is respectively connected with the second phase modulator and the Faraday rotator, and the second phase modulator is connected with the Faraday rotator; the first polarization-preserving beam splitter, the second phase modulator and the Faraday rotator are connected through polarization-preserving optical fibers;
the safety detection light path module comprises a second circulator, a second polarization-maintaining beam splitter, a third beam splitter, a fourth beam splitter, a third phase modulator, a third single-photon detector and a fourth single-photon detector; the second circulator is respectively connected with the second polarization-maintaining beam splitter and the third single-photon detector through single-mode fibers; the third polarization-maintaining beam splitter is connected with the fourth single-photon detector through a single-mode fiber; the second polarization-maintaining beam splitter is connected with the third beam splitter and the fourth beam splitter, the third beam splitter is respectively connected with the fourth beam splitter, the third phase modulator and the third polarization-maintaining beam splitter, and the fourth beam splitter is respectively connected with the third phase modulator and the third polarization-maintaining beam splitter; the second circulator, the third beam splitter, the fourth beam splitter, the third phase modulator and the third polarization-maintaining beam splitter are all connected through polarization-maintaining optical fibers;
the filter coupler is connected with the first polarization-preserving beam splitter through a single-mode fiber delay line, and the filter coupler is connected with the second circulator through a single-mode fiber;
the quantum channel is a single-mode fiber, and the second beam splitter in the communication receiving module is connected with the filter coupler in the communication transmitting module through the single-mode fiber.
2. The quantum communication system of claim 1, wherein: 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.
3. A method for quantum secure direct communication using the quantum communication system of claim 1, the method comprising:
step S1, the laser source emits a light beam, the light beam forms a single photon pulse through the attenuator, and the single photon pulse is divided into two beams after reaching the first beam splitter; one beam of light is transmitted to the second beam splitter along the short arm and is set as P 1 short (ii) a The other beam of light is transmitted to the second beam splitter after being modulated in phase by the first phase modulator along the long arm, and is set as P 1 long (ii) a The second beam splitter combines the two beams of light into one beam;
step S2, the second beam splitter sends the light synthesized into one beam to the filter coupler, the filter coupler divides the light beam into two beams, one beam of light is sent to the safety detection light path module, and the other beam of light is sent to the coding light path module along the single-mode optical fiber delay line;
step S3, the light beam reaching the safety detection optical path module passes through the second circulator and is then split into two orthogonal polarization components by the second polarization maintaining beam splitter, wherein the horizontal polarization state light beam is set as P 2H With the vertically polarized beam set to P 2V ；
Horizontal polarized light beam P 2H Transmitted to the third beam splitter and split into two beams by the third beam splitter, one beam is transmitted to the fourth beam splitter along a short arm and is set as P 2H short The other beam is transmitted to the fourth beam splitter after being modulated in phase by the third phase modulator along the long arm, and is set as P 2H long (ii) a Light beam P 2H short And a light beam P 2H long When the light passes through the fourth beam splitter, the light is split into two beams, one beam is reflected to the second polarization-preserving beam splitter, and the other beam is transmitted to the third polarization-preserving beam splitter;
vertically polarized light beam P 2V Transmitted to the fourth beam splitter and split into two beams by the fourth beam splitter, one beam is transmitted to the third beam splitter along a short arm and is set as P 2V short The other beam passes through the third phase modulator along the long arm to be transmitted to the third beam splitter after being modulated in phase, and the modulation phase is set as P 2V long (ii) a Light beam P 2V short And a light beam P 2V long When passing through the third beam splitter, the light beams are respectively split 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;
if the detected bit error rate is higher than or equal to a preset threshold value, the coded light path does not encode information, and if the detected bit error rate is smaller than the preset threshold value, the coded light path executes the coded information;
step S4, the light beam arriving at the encoding light 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 With the vertically polarized beam set to P 1V ；
Horizontal polarized light beam P 1H Sequentially passes through the second phase modulator and the Faraday rotator, and the light beam is rotated by 90 degrees by the Faraday rotator to be set as P 1HV And then reflected back to the first polarizing beamsplitter; vertically polarized light beam P 1V Passes through the Faraday rotator and is rotated by 90 DEG to be set as P 1VH Then through the second phase modulator and then reflected back to the first polarization-preserving beam splitter; light beam P 1HV And a light beam P 1VH The first polarization-preserving beam splitter is combined into a beam of light, the light is reflected back to the second beam splitter along a light path, the second beam splitter splits the light into two beams of light, one beam of light is transmitted back to the first beam splitter along a short arm and is set as P Return to the short The other beam is transmitted back to the first beam splitter after passing through the first phase modulator, and is set as P Back length And the interference result of the two beams of light passing through the first beam splitter is detected by the second single-photon detector.
4. The method of claim 3, wherein in step S1: the single photon pulse satisfies Poisson distribution; light beam P 1 long For use by the first phase modulator
Randomly modulating the phase in four states; light beam P 1 short Specific beam P 1 long First reaches the second beam splitter, which splits the beam P 1 short And a light beam P 1 long End-to-end into a beam of light.
5. The method of claim 3, wherein in step S2: the light beam sent to the safety detection light path module arrives firstly, and the light beam sent to the coding light path module arrives later.
6. The method of claim 3, wherein in step S3: light beam P 2H long And a light beam P 2V long Are all phase-locked by the third phase while passing through the third phase modulator, respectivelyFor modulators
The two states randomly modulate each photon of the beam.
7. The method of 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 between the two interfering beams is
The third single-photon detector responds, and if the phase difference is 0, the fourth single-photon detector responds; and if the detection bit error rate is higher than or equal to a preset threshold value, the coding light path does not code information, and if the detection bit error rate is smaller than the preset threshold value, the coding light path executes coding information.
8. The method of claim 3, wherein in step S4:
light beam P 1H And a light beam P 1VH While passing through the second phase modulator, each photon is modulated by the second phase modulator with two states {0, pi };
light beam P 1HV And a light beam P 1VH The first polarization-preserving beam splitter is combined into a beam of light, the light is reflected back to the second beam splitter along a light path, the second beam splitter splits the light into two beams of light, one beam of light is transmitted back to the first beam splitter along a short arm and is set as P Return to the short The other beam is transmitted back to the first beam splitter after passing through the first phase modulator, and is set as P Back length The two beams of light interfere with each other when passing through the first beam splitter, if the phase difference of the two beams of light is pi, the first single-photon detector responds, and if the phase difference of the two beams of light is 0, the second single-photon detector responds, and the communication receiving module can know the operation and the quantum bit executed by the communication sending module on the beams of light.
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