A sagnac loop-based intensity-modulated polarization encoding quantum key distribution system

Abstract

The application discloses a kind of intensity modulation polarization encoding quantum key distribution systems based on sagnac ring, including communication end Bob and communication end Alice, the communication end Bob and communication end Alice are connected by optical fiber channel;The laser in the communication end Bob generates optical signal into optical isolator and exports linearly polarized signal parallel to cat eye direction into first polarization maintaining beam splitter;First polarization maintaining beam splitter exports linearly polarized signal parallel to cat eye direction into communication end Alice;Communication end Alice divides the linearly polarized signal parallel to cat eye direction into first linearly polarized signal and second linearly polarized signal with equal intensity and carries out intensity modulation, and finally polarized state signal is output into communication end Bob by the beam combination superposition of third polarization beam splitter after being combined, first single-photon detector to fourth single-photon detector in communication end Bob carries out measurement.The polarization encoding and quantum state modulation of signal are realized by one intensity modulator in the application, and the equipment cost of QKD is reduced.

Description

Technical Field

[0001] This invention relates to the field of quantum key distribution communication based on optical fiber networks, and specifically to an intensity modulation polarization coding quantum key distribution system based on a Sagnac ring. Background Technology

[0002] Quantum key distribution (QKD) is a novel symmetric encryption communication technology whose security is based on the fundamental principles of quantum mechanics. Compared to widely used traditional asymmetric encryption methods such as the RSA algorithm, QKD technology can resist attacks from quantum computers with powerful parallel computing capabilities, safeguarding secure communication in the future.

[0003] QKD can be divided into two types based on the transmission channel: fiber optic QKD and free space QKD.

[0004] Due to the inherent loss of optical fiber channels (0.2dB / km@1550nm), the attenuation of optical signals over long distances in optical fibers is too great, limiting the communication distance of fiber-optic QKD. Currently, no experiments have been able to break through transmission distances exceeding 1000km. However, QKD based on free-space channels has already achieved key distribution over 1200km.

[0005] Researchers Tang Zhilie et al. proposed a phase modulation polarization control QKD system, and Guo Banghong et al. proposed a joint modulation QKD system. They are based on the phase modulation polarization control or phase polarization joint modulation principle of non-equidistant interferometers. The plug-and-play system based on the Sagnac ring has the advantages of simple structure and good stability.

[0006] Therefore, it is necessary to improve upon the shortcomings of existing technologies and propose an intensity modulation polarization coding quantum key distribution system based on the sagnac ring. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of existing technologies and to solve the stability problem of transmission in optical fiber channels. It proposes an intensity modulation polarization coding quantum key distribution system based on a sagnac ring.

[0008] The method of this invention is achieved through the following technical solution:

[0009] An intensity-modulated polarization-coded quantum key distribution system based on a Sagnac ring, the system comprising a communication terminal Bob and a communication terminal Alice, which are connected via an optical fiber channel;

[0010] The communication terminal Bob includes a laser, an optical isolator, a first polarization-maintaining beam splitter, a polarization controller, a second polarization-maintaining beam splitter, a first polarization beam splitter, a second polarization beam splitter, and a first to a fourth single-photon detector.

[0011] The laser, optical isolator, and first polarization-maintaining beam splitter are connected in sequence. The output port of the first polarization-maintaining beam splitter is connected to the second polarization-maintaining beam splitter through a polarization controller. The output port of the second polarization-maintaining beam splitter is connected to the first polarization beam splitter and the second polarization beam splitter, respectively. The first polarization beam splitter is connected to the first single-photon detector and the second single-photon detector, respectively. The second polarization beam splitter is connected to the third single-photon detector and the fourth single-photon detector, respectively.

[0012] The communication terminal Alice includes a third polarization beamsplitter, an intensity modulator, a fourth polarization beamsplitter, and a Faraday mirror; the first output ports of the third and fourth polarization beamsplitters are respectively connected to the two ports of the intensity modulator; the second output ports of the third and fourth polarization beamsplitters are connected through polarization-maintaining fiber to form a closed loop; and the input port of the fourth polarization beamsplitter is connected to the Faraday mirror through an optical signal.

[0013] The communication terminal Bob is used to generate optical signals that are input to the communication terminal Alice;

[0014] The communication terminal Alice is used to modulate the optical signal with polarization state and weak coherence state before inputting it to the communication terminal Bob for measurement.

[0015] Preferably, the laser in the communication terminal Bob generates an optical signal that enters an optical isolator. The optical isolator filters out linearly polarized signals perpendicular to the cat's eye direction and outputs linearly polarized signals parallel to the cat's eye direction, which enter the first polarization-maintaining beam splitter. The linearly polarized signal is then input in reverse through the input port to the first polarization-maintaining beam splitter and output to the communication terminal Alice.

[0016] Preferably, the output port of the optical isolator is parallel to the key of the fiber optic connector of the polarization-maintaining fiber;

[0017] The cat's-eye direction of the two output ports of the first polarization-maintaining beam splitter is parallel to the key of the fiber optic connector of the polarization-maintaining fiber, and the cat's-eye direction of the input port is at a 45° angle to the key of the fiber optic connector of the polarization-maintaining fiber.

[0018] Preferably, after the signal enters the third polarization beamsplitter, it is split into a first linearly polarized signal and a second linearly polarized signal with equal intensity.

[0019] The first linearly polarized signal is output from the first output port of the third polarization beam splitter and enters the intensity modulator for intensity modulation processing. After being reflected by the Faraday mirror through the fourth polarization beam splitter, it becomes a linearly polarized signal A1. The linearly polarized signal A1 is input in the forward direction to the fourth polarization beam splitter and outputs a linearly polarized signal A2. The linearly polarized signal A2 is input in the reverse direction from the second output port of the third polarization beam splitter and outputs a linearly polarized signal A3.

[0020] The second linearly polarized signal is output from the second output port of the polarization beam splitter and input to the second output port of the fourth polarization beam splitter through the polarization-maintaining fiber. After being input in reverse to the fourth polarization beam splitter, the linearly polarized signal B1 is output. The linearly polarized signal B1 is reflected by the Faraday mirror and output as a linearly polarized signal B2. The linearly polarized signal B2 is input to the intensity modulator for intensity modulation processing. Finally, it is input in reverse from the first output port of the third polarization beam splitter and output as a linearly polarized signal B3.

[0021] The two linearly polarized signals, A3 and B3, enter the third polarization beam splitter for beam combining and superposition, outputting the final polarization state signal which then enters the communication terminal Bob.

[0022] Preferably, the cat's-eye direction of the input ports of the third and fourth polarization beam splitters is parallel to the key of the fiber optic connector of the polarization-maintaining fiber, the cat's-eye direction of the first output port is parallel to the key of the fiber optic connector of the polarization-maintaining fiber, and the cat's-eye direction of the second output port is perpendicular to the key of the fiber optic connector of the polarization-maintaining fiber.

[0023] The input port of the intensity modulator is aligned with the key of the fiber optic connector, and the output port is aligned with the key of the polarization-maintaining fiber optic connector.

[0024] Preferably, the intensity modulation process is as follows:

[0025] When the intensity of linearly polarized signal A3 is a1 and the intensity of linearly polarized signal B3 is a2, and assuming the phase difference between linearly polarized signal A3 and linearly polarized signal B3 is δ, the polarization trajectory of the two signals after being combined satisfies the following formula:

[0026]

[0027] Where δ represents the phase difference between the two signals, E x and E y A and B represent the electric field components parallel and perpendicular to the cat's eye, respectively, and a1 and a2 represent the intensity of the linearly polarized signal A3 and the intensity of the linearly polarized signal B3, respectively.

[0028] Preferably, when the phase difference δ between the linearly polarized signal A3 and the linearly polarized signal B3 is 0, the polarization direction of the combined signal is... The formula is as follows:

[0029]

[0030] in, The polarization direction of the final polarization state signal is indicated by a1 and a2, which represent the intensity of the linearly polarized signal A3 and the intensity of the linearly polarized signal B3, respectively.

[0031] Preferably, after the final polarization state signal enters the communication terminal Bob, it is first split in the forward direction by the first polarization-maintaining beam splitter. The signal is then output from the second output port of the first polarization-maintaining beam splitter and enters the polarization controller for basis vector conversion to obtain a right-angle basis or diagonal baseline polarization signal. Then, the linearly polarized signal enters the second polarization-maintaining beam splitter for beam splitting and outputs the first and second signals, which then enter the first and second polarization beam splitters, respectively. The output linearly polarized signals, which are horizontal and vertical to the cat's eye direction, enter the first to fourth single-photon detectors for measurement.

[0032] The beneficial effects of this invention are as follows:

[0033] This invention is based on the phase self-compensation principle of the Sagnac ring, which can achieve the stability of polarization state transmission in optical fiber channels. Based on the superposition principle of vertical polarization states, this invention realizes the polarization encoding of the signal through intensity modulation. Moreover, the polarization encoding and quantum state modulation of the signal are realized through an intensity modulator, which reduces the equipment cost of QKD. Attached Figure Description

[0034] Figure 1 This is a system block diagram of the present invention;

[0035] Figure 2 This is a schematic diagram of the cross-section of the cat-eye polarization-maintaining optical fiber and optical fiber connector of the present invention.

[0036] The components in the attached diagram are labeled as follows: Laser-01, Optical Isolator-02, First Polarization Maintaining Beam Splitter-03, Fiber Optic Channel-04, Third Polarization Beam Splitter-05, Intensity Modulator-06, Fourth Polarization Beam Splitter-07, Faraday Mirror-08, Polarization Maintaining Fiber-09, Polarization Controller-10, Second Polarization Maintaining Beam Splitter-11, First Polarization Beam Splitter-12, Second Polarization Beam Splitter-13, First Single-Photon Detector-14, Second Single-Photon Detector-15, Third Single-Photon Detector-16, Fourth Single-Photon Detector-17; Cat's Eye-21, Key-22. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the embodiments. However, the scope of protection of this invention is not limited to the specific embodiments described below.

[0038] An intensity-modulated polarization-coded quantum key distribution system based on a Sagnac ring, such as... Figure 1 As shown, the system includes a communication terminal Bob and a communication terminal Alice, which are connected via fiber optic channel 04.

[0039] The communication terminal Bob includes a laser 01, an optical isolator 02, a first polarization-maintaining beam splitter 03, a polarization controller 10, a second polarization-maintaining beam splitter 11, a first polarization beam splitter 12, a second polarization beam splitter 13, a first single-photon detector 14, a second single-photon detector 15, a third single-photon detector 16, and a fourth single-photon detector 17.

[0040] The laser 01, optical isolator 02, and first polarization-maintaining beam splitter 03 are connected in sequence. The output port of the first polarization-maintaining beam splitter 03 is connected to the second polarization-maintaining beam splitter 11 through polarization controller 10. The output port of the second polarization-maintaining beam splitter 11 is connected to the first polarization beam splitter 12 and the second polarization beam splitter 13, respectively. The first polarization beam splitter 12 is connected to the first single-photon detector 14 and the second single-photon detector 15, respectively. The second polarization beam splitter 13 is connected to the third single-photon detector 16 and the fourth single-photon detector 17, respectively.

[0041] The communication terminal Alice includes a third polarization beamsplitter 05, an intensity modulator 06, a fourth polarization beamsplitter 07, and a Faraday mirror 08. The first output ports ① of the third polarization beamsplitter 05 and the fourth polarization beamsplitter 07 are respectively connected to the two ports of the intensity modulator 06. The second output ports ② of the third polarization beamsplitter 05 and the fourth polarization beamsplitter 07 are connected through polarization-maintaining fiber 09 to form a closed loop. The input port of the fourth polarization beamsplitter 07 is connected to the Faraday mirror 08 through an optical signal.

[0042] The communication terminal Bob is used to generate optical signals that are input to the communication terminal Alice;

[0043] The communication terminal Alice is used to modulate the optical signal with polarization state and weak coherence state before inputting it to the communication terminal Bob for measurement.

[0044] The working principle and process of the intensity modulation polarization coding quantum key distribution system based on the Sagnac ring in this embodiment are as follows:

[0045] like Figure 1-2As shown, the laser 01 in the communication terminal Bob generates an optical signal that enters the optical isolator 02. The optical isolator 02 filters out the linearly polarized signal perpendicular to the direction of the cat's eye 21 and outputs a linearly polarized signal parallel to the direction of the cat's eye, which enters the first polarization-maintaining beam splitter 03. At this time, the key 22 of the optical isolator is parallel to the direction of the cat's eye 21, i.e. Figure 2 As shown, when θ = 0°;

[0046] The first polarization-maintaining beam splitter 03 outputs a linearly polarized signal that is horizontal to the direction of the cat's eye through its input port and enters the communication terminal Alice. The first polarization-maintaining beam splitter 03 is a 10:90 beam splitter. 10% of the energy of the signal input from the input port of the first polarization-maintaining beam splitter 03 will be output from port ① of the first polarization-maintaining beam splitter 03, and 90% of the energy will be output from port ② of the first polarization-maintaining beam splitter 03. The linearly polarized signal that is parallel to the direction of the cat's eye is output from the input port of the first polarization-maintaining beam splitter 03 in reverse direction and is transmitted to Alice through the optical fiber channel.

[0047] The linearly polarized signal, parallel to the cat's eye direction, is split into a first linearly polarized signal and a second linearly polarized signal of equal intensity after entering the third polarization beam splitter 05 in Alice. Specifically, the input port key of the third polarization beam splitter 05 is at a 45° angle to the cat's eye direction. Figure 2 In the case of θ = 45°, the key keys of output ports ① and ② of the third polarization beam splitter 05 are parallel to the direction of the cat's eye.

[0048] The first linearly polarized signal is output from the first output port ① of the third polarization beam splitter 05 and enters the intensity modulator 06 for intensity modulation processing. After being reflected by the Faraday mirror 08 through port ① of the fourth polarization beam splitter 07, the polarization state of the first linearly polarized signal is rotated by 90°. The Faraday mirror 08 outputs a linearly polarized signal A1 perpendicular to the cat's eye direction. The linearly polarized signal A1 is input in the forward direction to the fourth polarization beam splitter 07 and outputs a linearly polarized signal A2 parallel to the cat's eye direction. The linearly polarized signal A2 is output in the reverse direction from the second output port ② of the third polarization beam splitter 05 and input to the third polarization beam splitter 05 through the polarization-maintaining fiber 09 and outputs a linearly polarized signal A3 perpendicular to the cat's eye direction.

[0049] The second linearly polarized signal is output from the second output port ② of the polarization beam splitter 05 and input to the second output port ② of the fourth polarization beam splitter 07 through the polarization-maintaining fiber. After input to the fourth polarization beam splitter 07, a linearly polarized signal B1 perpendicular to the cat's eye direction is output. The linearly polarized signal B1 is reflected by the Faraday mirror 08. The polarization state of the linearly polarized signal B1 is rotated by 90°. Then, the Faraday mirror 08 outputs a linearly polarized signal B2 parallel to the cat's eye direction. The linearly polarized signal B2 is processed by the intensity modulator 06. Finally, it is input in reverse from the first output port of the third polarization beam splitter 05 and outputs a linearly polarized signal B3 parallel to the cat's eye direction.

[0050] The two mutually perpendicular linearly polarized signals A3 and B3 enter the third polarization beam splitter 05 for beam combining and superposition, outputting the final polarization state signal, which then enters the communication terminal Bob.

[0051] Specifically, the intensity modulation processing is as follows:

[0052] Two signals pass through intensity modulator 06 at different times, either forward or reverse. By applying a time-division modulating signal to intensity modulator 06, the two signals are modulated into output signals of different intensities as they pass through it. When the intensity of linearly polarized signal A3 is... The intensity of the linearly polarized signal B3 is At this time, the two signals have stable intensity and phase difference, and they superimpose to form a new polarization state signal; the degree of polarization of the final polarization state signal after superposition is 100%, and the polarization state is related to the intensity and phase difference of the two superimposed signals. Let the phase difference δ between the linearly polarized signal A3 and the linearly polarized signal B3 be, and after the two signals are combined, the polarization trajectory of the signal satisfies the following formula:

[0053]

[0054] Where δ represents the phase difference between the two signals, E x and E y These represent the linear polarization components parallel and perpendicular to the cat's eye, respectively. and These represent the intensity of linearly polarized signal A3 and the intensity of linearly polarized signal B3, respectively.

[0055] When the phase difference δ between the linearly polarized signal A3 and the linearly polarized signal B3 is 0, the polarization direction of the final polarized signal output by the superposition of the two signals is as follows: The polarization direction The formula is as follows:

[0056]

[0057] in, Indicates the polarization direction of the final polarization state signal. and These represent the intensity of linearly polarized signal A3 and the intensity of linearly polarized signal B3, respectively.

[0058] In this embodiment, the intensity modulator 06, based on the key to be loaded, independently modulates two signals passing through it at different times (forward or reverse) using a time-division multiplexing method. According to the B92 protocol, when the system needs to modulate two non-orthogonal quantum states, let the linear polarization state of signal 1 be 22.5° and the linear polarization state of signal 2 be 67.5°, and the intensity of signal 1 be... The strength of signal 2 is ;

[0059] When the linear polarization state of modulation signal 1 is 22.5°, calculate the ratio H of the intensities of the two signal beams before superposition. When modulated to a 67.5° linear polarization state, Specifically, polarization modulation of the signal is achieved by controlling the ratio H of the intensities of the two signal beams before superposition, outputting a linearly polarized state of 22.5° or 67.5°. Furthermore, this invention also achieves quantum state modulation of the signal by controlling a predetermined value of the intensities of the two signal beams, outputting a signal state, a decoy state, or a vacuum state.

[0060] This invention reduces the equipment cost of QKD by implementing polarization encoding and quantum state modulation of the signal through an intensity modulator.

[0061] The third polarization beam splitter 05 combines and superimposes the two beams to output the final polarization state signal, which then enters the communication terminal Bob. The signal is first forward-biased through the first polarization-maintaining beam splitter 03, and then output from the second output port ② of the first polarization-maintaining beam splitter 03 to the polarization controller 10 for basis vector conversion to obtain linearly polarized signals. For example, linearly polarized signals of 22.5° and 67.5° are converted into corresponding linearly polarized signals of 0° and 45°. The second polarization-maintaining beam splitter 11 is a 50:50 beam splitter used for passive basis vector selection.

[0062] The linearly polarized signal enters the second polarization-maintaining beam splitter 12 for beam splitting, and then outputs the first signal and the second signal, which enter the first polarization beam splitter 12 and the second polarization beam splitter 13 respectively.

[0063] After the first signal enters the first polarization beam splitter 12, the component parallel to the cat's eye direction is output from the output port ① of the first polarization beam splitter 12 and then enters the second single-photon detector 15 for measurement; the component perpendicular to the cat's eye direction is output from the output port ② of the first polarization beam splitter 12 and then enters the first single-photon detector 14 for measurement; wherein, the cat's eye of the input port, output port ① and output port ② of the first polarization beam splitter 12 is parallel to the key.

[0064] After the second signal enters the second polarization beam splitter 13, the component parallel to the cat's eye direction is output from the output port ① of the first polarization beam splitter 13 and then enters the fourth single-photon detector 17 for measurement; the component perpendicular to the cat's eye direction is output from the output port ② of the first polarization beam splitter 13 and then enters the third single-photon detector 16 for measurement; wherein, the input port of the first polarization beam splitter 13 is at 45° with the key, and the cat's eye of both its output port ① and output port ② is parallel to the key.

[0065] This invention is based on the phase self-compensation principle of the sagnac ring, which can achieve the stability of polarization state transmission in optical fiber channels.

[0066] This invention is based on the superposition principle of vertical polarization states and realizes the polarization encoding of the signal through intensity modulation. Moreover, the polarization encoding and quantum state modulation of the signal are realized through an intensity modulator, which reduces the equipment cost of QKD.

[0067] Based on the disclosure and teachings of the foregoing specification, those skilled in the art can make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the specific embodiments disclosed and described above, and some modifications and changes to the invention should also fall within the protection scope of the claims of the present invention. Furthermore, although some specific terms are used in this specification, these terms are only for convenience of explanation and do not constitute any limitation on the invention.

Claims

1. A intensity-modulated polarization-coded quantum key distribution system based on a Sagnac ring, characterized in that, The system includes a communication terminal Bob and a communication terminal Alice, which are connected via a fiber optic channel. The communication terminal Bob includes a laser, an optical isolator, a first polarization-maintaining beam splitter, a polarization controller, a second polarization-maintaining beam splitter, a first polarization beam splitter, a second polarization beam splitter, and a first to a fourth single-photon detector. The laser, optical isolator, and first polarization-maintaining beam splitter are connected in sequence. The output port of the first polarization-maintaining beam splitter is connected to the second polarization-maintaining beam splitter through a polarization controller. The output port of the second polarization-maintaining beam splitter is connected to the first polarization beam splitter and the second polarization beam splitter, respectively. The first polarization beam splitter is connected to the first single-photon detector and the second single-photon detector, respectively. The second polarization beam splitter is connected to the third single-photon detector and the fourth single-photon detector, respectively. The communication terminal Alice includes a third polarization beamsplitter, an intensity modulator, a fourth polarization beamsplitter, and a Faraday mirror; the first output ports of the third and fourth polarization beamsplitters are respectively connected to the two ports of the intensity modulator; the second output ports of the third and fourth polarization beamsplitters are connected through polarization-maintaining fiber to form a closed loop; and the input port of the fourth polarization beamsplitter is connected to the Faraday mirror through an optical signal. The communication terminal Bob is used to generate optical signals that are input to the communication terminal Alice; The communication terminal Alice is used to modulate the optical signal with polarization state and weak coherence state before inputting it to the communication terminal Bob for measurement.

2. The intensity-modulated polarization-coded quantum key distribution system based on a Sagnac ring according to claim 1, characterized in that, The laser in the communication terminal Bob generates an optical signal that enters an optical isolator. The optical isolator filters out linearly polarized signals perpendicular to the cat's eye direction and outputs linearly polarized signals parallel to the cat's eye direction, which enter the first polarization-maintaining beam splitter. The linearly polarized signal is then input in reverse through the input port to the first polarization-maintaining beam splitter and output to the communication terminal Alice.

3. The intensity-modulated polarization-coded quantum key distribution system based on a Sagnac ring according to claim 2, characterized in that, The output port of the optical isolator is parallel to the key of the fiber optic connector of the polarization-maintaining fiber. The cat's-eye direction of the two output ports of the first polarization-maintaining beam splitter is parallel to the key of the fiber optic connector of the polarization-maintaining fiber, and the cat's-eye direction of the input port is at a 45° angle to the key of the fiber optic connector of the polarization-maintaining fiber.

4. The intensity-modulated polarization-coded quantum key distribution system based on a Sagnac ring according to claim 2, characterized in that, After entering the third polarization beam splitter, the signal is split into a first linearly polarized signal and a second linearly polarized signal with equal intensity. The first linearly polarized signal is output from the first output port of the third polarization beam splitter and enters the intensity modulator for intensity modulation processing. After being reflected by the Faraday mirror through the fourth polarization beam splitter, it becomes a linearly polarized signal A1. The linearly polarized signal A1 is input in the forward direction to the fourth polarization beam splitter and outputs a linearly polarized signal A2. The linearly polarized signal A2 is input in the reverse direction from the second output port of the third polarization beam splitter and outputs a linearly polarized signal A3. The second linearly polarized signal is output from the second output port of the polarization beam splitter and input to the second output port of the fourth polarization beam splitter through the polarization-maintaining fiber. After being input in reverse to the fourth polarization beam splitter, the linearly polarized signal B1 is output. The linearly polarized signal B1 is reflected by the Faraday mirror and output as a linearly polarized signal B2. The linearly polarized signal B2 is input to the intensity modulator for intensity modulation processing. Finally, it is input in reverse from the first output port of the third polarization beam splitter and output as a linearly polarized signal B3. The two linearly polarized signals A3 and B3 are combined by a third polarization beam splitter, and the final polarization signal is output to the communication terminal Bob.

5. The intensity-modulated polarization-coded quantum key distribution system based on a Sagnac ring according to claim 4, characterized in that: The input port cat's eye direction of the third polarization beam splitter and the fourth polarization beam splitter is parallel to the key of the fiber optic connector of the polarization-maintaining fiber, the output port one cat's eye direction is parallel to the key of the fiber optic connector of the polarization-maintaining fiber, and the output port two cat's eye direction is perpendicular to the key of the fiber optic connector of the polarization-maintaining fiber. The input port of the intensity modulator is aligned with the key of the fiber optic connector, and the output port is aligned with the key of the polarization-maintaining fiber optic connector.

6. The intensity-modulated polarization-coded quantum key distribution system based on a Sagnac ring according to claim 3, characterized in that... The intensity modulation process is as follows: When the intensity of the linearly polarized signal A3 is The intensity of the linearly polarized signal B3 is When the phase difference between the linearly polarized signal A3 and the linearly polarized signal B3 is δ, the polarization trajectory of the two signals after they are combined satisfies the following formula: Where δ represents the phase difference between the two signals, E x and E y Let represent the electric field components parallel and perpendicular to the cat's eye, respectively. and These represent the intensity of linearly polarized signal A3 and the intensity of linearly polarized signal B3, respectively.

7. The intensity-modulated polarization-coded quantum key distribution system based on a Sagnac ring according to claim 4, characterized in that, When the phase difference δ between the linearly polarized signal A3 and the linearly polarized signal B3 is 0, then the polarization direction of the combined signal is... The formula is as follows: in, Indicates the polarization direction of the final polarization state signal. and These represent the intensity of linearly polarized signal A3 and the intensity of linearly polarized signal B3, respectively.

8. The intensity-modulated polarization-coded quantum key distribution system based on a Sagnac ring according to claim 3, characterized in that, After the final polarization signal enters the communication terminal Bob, it is first split in the forward direction by the first polarization-maintaining beam splitter. The signal is then output from the second output port of the first polarization-maintaining beam splitter and enters the polarization controller for basis vector conversion to obtain linearly polarized signals of the right angle basis or diagonal basis. Then, the linearly polarized signal enters the second polarization-maintaining beam splitter for beam splitting and outputs the first and second signals, which enter the first and second polarization beam splitters, respectively. The output linearly polarized signals, which are horizontal and vertical to the cat's eye direction, enter the first to fourth single-photon detectors for measurement.

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