Fluorescence reflection characteristic-based information extraction optimization method for polarization coding quantum key distribution system

Abstract

A fluorescence reflection characteristic information extraction-based optimization method for a quantum key distribution system for polarization coding comprises the following steps: arranging a circulator, a 50: 50 optical beam splitter, a first polarization controller, a second polarization controller, a first polarization beam splitter and a second polarization beam splitter between a QKD system sending end and a QKD system first receiving end; the first to fourth superconducting single-photon detectors and the light path are connected with the optical fiber; transmitting end signal light photons are transmitted from the first port to the second port, but cannot be transmitted from the first port to the third port; fluorescent photons generated by an avalanche single-photon detector in the first receiving end are transmitted to the third port from the second port and cannot be transmitted to the first port from the third port; according to the invention, on the basis of the fluorescence reflection characteristic of the BB84 protocol polarization coding quantum key distribution system, the problem that the polarization discrimination degree of the second receiving end is not high due to inconsistent fluorescence wavelengths is optimized to the greatest extent.

Description

Technical Field

[0001] The present invention belongs to the technical field of quantum key distribution (QKD), and relates to an information extraction method based on fluorescence reflection characteristics at a receiving end of a polarization-coded QKD system. Background Art

[0002] In the information age, information security is of vital importance to both individuals and countries. In order to cope with the security threat of quantum computers to the basic system of classical cryptography, replacing the existing classical cryptography system with a "quantum-safe" cryptography system will be an important strategic measure.

[0003] CN2018104751296 relates to a quantum key distribution method and a quantum key distribution network system. The method first establishes a central end for quantum communication with each user end, and then the central end and each user end adopt a key distribution scheme based on the Blom scheme and QKD. The central end is one or more; a quantum channel can be established between each pair of central ends, and the QKD protocol is executed between the central ends to resist DoS attacks; further, a block group design can be used to form a quantum key distribution network. When the key distribution network is attacked by quantum denial of service, the present invention proposes corresponding key distribution schemes for different types of damage caused by DoS attacks. The present invention can reduce the cost of quantum key distribution and improve the security and scalability of quantum key distribution.

[0004] CN202310066882.0 provides a quantum key distribution method and a quantum key distribution system. The method includes emitting an initial laser signal using a phase-locked light source, wherein the phase difference of the initial laser signals of the multiple quantum state preparation sending ends is a preset value; processing the initial laser signal using a chopper to obtain a transition pulse laser signal; phase modulating the transition pulse laser signal using a first phase modulator to obtain a target pulse laser signal, wherein the phase modulation includes a randomized phase and a bit-encoded phase; sending a target pulse laser signal from the multiple quantum state preparation sending ends to a measuring end, so that the measuring end generates a detection result according to the multiple target pulse laser signals; determining a target quantum key between the multiple quantum state preparation sending ends according to the bit encoding and the detection result sent by the measuring end.

[0005] CN202310866750.6 is suitable for establishing a quantum key distribution device and method for a global quantum key distribution network, the quantum key distribution device comprising: an input signal path, the input signal path is suitable for receiving a plurality of optical input signals from a transmitter device; an output signal path, the output signal path is connected to the input signal path at a first end and is suitable for transmitting a plurality of first optical output signals at a second end opposite to the first end; and a detector signal path, the detector signal path is connected to the input signal path at a third end and includes a photon detector device at a fourth end opposite to the third end. The photon detector device is suitable for detecting the optical input signal from the transmitter device, and the quantum key distribution device is suitable for establishing a shared quantum cryptographic key with the transmitter device based on the detected optical input signal. The input signal path forms an input path of a Mach-Zehnder interferometer, and the output signal path and the detector signal path form an output path of the Mach-Zehnder interferometer.

[0006] Quantum key distribution has attracted much attention because it is a key transmission method that is theoretically unconditionally secure. "Unconditionally secure" refers to security that can be proven by information theory: without imposing any restrictions on the computing resources or operating technologies that can be used by the second receiving end, its security can be theoretically proven from the perspective of information theory. However, since the various software and hardware required by the QKD system still deviate from the ideal situation, there are many possible information leaks in the QKD system in actual use, which may be exploited by the second receiving end.

[0007] The basic operation process of the QKD system based on polarization encoding is as follows: Alice, the transmitter, prepares a single-photon laser pulse and randomly encodes it into one of the four polarization states. Here, four polarization states |H>, |V>, |+>, |-> are selected. The horizontal polarization state |H> and the vertical polarization state |V> are a set of bases, one of which represents 0 and the other 1; the π / 4 polarization state |+> and -π / 4 polarization state |-> are another set of bases, which are also 0 and 1 respectively. The first receiving end Bob randomly uses a set of bases to measure the photons. If the wrong basis is used for measurement, there is a 1 / 2 probability of getting an incorrect result. Then the basis selection information of Alice or Bob will be made public, and the information of selecting the same basis for measurement will be retained.

[0008] The end of the first receiving end uses an avalanche single photon detector to measure and extract information. In actual implementation, after the avalanche single photon detector of the receiving unit receives a photon, an avalanche current will be generated when the avalanche photodiode reaches the breakdown voltage. When the avalanche current is formed, the recombination of hole-electron pairs will be accompanied by the generation of fluorescent photons. These fluorescent photons will be emitted back into the optical path, and will carry polarization information after passing through the polarization device of the first receiving end, thereby causing information leakage.

[0009] As shown in the figure, in a QKD system based on polarization encoding, the receiver (Bob) decodes the signal photons emitted by the sender (Alice) through a polarization device, receives the photons carrying polarization information from the sender, and obtains the key. After the avalanche single-photon detector of the receiver responds, the generated fluorescent photons are transmitted in the reverse direction, and carry the corresponding polarization information when passing through Bob's polarization device in the reverse direction. The possible second receiving end can intercept the fluorescent photons transmitted in the reverse direction in the optical path by adding a circulator in the optical path. After being received by the second receiving end, these fluorescent photons will first pass through a 50:50 beam splitter and randomly select a basis with a probability of 1 / 2. The selection results of the two groups of bases correspond to the two output ends of the 50:50 beam splitter. Subsequently, polarization calibration is performed through a polarization controller, and the polarization state of the fluorescent photons is measured in the polarization beam splitter, and finally received by a superconducting single-photon detector. Thus, the purpose of obtaining and decoding fluorescence is achieved. The reverse fluorescence will carry time and polarization information, and the reception and decoding of fluorescent photons will not generate additional bit error rate during the communication process. Summary of the invention

[0010] The present invention provides an information extraction optimization method based on fluorescence reflection characteristics for a polarization-encoded quantum key distribution system, which realizes the acquisition and decoding of reflected fluorescence generated by an avalanche single-photon detector at a receiving end in the QKD system.

[0011] In order to solve the above technical problems, the technical solution adopted by the present invention is: an information extraction optimization method based on fluorescence reflection characteristics for a polarization-coded quantum key distribution system, an information extraction device based on fluorescence reflection characteristics is provided between the QKD system transmitting end and the QKD system first receiving end, the device comprising: after the circulator is connected to the 50:50 optical beam splitter, the first parallel path is connected in sequence to the first polarization controller, the first polarization beam splitter, and then the first superconducting single-photon detector and the second superconducting single-photon detector in parallel; the second parallel path after the 50:50 optical beam splitter is connected to the second polarization controller, the second polarization beam splitter, and then the third superconducting single-photon detector and the fourth superconducting single-photon detector in parallel, all of which are connected by optical fibers;

[0012] The circulator has three ports, the first port of the circulator is connected to the side of the channel close to the transmitting end, the second port of the circulator is connected to the side of the channel close to the first receiving end, and the third port of the circulator is connected to a possible second receiving end; the signal light photons of the transmitting end are transmitted from the first port to the second port, but cannot be transmitted from the first port to the third port; the fluorescent photons generated by the avalanche single photon detector in the first receiving end are transmitted from the second port to the third port, and cannot be transmitted from the third port to the first port;

[0013] The 50:50 optical beam splitter includes the following features: the optical beam splitter has an input port and two output ports, and the two output ports output the photons input from the input port at a fractional ratio of 50:50; the input port of the beam splitter is connected to the third port of the circulator, the first output port is sequentially connected to the first polarization controller and the first polarization beam splitter; the second output port is sequentially connected to the second polarization controller and the second polarization beam splitter;

[0014] The first and second polarization beam splitters include the following features: the first and second polarization beam splitters are used to measure polarization states under two different bases, namely, a horizontal base ([0°], [90°]) and a diagonal base ([45°], [135°]); the polarization beam splitter includes three ports, and the polarization beam splitter distinguishes the polarization states of the fluorescent photons received at the first port, and the photons with the polarization state |V> are output at the second port, and the photons with the polarization state |H> are output at the third port; the first port of the first polarization beam splitter is connected to the first polarization controller, and the second and third ports are connected to the first superconducting single photon detector and the second superconducting single photon detector, respectively; the first port of the second polarization beam splitter is connected to the second polarization controller, and the second and third ports are connected to the third superconducting single photon detector and the fourth superconducting single photon detector, respectively;

[0015] The method for extracting and optimizing information based on fluorescence reflection characteristics comprises the following steps:

[0016] Step 1: Use the FDTD platform to digitally model the physical structure of the pn junction in the avalanche single-photon detector at the first receiving end, simulate the avalanche process of the avalanche single-photon detector, monitor the secondary photon emission when the pn junction avalanche effect occurs, and fit the spectrum of the fluorescence photons;

[0017] Step 2: Digital modeling of the first and second three-ring polarization controllers:

[0018] Calculate the phase delay of the polarization controller propeller change:

[0019]

[0020] In the formula is the phase delay, a is a constant (quartz fiber a is equal to 0.133), N is the number of turns, d is the fiber cladding diameter, λ is the wavelength, and D is the diameter of the coil;

[0021] Calculate the Jones matrix for the effect of the three propellers on polarization:

[0022]

[0023] Where U θis the Jones matrix of the polarization controller propeller, θ1, θ2, and θ3 are respectively related to the rotation angles of the three propellers; by cascading the matrices of the three propellers, the simulation model of the three-ring polarization controller can be simulated;

[0024] Step 3: Digital modeling of the polarization beam splitter: According to the wavelength-sensitive characteristics of the polarization beam splitter, the two polarization states of photons of each wavelength correspond to different transmittances on the two output channels. The transmittance parameters of the two channels of the polarization beam splitter at different wavelengths are written into the simulation program, and two curves of transmittance varying with wavelength are fitted;

[0025] Step 4: The fluorescence spectrum described in step 1 is used as input, and the photons of each wavelength are assumed to be in a specific polarization state, which is recorded as the initial state; the process of reverse propagation of the fluorescence photons from the first receiving end in the optical path is simulated;

[0026] ① Apply the result in step 3 to the initial state, and the calculated result is recorded as the first impact state;

[0027] ② Apply the matrix of step 2 to the first impact state, and the calculation result is recorded as the second impact state;

[0028] ③ Apply the result in step 3 to the second impact state, and the calculated result is recorded as the third impact state;

[0029] ④ Apply the matrix of step 2 to the third impact state, and the calculation result is recorded as the final state;

[0030] Step 5: Project and measure the photons of each wavelength, calculate the measurement probabilities of the two polarization states, and then weight the number of photons at different wavelengths according to the spectrum of the final state to obtain the weighted measurement probabilities of the two polarization states. The ratio of the two is the simulation result of the polarization discrimination of fluorescence decoding by the second receiving end, which is recorded as the target polarization discrimination;

[0031] Step 6: The third port of the circulator is connected to the second receiving end, that is, the optical fiber channel between the QKD system transmitting end and the QKD system first receiving end.

[0032] The transmitter transmits a fixed polarization state (|H> / |V>) signal light photon in the first group of horizontal bases to the first receiving end. Its polarization state is recorded as the polarization state to be calibrated. The polarization states of the two groups of bases are different, and there is no difference in optical path construction and frequency.

[0033] Step 7: Turn on the fluorescence detection device, and turn on the first and second superconducting single photon detectors (with the same quantum P-SPD-10S). When the fluorescence photons reach the superconducting single photon detectors, the superconducting single photon detectors can respond in real time.

[0034] Step 8: Calibrate the optical path of the first group of bases at the second receiving end; set the polarization controller of the second receiving end corresponding to the base of the signal light polarization state, observe the detection counts of the two superconducting single-photon detectors under this base, and adjust the detection efficiency of one superconducting single-photon detector connected to it to be maximum and the detection efficiency of the other to be minimum by changing the angle of the polarization controller; the fluorescence polarization state received by the superconducting single-photon detector with the maximum detection efficiency is the polarization state to be calibrated, and the fluorescence polarization state received by the superconducting single-photon detector with the minimum detection efficiency is another polarization state in this group of bases; the ratio of the maximum detection efficiency to the minimum detection efficiency of the superconducting single-photon detector is recorded as the actual polarization discrimination, and when the actual polarization discrimination is within the range of (0.9 to 1.1) times the target polarization discrimination, it is determined that the polarization calibration is successful;

[0035] In step 9: the QKD system transmits and receives a fixed polarization state (|A> / |D>) in another set of diagonal bases, repeats step 8, turns on the third and fourth superconducting single-photon detectors (with the same quantum P-SPD-10S), and completes the polarization calibration of the other set of bases at the second receiving end.

[0036] Preferably, the setting parameters of the pn junction in the FDTD platform in step 1 are actual data of the pn junction of the avalanche single-photon detector in the actual QKD system, including material selection, material thickness, bias voltage, etc.

[0037] Preferably, the propeller setting parameters of the polarization controller in step 2 are actual parameters of the polarization controller in an actual QKD system, including optical fiber material, winding diameter, and number of windings.

[0038] Preferably, the transmittances of the two polarization states in the two channels at different wavelengths in step 3 are transmittance parameters of the two channels of the polarization beam splitter actually used in the fluorescence extraction device.

[0039] The present invention proposes an information extraction optimization method based on fluorescence reflection characteristics of a quantum key distribution (QKD) system. A digital modeling method is used to simulate the process of fluorescent photons from a first receiving end to a second receiving end in an optical path, and the polarization state discrimination degree of the fluorescent photons by the second receiving end is analyzed and calculated, and the second receiving end is polarized and calibrated based on this target polarization discrimination degree. The second receiving end adopts a superconducting single-photon detector, and by taking advantage of its low dark count, the influence of the dark count on the polarization calibration stage is greatly reduced, thereby improving the accuracy of the polarization calibration of the second receiving end. Based on the fluorescence reflection characteristics of the BB84 protocol polarization-encoded quantum key distribution system, the present invention optimizes the problem of low polarization discrimination of the second receiving end due to inconsistent fluorescence wavelengths to the greatest extent, thereby improving the information extraction capability of fluorescent photons.

[0040] The information extraction optimization method includes the use of a digital simulation method, which can theoretically analyze and calculate the upper limit of the polarization discrimination of the second receiving end for the fluorescent photons, providing a reference basis for the second receiving end in the polarization calibration stage. In addition, a superconducting single-photon detector is used in the information extraction device based on the fluorescence reflection characteristics to detect the fluorescent photons. This method effectively reduces the dark count effect of the detector and directly adjusts the polarization controller of the second receiving end through the real-time fluorescence quantity, so that the polarization discrimination of the second receiving end can be adjusted to the optimal level to the greatest extent.

[0041] Beneficial effects of the present invention: The fluorescence detection system can effectively improve the polarization discrimination of the second receiving end when decoding fluorescent photons. When using a traditional avalanche single-photon detector to detect fluorescence, since the avalanche single-photon detector itself has a high dark count, it is difficult to distinguish the number of fluorescent photons from the dark count during the polarization calibration stage. The use of a superconducting single-photon detector can greatly reduce the influence of the dark count, providing a guarantee for the polarization calibration of the second receiving end. Using the fluorescence contrast modeling and simulation method, the upper limit of the second receiving end's decoding of fluorescent photons can be theoretically analyzed, providing a setting target for the polarization calibration stage. The present invention can be used to optimize the information decoding of the second receiving end in the fluorescence reflection characteristics, thereby obtaining the highest information extraction rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0043] Figure 1 The system diagram of the present invention includes: 1. QKD system transmitter, 2. QKD system first receiver, 3. circulator, 4. 50:50 beam splitter, 5. first polarization controller, 6. second polarization controller, 7. first polarization beam splitter, 8. second polarization beam splitter, 9. first superconducting single photon detector, 10. second superconducting single photon detector, 11. third superconducting single photon detector, 12. fourth superconducting single photon detector. The above 3 to 12 components are for receiving and decoding fluorescent photons. DETAILED DESCRIPTION

[0044] The present invention is further explained below in conjunction with the accompanying drawings and specific embodiments. It should be understood that these examples are only used to illustrate the present invention and are not used to limit the scope of the present invention. After reading the present invention, modifications to various equivalent forms of the present invention by those skilled in the art all fall within the scope defined by the claims attached to this application. In order to facilitate users to better use the information extraction device, the actual use process is described here in conjunction with the schematic diagram of the information extraction device and the specific diagrams of each module.

[0045] As shown in the figure, when the QKD system based on BB84 protocol polarization encoding is running, the transmitter 1 transmits signal light photons to the first receiver 2. After the avalanche single-photon detector in the first receiver 2 receives the signal light photons and responds, it generates fluorescent photons and carries polarization information into the channel, which is intercepted by the second receiver through the circulator 3. The basis is selected in the beam splitter 4 and output at one of the two output ports with a probability of 50%. Polarization calibration is performed through the polarization controller 5 / 6 to correct the drift of polarization in the optical path. When passing through the polarization beam splitter 7 / 8, if the correct basis is selected, its polarization state will be correctly measured by the superconducting single-photon detector. If the wrong basis is selected, there will be a 50% probability of obtaining the correct measurement result.

[0046] In the traditional detection method of reflected fluorescence, the second receiving end usually uses an avalanche single-photon detector to detect fluorescent photons, but the dark count of the avalanche single-photon detector is high and the number of fluorescent photons is low. Usually, the number of fluorescent photons is much lower than the dark count of the detector. In addition, since the wavelength of the generated fluorescent photons is not fixed, its spectrum is distributed in a certain frequency domain. When passing through optical devices such as polarization controllers and polarization beam splitters in the optical path, these optical devices are sensitive to wavelength, which affects the polarization of fluorescent photons when passing through these optical devices. Changes, resulting in the second receiving end's low polarization discrimination when decoding fluorescent photons. Due to these reasons, it is difficult for the second receiving end to set the polarization controller to an optimal range during the polarization calibration stage in traditional fluorescence detection methods. Therefore, in order to improve the second receiving end's discrimination of the polarization state of fluorescent photons, the following information extraction optimization method is invented.

[0047] First, the upper limit of the discrimination degree of the second receiving end for the polarization state of the fluorescent photons is theoretically calculated to provide the target polarization discrimination degree for calibration in the polarization calibration phase, and then the polarization calibration is performed. The information extraction device equipped with a superconducting single-photon detector is used to optimize the decoding ability of the second receiving end for the fluorescent photons to the greatest extent. The steps are as follows:

[0048] Step 1: Use the FDTD platform to digitally model the physical structure of the pn junction in the avalanche single-photon detector at the first receiving end, simulate the avalanche process of the avalanche single-photon detector, monitor the secondary photon emission of the pn junction when the avalanche effect occurs, and fit the spectrum of the fluorescence photons.

[0049] Step 2: Digital modeling of the three-ring polarization controller:

[0050] Calculate the phase delay of the polarization controller propeller change:

[0051]

[0052] In the formula is the phase delay, a is a constant (for quartz fiber a is equal to 0.133), N is the number of turns, d is the fiber cladding diameter, λ is the wavelength, and D is the diameter of the coil.

[0053] Calculate the Jones matrix for the effect of the three propellers on polarization:

[0054]

[0055] Where U θ is the Jones matrix of the polarization controller propeller, and θ1, θ2, and θ3 are related to the rotation angles of the three propellers, respectively. By cascading the matrices of the three propellers, the simulation model of the three-ring polarization controller can be simulated. The three-ring polarization controller (THORLABS-FPC030) refers to the formation of independent wave plates by winding the optical fiber on two or three circular disks to produce stress birefringence, thereby changing the polarization state of light transmitted in a single-mode optical fiber. The fast axis of the optical fiber is located in the plane of the winding disk, so the polarization state of any input light can be adjusted by the propeller. We can provide polarization controllers with three-ring configurations, with winding disks of 27mm or 56mm in diameter; we can also provide controllers with mini dual-ring configurations, with winding disks of 18mm in diameter.

[0056] Step 3: Digital modeling of the polarization beam splitter: According to the wavelength-sensitive characteristics of the polarization beam splitter, the two polarization states of photons of each wavelength correspond to different transmittances on the two output channels. The transmittance parameters of the two channels of the polarization beam splitter at different wavelengths are written into the simulation program, and two curves of transmittance changing with wavelength are fitted.

[0057] Step 4: Take the fluorescence spectrum described in step 1 as input, assume that the photons of each wavelength are in a specific polarization state, record it as the initial state, and simulate the process of fluorescence photons propagating in the optical path from the first receiving end in the reverse direction.

[0058] ① Apply the result in step 3 to the initial state, and the calculated result is recorded as the first impact state;

[0059] ② Apply the matrix of step 2 to the first impact state, and the calculation result is recorded as the second impact state;

[0060] ③ Apply the result in step 3 to the second impact state, and the calculated result is recorded as the third impact state;

[0061] ④ Apply the matrix of step 2 to the third impact state, and record the calculation result as the final state.

[0062] Step 5: Perform projection measurement on photons of each wavelength, calculate the measurement probabilities of the two polarization states, and then weight the number of photons at different wavelengths according to the spectrum of the final state to obtain the weighted measurement probabilities of the two polarization states. The ratio of the two is the simulation result of the polarization discrimination of fluorescence decoding by the second receiving end, which is recorded as the target polarization discrimination.

[0063] Step 6: The third port of the circulator is connected to the second receiving end, that is, the optical fiber channel between the QKD system transmitting end and the QKD system first receiving end. The transmitting end transmits a fixed polarization state (|H> / |V>) signal light photon in the first group of horizontal bases to the first receiving end. Its polarization state is recorded as the polarization state to be calibrated.

[0064] Step 7: Turn on the fluorescence detection device, and turn on the first and second superconducting single photon detectors (with the same quantum P-SPD-10S). When the fluorescence photons reach the superconducting single photon detectors, the superconducting single photon detectors can respond in real time.

[0065] Step 8: Calibrate the optical path of the first group of bases at the second receiving end; set the polarization controller of the second receiving end corresponding to the base of the signal light polarization state, observe the detection counts of the two superconducting single-photon detectors under this base, and adjust the detection efficiency (detection reading) of one superconducting single-photon detector connected to it to be maximum and the detection efficiency of the other to be minimum by changing the angle of the polarization controller; the fluorescence polarization state received by the superconducting single-photon detector with the maximum detection efficiency is the polarization state to be calibrated, and the fluorescence polarization state received by the superconducting single-photon detector with the minimum detection efficiency is another polarization state in this group of bases; the ratio of the maximum detection efficiency to the minimum detection efficiency of the superconducting single-photon detector is recorded as the actual polarization discrimination, and when the actual polarization discrimination is within the range of (0.9 to 1.1) times the target polarization discrimination, it is determined that the polarization calibration is successful;

[0066] In step 9: the QKD system transmits and receives a fixed polarization state (|A> / |D>) in another set of diagonal bases, repeats step 8, turns on the third and fourth superconducting single-photon detectors (with the same quantum P-SPD-10S), and completes the polarization calibration of the other set of bases at the second receiving end.

[0067] The FDTD platform (software) is a tool widely used in the field of optical and electromagnetic simulation. It simulates the propagation, scattering, reflection and refraction of light in the medium by solving the Maxwell equations.

[0068] If an avalanche single-photon detector is used to receive fluorescent photons at the second receiving end, there are two methods for polarization calibration. The first method is similar to this solution, which controls the polarization controller in real time by the number of fluorescent light. However, the disadvantage of this method is that the dark count of the avalanche single-photon detector is too high and the dark count changes in real time, making it difficult to distinguish fluorescent photons from the dark count. Therefore, the feasibility of this method is not high. Another method is to input the laser in reverse at the output port of the polarization splitter at the first receiving end, and then adjust the polarization controller according to the detection efficiency of the two corresponding detectors at the second receiving end. The disadvantage of this method is that the wavelength of the input light source can only be fixed, while the wavelength of fluorescent light is distributed in a certain frequency domain. Even if a considerable polarization discrimination is obtained when the laser is irradiated in reverse for polarization calibration, the actual decoding effect of fluorescent light is not good.

[0069] Therefore, the scheme of the present invention aims at the characteristics of fluorescence and makes up for the deficiencies in decoding fluorescence polarization information. Due to the uncertainty of fluorescence wavelength, it is difficult to ensure the optimal effect of polarization calibration, so the upper limit of information extraction needs to be determined by data simulation, so as to determine the target polarization discrimination for polarization calibration. Taking advantage of the low dark count of superconducting single-photon detectors, polarization calibration is performed in real time according to the count feedback of fluorescence, which effectively avoids the drawbacks of traditional methods. The use of the scheme of the present invention can avoid the adverse effects of detector dark counts to the greatest extent, thereby optimizing the information extraction based on fluorescence reflection characteristics of the polarization-encoded quantum key distribution system to the greatest extent.

[0070] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, the elements defined by the sentence "comprise a ..." do not exclude the existence of other identical elements in the process, method, article or device including the elements.

[0071] Each embodiment in this specification is described in a related manner, and the same or similar parts between the embodiments can be referenced to each other, and each embodiment focuses on the differences from other embodiments.

[0072] The above description is only a preferred embodiment of the present invention and is not intended to limit the protection scope of the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention are included in the protection scope of the present invention.

Claims

1. An optimization method for extracting information based on fluorescence reflection characteristics for a polarization-encoded quantum key distribution system, characterized in that: The fluorescence reflection characteristic information extraction device includes: a circulator, a 50:50 optical beam splitter, a first polarization controller, a second polarization controller, a first polarization beam splitter, a second polarization beam splitter, a first superconducting single photon detector, a second superconducting single photon detector, a third superconducting single photon detector, a fourth superconducting single photon detector and an optical fiber for connecting the optical path between the QKD system transmitting end and the QKD system first receiving end; The circulator has three ports, the first port of the circulator is connected to the side of the channel close to the transmitting end, the second port of the circulator is connected to the side of the channel close to the first receiving end, and the third port of the circulator is connected to the second receiving end; the signal light photons of the transmitting end are transmitted from the first port to the second port, but cannot be transmitted from the first port to the third port; the fluorescent photons generated by the avalanche single photon detector in the first receiving end are transmitted from the second port to the third port, and cannot be transmitted from the third port to the first port; The 50:50 optical beam splitter includes the following features: the optical beam splitter has an input port and two output ports, and the two output ports output the photons input from the input port at a fractional ratio of 50:50; the input port of the beam splitter is connected to the third port of the circulator, the first output port is sequentially connected to the first polarization controller and the first polarization beam splitter; the second output port is sequentially connected to the second polarization controller and the second polarization beam splitter; The first and second polarization beam splitters include the following features: the first and second polarization beam splitters are used to measure polarization states under two different bases, namely, a horizontal base ([0°], [90°]) and a diagonal base ([45°], [135°]); the polarization beam splitter includes three ports, and the polarization beam splitter distinguishes the polarization states of the fluorescent photons received at the first port, and the photons with the polarization state |V> are output at the second port, and the photons with the polarization state |H> are output at the third port; the first port of the first polarization beam splitter is connected to the first polarization controller, and the second and third ports are connected to the first superconducting single photon detector and the second superconducting single photon detector respectively; the first port of the second polarization beam splitter is connected to the second polarization controller, and the second and third ports are connected to the third superconducting single photon detector and the fourth superconducting single photon detector respectively; The method for extracting and optimizing information based on fluorescence reflection characteristics includes the following steps: Step 1: Use the FDTD platform to digitally model the physical structure of the pn junction of the avalanche single photon detector at the first receiving end, simulate the avalanche process of the avalanche single photon detector, monitor the secondary photon emission when the pn junction avalanche effect occurs, and fit the spectrum of the fluorescence photons; Step 2: Digital modeling of the first and second three-ring polarization controllers: Calculate the phase delay of the polarization controller propeller change: In the formula is the phase delay, a is a constant (quartz fiber a is equal to 0.133), N is the number of turns, d is the fiber cladding diameter, λ is the wavelength, and D is the diameter of the coil; Calculate the Jones matrix for the effect of the three propellers on polarization: Where U θ is the Jones matrix of the polarization controller propeller, θ1, θ2, and θ3 are respectively related to the rotation angles of the three propellers; by cascading the matrices of the three propellers, the simulation model of the three-ring polarization controller can be simulated; Step 3: Digital modeling of the polarization beam splitter: According to the wavelength-sensitive characteristics of the polarization beam splitter, the two polarization states of photons of each wavelength correspond to different transmittances on the two output channels. The transmittance parameters of the two channels of the polarization beam splitter at different wavelengths are written into the simulation program, and two curves of transmittance varying with wavelength are fitted; Step 4: The fluorescence spectrum described in step 1 is used as input, and the photons of each wavelength are assumed to be in a specific polarization state, which is recorded as the initial state; the process of reverse propagation of the fluorescence photons from the first receiving end in the optical path is simulated; ① Apply the result in step 3 to the initial state, and the calculated result is recorded as the first impact state; ② Apply the matrix of step 2 to the first impact state, and the calculation result is recorded as the second impact state; ③ Apply the result in step 3 to the second impact state, and the calculated result is recorded as the third impact state; ④ Apply the matrix of step 2 to the third impact state, and the calculation result is recorded as the final state; Step 5: Project and measure the photons of each wavelength, calculate the measurement probabilities of the two polarization states, and then weight the number of photons at different wavelengths according to the spectrum of the final state to obtain the weighted measurement probabilities of the two polarization states. The ratio of the two is the simulation result of the polarization discrimination of fluorescence decoding by the second receiving end, which is recorded as the target polarization discrimination; Step 6, starting the QKD system, in the polarization calibration phase of the first group of bases, the transmitter transmits a fixed polarization state signal light photon in the first group of bases to the first receiving end; its polarization state is recorded as the polarization state to be calibrated; Step 7: Turn on the fluorescence detection device, and turn on the first and second superconducting single-photon detectors; Step 8: Calibrate the optical path of the first group of bases at the second receiving end; set the polarization controller of the second receiving end corresponding to the base of the signal light polarization state, observe the detection counts of the two superconducting single-photon detectors under this base, and adjust the detection efficiency of one superconducting single-photon detector connected to it to be maximum and the detection efficiency of the other to be minimum by changing the angle of the polarization controller; the fluorescence polarization state received by the superconducting single-photon detector with the maximum detection efficiency is the polarization state to be calibrated, and the fluorescence polarization state received by the superconducting single-photon detector with the minimum detection efficiency is another polarization state in this group of bases; the ratio of the maximum detection efficiency to the minimum detection efficiency of the superconducting single-photon detector is recorded as the actual polarization discrimination, and when the actual polarization discrimination is within the range of (0.9 to 1.1) times the target polarization discrimination, it is determined that the polarization calibration is successful; Step 9: The QKD system transmits and receives a fixed polarization state in another set of bases, repeats step 8, turns on the third and fourth superconducting single-photon detectors, and completes the polarization calibration of another set of bases at the second receiving end.

2. The information extraction optimization method based on fluorescence reflection characteristics of the polarization-coded quantum key distribution system according to claim 1 is characterized in that: The setting parameters of the pn junction in the FDTD platform in step 1 are the actual data of the avalanche single-photon detector pn junction in the actual QKD system, including material selection, material thickness, and bias voltage.

3. The information extraction optimization method based on fluorescence reflection characteristics of the polarization-coded quantum key distribution system according to claim 1 is characterized in that: The polarization controller propeller setting parameters in step 2 are the actual parameters of the polarization controller in the actual QKD system, including optical fiber material, winding diameter, and number of windings.

4. The information extraction optimization method based on fluorescence reflection characteristics of the polarization-coded quantum key distribution system according to claim 1 is characterized in that: The transmittances of the two polarization states in the two channels at different wavelengths in step 3 are transmittance parameters of the two channels of the polarization beam splitter actually used in the fluorescence extraction device.

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