Channel-associated local oscillator free space continuous variable quantum key distribution method and system

Abstract

The invention discloses a channel-associated local oscillator free space continuous variable quantum key distribution method, which comprises the following steps of: initializing a receiving end telescope and a transmitting end telescope, detecting the local oscillator light intensity of a receiving end, adjusting the telescopes to be in an optimal coupling efficiency state, and tracking the change of beacon light through a fine tracking and pointing system of the telescopes so as to maintain the stability of a communication link; a transmitting end performs heterodyne detection on 90% of thermal-state optical signals after polarization and beam splitting of a thermal-state light source, results X and P serve as local initial secret keys, the other 10% of thermal-state optical signals are attenuated and then combined with local oscillation light to be transmitted to a receiving end through a free space channel so as to perform homodyne or heterodyne detection, and corresponding initial continuous secret key data are obtained; and the receiving end performs data frame synchronization, phase compensation, parameter evaluation, error correction and confidentiality enhancement on the obtained initial continuous key data to obtain a binary bit key. The channel associated local oscillator free space CVQKD safe coding based on the thermal state source is realized, and the average key distribution rate under a-15dB attenuation channel reaches a thousand bps magnitude.

Description

Technical Field

[0001] The present invention belongs to the technical field of quantum key distribution, and particularly relates to a method and system for free-space continuous-variable quantum key distribution with an accompanying local oscillator. Background Art

[0002] Currently, most continuous-variable quantum key distribution systems (CVQKD) use amplitude and phase modulators to modulate and encode random key information generated by a quantum true random number generator onto weak coherent light for key distribution. For example, in Gaussian modulated quantum key distribution (GMCS-CVQKD), amplitude and phase modulators are used to modulate Gaussian-distributed quantum true random numbers onto the canonical components of weak coherent light and send them to the receiving party through an optical fiber or free-space channel. The discrete-variable quantum key modulation system (DVQKD) also realizes the modulation of a finite number of quantum states through amplitude and phase modulators and sends them to the receiving party for demodulation and detection. Both of these methods not only require a quantum true random number generator but also use active amplitude and phase modulators to achieve linear modulation of optical signals.

[0003] However, the operation of actual devices usually has non-linear effects, resulting in the inability to achieve linear modulation of the generated initial key information on the optical field components, thus affecting the system implementation.

[0004] In addition, in the field of quantum key distribution based on thermal light sources, whether it is a free-space channel or a free-space channel, there is no mature quantum key distribution scheme yet. Summary of the Invention

[0005] The purpose of the present invention is to provide a method and system for free-space continuous-variable quantum key distribution with an accompanying local oscillator, realizing secure key generation for free-space CVQKD with an accompanying local oscillator based on a thermal light source, and achieving an average key distribution rate of the order of thousands of bps in a -15 dB attenuation channel.

[0006] To solve the above problems, the technical solution of the present invention is as follows: A method for free-space continuous-variable quantum key distribution with an accompanying local oscillator, comprising: Initializing the telescopes at the receiving and transmitting ends, adjusting the telescopes to the optimal coupling efficiency state by detecting the intensity of the local oscillator light at the receiving end, and tracking the change of the beacon light through the fine tracking and aiming system of the telescopes to maintain the stability of the communication link; The transmitting end performs heterodyne detection on 90% of the thermally generated light signals after polarization splitting of the thermal light source, uses the results X and P as local initial keys, and attenuates the other 10% of the thermally generated light signals and combines them with the local oscillator light to be transmitted through the free-space channel to the receiving end for homodyne or heterodyne detection to obtain the corresponding initial continuous key data; The receiving end performs data frame synchronization, phase compensation, parameter evaluation, error correction, and confidentiality enhancement on the obtained initial continuous key data to obtain binary bit keys.

[0007] According to an embodiment of the present invention, the sending end amplifies the thermal optical signal generated by the ASE thermal light source and then passes it through a narrowband optical filter to obtain a thermal optical signal with wavelengths concentrated at 1550 nm. The thermal optical signal is divided into two beams by a 90:10 optical splitter. Among them, 90% of the thermal optical signal and a part of the local oscillator optical signal generated by a 1550 nm laser source that passes through a 50:50 optical splitter are simultaneously input into an optical mixer and then an external heterodyne detection is performed using a photodetector. Based on the detected regular components X and P, a local initial key is obtained; the other 10% of the thermal optical signal is attenuated by an optical attenuator and then combined with another part of the local oscillator optical signal through a polarization beam combiner and sent to the receiving end through a free space channel; The receiving end adjusts the polarization of the received optical signal and then performs polarization demultiplexing to obtain a thermal optical signal and a local oscillator optical signal; the local oscillator optical signal is amplified and then filtered using a 1550 nm narrowband filter to reduce ASE noise; the thermal optical signal is passed through a 99:1 optical splitter, and an optical power meter is used to measure a beam of thermal optical signal with a relatively small proportion as feedback for adjusting polarization to monitor polarization leakage; a beam of thermal optical signal with a relatively large proportion and the filtered local oscillator optical signal are simultaneously input into an optical mixer, and a homodyne detection is performed using a photodetector to obtain the regular component X or P of the thermal optical signal as the initial continuous key data.

[0008] According to an embodiment of the present invention, the sending end performs preparation noise control by adjusting the ASE thermal light source, optical amplifier, and optical attenuator, so that the preparation noise is less than 0.005. At this time, the modulation variance V A has a value range greater than 0 and less than 20, where V A = 0.5n0, and n0 is the average photon number of the thermal optical signal after passing through a band-pass filter.

[0009] According to an embodiment of the present invention, the receiving end performs frame synchronization on the initial continuous key data to obtain original synchronization data; Based on a machine learning model, phase compensation is performed on the original synchronization data; Parameter evaluation is performed on the original synchronization data after phase compensation to obtain corresponding over-noise and modulation variance parameters; Based on channel parameters including over-noise and modulation variance, the Holevo bound and the mutual information of legitimate communication parties are calculated to obtain the corresponding information compression rate, and the final key is output after confidentiality enhancement processing.

[0010] According to an embodiment of the present invention, performing phase compensation on the original synchronization data based on a machine learning model further includes: Before performing quantum key distribution, phase drift data is obtained through signal interaction between the sending end and the receiving end as training data for the machine learning model. Based on the training data, the recurrent neural network model RNN is trained to meet the preset requirements. The trained recurrent neural network model RNN is used to perform phase compensation on the original synchronization data.

[0011] According to an embodiment of the present invention, initializing the telescopes at the receiving and sending ends, and adjusting the telescopes to the optimal coupling efficiency state by detecting the local oscillator light intensity at the receiving end further includes: Initializing the telescope systems at the sending and receiving ends, including initializing the coarse tracking camera, fine tracking camera, and computer control system. The sending and receiving ends turn on their respective beacon lights and adjust them to an intensity that can be clearly displayed in the other party's camera; the sending end sends 1550nm local oscillator light. Based on the detection of the light intensity of the received 1550nm band, the telescope systems at the sending and receiving ends are adjusted until the received light intensity at the receiving end reaches the expected value and is marked as the optimal coupling point in the system. Turn on the automatic fine tracking functions at the sending and receiving ends to maintain the optimal coupling efficiency state.

[0012] According to an embodiment of the present invention, without time-division multiplexing, wavelength-division multiplexing, and pulse modulation, the sending and receiving ends achieve the interference of continuous hot-state signal light and local oscillator light signal, and use a photodetector and an oscilloscope to quickly obtain the initial continuous key data to achieve continuous variable quantum key distribution.

[0013] A free-space continuous variable quantum key distribution system with an accompanying local oscillator includes: An initialization module for initializing the telescopes at the receiving and sending ends, adjusting the telescopes to the optimal coupling efficiency state by detecting the local oscillator light intensity at the receiving end, and tracking the change of the beacon light through the fine tracking system of the telescope to maintain the stability of the communication link. A sending end execution module for performing heterodyne detection on 90% of the hot-state optical signal after polarization splitting of the hot-state light source, taking the results X and P as the local initial key, and attenuating the other 10% of the hot-state optical signal and combining it with the local oscillator light to be transmitted through the free-space channel to the receiving end for homodyne or heterodyne detection to obtain the corresponding initial continuous key data. A receiving end execution module for performing data frame synchronization, phase compensation, parameter evaluation, error correction, and privacy amplification on the obtained initial continuous key data to obtain a binary bit key.

[0014] According to an embodiment of the present invention, the transmitting end execution module includes an ASE hot light source, an optical amplifier, a narrowband optical filter, a 90:10 optical splitter, a 50:50 optical splitter, an optical mixer, a photodetector, an optical attenuator, and a polarization beam combiner; The hot optical signal generated by the ASE hot light source is amplified by the optical amplifier and then passes through the narrowband optical filter to obtain a hot optical signal with a wavelength concentrated at 1550 nm. The hot optical signal is divided into two beams by the 90:10 optical splitter. Among them, 90% of the hot optical signal and a part of the local oscillator optical signal generated by the 1550 nm laser source passing through the 50:50 optical splitter are simultaneously input into the optical mixer and then the heterodyne detection is performed using the photodetector. Based on the detected regular components X and P, the local initial key is obtained; the other 10% of the hot optical signal is attenuated by the optical attenuator and then combined with the other part of the local oscillator optical signal through the polarization beam combiner and sent to the receiving end through the free space channel.

[0015] According to an embodiment of the present invention, the receiving end execution module includes a polarization controller, a polarization beam splitter, an optical amplifier, a band-pass filter, a 99:1 optical splitter, an optical power meter, and a 90° optical mixer; The polarization controller and the polarization beam splitter perform polarization demultiplexing on the received optical signal after adjusting the polarization to obtain a hot optical signal and a local oscillator optical signal; the local oscillator optical signal is amplified by the optical amplifier and then filtered using a 1550 nm band-pass filter to reduce the ASE noise; the hot optical signal passes through the 99:1 optical splitter, and the optical power meter measures a beam of hot optical signal with a relatively small proportion as the feedback for adjusting the polarization to monitor the polarization leakage; a beam of hot optical signal with a relatively large proportion and the filtered local oscillator optical signal are simultaneously input into the 90° optical mixer, and the homodyne detection is performed using the photodetector to obtain the regular component X or P of the hot optical signal as the initial continuous key data.

[0016] Due to the adoption of the above technical solutions, the present invention has the following advantages and positive effects compared with the prior art: 1) In the method for free-space continuous-variable quantum key distribution with an accompanying local oscillator in an embodiment of the present invention, by initializing the telescopes at the receiving and transmitting ends, by detecting the intensity of the local oscillator light at the receiving end, the telescopes are adjusted to the state with the optimal coupling efficiency, and the change of the beacon light is tracked through the fine tracking and aiming system of the telescopes to maintain the stability of the communication link; the transmitting end performs heterodyne detection on 90% of the thermal light signals after polarization splitting of the thermal light source, takes the results X and P as the local initial key, and attenuates the other 10% of the thermal light signals and combines them with the local oscillator light to be transmitted through the free-space channel to the receiving end for homodyne or heterodyne detection to obtain the corresponding initial continuous key data; the receiving end performs data frame synchronization, phase compensation, parameter evaluation, error correction, and privacy amplification on the obtained initial continuous key data to obtain the binary bit key. It realizes secure key generation of free-space CVQKD with an accompanying local oscillator based on a thermal light source, and the average key distribution rate reaches the order of thousands of bps in a -15 dB attenuation channel.

[0017] 2) According to the calculation formula of the passively prepared excess noise in the present invention, it is obtained that when the beam splitting ratio of the beam splitter at the transmitting end changes, it will affect the passively prepared excess noise of the system. Therefore, using a 90:10 beam splitter can greatly reduce this part of the noise compared with a 50:50 beam splitter. Further, the transmitting end controls the prepared excess noise by adjusting the ASE thermal light source, optical amplifier, and optical attenuator, so that the prepared excess noise is less than 0.005, which can also reduce this part of the noise.

[0018] 3) When the receiving end performs phase compensation on the original synchronization data, compared with the compensation method of compensating the phase drift of the next period through the phase drift of the previous period of data, using an RNN to predict the next point by inputting the data of the previous period of time can better utilize the change law of the phase drift in the historical data for phase compensation. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a block diagram of a free-space continuous-variable quantum key distribution system with an accompanying local oscillator in an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0020] The following further elaborates in detail on a method and system for free-space continuous-variable quantum key distribution with an accompanying local oscillator proposed by the present invention in conjunction with the accompanying drawings and specific embodiments. According to the following description and the claims, the advantages and features of the present invention will be clearer.

[0021] This embodiment provides a method for free-space continuous-variable quantum key distribution with an accompanying local oscillator, which can not only generate quantum true random numbers using the inherent random fluctuations of a heat source, eliminating the need for a random number generator and amplitude and phase modulators, but also only adds some preparation excess noise, and these additional preparation excess noises can be suppressed by controlling the average photon number of the thermal light source, the signal light splitting ratio, and the attenuation coefficient in the optical path. It is worth noting that using a high-precision communication system, by increasing the beam splitter splitting ratio and the attenuation coefficient, the total excess noise of quantum key distribution based on a thermal light source is reduced; using a pilotless phase retrieval algorithm based on machine learning; using an EDFA to post-amplify the local oscillator light at the Bob end to make the detector reach the shot noise limit are the key technical breakthroughs of the quantum key distribution scheme for free-space channel transmission based on a thermal light source. This method uses the regular components generated by the inherent random fluctuations of a thermal light source to achieve random encoding of non-commuting optical field components in continuous-variable quantum cryptographic communication. Although it equivalently increases the preparation excess noise of some encoded signals, it can achieve continuous-variable quantum key distribution in a free-space channel without a quantum true random number generator and amplitude and phase modulators.

[0022] Among them, ASE (amplified spontaneous emission) is an amplified spontaneous emission heat source, EDFA (Er-Doped fiber amplifier) is an erbium-doped fiber amplifier, and Hom (Homdyne Detector) is a homodyne detector. The Alice end (transmitting end) internally includes a 50:50 beam splitter and a 90:10 beam splitter, which respectively divide the input local oscillator light and signal light into two paths. One path of the local oscillator light splitting and the stronger path of the signal light splitting pass through a 90° optical mixer and two balanced homodyne detectors for heterodyne detection. The Bob end (receiving end) internally includes a polarization controller for controlling the polarization state of the signal light, a polarization beam splitter for polarization demultiplexing, and a 99:1 beam splitter for monitoring polarization leakage. After the local oscillator light is amplified by an EDFA and filtered by a band-pass filter after splitting, it is subjected to heterodyne detection with the signal light.

[0023] Specifically, the method for free-space continuous-variable quantum key distribution with an accompanying local oscillator includes the following steps: S1: Initialize the telescopes at the receiving and transmitting ends. By detecting the intensity of the local oscillator light at the receiving end, adjust the telescopes to the optimal coupling efficiency state, and track the changes of the beacon light through the fine tracking and aiming system of the telescopes to maintain the stability of the communication link; S2: The transmitting end performs heterodyne detection on 90% of the thermally polarized light signals after beam splitting of the thermal light source, takes the results X and P as the local initial keys, and attenuates the other 10% of the thermally polarized light signals and combines them with the local oscillator light to be transmitted through the free-space channel to the receiving end for homodyne or heterodyne detection to obtain the corresponding initial continuous key data; S3: The receiving end performs data frame synchronization, phase compensation, parameter evaluation, error correction, and privacy amplification on the obtained initial continuous key data to obtain binary bit keys.

[0024] This method directly uses a thermal light source to implement continuous-variable quantum key distribution in a free-space channel without intensity and phase modulators and a random number source, reducing the implementation complexity of the continuous-variable quantum key distribution system while achieving continuous-variable quantum key distribution with a high key rate.

[0025] Please refer to Figure 1 , the sending end optically amplifies the thermal optical signal generated by the ASE thermal light source and then passes it through a narrowband optical filter to obtain a thermal optical signal with a wavelength concentrated at 1550 nm. The thermal optical signal is divided into two beams by a 90:10 optical beam splitter. Among them, 90% of the thermal optical signal and a part of the local oscillator optical signal generated by a 1550 nm laser source passing through a 50:50 optical beam splitter are simultaneously input into an optical mixer and then an external heterodyne detection is performed using a photodetector. Based on the detected quadrature components X and P, a local initial key is obtained; the other 10% of the thermal optical signal is attenuated by an optical attenuator and then combined with the other part of the local oscillator optical signal through a polarization beam combiner and sent to the receiving end through a free-space channel; The receiving end adjusts the polarization of the received optical signal and then performs polarization demultiplexing to obtain a thermal optical signal and a local oscillator optical signal; the local oscillator optical signal is amplified and then filtered using a 1550 nm narrowband filter to reduce the ASE noise; the thermal optical signal is passed through a 99:1 optical beam splitter, and an optical power meter is used to measure a beam of thermal optical signal with a relatively small proportion as a feedback for adjusting the polarization to monitor the polarization leakage; a beam of thermal optical signal with a relatively large proportion and the filtered local oscillator optical signal are simultaneously input into an optical mixer, and a homodyne detection is performed using a photodetector to obtain the quadrature component X or P of the thermal optical signal as the initial continuous key data.

[0026] In the local oscillator system along with the signal, the local oscillator optical signal is sent from the sending end to the receiving end through polarization multiplexing along with the signal optical signal. Due to the large attenuation of the external field channel, the intensity of the local oscillator optical signal received at the receiving end is very low. Since the intensity of the electrical signal finally detected by coherent detection is directly related to the intensity of the local oscillator optical signal, the shot noise limit cannot be reached under the condition of a weak local oscillator optical signal, that is, the quantum signal cannot be detected. Therefore, it is necessary to amplify and filter the local oscillator optical signal at the receiving end.

[0027] In step S1, the telescopes at the receiving and sending ends are initialized. By detecting the intensity of the local oscillator optical signal at the receiving end, the telescopes are adjusted to the optimal coupling efficiency state, and the change of the beacon optical signal is tracked through the fine tracking and aiming system of the telescopes to maintain the stability of the communication link. Further, it includes: Step S1.1: The sender Alice and the receiver Bob initialize their respective telescope systems, including initializing the coarse tracking camera, the fine tracking camera, and the computer control system.

[0028] Step S1.2: The sender Alice and the receiver Bob turn on their respective beacon lights and adjust them to an appropriate intensity that can be clearly displayed in each other's cameras. Alice turns on the local oscillator light at 1550 nm.

[0029] Step S1.3: The sender Alice and the receiver Bob adjust their respective systems by the receiver Bob detecting the intensity of the received light in the 1550 nm band until the intensity of the received light by Bob reaches the expected value, and mark this position in the system as the optimal coupling point for this experiment. Both sides turn on the automatic fine tracking function of their respective telescopes to ensure that the system can maintain a high coupling efficiency state in subsequent experiments.

[0030] Generally, in the laboratory, the coupling efficiency at both ends is optimized by manually adjusting the angle of the coupler, and this coupling efficiency can remain stable for a long time. However, in the field, due to the complex optical path and large channel length in the coupling device, the influence of atmospheric turbulence, mechanical vibration, thermal expansion and contraction, etc. causes the channel coupling efficiency to fluctuate more significantly. Therefore, the two telescopes need to use automatic fine tracking to adjust in real time.

[0031] In step S2, the sender Alice performs heterodyne detection on half of the thermal light signal after polarization splitting of the thermal light source, takes the results X and P as the local initial key, attenuates the other half of the thermal light signal and combines it with the local oscillator light, and then transmits it through the free space channel and is detected by the receiver Bob through homodyne or heterodyne detection to obtain the corresponding initial continuous key data.

[0032] Specifically, it includes the following steps: Step S2.1: The sender Alice and the receiver Bob perform communication initialization on the free space continuous variable quantum key distribution system with an in-path local oscillator based on a thermal light source, including initializing the ASE thermal light source, optical amplifier, optical bandpass filter, polarization beam splitter, 90° optical mixer, photodetector, and control circuit in the system; Step S2.2: The hot-state optical signal generated by the ASE hot-state light source at the Alice side is amplified by an optical amplifier and then passes through a narrowband optical filter to obtain a hot-state optical signal with wavelengths concentrated at 1550 nm. Then it is split into two beams by a 90:10 optical splitter. The stronger beam and a part of the local oscillator optical signal generated by a 1550-nm laser source passing through a 50:50 optical splitter are simultaneously input into an optical mixer, and then heterodyne detection is performed using a photodetector. The regular components X and P of the detection result are used to obtain the initial key Key1. The other weaker beam is attenuated by an optical attenuator, combined with the other part of the local oscillator optical signal through a polarization beam combiner, and then sent to the receiver Bob through a free-space channel; Step S2.3: At the receiver Bob side, the optical signal sent by Alice is adjusted in polarization and then polarization demultiplexing is performed to obtain the hot-state optical signal and the local oscillator optical signal. An erbium-doped fiber amplifier (EDFA) is used to amplify the local oscillator optical signal, and the output light is filtered using a 1550-nm narrowband filter to reduce ASE noise. The hot-state optical signal is passed through a 99:1 optical splitter, and an optical power meter is used to measure the smaller proportion of the hot-state optical signal as the feedback for adjusting polarization to monitor polarization leakage; the larger proportion of the hot-state optical signal and the local oscillator optical signal are simultaneously input into an optical mixer, and homodyne detection is performed using a photodetector to obtain the regular component X or P of the received hot-state optical signal as the initial key data Key2.

[0033] Further, step S2.2 includes: Step S2.2.1: The sender Alice performs preparation noise control by adjusting the ASE light source, the optical amplifier, and the optical attenuator, so that the preparation noise is less than 0.005. At this time, the modulation variance V A has a value range greater than 0 and less than 20, where V A =0.5n0, and n0 is the average photon number of the hot-state optical signal after passing through the bandpass filter; Step S2.2.2: The sender Alice retains the stronger half of the hot optical signal split by the 90:10 optical splitter locally, and simultaneously inputs it into a Hybrid mixer together with a part of the 1550-nm local oscillator optical signal generated by the laser. The interference of the two continuous optical signals is realized, and the initial key data X is obtained through a photodetector, which is the value of the regular components X and P of the hot-state optical signal. The other path is combined with the local oscillator through a polarization beam combiner and then sent to the receiver Bob together.

[0034] In the above step S2, according to the formula for calculating the passively prepared noise: where V A is the modulation variance, v ax is the axial velocity, ηax is the axial length, and η0 and η1 are the quantum efficiencies at different stages. It can be seen from this that when the beam splitting ratio of the above beam splitter changes, it will affect the system's passive preparation of excess noise. Therefore, a 90:10 beam splitter can greatly reduce this part of the noise compared to a 50:50 beam splitter. Similarly, by adjusting the ASE light source, optical amplifier, and optical attenuator to control the excess noise during preparation so that the excess noise is less than 0.005 is also a measure to reduce this part of the noise.

[0035] In step S3, the receiver Bob performs data frame synchronization, phase compensation, parameter evaluation, error correction, and secrecy enhancement on the obtained initial continuous key data to obtain a binary bit key, which further includes: Step S3.1: The receiver Bob and the sender Alice perform frame synchronization of the initial continuous key data to obtain the original synchronization data; Step S3.2: The receiver Bob and the sender Alice perform machine learning-based phase compensation on the original synchronization data; Step S3.3: The sender Alice and the receiver Bob publish part of the initial key data for parameter evaluation to obtain the excess noise and modulation variance parameters; Step S3.4: The receiver Bob calculates the Holevo bound and the mutual information of the legitimate communication parties through the channel parameters to obtain the information compression rate, and outputs the final key through secrecy enhancement.

[0036] Specifically, in the step S3.2: Step S3.2.1: For each experiment, the receiver Bob and the sender Alice need to send known deterministic signals before conducting the quantum key distribution experiment to obtain sufficient phase drift data. Use a recurrent neural network (RNN) to train these phase drift data as a training set. This RNN model is a relatively mature existing model, and its specific structure and principle will not be introduced in detail here.

[0037] Step S3.2.2: The sender Alice discloses part of the data in the original synchronization data, and the receiver Bob uses the trained RNN model to predict the phase drift based on the disclosed data and perform corresponding compensation.

[0038] Compared with simply compensating for the phase drift of the next period through the phase drift of the previous period of data, this method uses an RNN to predict the next point by inputting the data of the previous period of time, and can better utilize the change law of the phase drift in the historical data for phase compensation.

[0039] In the above-described accompanying local oscillator free-space continuous-variable quantum key distribution method, without time-division multiplexing, wavelength-division multiplexing, or pulse modulation, the transmitting end and the receiving end achieve the interference of continuous thermal state signal light and local oscillator light signal, and use a photodetector and an oscilloscope to rapidly obtain initial continuous key data, thereby realizing continuous-variable quantum key distribution. The average key distribution rate can reach the order of kilobits per second in a -15 dB attenuation channel; moreover, the over-noise model is improved, over-noise suppression based on the thermal state light source scheme is achieved, and a major problem in continuous-variable quantum key distribution based on the thermal state light source is solved; furthermore, by using the local oscillator post-amplification technology, the detection at the receiving end under weak local oscillator light conditions can reach the shot noise limit, the requirement for the intensity of the transmitted local oscillator light is reduced, the local oscillator light leakage noise is further suppressed, and the application scenarios of the system are greatly expanded.

[0040] Based on the same concept, this embodiment also provides an accompanying local oscillator free-space continuous-variable quantum key distribution system, including: An initialization module, configured to initialize the telescopes at the receiving and transmitting ends, adjust the telescopes to the optimal coupling efficiency state by detecting the intensity of the local oscillator light at the receiving end, and track the change of the beacon light through the fine tracking and aiming system of the telescopes to maintain the stability of the communication link; A transmitting end execution module, configured to perform heterodyne detection on 90% of the thermal state light signal after polarization splitting of the thermal state light source, use the results X and P as local initial keys, and attenuate the other 10% of the thermal state light signal and then combine it with the local oscillator light to be transmitted through the free-space channel to the receiving end for homodyne or heterodyne detection to obtain corresponding initial continuous key data; A receiving end execution module, configured to perform data frame synchronization, phase compensation, parameter evaluation, error correction, and privacy amplification on the obtained initial continuous key data to obtain binary bit keys.

[0041] Please refer to Figure 1 , the transmitting end execution module includes an ASE thermal state light source, an optical amplifier, a narrowband optical filter, a 90:10 optical splitter, a 50:50 optical splitter, an optical mixer, a photodetector, an optical attenuator, and a polarization beam combiner. The thermal state light signal generated by the ASE thermal state light source is amplified by the optical amplifier and then passes through the narrowband optical filter to obtain a thermal state light signal with wavelengths concentrated at 1550 nm, and it is divided into two beams by the 90:10 optical splitter. Among them, 90% of the thermal state light and a part of the local oscillator light signal generated by a 1550 nm laser source passing through the 50:50 optical splitter are simultaneously input into the optical mixer and then heterodyne detection is performed using the photodetector. Based on the detected regular components X and P, local initial keys are obtained; the other 10% of the thermal state light is attenuated by the optical attenuator and then combined with the other part of the local oscillator light signal through the polarization beam combiner and sent to the receiving end through the free-space channel.

[0042] The receiving-end execution module includes a polarization controller, a polarization beam splitter, an optical amplifier, a band-pass filter, a 99:1 optical splitter, an optical power meter, and a 90° optical mixer. The polarization controller and the polarization beam splitter perform polarization demultiplexing on the received optical signal after adjusting the polarization to obtain a thermal-state optical signal and a local oscillator optical signal; the local oscillator optical signal is amplified by the optical amplifier and then filtered by a 1550 nm band-pass filter to reduce ASE noise; the thermal-state optical signal passes through the 99:1 optical splitter, and the optical power meter measures a beam of thermal-state optical signal with a smaller proportion as the feedback for adjusting polarization to monitor polarization leakage; a beam of thermal-state optical signal with a larger proportion and the filtered local oscillator optical signal are simultaneously input into the 90° optical mixer, and homodyne detection is performed using a photodetector to obtain the canonical component X or P of the thermal-state optical signal as the initial continuous key data.

[0043] The in-path local oscillator free-space continuous variable quantum key distribution system can be implemented by executing the process steps of the above-mentioned in-path local oscillator free-space continuous variable quantum key distribution method.

[0044] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings, but the present invention is not limited to the above embodiments. Even if various changes are made to the present invention, provided that these changes fall within the scope of the claims of the present invention and their equivalent technologies, they still fall within the protection scope of the present invention.

Claims

1. A method for free-space continuous-variable quantum key distribution with an accompanying local oscillator, characterized in that, Including: Initialize the telescopes at the transmitting and receiving ends. By detecting the local oscillator light intensity at the receiving end, adjust the telescopes to the optimal coupling efficiency state, and track the changes of the beacon light through the fine tracking and aiming system of the telescopes to maintain the stability of the communication link; The transmitting end performs heterodyne detection on 90% of the thermal light signals after polarization splitting of the thermal light source in the hot state, takes the results X and P as the local initial key, and attenuates the other 10% of the thermal light signals and then combines them with the local oscillator light and transmits them through the free space channel to the receiving end for homodyne or heterodyne detection to obtain the corresponding initial continuous key data; The receiving end performs data frame synchronization, phase compensation, parameter evaluation, error correction, and privacy amplification on the obtained initial continuous key data to obtain the binary bit key.

2. The method for free-space continuous-variable quantum key distribution with an accompanying local oscillator according to claim 1, wherein The transmitting end amplifies the thermal light signal generated by the ASE thermal light source and then passes it through a narrowband optical filter to obtain a thermal light signal with a wavelength concentrated at 1550 nm, and divides it into two beams through a 90:10 optical splitter. Among them, 90% of the thermal light and a part of the local oscillator light signal generated by the 1550 nm laser source passing through a 50:50 optical splitter are simultaneously input into an optical mixer and then a photodetector is used to perform heterodyne detection. Based on the detected regular components X and P, obtain the local initial key; the other 10% of the thermal light is attenuated by an optical attenuator and then combined with another part of the local oscillator light signal through a polarization beam combiner and sent to the receiving end through the free space channel; The receiving end adjusts the polarization of the received optical signal and then performs polarization demultiplexing to obtain the thermal light signal and the local oscillator light signal; amplifies the local oscillator light signal and then filters it using a 1550 nm narrowband filter to reduce the ASE noise; passes the thermal light signal through a 99:1 optical splitter, and uses an optical power meter to measure a beam of thermal light signal with a smaller proportion as the feedback for adjusting the polarization to monitor the polarization leakage; simultaneously inputs a beam of thermal light signal with a larger proportion and the filtered local oscillator light signal into an optical mixer, and uses a photodetector to perform homodyne detection to obtain the regular component X or P of the thermal light signal as the initial continuous key data.

3. The method for free-space continuous-variable quantum key distribution with an accompanying local oscillator according to claim 2, characterized in that, The transmitting end performs preparation over-noise control by adjusting the ASE hot-state light source, optical amplifier, and optical attenuator, so that the preparation over-noise is less than 0.

005. At this time, the modulation variance V A has a value range greater than 0 and less than 20, where V A = 0.5n0, and n0 is the average number of photons of the hot-state optical signal after passing through the band-pass filter.

4. The method for free-space continuous-variable quantum key distribution with in-band local oscillator according to claim 1, wherein The receiving end performs frame synchronization on the initial continuous key data to obtain the original synchronous data; Based on the machine learning model, perform phase compensation on the original synchronous data; Perform parameter evaluation on the phase-compensated original synchronous data to obtain the corresponding excess noise and modulation variance parameters; Based on the channel parameters including excess noise and modulation variance, calculate the Holevo bound and the mutual information of the legitimate communication parties, obtain the corresponding information compression rate, and output the final key after privacy amplification processing.

5. The method for free-space continuous-variable quantum key distribution with in-band local oscillator as claimed in claim 4, wherein Based on the machine learning model, the phase compensation for the original synchronous data further includes: Before performing quantum key distribution, obtain the phase drift data through the signal interaction between the transmitting end and the receiving end as the training data of the machine learning model; Based on the training data, train the recurrent neural network model RNN to meet the preset requirements; Adopt the trained recurrent neural network model RNN to perform phase compensation on the original synchronous data.

6. The method for free-space continuous-variable quantum key distribution with an accompanying local oscillator as claimed in claim 1, wherein Initializing the telescopes at the transmitting and receiving ends and adjusting the telescopes to the optimal coupling efficiency state by detecting the local oscillator light intensity at the receiving end further includes: Initialize the telescope systems of the transmitter and the receiver, including initializing the coarse tracking camera, the fine tracking camera, and the computer control system; The transmitter and the receiver turn on their respective beacon lights and adjust them to an intensity that can be clearly displayed in the other's camera; the transmitter sends 1550 nm local oscillator light; Based on the detection of the intensity of the 1550 nm band light received, adjust the telescope systems of the transmitter and the receiver until the received light intensity at the receiver reaches the expected value and mark it as the optimal coupling point in the system; Turn on the automatic fine tracking functions of the transmitter and the receiver to maintain the optimal coupling efficiency state.

7. The method for free-space continuous-variable quantum key distribution with an accompanying local oscillator as claimed in claim 1, wherein Without time division multiplexing, wavelength division multiplexing, and pulse modulation, the transmitter and the receiver achieve the interference of the continuous hot state signal light and the local oscillator light signal, and use a photodetector and an oscilloscope to quickly obtain the initial continuous key data to achieve continuous variable quantum key distribution.

8. An in-band local oscillator free-space continuous variable quantum key distribution system, characterized in that, Including: An initialization module for initializing the telescopes at the receiving and transmitting ends, adjusting the telescopes to the optimal coupling efficiency state by detecting the intensity of the local oscillator light at the receiving end, and tracking the change of the beacon light through the fine tracking system of the telescope to maintain the stability of the communication link; A transmitter execution module for performing heterodyne detection on 90% of the hot state optical signals after polarization splitting of the hot state light source, taking the results X and P as the local initial key, and attenuating the other 10% of the hot state optical signals and then combining them with the local oscillator light and transmitting them through the free space channel to the receiving end for homodyne or heterodyne detection to obtain the corresponding initial continuous key data; A receiver execution module for performing data frame synchronization, phase compensation, parameter evaluation, error correction, and privacy amplification on the obtained initial continuous key data to obtain the binary bit key.

9. The in-band local oscillator free-space continuous variable quantum key distribution system according to claim 8, characterized in that, The transmitter execution module includes an ASE hot state light source, an optical amplifier, a narrowband optical filter, a 90:10 optical splitter, a 50:50 optical splitter, an optical mixer, a photodetector, an optical attenuator, and a polarization optical combiner; The hot state optical signal generated by the ASE hot state light source passes through an optical amplifier and then through a narrowband optical filter to obtain a hot state optical signal with a wavelength concentrated at 1550 nm, and it is divided into two beams by a 90:10 optical splitter. Among them, 90% of the hot state light and a part of the local oscillator light signal generated by a 1550 nm laser source passing through a 50:50 optical splitter are simultaneously input into the optical mixer and then heterodyne detection is performed using a photodetector. Based on the detected results of the regular components X and P, the local initial key is obtained; the other 10% of the hot state light is attenuated by the optical attenuator and then combined with the other part of the local oscillator light signal through the polarization optical combiner and then sent to the receiving end through the free space channel.

10. The free-space continuous-variable quantum key distribution system with an accompanying local oscillator as claimed in claim 8, wherein The receiver execution module includes a polarization controller, a polarization beam splitter, an optical amplifier, a band-pass filter, a 99:1 optical splitter, an optical power meter, and a 90° optical mixer; The polarization controller and the polarization beam splitter perform polarization demultiplexing on the received optical signal after adjusting its polarization to obtain a hot state optical signal and a local oscillator optical signal; the local oscillator optical signal is amplified by an optical amplifier and then filtered by a 1550 nm band-pass filter to reduce ASE noise; the hot state optical signal passes through a 99:1 optical splitter, and an optical power meter is used to measure a beam of hot state optical signal with a smaller proportion as the feedback for adjusting polarization to monitor polarization leakage; a beam of hot state optical signal with a larger proportion and the filtered local oscillator optical signal are simultaneously input into a 90° optical mixer, and a homodyne detection is performed by using a photodetector to obtain the regular component X or P of the hot state optical signal as the initial continuous key data.

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