All-day free space continuous variable quantum key distribution method and system

Abstract

The invention provides an all-time free space continuous variable quantum key distribution method and system, and the method comprises the steps: a sender Alice prepares a Gaussian modulation coherent state and adjusts the collimation of a free space channel, then transmits the modulated coherent state to a receiver Bob through the free space channel, and the Bob carries out the demodulation detection after receiving a signal, then partial light splitting is carried out on the local oscillation light to realize real-time monitoring so as to obtain a channel transmittance value of each corresponding pulse and a corresponding shot noise value, and a quantum signal path peak value submerged in noise is obtained by using a fixed time delay difference between a quantum signal path and a local oscillation path so as to obtain preliminary continuous key data; the Bob groups the received continuously distributed quantum key data in combination with channel transmittance data monitored in real time, and then processes each group of quantum key data to finally generate a security key. According to the invention, the safe communication distance of the free space CVQKD system is prolonged to 9.6 kilometers from the past maximum 860 meters, and the free space CVQKD system can be used all day long.

Description

Technical Field

[0001] The present invention relates to the field of quantum key distribution technology, and more specifically, to a method and system for all-day free-space continuous variable quantum key distribution. In particular, it relates to an all-day free-space continuous variable quantum key distribution (CVQKD) method based on Gaussian modulated coherent states. Background Art

[0002] CVQKD provides a secure method for sharing secret keys, allowing two geographically separated communicating parties, Alice and Bob, to obtain the secret key via quantum and authenticated classical channels. The key information is typically encoded in orthogonal components of a coherent state using standard telecommunications components. Bob can detect the key information using a homodyne or heterodyne detector. Therefore, CVQKD inherits the cost-effectiveness and high-speed advantages of coherent optical communications. Furthermore, under ideal conditions, CVQKD can reach the PLOB bound, the key capacity limit for relay-free quantum communication. To date, the Gaussian modulated coherent state CVQKD protocol has been proven secure, resisting both collective and coherent attacks, both with and without finite-size effects. Because it does not require single-photon sources and detectors, and continuous modulation offers greater channel capacity, it is well integrated with traditional optical signals. Therefore, CVQKD offers several advantages over DVQKD in these areas.

[0003] To date, research on CVQKD has primarily focused on optical fiber channels, where the channel transmittance is assumed to be a relatively stable fixed value. Free-space CVQKD research is relatively limited, primarily focusing on theoretical and experimental verification in the laboratory or over short distances. The feasibility of free-space and satellite-based CVQKD has been theoretically investigated, and some experimental studies have been conducted on the transmission of continuous-variable quantum signals over free-space and satellite-to-ground links. In particular, the effective resistance of free-space CVQKD to background noise has been demonstrated both theoretically and experimentally. Furthermore, a series of key techniques for free-space CVQKD have been proposed and verified, including parameter estimation, phase compensation, polarization calibration, frame synchronization, and data acquisition schemes. Secure key distribution has been further demonstrated over indoor foggy and turbulent channels, and over outdoor free-space atmospheric channels with polarization encoding and quadrature encoding. However, these reported free-space CVQKD experiments were either conducted in laboratories or in short-distance field environments (the longest reported field free-space secure coding is 860 meters, using polarization encoding, see reference npj Quantum Information (2025) 11:52). As the transmission distance increases, the transmission attenuation and its jitter will increase significantly, and the induced changes in the intensity, phase, and polarization of the quantum signal will become more intense. Therefore, it is difficult to accurately obtain and reconstruct the encoded orthogonal components of each quantum state, the transmission attenuation experienced, and the associated shot noise, which will inevitably lead to a large amount of excess noise. This is the main factor hindering the realization of long-distance free-space CVQKD field tests.

[0004] Patent document CN110113163A discloses a free-space continuous variable quantum key distribution method and system, comprising: modulating quantum signal light using Gaussian modulation, coupling the modulated quantum signal light with local oscillator light to obtain coupled light, and transmitting the coupled light through a free-space channel; receiving the coupled light, separating the quantum signal light from the local oscillator light in the coupled light; obtaining partially separated local oscillator light, and correcting the separated quantum signal light based on the wavefront phase of the local oscillator light. However, this patent document cannot achieve long-distance CVQKD.

[0005] To achieve long-distance CVQKD, this paper proposes a series of key technologies for Gaussian-modulated coherent state CVQKD in free-space channels, including high-precision manipulation of continuous variable quantum states independent of channel jitter, high-precision quantum signal acquisition and processing, efficient free-space ATP, and data post-processing in jittered channels. It is worth noting that these key technologies are not simply superimposed, but rather represent a comprehensive consideration of CVQKD systems and breakthroughs in technical bottlenecks. To date, no international CVQKD experimental systems with kilometer-scale free-space transmission distances have been reported. The fundamental bottlenecks are twofold: noise suppression of coherent optical signals and detection and processing of signals with significantly jittered transmission signals by the optical path encoding and decoding module during the initial key distribution phase in channels with significantly jittered transmission rates; and efficient negotiation of the initial key in channels with significantly jittered transmission rates during the data post-processing phase. The former is because free-space channels are more unstable than optical fiber channels, and large fluctuations in transmittance will introduce more excess noise, including the difficulty in controlling parameters such as transmittance, polarization, and phase. The latter is that under free-space channel transmission, the signal-to-noise ratio will fluctuate greatly and change extremely quickly, and efficient negotiation in this scenario is a problem.

[0006] In summary, implementing free-space CVQKD requires not only suppressing excess noise and effectively extracting and processing transmittance jitter signals during the initial key distribution phase at the front end, but also considering efficient data negotiation algorithms for signals transmitted through the jittered channel at the back end. Our introduction of high-precision manipulation of continuous variable quantum states independent of channel jitter, high-precision quantum signal acquisition and processing, and efficient free-space ATP technology suppresses excess noise in the CVQKD system while enabling detection and processing of signals with significant transmittance jitter. Furthermore, our post-processing of data under the jittered channel extracts more of the final key information, ultimately enabling CVQKD over a secure transmission distance of 9.6 km over a free-space channel. Summary of the Invention

[0007] In view of the defects in the prior art, the purpose of the present invention is to provide a 24 / 7 free-space continuous variable quantum key distribution method and system.

[0008] According to the present invention, a full-time free-space continuous variable quantum key distribution method is provided, comprising:

[0009] Step S1: After the sender Alice prepares a Gaussian modulated coherent state and adjusts the collimation of the free-space channel, she transmits the modulated coherent state to the receiver Bob through the free-space channel. Bob receives the signal and performs demodulation detection.

[0010] Step S2: After demodulation at Bob's end, the local oscillator light is partially split to achieve real-time monitoring to obtain the channel transmittance value and corresponding shot noise value of each corresponding pulse. The fixed delay difference between the quantum signal path and the local oscillator path is used to obtain the peak value of the quantum signal path submerged in the noise, and the preliminary continuous key data is obtained.

[0011] Step S3: Bob groups the received continuously distributed quantum key data based on the real-time monitored channel transmittance data, and then performs data correction, parameter evaluation, error correction, and confidentiality enhancement on each group of quantum key data to ultimately generate a secure key.

[0012] Preferably, the step S1 includes:

[0013] Step S1.1: The sender Alice and the receiver Bob initialize the CVQKD system, including setting up the signal source, random number generator (RNG), modem, detector, and control circuit in the system.

[0014] Step S1.2: Alice chops the laser light using a coherent light source and uses an optical beam splitter to split the light into signal light and local oscillator light. She then Gaussian modulates the signal light and uses "pilot-quantum signal" time division multiplexing. Simultaneously, the pilot light, quantum signal, and local oscillator light are time-division and polarization-multiplexed.

[0015] Step S1.3: Alice and Bob use the acquisition, tracking, and targeting (ATP) technique to collimate the free-space channel and send the signal light and the local oscillator light to Bob through the channel.

[0016] Preferably, an isolator and a photodetector PD or an optical power meter are provided in the optical path between Alice and Bob to monitor the intensity of the local oscillator light.

[0017] Preferably, the step S1.2 includes:

[0018] Step S1.2.1: Alice uses the in-phase and quadrature IQ modulators to code and modulate the regular components X and P of the coherent state, so that the signal light has a mean of zero and a variance of V. A Gaussian distribution;

[0019] Step S1.2.2: Alice performs time-division polarization multiplexing on the quantum signal light, pilot signal light and local oscillator light and sends them to Bob through a free-space channel, where the quantum signal light and pilot signal are horizontally polarized and the local oscillator light is vertically polarized.

[0020] Preferably, step S2 includes:

[0021] Step S2.1: After polarization compensation and time and polarization demultiplexing, Bob splits the local oscillator light, signal light, and pilot signal.

[0022] Step S2.2: Peak sampling is performed on the output of the homodyne detector to obtain peak point data. The delay of the local oscillator light and the signal light is calibrated in advance. The peak point of the electrical signal pulse output by the local oscillator light in step S2.1 is used to determine the peak point position of the quantum signal, thereby achieving stable trigger sampling of the peak point of the weak quantum signal.

[0023] Step S2.3: Using the real-time monitoring value of the free-space channel transmittance variation obtained in step S2.1, and the relationship curve between the local oscillator light intensity and shot noise calibrated in advance by the system, the shot noise value corresponding to each corresponding pulse is obtained in real time.

[0024] Preferably, in step S2.1, a small portion of the local oscillator light and the signal light are divided by the pilot signal after power monitoring to eliminate uncertainty introduced by channel jitter, and the division output value is fed back to the polarization controller to ensure the accuracy of polarization compensation;

[0025] After the remaining local oscillator light is selected by the PM for measurement basis, part of the light is separated and passed through the PD for local oscillator light intensity monitoring, outputting electrical signal pulses. The channel transmittance value experienced by each quantum and pilot signal light pulse is obtained in real time through data acquisition. The remaining main part of the local oscillator light, the remaining quantum signal, and the main part of the pilot signal enter the homodyne detector to realize signal measurement.

[0026] Preferably, the detection process of the homodyne detector is: by modulating the phase of the local oscillator light by 0 degrees or 90 degrees, inputting the time-division multiplexed quantum signal and pilot signal into two interfaces of a 50:50 beam splitter, and then connecting to a calibrated homodyne detector for detection.

[0027] Preferably, step S3 includes:

[0028] Step S3.1: Bob and Alice pre-process the initial continuous key data and perform phase compensation signal by signal through data post-processing using pilot signals;

[0029] Step S3.2: Alice and Bob set different transmittance grouping intervals based on the channel transmittance data of each monitored quantum signal and pilot signal pulse, and group all the initial key data. For each grouped data, Alice and Bob publish part of the initial continuous key data for parameter evaluation, thereby obtaining parameters including signal noise, modulation variance, and channel transmittance.

[0030] Step S3.3: Bob uses an efficient multi-dimensional negotiation algorithm based on LDPC coding to encode the remaining initial key data after phase compensation and sends it to Alice. Alice performs LDPC decoding and finally obtains a consistent binary shared key string.

[0031] Step S3.4: Alice and Bob calculate the mutual information of each grouped data block, including the Holevo bound and the legal communicating parties, based on the parameters evaluated in step S3.2. They then obtain the information compression ratio of each group of data and finally generate multiple sets of security keys through confidentiality enhancement.

[0032] Preferably, the phase compensation of each signal includes: at the transmitting end, the original pilot signal sent is a quadrature phase shift keying QPSK signal, and the angle between the four quadrants in the signal constellation diagram and the coordinate axis is 45° clockwise, and the angle is recorded as φ c ;

[0033] The transmission frequency of the pilot is the same as that of the signal, and is cyclically transmitted in the order of Q4, Q3, Q2, and Q1, where Q represents the quadrant in the constellation diagram where the signal is located.

[0034] According to the present invention, a 24 / 7 free-space continuous variable quantum key distribution system is provided, comprising:

[0035] Module M1: After the sender Alice prepares a Gaussian modulated coherent state and adjusts the collimation of the free-space channel, she transmits the modulated coherent state to the receiver Bob through the free-space channel. Bob receives the signal and performs demodulation detection.

[0036] Module M2: After demodulation at Bob's end, it partially splits the local oscillator light to implement real-time monitoring to obtain the channel transmittance value and corresponding shot noise value of each corresponding pulse. It also uses the fixed delay difference between the quantum signal path and the local oscillator path to obtain the peak value of the quantum signal path submerged in the noise, and obtains the preliminary continuous key data.

[0037] Module M3: Bob groups the received continuously distributed quantum key data based on the real-time monitored channel transmittance data. He then performs data correction, parameter evaluation, error correction, and confidentiality enhancement on each group of quantum key data to ultimately generate a secure key.

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] 1. The method provided by the present invention can extend the safe communication distance of free-space CVQKD based on Gaussian modulated coherent states to 9.6 km, which is currently the longest safe communication distance of free-space CVQKD in the world.

[0040] 2. The present invention can eliminate channel jitter by separately splitting the local oscillator light and signal light after demultiplexing at the receiving end, and then performing power monitoring and division operations. It can also use a commercial electrically controlled polarization control module to achieve real-time high extinction ratio dynamic polarization control in a free-space jitter transmittance channel, effectively reducing the excess noise introduced by local oscillator light leakage.

[0041] 3. The present invention performs spectroscopic monitoring of local oscillator light. Since the local oscillator and the quantum and pilot signal are polarization multiplexed (timing is consistent), the channel transmittance experienced by each quantum and pilot signal pulse can be monitored in real time, thereby achieving accurate data grouping and subsequent parameter evaluation. At the same time, the obtained local oscillator light intensity also realizes real-time monitoring and acquisition of shot noise.

[0042] 4. The present invention performs homodyne detection on the quantum and pilot signals and outputs peak sampling of the signal. It utilizes the strong light after local oscillator splitting to locate the peak sampling. Then, based on the fixed time delay between the local oscillator and the quantum signal, the peak values ​​of the weak quantum and pilot signals submerged in the noise can be accurately obtained.

[0043] 5. The principle of the high-precision signal-by-signal phase compensation algorithm based on data post-processing in the present invention is based on the consistency of phase jitter in the interval timing. It uses a stronger known pilot signal to evaluate the channel phase jitter and compensates the phase jitter value to the adjacent quantum signal, which can reduce the excess noise introduced by phase jitter to an extremely low point.

[0044] 6. The multi-dimensional data negotiation algorithm of the present invention based on transmittance-grouped LDPC codes groups the received data with a wide range of transmittance jitter, and performs parameter evaluation and subsequent negotiation and confidentiality enhancement separately. This method minimizes the additional excess noise introduced by channel jitter and is applicable to free-space channels with large transmittance jitter. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Other features, objects and advantages of the present invention will become more apparent upon reading the detailed description of non-limiting embodiments with reference to the following drawings:

[0046] Figure 1 Schematic diagram of the flow of the all-day free-space CVQKD system based on Gaussian modulated coherent states;

[0047] Figure 2 This is the optical path principle diagram of the all-day free-space CVQKD technology based on Gaussian modulated coherent states. In the figure: CWlaser is a continuous laser, IQ is an in-phase orthogonal modulator, AM is an intensity modulator, PM is a phase modulator, and PD is a photodetector. DETAILED DESCRIPTION

[0048] The present invention will be described in detail below with reference to specific embodiments. The following examples will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those skilled in the art, several changes and improvements can be made without departing from the scope of the present invention. These all fall within the scope of protection of the present invention.

[0049] The present invention improves the CVQKD system's over-noise suppression capability and post-processing efficiency by introducing a number of key technologies, including high-precision manipulation technology of continuous variable quantum states that is independent of channel jitter, high-precision quantum signal acquisition and processing technology, efficient free-space ATP technology, and data post-processing technology under jitter channels, thereby achieving CVQKD with a safe transmission distance of 9.6 km under a standard free-space channel.

[0050] Example 1

[0051] According to the present invention, a free-space continuous variable quantum key distribution method is provided. Figure 1 and Figure 2 Shown, including:

[0052] Step S1: After the sender Alice prepares a Gaussian modulated coherent state and adjusts the collimation of the free-space channel, she transmits the modulated coherent state to the receiver Bob through the free-space channel. After receiving the signal, Bob performs demodulation detection.

[0053] The step S1 comprises:

[0054] Step S1.1: Sender Alice and receiver Bob initialize the CVQKD system, including setting up the signal source, random number generator (RNG), modem, detector, and control circuitry. An isolator and a photodetector (PD), or optical power meter, are placed in the optical path between Alice and Bob to monitor the intensity of the local oscillator light.

[0055] Step S1.2: Alice chops the laser light using a coherent light source and uses an optical beam splitter to separate the light into signal light and local oscillator light. The signal light is then Gaussian modulated and time-division multiplexed using a "pilot-quantum signal" method. Simultaneously, the pilot signal, quantum signal, and local oscillator light are time-division and polarization-multiplexed. Here, the pilot signal is approximately 40 dB stronger than the quantum signal. The alternating pilot and quantum signal method used here is because atmospheric turbulence in the free-space channel significantly affects the phase of the quantum signal, requiring the use of sequentially adjacent pilot signals for high-precision phase drift estimation and compensation of the quantum signal. Step S1.2 includes:

[0056] Step S1.2.1: Alice uses an in-phase and quadrature (IQ) modulator to code and modulate the regular components X and P of the coherent state so that the signal light has a mean of zero and a variance of VA Gaussian distribution, where V A The value range is greater than 0 and less than 10.

[0057] Step S1.2.2: Alice performs time-division polarization multiplexing on the quantum signal light, pilot signal light and local oscillator light and sends them to Bob through a free-space channel, where the quantum signal light and pilot signal are horizontally polarized and the local oscillator light is vertically polarized.

[0058] Step S1.3: Alice and Bob use the acquisition, tracking, and targeting (ATP) technology to collimate the free-space channel and send the signal light and the local oscillator light to Bob through the channel.

[0059] Step S2: To simultaneously monitor the free-space channel transmittance and peak sampling of the quantum signal path in real time, it is necessary to partially split the local oscillator light after demodulation at the Bob end to monitor the channel transmittance and shot noise values ​​of each corresponding pulse. The fixed time delay difference between the quantum signal path and the local oscillator path is used to obtain the peak value of the quantum signal path submerged in the noise, thereby obtaining preliminary continuous key data. Step S2 includes:

[0060] Step S2.1: After polarization compensation and time and polarization demultiplexing, Bob splits the local oscillator light, signal light and pilot signal. Specifically, a small part of the local oscillator light, signal light and pilot signal are divided after power monitoring to eliminate the uncertainty introduced by channel jitter, and the division output value is fed back to the polarization controller to ensure the accuracy of polarization compensation. The remaining local oscillator light is selected by the PM for measurement basis, and then part of the light is separated and passed through the PD for local oscillator light intensity monitoring, and an electrical signal pulse is output. The channel transmittance value experienced by each quantum and pilot signal light pulse is obtained in real time through data acquisition. The remaining main part of the local oscillator light and the remaining quantum signal and the main part of the pilot signal enter the zero-difference detector to realize signal measurement. It should be pointed out that the power monitoring after the signal light is split here improves the accuracy of polarization feedback control and is the core of achieving high-precision polarization feedback.

[0061] The detection process of the homodyne detector is as follows: by modulating the phase of the local oscillator light by 0 degrees or 90 degrees, inputting the two interfaces of the 50:50 beam splitter with the time-division multiplexed quantum signal and the pilot signal, and then connecting to the calibrated homodyne detector for detection. The homodyne detector is a prior art. For example, those skilled in the art can realize the homodyne detector by studying the paper “Duan Huang, Erhu Han, Weiqi Liu, Dakai Lin, Chao Wang, Peng Huang, Guihua Zeng, Experimental realization of a photon-number-resolving homodyne detector, Conference on Lasers and Electro-Optics (ClEO: 2014), 8-13 June 2014, San Jose, USA”. In this paper, “homodyne detector” is referred to as “photon-number-resolving homodyne”.

[0062] The calibrated homodyne detector is a prior art detector, such as the product PDB450C of Thorlabs.

[0063] Step S2.2: To accurately obtain the canonical position X or canonical momentum P of the pilot and quantum signals, peak sampling of the homodyne detector output is required to obtain peak point data. Because the transmittance of the free-space channel changes rapidly in real time, the instantaneous attenuation changes of the channel may mask the peak of the detector output electrical signal, making it impossible to determine the true peak point location. This is fundamentally different from the stable transmittance of the optical fiber channel. To this end, the local oscillator light and signal light delays can be calibrated in advance, and the peak point of the electrical signal pulse output by the local oscillator light in step S2.1 can be used to determine the peak point location of the quantum signal, achieving stable triggered sampling of the peak point of the weak quantum signal.

[0064] Step S2.3: Because jitter in the free-space channel transmittance directly affects the system's shot noise value, which is directly related to subsequent system parameter evaluation and the security key rate, this is fundamentally different from the stable transmittance of a fiber channel. The real-time monitoring value of the free-space channel's varying transmittance obtained in Step S2.1 can be used, along with the system's pre-calibrated LO intensity-shot noise curve, to obtain the shot noise value corresponding to each corresponding pulse in real time.

[0065] Polarization demultiplexing utilizes dynamic polarization control technology. At the receiving end, Bob uses two photodetectors in an optical power meter to collect and monitor the 10% quantum signal and pilot signal, along with the 5% total received signal, after being split by the polarization beam splitter. After the optical power monitoring module measures the input signals from the two monitoring optical paths, it divides the power of the signal light by the local oscillator (LO) and then feeds this power back to the electrically controlled polarization control module. This module then adjusts the polarization in real time. When the output feedback signal, which is independent of channel jitter, reaches its maximum value, the polarization extinction ratio of the polarization demultiplexing system also reaches its maximum value.

[0066] Step S3: Bob groups the received continuously distributed quantum key data based on the real-time channel transmittance data, and then performs data correction, parameter evaluation, error correction, and confidentiality enhancement on each group of quantum key data to ultimately generate a secure key. Step S3 includes:

[0067] Step S3.1: Bob and Alice pre-process the initial continuous key data and use the pilot signal to perform phase compensation signal by signal through data post-processing. The phase compensation technology for each signal is as follows: At the transmitting end, the original pilot signal we send is a quadrature phase shift keying (QPSK) signal. The four quadrants in the constellation diagram of this signal have a clockwise angle of 45° with the coordinate axis. This angle is denoted as φ. c The transmission frequency of the pilot is the same as that of the signal, and is transmitted cyclically in the order of Q4, Q3, Q2, and Q1 (Q represents the quadrant where the signal is located in the constellation diagram). Assume that the number of pilots in a data frame is N p When the pilot signal reaches the receiving end, φ c is no longer 45°. In a data frame, φ c becomes φ c1 、φ c2 、φ c3 ,...,φ cN Each φ c Corresponding to a set of pilot coordinate values, such as φ ci Corresponding to x i pilot ,p i pilot We use homodyne detection to measure only the pilot signal x i pilot Component. The measured values ​​of the first five pilot signals are sinφ c1 、-cosφ c2 、-sinφ c3 、cosφ c4 and sinφ c5 , and the p of the first four pilot signals i pilot The value should be -cosφc1 、-sinφ c2 、cosφ c3 and sinφ c4 Each p i The mathematical form of the next pilot signal x i The mathematical form corresponds to φ. ci With φ c(i+1) The sampling time difference between them is 0.4μs, so φ ci Approximately equal to φ c(i+1) Based on this assumption, the measured value x of each pilot signal is i It can be approximated by the p of the previous pilot signal i Value is represented by x i and p i The phase angle can be determined, and by comparing these values ​​with those of the original QPSK signal, the phase changes introduced by the channel can be calculated.

[0068] Step S3.2: Alice and Bob set different transmittance grouping intervals based on the channel transmittance data of each monitored quantum signal and pilot signal pulse, and group all the initial key data. For each grouped data, Alice and Bob publish a portion of the initial continuous key data for parameter evaluation, thereby obtaining parameters such as signal noise, modulation variance, and channel transmittance.

[0069] Step S3.3: Bob encodes the remaining initial key data after phase compensation using an efficient multi-dimensional negotiation algorithm based on LDPC coding and sends it to Alice. Alice performs LDPC decoding, ultimately obtaining a consistent binary shared key string. The efficient multi-dimensional negotiation algorithm based on LDPC coding is a prior art. Those skilled in the art can implement the efficient multi-dimensional negotiation algorithm based on LDPC coding by studying the prior art, for example, by studying the paper “Kunz-Jacques S., Leverrier A. & Jouguet P. Long-distance continuous-variable quantum key distribution with a Gaussian modulation. Phys. Rev. A. 84, 062317 (2011).” In this paper, “multi-dimensional negotiation algorithm” is referred to as “Multidimensional reconciliation.”

[0070] Step S3.4: Alice and Bob calculate the Holevo bound and the mutual information between legitimate communicating parties for each data block based on the parameters evaluated in step S3.2. This calculation yields the information compression ratio for each data block, and ultimately generates multiple sets of security keys through privacy enhancement. The method for calculating the compression ratio for each data block is well known in the art. For example, those skilled in the art can refer to the paper "Weedbrook, C. et al. Gaussian quantum information. Rev. Mod. Phys. 84, 621 (2012)" for implementation.

[0071] Example 2

[0072] In a preferred embodiment:

[0073] The sender Alice and the receiver Bob initialize the communication of the CVQKD system, including initializing the signal source, random number generator (RNG), modem, detector, and control circuit in the CVQKD system. The sender Alice uses two high extinction ratio intensity modulators (AM) to cut the continuous laser to generate a 5MHz pulse signal, and uses a beam splitter to divide the optical path into the local oscillator optical path and the signal optical path. In the signal optical path, Alice uses the IQ modulator to code and modulate the coherent state regular components X and P, so that the signal light obeys a zero mean and a variance of V. A Alice then uses polarization and time multiplexing technology to send the signal light and local oscillator light to Bob through the same free space channel.

[0074] At the receiving end, Bob first monitors the power of the LO light, quantum light, and pilot signal after splitting them, then performs a division operation. This provides feedback to the PCD-M02's electrically controlled polarization control module for high-precision dynamic polarization control. Polarization and time demultiplexing is then performed. Channel transmittance is monitored by performing LO basis selection and splitting on the LO light. A homodyne detector is used to interfere with the remaining signal light and pilot signal with the LO light to measure the component X or P. In this experimental scheme, isolators and PDs are added to the optical path to monitor LO light intensity to prevent Trojan horse attacks, LO light jitter attacks, and shot noise variance scaling attacks. The system can also defend against other attacks by adding filters, such as wavelength-based attacks.

[0075] The first important reason why the free-space CVQKD scheme can achieve a transmission distance of 9.6 km is that the system uses free-space channel dynamic polarization control technology and signal-by-signal phase compensation technology based on data post-processing to effectively control excess noise, so that the excess noise introduced by polarization jitter and phase jitter under the condition of channel transmittance jitter is reduced to a very low level; the second important reason is that the data is grouped according to the channel transmittance, which greatly reduces the channel jitter excess noise; the third important reason is the use of a high-efficiency multi-dimensional negotiation algorithm based on LDPC coding of transmittance grouping, which can greatly improve the system error correction efficiency and negotiation performance, thereby increasing the safe transmission distance; the fourth important reason is the use of efficient ATP technology to improve the efficiency of optical signal transmission in the free-space channel.

[0076] The following specifically analyzes the suppression of excessive noise introduced into the CVQKD system by dynamic polarization control, a signal-by-signal phase compensation scheme based on data post-processing, and data grouping based on channel transmittance.

[0077] First, consider the suppression of excess noise using dynamic polarization control technology in a free-space channel. Due to the limitation of the extinction ratio, residual photons still exist between pulses after the local oscillator light is chopped. These residual photons will affect the signal due to polarization leakage, thus generating polarization leakage noise. At the channel input, the average number of photons in the local oscillator light is:

[0078]

[0079] Where h is Planck's constant, c is the speed of light, is the local oscillator optical power at the transmitter, is the pulse frequency of the local oscillator light, λ 1550 is the wavelength. Considering that the average polarization isolation is about 35dB and the local oscillator pulse extinction ratio is greater than 90dB, the total extinction ratio of the system R e About 125dB. Polarization leakage noise ε PL It can be expressed as:

[0080]

[0081] Here, SNU represents the shot noise unit.

[0082] Secondly, in order to control the excess noise introduced by phase drift, we introduce a signal-by-signal phase compensation scheme based on data post-processing. est The expression is:

[0083] V est =V error +V drift +V channel

[0084] Among them, V drift Reflects the relative phase drift between the signal and pilot generating laser. Since the signal and pilot in this system originate from the same laser, V drift Can be regarded as 0. V channel Characterizes the relative phase drift caused by the cumulative phase difference during the propagation of the signal and the pilot. It is generally believed that it is mainly determined by the optical path difference between the two. This system uses time division multiplexing technology, and there is no optical path difference between the signal and the pilot, so V channel It can also be regarded as 0. V error The expression is:

[0085]

[0086] Where E R represents the amplitude of the pilot signal. In this design, the pilot power is 40dB higher than the signal power. The χ parameter under homodyne detection conditions can be expressed as:

[0087]

[0088] Where T is the total intensity transmittance of the channel, ε A is the total channel excess noise variance converted to the transmitter input. At a transmission distance of 9.6 km, the estimated phase excess noise is:

[0089]

[0090] Secondly, considering the characteristic of atmospheric channels that transmit large jitter, the data is grouped into sub-channels with a transmittance of 0.2 dB within the [15, 23] dB attenuation range. The excess noise caused by channel jitter is estimated, and the average value is calculated as:

[0091]

[0092] Where T represents the channel transmittance, is the corresponding modulation variance.

[0093] Example 3

[0094] In another preferred embodiment, the all-day free-space CVQKD based on Gaussian modulated coherent states comprises the following steps:

[0095] (1) Continuous variable initial key distribution phase: First, the sender Alice generates pulsed coherent light through a high extinction ratio modulator and a continuous light source, and divides the light into a weak signal light, a slightly stronger pilot signal, and a strong local oscillator light. Then Alice encodes a string of Gaussian random numbers on the signal light, uses the encoded coherent state signal light as the signal state, and modulates the known pilot signal at the same time. It is then time-division multiplexed and polarization-multiplexed with the local oscillator light signal and sent to Bob through efficient ATP technology. Bob first performs dynamic polarization control on the received signal state through splitting, power monitoring, and feedback operations, and then further monitors the channel transmittance and shot noise of the local oscillator light splitting in real time, and detects the remaining signal light and pilot signal using a homodyne detector to obtain the initial information.

[0096] (2) Continuous Data Post-Processing: Bob first filters the initial information he obtained with Alice using the measurement basis. He then preprocesses the filtered data, using pilot signals to compensate for phase drift during the channel process. The initial key data shared by Alice and Bob is then grouped using the monitored channel transmittance. After parameter evaluation, data negotiation is performed using a high-efficiency negotiation algorithm. Finally, a binary bit string is output for each grouped data block using confidentiality enhancement.

[0097] Through the above process, the excess noise introduced by polarization, phase jitter, and local oscillator light leakage can be reduced to below the safe limit of free-space communication, and finally CVQKD can be achieved in a 9.6 km long free-space channel in an actual atmospheric environment.

[0098] Example 4

[0099] The present invention also provides a 24 / 7 free-space continuous variable quantum key distribution system. The 24 / 7 free-space continuous variable quantum key distribution system can be implemented by executing the process steps of the 24 / 7 free-space continuous variable quantum key distribution method. That is, those skilled in the art can understand the 24 / 7 free-space continuous variable quantum key distribution method as a preferred embodiment of the 24 / 7 free-space continuous variable quantum key distribution system.

[0100] According to the present invention, a 24 / 7 free-space continuous variable quantum key distribution system is provided, comprising:

[0101] Module M1: After the sender Alice prepares the Gaussian modulated coherent state and adjusts the collimation of the free-space channel, the modulated coherent state is transmitted to the receiver Bob through the free-space channel. After receiving the signal, Bob performs demodulation detection. The module M1 includes: Module M1.1: The sender Alice and the receiver Bob initialize the communication of the CVQKD system, including setting the signal source, random number generator RNG, modem, detector and control circuit in the system. Module M1.2: Alice chops the laser through a coherent light source, and uses an optical beam splitter to divide the light into signal light and local oscillator light, and then Gaussian modulates the signal light and uses "pilot-quantum signal" time division multiplexing. At the same time, the pilot and quantum signal and local oscillator light are time-division and polarization-multiplexed. The module M1.2 includes: Module M1.2.1: Alice uses in-phase and orthogonal IQ modulators to encode and modulate the regular components X and P of the coherent state, so that the signal light has a mean of zero and a variance of V A Gaussian distribution. Module M1.2.2: Alice transmits the quantum signal light, pilot signal light, and local oscillator light to Bob through time-division polarization multiplexing via a free-space channel. The quantum signal light and pilot signal light are horizontally polarized, while the local oscillator light is vertically polarized. Module M1.3: Alice and Bob use the capture, tracking, and targeting (ATP) technique to collimate the free-space channel and transmit the signal light and local oscillator light to Bob through this channel.

[0102] An isolator and a photodetector PD or optical power meter are provided in the optical path between Alice and Bob to monitor the intensity of the local oscillator light.

[0103] Module M2: After demodulation at Bob's end, partial splitting of the local oscillator light is performed to achieve real-time monitoring to obtain the channel transmittance value and corresponding shot noise value of each corresponding pulse. The fixed delay difference between the quantum signal path and the local oscillator path is used to obtain the peak value of the quantum signal path submerged in the noise, and the preliminary continuous key data is obtained. The module M2 includes:

[0104] Module M2.1: After polarization compensation and time and polarization demultiplexing, Bob splits the local oscillator (LO) light, the signal light, and the pilot signal. A small portion of the LO light, the signal light, and the pilot signal undergo power monitoring and division to eliminate uncertainty introduced by channel jitter. The division output is fed back to the polarization controller to ensure polarization compensation accuracy. The remaining LO light undergoes measurement basis selection through the PM, after which a portion is split and passed through the PD for LO light intensity monitoring. An electrical signal pulse is output, and data acquisition is used to obtain the channel transmittance values ​​experienced by each quantum and pilot signal pulse in real time. The remaining main portion of the LO light, the remaining quantum signal, and the main portion of the pilot signal are fed into a homodyne detector for signal measurement. The homodyne detector performs detection by modulating the LO light phase by 0 or 90 degrees. This phase is then fed into two interfaces of a 50:50 beam splitter along with the time-division-multiplexed quantum signal and pilot signal. The detector then connects to a calibrated homodyne detector for detection.

[0105] Module M2.2: Peak sampling is performed on the output of the homodyne detector to obtain peak data. The local oscillator (LO) and signal light delays are pre-calibrated. The peak point of the electrical signal pulse output by the LO in module M2.1 is used to determine the peak position of the quantum signal, achieving stable trigger sampling of the peak point of the weak quantum signal. Module M2.3: Utilizing the real-time monitoring value of the free-space channel transmittance obtained in Module M2.1 and the system's pre-calibrated LO intensity-shot noise curve, the shot noise value corresponding to each corresponding pulse is obtained in real time.

[0106] Module M3: Bob groups the received continuous distributed quantum key data in combination with the real-time monitored channel transmittance data, and then performs data correction, parameter evaluation, error correction and confidentiality enhancement on each group of quantum key data to finally generate a secure key. The module M3 includes: Module M3.1: Bob and Alice pre-process the initial continuous key data and use the pilot to perform phase compensation for each signal through data post-processing. The phase compensation for each signal includes: at the transmitting end, the original pilot signal sent is a quadrature phase shift keying QPSK signal, and the angle between the four quadrants in the signal constellation diagram and the coordinate axis is 45° clockwise, and this angle is recorded as φ c The transmission frequency of the pilot is the same as that of the signal and is cyclically transmitted in the order of Q4, Q3, Q2, and Q1, where Q represents the quadrant in the constellation diagram where the signal is located.

[0107] Module M3.2: Alice and Bob set different transmittance grouping intervals based on the channel transmittance data of each monitored quantum signal and pilot signal pulse, and group all the initial key data. For each grouped data, Alice and Bob publish part of the initial continuous key data for parameter evaluation, thereby obtaining parameters including signal noise, modulation variance, and channel transmittance.

[0108] Module M3.3: Bob uses an efficient multi-dimensional negotiation algorithm based on LDPC coding to encode the remaining initial key data after phase compensation and sends it to Alice. Alice performs LDPC decoding and finally obtains a consistent binary shared key string.

[0109] Module M3.4: Alice and Bob calculate the Holevo bound and the mutual information of the legitimate communicating parties for each block of data based on the parameters evaluated in Module M3.2. This calculation then determines the information compression ratio for each set of data and ultimately generates multiple sets of security keys through confidentiality enhancement.

[0110] Those skilled in the art will appreciate that, in addition to implementing the system and its various devices, modules, and units provided by the present invention in purely computer-readable program code, it is entirely possible to implement the same functions of the system and its various devices, modules, and units provided by the present invention in the form of logic gates, switches, application-specific integrated circuits, programmable logic controllers, and embedded microcontrollers by logically programming the method steps. Therefore, the system and its various devices, modules, and units provided by the present invention can be considered a hardware component, and the devices, modules, and units included therein for implementing various functions can also be considered as structures within the hardware component; the devices, modules, and units for implementing various functions can also be considered as both software modules implementing the method and structures within the hardware component.

[0111] The above describes specific embodiments of the present invention. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art may make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. The embodiments of this application and the features in the embodiments may be combined with each other in any manner unless there is a conflict.

Claims

1. A free-space continuous variable quantum key distribution method, characterized in that: include: Step S1: After the sender Alice prepares a Gaussian modulated coherent state and adjusts the collimation of the free-space channel, she transmits the modulated coherent state to the receiver Bob through the free-space channel. Bob receives the signal and performs demodulation detection. Step S2: After demodulation at Bob's end, the local oscillator light is partially split to achieve real-time monitoring to obtain the channel transmittance value and corresponding shot noise value of each corresponding pulse. The fixed delay difference between the quantum signal path and the local oscillator path is used to obtain the peak value of the quantum signal path submerged in the noise, and the preliminary continuous key data is obtained. Step S3: Bob groups the received continuously distributed quantum key data based on the real-time monitored channel transmittance data, and then performs data correction, parameter evaluation, error correction, and confidentiality enhancement on each group of quantum key data to ultimately generate a secure key.

2. The all-day free-space continuous variable quantum key distribution method according to claim 1, characterized in that: The step S1 comprises: Step S1.1: The sender Alice and the receiver Bob initialize the CVQKD system, including setting up the signal source, random number generator (RNG), modem, detector, and control circuit in the system. Step S1.2: Alice chops the laser light using a coherent light source and uses an optical beam splitter to split the light into signal light and local oscillator light. She then Gaussian modulates the signal light and uses "pilot-quantum signal" time division multiplexing. Simultaneously, the pilot light, quantum signal, and local oscillator light are time-division and polarization-multiplexed. Step S1.3: Alice and Bob use the acquisition, tracking, and targeting (ATP) technique to collimate the free-space channel and send the signal light and the local oscillator light to Bob through the channel.

3. The all-day free-space continuous variable quantum key distribution method according to claim 1, characterized in that: An isolator and a photodetector PD or optical power meter are provided in the optical path between Alice and Bob to monitor the intensity of the local oscillator light.

4. The all-day free-space continuous variable quantum key distribution method according to claim 2, characterized in that: The step S1.2 includes: Step S1.2.1: Alice uses the in-phase and quadrature IQ modulators to code and modulate the regular components X and P of the coherent state, so that the signal light has a mean of zero and a variance of V. A Gaussian distribution; Step S1.2.2: Alice performs time-division polarization multiplexing on the quantum signal light, pilot signal light and local oscillator light and sends them to Bob through a free-space channel, where the quantum signal light and pilot signal are horizontally polarized and the local oscillator light is vertically polarized.

5. The all-day free-space continuous variable quantum key distribution method according to claim 1, characterized in that: The step S2 comprises: Step S2.1: After polarization compensation and time and polarization demultiplexing, Bob splits the local oscillator light, signal light, and pilot signal. Step S2.2: Peak sampling is performed on the output of the homodyne detector to obtain peak point data. The delay of the local oscillator light and the signal light is calibrated in advance. The peak point of the electrical signal pulse output by the local oscillator light in step S2.1 is used to determine the peak point position of the quantum signal, thereby achieving stable trigger sampling of the peak point of the weak quantum signal. Step S2.3: Using the real-time monitoring value of the free-space channel transmittance variation obtained in step S2.1, and the relationship curve between the local oscillator light intensity and shot noise calibrated in advance by the system, the shot noise value corresponding to each corresponding pulse is obtained in real time.

6. The all-day free-space continuous variable quantum key distribution method according to claim 5, characterized in that: In step S2.1, a small portion of the local oscillator light and the signal light are divided by the pilot signal after power monitoring to eliminate uncertainty introduced by channel jitter, and the division output value is fed back to the polarization controller to ensure the accuracy of polarization compensation; After the remaining local oscillator light is selected by the PM for measurement basis, part of the light is separated and passed through the PD for local oscillator light intensity monitoring, outputting electrical signal pulses. The channel transmittance value experienced by each quantum and pilot signal light pulse is obtained in real time through data acquisition. The remaining main part of the local oscillator light, the remaining quantum signal, and the main part of the pilot signal enter the homodyne detector to realize signal measurement.

7. The all-day free-space continuous variable quantum key distribution method according to claim 6, characterized in that: The detection process of the homodyne detector is as follows: by modulating the phase of the local oscillator light by 0 degrees or 90 degrees, inputting the two interfaces of the 50:50 beam splitter with the time-division multiplexed quantum signal and pilot signal, and then connecting to the calibrated homodyne detector for detection.

8. The all-day free-space continuous variable quantum key distribution method according to claim 1, characterized in that: The step S3 comprises: Step S3.1: Bob and Alice pre-process the initial continuous key data and perform phase compensation signal by signal through data post-processing using pilot signals; Step S3.2: Alice and Bob set different transmittance grouping intervals based on the channel transmittance data of each monitored quantum signal and pilot signal pulse, and group all the initial key data. For each grouped data, Alice and Bob publish part of the initial continuous key data for parameter evaluation, thereby obtaining parameters including signal noise, modulation variance, and channel transmittance. Step S3.3: Bob uses an efficient multi-dimensional negotiation algorithm based on LDPC coding to encode the remaining initial key data after phase compensation and sends it to Alice. Alice performs LDPC decoding and finally obtains a consistent binary shared key string. Step S3.4: Alice and Bob calculate the mutual information of each grouped data block, including the Holevo bound and the legal communicating parties, based on the parameters evaluated in step S3.

2. They then obtain the information compression ratio of each group of data and finally generate multiple sets of security keys through confidentiality enhancement.

9. The all-day free-space continuous variable quantum key distribution method according to claim 8, characterized in that: The phase compensation of each signal includes: at the transmitting end, the original pilot signal sent is a quadrature phase shift keying QPSK signal, and the angle between the four quadrants in the signal constellation diagram and the coordinate axis is 45° clockwise, and the angle is recorded as φ c ; The transmission frequency of the pilot is the same as that of the signal, and is cyclically transmitted in the order of Q4, Q3, Q2, and Q1, where Q represents the quadrant in the constellation diagram where the signal is located.

10. A 24 / 7 free-space continuous variable quantum key distribution system, characterized in that: include: Module M1: After the sender Alice prepares a Gaussian modulated coherent state and adjusts the collimation of the free-space channel, she transmits the modulated coherent state to the receiver Bob through the free-space channel. Bob receives the signal and performs demodulation detection. Module M2: After demodulation at Bob's end, it partially splits the local oscillator light to implement real-time monitoring to obtain the channel transmittance value and corresponding shot noise value of each corresponding pulse. It also uses the fixed delay difference between the quantum signal path and the local oscillator path to obtain the peak value of the quantum signal path submerged in the noise, and obtains the preliminary continuous key data. Module M3: Bob groups the received continuously distributed quantum key data based on the real-time monitored channel transmittance data. He then performs data correction, parameter evaluation, error correction, and confidentiality enhancement on each group of quantum key data to ultimately generate a secure key.

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