Continuous variable quantum key distribution method and system based on free space channel

Abstract

The invention relates to the technical field of quantum communication, and discloses a continuous variable quantum key distribution method and system based on a free space channel. The method is applied to each of two communication parties, and a traveling wave parameter amplifier is arranged in each of the two communication parties. Comprising the following steps: generating a microwave dual-mode compression state through a traveling wave parametric amplifier; heterodyne detection is applied to the microwave signal of one mode in the microwave dual-mode compression state, so that the microwave signal of the other mode is mapped to a coherent state, and a coherent state microwave signal is formed; the coherent state microwave signals are sent to a free space quantum channel, so that third-party equipment obtains the coherent state microwave signals corresponding to the two communication parties from the free space quantum channel, and continuous variable Bell detection is carried out after the coherent state microwave signals corresponding to the two communication parties are mixed; the continuous variable Bell detection result of the third-party device is obtained, the secret key is extracted from the continuous variable Bell detection result, and the method can be used for all-weather quantum communication.

Description

Technical Field

[0001] The present invention relates to the technical field of quantum communication, and particularly to a continuous variable quantum key distribution method and system based on a free space channel. Background Art

[0002] Quantum Key Distribution (QKD) is a type of quantum communication technology. Quantum key distribution transmits information by encoding information in the quantum states of particles. Since the quantum states of the encoding particles in quantum key distribution follow the uncertainty principle and the no-cloning theorem of quantum mechanics, even if an eavesdropper has infinite computing power, it is impossible to eavesdrop on information without being detected, which ensures the unconditional security of quantum key distribution.

[0003] Quantum key distribution has two types: continuous variable quantum key distribution (CVQKD) and discrete variable quantum key distribution (DVQKD). Among them, CVQKD encodes key bits by using continuous variables of light, such as amplitude, phase, etc., and DVQKD encodes key bits by using discrete attributes of light particles. Compared with DVQKD, CVQKD allows multiple bits to be transmitted within a period of time by using the characteristics of continuous variables, and the key transmission rate is much higher than that of DVQKD.

[0004] CVQKD can be implemented through two communication media: free space and optical fiber channels. Among them, free space means transmitting optical quantum signals through the atmosphere or vacuum. Optical fiber channel means transmitting optical quantum information through an optical fiber as a quantum communication channel. Compared with CVQKD based on optical fiber channels, free space CVQKD has the advantages of achieving longer-distance communication by bypassing optical fiber losses and being easier to deploy.

[0005] However, free space CVQKD faces challenges such as absorption, heat dissipation, turbulence, and thermal noise in the atmosphere seriously affecting quantum signals, and weather conditions affecting communication channels. Summary of the Invention

[0006] The purpose of the present invention is to provide a continuous variable quantum key distribution method and system based on a free space channel, which can solve the technical problems that free space CVQKD faces, such as absorption, heat dissipation, turbulence, and thermal noise in the atmosphere seriously affecting quantum signals, and weather conditions affecting communication channels.

[0007] To solve the above technical problems, an embodiment of the present invention provides a continuous variable quantum key distribution method based on a free space channel, which is applied to each of the two communication parties. A traveling wave parametric amplifier is provided inside each of the two communication parties. The method includes the following steps: By applying a strong pump signal to the traveling wave parametric amplifier, the traveling wave parametric amplifier uses the nonlinear effect to transfer the energy of the strong pump signal to two modes, and compresses the signals of the two modes to generate a microwave two-mode squeezed state. Apply heterodyne detection to the microwave signal of one mode in the microwave two-mode squeezed state to map the microwave signal of the other mode to a coherent state, forming a coherent state microwave signal. Send the coherent state microwave signal to the free space quantum channel, so that a third-party device can obtain the coherent state microwave signals corresponding to the two communication parties from the free space quantum channel, and after mixing the coherent state microwave signals corresponding to the two communication parties, perform continuous variable Bell detection. Obtain the result of the continuous variable Bell detection by the third-party device, and extract the key from the result of the continuous variable Bell detection.

[0008] Optionally, the two communication parties include a first communication party and a second communication party. The amplitudes of the coherent state microwave signals of the first communication party and the second communication party are respectively: , ; In the formula, and are two canonical components of the coherent state microwave signal of the first communication party, and are two canonical components of the coherent state microwave signal of the second communication party, is the imaginary unit.

[0009] Optionally, the continuous variable Bell detection is used to detect the component and the component to obtain the detection result .

[0010] Optionally, the extracting the key from the result of the continuous variable Bell detection includes: Taking to decode the result of the continuous variable Bell detection to obtain the amplitudes and of the coherent state microwave signals corresponding to the two communication parties respectively, and extracting the key in combination with the amplitudes of the coherent state microwave signals corresponding to the two communication parties; In the formula, is the conjugate of .

[0011] Optionally, after extracting the key from the results of the continuous-variable Bell detection, the following steps are further included: According to the results of the continuous-variable Bell detection, calculate the key rate through the following formula: ; In the formula, represents the reconciliation efficiency, represents the mutual information between the two communicating parties, represents the Holevo bound of the eavesdropper.

[0012] Optionally, the sending of the coherent-state microwave signal to the free-space quantum channel includes: Couple the coherent-state microwave signal to the free-space quantum channel through a microwave antenna; wherein, the microwave antenna is modeled as a transmission circuit with spatially varying resistance, and is used to connect the traveling-wave parametric amplifier and the free-space quantum channel to couple the coherent-state microwave signal to the free-space quantum channel.

[0013] Optionally, both of the two communicating parties are in a low-temperature environment.

[0014] An embodiment of the present invention further provides a continuous-variable quantum key distribution system based on a free-space channel, including: two communicating parties and a third-party device, and a traveling-wave parametric amplifier is arranged inside each of the two communicating parties; Each of the two communicating parties is used to apply a strong pump signal to its own traveling-wave parametric amplifier; The traveling-wave parametric amplifier is used to transfer the energy of the strong pump signal to two modes by using the nonlinear effect, and compress the signals of the two modes to generate a microwave two-mode squeezed state; Each of the two communicating parties is further used to apply heterodyne detection to the microwave signal of one mode in the microwave two-mode squeezed state to map the microwave signal of the other mode to a coherent state, form a coherent-state microwave signal, and send the coherent-state microwave signal to the free-space quantum channel; The third-party device is used to obtain the coherent-state microwave signals respectively corresponding to the two communicating parties from the free-space quantum channel, and perform continuous-variable Bell detection after mixing the coherent-state microwave signals respectively corresponding to the two communicating parties; Each of the two communicating parties is further used to obtain the results of the continuous-variable Bell detection performed by the third-party device, and extract the key from the results of the continuous-variable Bell detection.

[0015] The continuous-variable quantum key distribution method provided by the present invention has at least the following beneficial effects: A microwave two-mode squeezed state is generated by using a new instrument, a traveling-wave parametric amplifier, and a coherent-state microwave signal is sent to a free-space quantum channel. Then, after the coherent-state microwave signals corresponding to both communication parties on the free-space quantum channel are mixed, the result of continuous-variable Bell detection is used to extract the key. That is, quantum information is transmitted over the free-space quantum channel with a microwave signal as the communication carrier. The microwave signal has lower loss in free space, is insensitive to environments such as fog, rain, and snow, and can be used for all-weather quantum communication, avoiding the influence of the atmosphere and weather on the free-space communication channel.

[0016] Moreover, both communication parties use a third-party device to mix the coherent-state microwave signals corresponding to both communication parties on the free-space quantum channel and then use the result of continuous-variable Bell detection to extract the key. Even if this third-party device is controlled by an eavesdropper, as long as the eavesdropper maintains the working mode of the third-party device, both communication parties can share part of the data on the classical public channel and, based on this, reconstruct the communication data of both communication parties and then extract the key, which is equivalent to adopting the measurement-device-independent quantum key distribution method MDI-QKD. That is, an untrusted third party interferes with and measures the quantum signals of both communication parties and sends the measurement results to both communication parties, achieving resistance to measurement-device attacks. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] One or more embodiments are exemplarily illustrated by the pictures in the corresponding drawings. These exemplary illustrations do not limit the embodiments.

[0018] Figure 1 is a flowchart of a continuous-variable quantum key distribution method based on a free-space channel according to an embodiment of the present invention; Figure 2 is a schematic diagram of the internal structure of a traveling-wave parametric amplifier according to an embodiment of the present invention; Figure 3 is a schematic diagram of the relationship between the key rate and distance under different weather conditions according to an embodiment of the present invention; Figure 4 is a schematic diagram of the implementation process of a continuous-variable quantum key distribution method based on a free-space channel according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0019] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the embodiments of the present invention will be described in detail below with reference to the accompanying drawings. However, those of ordinary skill in the art can understand that in the embodiments of the present invention, many technical details are provided to help readers better understand the present invention. However, even without these technical details and various changes and modifications based on the following embodiments, the technical solutions claimed in the present invention can still be implemented. The following division of each embodiment is for convenience of description and should not constitute any limitation on the specific implementation of the present invention. Each embodiment can be combined and cross-referenced with each other on the premise of no contradiction.

[0020] An embodiment of the present invention relates to a continuous variable quantum key distribution method based on a free space channel, which is applied to each of the two communication parties performing quantum communication, and a traveling wave parametric amplifier is provided inside each of the two communication parties.

[0021] The implementation details of the continuous variable quantum key distribution method based on the free space channel in this embodiment will be specifically described below. The following content is only the implementation details provided for convenience of understanding and is not necessary for implementing this solution. The specific process of the continuous variable quantum key distribution method based on the free space channel in this embodiment can be as Figure 1 shown and includes:

[0022] Step 101: By applying a strong pump signal to the traveling wave parametric amplifier, the traveling wave parametric amplifier uses the nonlinear effect to transfer the energy of the strong pump signal to two modes, and compresses the signals of the two modes to generate a microwave two-mode squeezed state.

[0023] Specifically, the traveling wave parametric amplifier (TWPA) is provided inside each of the two communication parties, and its internal structure is as Figure 2 shown. The TWPA can transfer the energy to two modes through parametric amplification by applying a strong pump signal with a frequency of in a low-temperature environment. These two modes are called the signal mode and the idler mode, and the frequencies of these two signal states satisfy , and correspond to the frequencies of the two modes respectively. The TWPA performs compression operations on the signal state and the idler state to generate a microwave two-mode squeezed state, that is, an EPR state.

[0024] For example, in a low-temperature environment, the two communication parties generate a microwave EPR state by injecting a strong pump signal into the TWPA , in CVQKD, the information of the microwave two-mode squeezed state can be described by its covariance matrix, and the covariance matrix of the microwave two-mode squeezed state is as follows: ; In the formula, represents the variance of the EPR state, represents the identity matrix of is the matrix .

[0025] In one example, both communication parties are in a low-temperature environment. When generating a microwave two-mode squeezed state by applying a strong pump signal to a traveling-wave parametric amplifier in a low-temperature environment, the generated noise is small, meeting the requirements of CVQKD.

[0026] Step 102, perform heterodyne detection on the microwave signal of one mode in the microwave two-mode squeezed state to map the microwave signal of the other mode to a coherent state, forming a coherent-state microwave signal.

[0027] Specifically, the heterodyne detector is also installed inside each of the two communication parties. Since the microwave signals of the two modes in the two-mode squeezed state are entangled with each other, each of the two communication parties can perform heterodyne detection on the microwave signal of one mode in the generated two-mode microwave signals through the heterodyne detector. Due to its entanglement property with the microwave signal of the other mode, the microwave signal of the other mode can be mapped to a coherent state, and the coherent-state microwave signal is sent to the free-space quantum channel. Among them, the heterodyne detector specifically mixes the input microwave state and the local microwave vacuum state and injects them into a balanced coupler, and then performs quadrature homodyne detection on the two microwave states emitted by the balanced coupler to simultaneously obtain the canonical components p and q of the input microwave state.

[0028] For example, by performing heterodyne detection on one mode of the EPR state generated by TWPA, the other mode is mapped to a coherent state, and its amplitude , , obeys a Gaussian distribution with a variance of 1 and a mean of zero. Then for the two communication parties (including communication party A and communication party B), the amplitudes of the coherent-state microwave signals of communication party A and communication party B can be respectively: , .

[0029] In the formula, and represent the two canonical components of the coherent-state microwave signal of communication party A, and representing two canonical components of the coherent-state microwave signal of communication party B where \(i\) is the imaginary unit

[0030] Step 103: Transmit the coherent-state microwave signal to the free-space quantum channel, enabling a third-party device to obtain the coherent-state microwave signals corresponding to the two communication parties from the free-space quantum channel. After mixing the coherent-state microwave signals corresponding to the two communication parties, perform continuous-variable Bell detection

[0031] In one example, assume that each of the two communication parties is in a low-temperature environment, such that the TWPA generates microwave signals in the low-temperature environment. Additionally, each of the two communication parties has a microwave antenna internally. The microwave antenna can serve as a microwave interface that couples the microwave state generated in the low-temperature environment to the free-space channel. The microwave antenna can be modeled as a transmission circuit with impedance varying with space, connecting a 50Ω low-temperature circuit to a 377Ω quantum channel, that is, connecting the traveling-wave parametric amplifier in the low-temperature environment to the free-space quantum channel. Thus, the two communication parties can respectively couple the coherent-state microwave signals generated in the low-temperature environment to the free-space quantum channel through their respective microwave antennas

[0032] wherein, the microwave antenna is characterized by its microwave gain , where \(\eta\) is the radiation efficiency, used to represent antenna loss, \(D\) is the antenna directivity, representing the ability of the antenna to focus the transmitted power in a specific direction, and , \(\lambda\) represents the wavelength \(A_p\) is the physical aperture area, determined by the size and shape of the antenna \(\eta_a\) is the aperture efficiency, which can be expressed as .

[0033] The microwave gain of the microwave antenna can compensate for the path loss during the propagation of the microwave state in free space. The path loss is the geometric attenuation experienced by the quantum signal during propagation in free space, and quantifies the portion of the initial signal power lost during the communication process. The path loss of microwaves with different frequencies can be expressed using the Friis transmission formula:

[0034] ; wherein, \(G_t\) and \(G_r\) represent the gains of the transmitting antenna and the receiving antenna respectively \(\lambda\) represents the microwave wavelength \(d\) represents the distance of microwave transmission

[0035] Therefore, even in free space where the path loss of microwaves is huge, this embodiment can mitigate this part of the path loss through a high-gain microwave antenna

[0036] In addition, weather factors can also affect the path loss of microwave states during propagation in free space. Table 1 shows the path loss after compensation by microwave antennas under different weather conditions: Table 1 The third-party device (i.e., the intermediate detection device) can be composed of a microwave antenna, a homodyne detector, and a balanced beam splitter. It receives the microwave quantum states (i.e., coherent-state microwave signals) sent by both communication parties in the free-space quantum channel through the microwave antenna, mixes the two quantum states in the balanced beam splitter, and then performs conjugate homodyne detection, i.e., continuous-variable Bell detection (CV Bell). Finally, the detection results are sent to both communication parties through the classical public channel, enabling both communication parties to obtain the results of continuous-variable Bell detection.

[0037] Among them, CV Bell detection is equivalent to detecting the components , , and broadcasts the final detection result to both communication parties through the classical public channel.

[0038] Step 104: Obtain the results of continuous-variable Bell detection by the third-party device and extract the secret key from the results of continuous-variable Bell detection.

[0039] Specifically, both communication parties receive the detection results of the intermediate detection device , in order to decode the results of continuous-variable Bell detection to obtain the amplitudes and of the coherent-state microwave signals corresponding to both communication parties respectively, and extract the secret key in combination with the amplitudes of the coherent-state microwave signals corresponding to both communication parties respectively; among them, is the conjugate of .

[0040] Each party among the communication parties extracts the secret key through a post-processing process (including error correction and privacy amplification). The post-processing process is a general stage of the CVQKD system, usually including error correction and privacy amplification, and is used to extract the final secure secret key. This embodiment adopts the same post-processing process as the conventional CVQKD system. This process does not belong to the core solution of this scheme and will not be elaborated here.

[0041] On this basis, this embodiment provides a schematic diagram of the relationship between the secret key rate and the distance (the distance between the two communication parties) under different weather conditions and a schematic diagram of the implementation process of the continuous-variable quantum key distribution method based on the free-space channel, as shown in Figure 3 and Figure 4 .

[0042] The secret key rate formula is , For coordination efficiency, is the mutual information between the two communication parties, the Holevo bound of the eavesdropper Charlie. During this process, even if the intermediate detection device is controlled by the eavesdropper Charlie, as long as the eavesdropper maintains the working mode of the intermediate detection device, the two communication parties can share part of the data on the classical public channel and reconstruct the probability distributions of the amplitudes and the detection results of the two communication parties, and extract the key through the first and second moments of the probability distribution to obtain the key rate.

[0043] Therefore, this embodiment for realizing measurement device-independent quantum key distribution (Measurement Device Independent, MDI)-QKD maintains the following assumptions: when the eavesdropper controls the intermediate detection device, it still maintains the working mode of the intermediate detection device - the microwave states sent by the two communication parties to the intermediate detection device are mixed in a balanced beam splitter and conjugate homodyne detected.

[0044] In this embodiment, a new instrument, a traveling-wave parametric amplifier, is used to generate a microwave two-mode squeezed state, and the coherent-state microwave signal is sent to the free-space quantum channel. Then, after the coherent-state microwave signals corresponding to the two communication parties on the free-space quantum channel are mixed, the key is extracted from the results of continuous-variable Bell detection. That is, the microwave signal is used as the communication carrier to transmit quantum information on the free-space quantum channel. The microwave signal has lower loss in free space, is insensitive to environments such as fog, rain, and snow, and can be used for all-weather quantum communication, avoiding the influence of the atmosphere and weather on the free-space communication channel.

[0045] Moreover, using the continuous-variable quantum key distribution method based on the free-space channel allows multiple bits to be transmitted within a period of time, improving the key transmission efficiency.

[0046] Furthermore, the two communication parties use a third-party device to mix the coherent-state microwave signals corresponding to the two communication parties on the free-space quantum channel and then extract the key from the results of continuous-variable Bell detection. Even if this third-party device is controlled by the eavesdropper, as long as the eavesdropper maintains the working mode of the third-party device, the two communication parties can share part of the data on the classical public channel and based on this reconstruct the communication data of the two communication parties, and then extract the key, which is equivalent to adopting the measurement device-independent quantum key distribution method MDI-QKD, that is, the quantum signals of the two communication parties are interferometrically measured by an untrusted third party and the measurement results are sent to the two communication parties, realizing resistance to measurement device attacks.

[0047] The step divisions of the above various methods are only for clear description. When implemented, they can be combined into one step or some steps can be split into multiple steps. As long as the same logical relationship is included, they are all within the protection scope of the present invention.

[0048] Another embodiment of the present invention relates to a continuous variable quantum key distribution system based on a free space channel. The implementation details of the continuous variable quantum key distribution system based on the free space channel in this embodiment will be specifically described below. The following content is only the implementation details provided for convenient understanding and is not necessary for implementing this solution. The continuous variable quantum key distribution system based on the free space channel in this embodiment includes: two communication parties and a third-party device. Each of the two communication parties is internally provided with a traveling wave parametric amplifier.

[0049] Each of the two communication parties is used to apply a strong pump signal to its respective traveling wave parametric amplifier; The traveling wave parametric amplifier is used to transfer the energy of the strong pump signal to two modes by using the nonlinear effect, and compress the signals of the two modes to generate a microwave two-mode squeezed state; Each of the two communication parties is further used to apply heterodyne detection to the microwave signal of one mode in the microwave two-mode squeezed state, so as to map the microwave signal of the other mode to a coherent state, form a coherent state microwave signal, and send the coherent state microwave signal to the free space quantum channel; The third-party device is used to obtain the coherent state microwave signals respectively corresponding to the two communication parties from the free space quantum channel, and after mixing the coherent state microwave signals respectively corresponding to the two communication parties, perform continuous variable Bell detection; Each of the two communication parties is further used to obtain the result of the continuous variable Bell detection performed by the third-party device, and extract the key from the result of the continuous variable Bell detection.

[0050] It is not difficult to find that this embodiment is a system embodiment corresponding to the above method embodiment, and this embodiment can be implemented in cooperation with the above method embodiment. The relevant technical details and technical effects mentioned in the above embodiment are still valid in this embodiment. To avoid repetition, they will not be elaborated here. Correspondingly, the relevant technical details mentioned in this embodiment can also be applied to the above embodiment.

[0051] It is worth mentioning that each module involved in this embodiment is a logical module. In practical applications, a logical unit can be a physical unit, a part of a physical unit, or can be implemented by a combination of multiple physical units. In addition, in order to highlight the innovative part of the present invention, units not closely related to solving the technical problems proposed by the present invention are not introduced in this embodiment, but this does not mean that there are no other units in this embodiment.

[0052] Those of ordinary skill in the art can understand that the above embodiments are specific embodiments for implementing the present invention. In actual applications, various changes can be made in form and details without departing from the spirit and scope of the embodiments of the present invention. Any person skilled in the art can make various changes and modifications without departing from the spirit and scope of the embodiments of the present invention. Therefore, the protection scope of the embodiments of the present invention should be subject to the scope defined by the claims.

Claims

1. A continuous variable quantum key distribution method based on a free space channel, characterized in that: Applied to each of the two communicating parties, each of the two communicating parties is internally provided with a traveling wave parametric amplifier; The method comprises: By applying a strong pump signal to the traveling wave parametric amplifier, the traveling wave parametric amplifier uses a nonlinear effect to transfer the energy of the strong pump signal to two modes, and compresses the signals of the two modes to generate a microwave dual-mode compressed state; Applying heterodyne detection to a microwave signal of one mode in the microwave dual-mode squeezed state to map the microwave signal of another mode to a coherent state to form a coherent state microwave signal; Sending a coherent microwave signal to a free-space quantum channel so that a third-party device can obtain the coherent microwave signals corresponding to the two communicating parties from the free-space quantum channel, and after mixing the coherent microwave signals corresponding to the two communicating parties, perform continuous variable Bell detection; A result of continuous variable Bell detection performed by a third-party device is obtained, and a key is extracted from the result of the continuous variable Bell detection.

2. The continuous variable quantum key distribution method based on free space channel according to claim 1, characterized in that: The communicating parties include a first communicating party and a second communicating party, and the amplitudes of the coherent microwave signals of the first communicating party and the second communicating party are respectively: , ; In the formula, and are the two regular components of the coherent microwave signal of the first communication party, and are the two regular components of the coherent microwave signal of the second communication party, Is an imaginary unit.

3. The continuous variable quantum key distribution method based on free space channel according to claim 2, characterized in that: The continuous variable Bell test is used to and quantity Conduct the test and obtain the test results .

4. The continuous variable quantum key distribution method based on free space channel according to claim 3, characterized in that: The step of extracting a key from a result of a continuous variable Bell detection comprises: by Decode the results of the continuous variable Bell detection to obtain the amplitudes of the coherent microwave signals corresponding to the two communicating parties. and , and extract the key by combining the amplitudes of the coherent microwave signals corresponding to the two communicating parties; In the formula, for The conjugation of.

5. The continuous variable quantum key distribution method based on free space channel according to claim 1, characterized in that: After extracting the key from the result of the continuous variable Bell detection, the method further comprises: According to the result of continuous variable Bell detection, the key rate is calculated by the following formula: ； In the formula, represents the coordination efficiency, represents the mutual information between the two communicating parties, Indicates the Holevo bound of the eavesdropper.

6. The continuous variable quantum key distribution method based on free space channel according to claim 1, characterized in that: The sending of the coherent microwave signal to the free space quantum channel comprises: Coherent state microwave signals are coupled to a free space quantum channel via a microwave antenna; Among them, the microwave antenna is modeled as a transmission circuit with spatially varying impedance, which is used to connect the traveling wave parametric amplifier and the free-space quantum channel to couple the coherent microwave signal to the free-space quantum channel.

7. The continuous variable quantum key distribution method based on a free space channel according to any one of claims 1 to 6, characterized in that: Both communicating parties are in a low temperature environment.

8. A continuous variable quantum key distribution system based on a free space channel, characterized in that: The system comprises: two communicating parties and a third-party device, each of the two communicating parties being internally provided with a traveling wave parametric amplifier; Each of the two communicating parties is used to apply a strong pump signal to the respective traveling wave parametric amplifier; The traveling wave parametric amplifier is used to transfer the energy of the strong pump signal to the two modes by utilizing the nonlinear effect, and compress the signals of the two modes to generate a microwave dual-mode compressed state; Each of the two communicating parties is further used to apply heterodyne detection to a microwave signal of one mode in the microwave dual-mode compressed state to map a microwave signal of another mode to a coherent state to form a coherent state microwave signal, and send the coherent state microwave signal to a free space quantum channel; The third-party device is used to obtain the coherent state microwave signals corresponding to the two communicating parties from the free space quantum channel, and perform continuous variable Bell detection after mixing the coherent state microwave signals corresponding to the two communicating parties; Each of the two communicating parties is also used to obtain the result of the continuous variable Bell detection performed by a third-party device and extract the key from the result of the continuous variable Bell detection.

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