Free space continuous variable quantum key distribution dynamic polarization control method and system

Abstract

The invention provides a free space continuous variable quantum key distribution dynamic polarization control method and system. The dynamic polarization control method comprises the following steps: receiving mixed light transmitted by a free channel through telescope coupling; mixed light received by a telescope is input into a dynamic polarization controller and is adjusted by the dynamic polarization controller, and output light is subjected to light beam separation through a beam splitter; power parameters of the output light are obtained through the optical power meter, feedback signals are generated according to the power parameters, the feedback signals are input into the dynamic polarization controller, and polarization state adjustment is conducted on the output light. According to the continuous variable quantum key distribution system, light splitting feedback and monitoring based on the optical power meter are carried out on weak signal light, the influence of space link transmittance jitter can be effectively resisted, then stable polarization control is achieved, and the robustness of the continuous variable quantum key distribution system in a free space scene is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of quantum secure communication, in particular to a free-space continuous-variable quantum key distribution dynamic polarization control method and system. BACKGROUND

[0002] With the development of information age, the amount of information transmission is rapidly increasing, accompanied by the increasing demand for information security in information technology. With the increase of computer computing power, the risk of encrypted system being cracked is gradually increasing, and the existing security embodiment is facing a major challenge. Since quantum key distribution technology can physically guarantee the unconditional security of communication, it has attracted the attention of many researchers. Quantum key distribution technology can be mainly divided into two categories: discrete variable quantum key distribution and continuous variable quantum key distribution technology, in which continuous variable quantum key distribution mainly uses optical fiber transmission scheme. In recent years, free-space continuous variable quantum key distribution technology has attracted attention due to its flexibility in link establishment and the ability to perform star-ground key distribution.

[0003] Compared with optical fiber channel, atmospheric channel is relatively complex and uncontrollable, so in practice, free-space continuous variable quantum key distribution needs to consider possible problems in all aspects as much as possible. Atmospheric channel mainly includes two problems, which are the influence of atmospheric channel effect and background noise. The atmospheric channel effect includes beam extinction and turbulence effect. On the one hand, beam extinction is caused by the absorption and scattering of molecules and aerosols to light, which mainly causes the intensity attenuation of the received light field. On the other hand, the turbulence caused by the random fluctuation of refractive index in the atmospheric channel can cause channel transmittance fluctuation (fading), communication interruption and excessive noise, etc. In addition to being affected by various atmospheric effects, it will also be affected by background radiation, such as sunlight. Background radiation usually causes background noise in the receiver's detector, affecting the detection of the signal, and thus affecting the final system performance.

[0004] According to whether the local oscillator light is transmitted, the continuous variable quantum key distribution system has two different implementations: on-the-way local oscillator scheme and local local oscillator scheme. In the on-the-way local oscillator scheme, the transmitter usually uses time division polarization multiplexing technology to couple the quantum signal light and the local oscillator light out of the channel; after the receiver receives the optical signal from the channel, the time division polarization demultiplexing technology is used to distinguish and output the quantum light and the local oscillator light to the demodulation and detection module for interference to obtain the encoded information (such as Figure 1). In the local oscillator scheme, although the transmitting end does not transmit the local oscillator light, in order to accurately obtain the frequency offset information between the non-identical lasers and the phase drift information introduced by the signal in the channel transmission, the transmitting end often transmits a reference light, and the reference light and the quantum signal light are coupled and transmitted to the channel through time division, polarization or frequency division multiplexing and the like. In order to meet the interference condition of the same polarization as the light source of the receiving end, the polarization demultiplexing or polarization control (as shown in Figure 2 and Figure 3 ) is often needed after the light signal is received from the channel. Therefore, dynamic polarization demultiplexing or control is a very important part of the continuous variable quantum key distribution system, and will directly affect the stability and performance of the system. However, due to the fading in the atmospheric channel, that is, the random fluctuation of the channel transmittance, as shown in Figure 4 and Figure 5 , the traditional light channel based on the splitting feedback method will no longer be applicable, which makes it a challenging problem to perform dynamic polarization control in this case. SUMMARY

[0005] In view of the defects in the prior art, the purpose of the present application is to provide a free space continuous variable quantum key distribution dynamic polarization control method and system.

[0006] According to the free space continuous variable quantum key distribution dynamic polarization control method provided by the present application, the method comprises the following steps:

[0007] Step S1: coupling and receiving mixed light from free channel transmission through a telescope;

[0008] Step S2: inputting the mixed light received by the telescope into a dynamic polarization controller, adjusting the mixed light through the dynamic polarization controller, and separating the output light through a beam splitter;

[0009] Step S3: acquiring the power parameter of the output light through an optical power meter, generating a feedback signal according to the power parameter, inputting the feedback signal into the dynamic polarization controller, and adjusting the polarization state of the output light.

[0010] Preferably, the mixed light comprises local oscillator light and signal light.

[0011] Alternatively, the mixed light comprises reference light and signal light.

[0012] Preferably, the optical power meter acquires the average power value in a certain time window through logarithmic amplification integration operation when sampling.

[0013] Preferably, the generation method of the feedback signal comprises:

[0014] Step S3.1: separating the output light through a first beam splitter to obtain a first light path and a second light path; the first light path is subjected to power detection by a first optical power meter, and the second light path is input into a polarization beam splitter;

[0015] Step S3.2: separating the second light path into local light and signal light with mutually perpendicular polarization states through the polarization beam splitter; the local light is transmitted into subsequent coherent detection;

[0016] Step S3.3: separating the signal light through a second beam splitter to obtain a third light path and a fourth light path, the third light path is subjected to power detection by a second optical power meter, and the fourth light path continues to be transmitted and finally interferes with the local light for signal measurement;

[0017] Step S3.4: subtracting the detection results of the first optical power meter and the second optical power meter to obtain a feedback signal.

[0018] Preferably, the separation ratio of the first beam splitter is 5:95, wherein the light path with lower power is the first light path, and the light path with higher power is the second light path.

[0019] Preferably, the separation ratio of the second beam splitter is 10:90, wherein the light path with lower power is the third light path, and the light path with higher power is the fourth light path.

[0020] Preferably, the detection result of the first optical power meter is T w T c (t)P w , wherein T w is the fixed transmission efficiency of the mixed light from the telescope to the first beam splitter at Bob's end, T c (t) is the transmittance of the free space channel, which changes randomly with time t, and P w is the mixed light power;

[0021] The detection result of the second optical power meter is , wherein T s is the dynamic transmission efficiency of the signal light from the telescope to the second beam splitter at Bob's end, which depends on the calibration polarization state, and P s is the fixed emission light power of the signal light;

[0022] The detection results of the two optical power meters are converted into results in units of dBm and subtracted as follows:

[0023]

[0024] , wherein is a fixed value.

[0025] Preferably, the formula result As a feedback signal, the dynamic polarization controller is driven to adjust when When the polarization demultiplexing of the dynamic polarization controller is completed, the polarization state of the output light is lower than the preset value.

[0026] Preferably, the local light and the signal light are generated by polarization time division multiplexing technology or same polarization state time division multiplexing technology.

[0027] According to the application, a free space continuous variable quantum key distribution dynamic polarization control system is provided, comprising:

[0028] Telescope: coupled to receive mixed light transmitted by a free space channel;

[0029] Dynamic polarization controller: for adjusting the polarization state of the output light according to the feedback signal, so as to reach the expected polarization state;

[0030] First beam splitter: for splitting the received light signal into two or more paths in a certain proportion, wherein the power of the split light is used for power monitoring;

[0031] Polarization beam splitter: for splitting the light beams with different polarization states;

[0032] Second beam splitter: for splitting the signal light after polarization beam splitting into two beams in a certain proportion, wherein the power of one path is used for signal measurement, and the power of the other path is used for power measurement;

[0033] First optical power meter: for measuring the power of the split light generated by the first beam splitter;

[0034] Second optical power meter: for measuring the power of the split light generated by the second beam splitter;

[0035] The free space continuous variable quantum key distribution dynamic polarization control system adopts the free space continuous variable quantum key distribution dynamic polarization control system method for dynamic polarization control.

[0036] Compared with the prior art, the application has the following beneficial effects:

[0037] 1、The application eliminates the influence of transmittance jitter on polarization control by performing split feedback on the received light and the demultiplexed weak signal light and performing subtraction operation on the power monitoring value, and performing division operation on the absolute power, thereby realizing stable polarization control and improving the robustness of the continuous variable quantum key distribution system in a free space scene.

[0038] 2、The application can increase the monitoring sensitivity by monitoring the weak light and based on the logarithmic amplification principle of the optical power meter, and the sensitivity of the weak light is higher than that of the strong light, which can effectively cope with the scene of rapid fading of the space channel.

[0039] 3. The logarithmic amplification and integration operation of the optical power meter can obtain the average power value in a certain time window, effectively smooth the random fluctuations of light intensity caused by atmospheric turbulence, weaken the influence of instantaneous noise on the detection result, avoid misjudging noise as signal, reduce the mutation of feedback signal, and improve the stability and reliability of feedback signal. BRIEF DESCRIPTION OF DRAWINGS

[0040] Other features, objects, and advantages of the application will become more apparent from the following detailed description of non-limiting embodiments, when read in conjunction with the accompanying drawings:

[0041] Figure 1 Schematic diagram of continuous variable quantum key distribution system based on local local oscillator architecture;

[0042] Figure 2 Schematic diagram of continuous variable quantum key distribution system based on local local oscillator architecture (reference light and signal light polarization multiplexing);

[0043] Figure 3 Schematic diagram of continuous variable quantum key distribution system based on local local oscillator architecture (reference light and signal light polarization multiplexing);

[0044] Figure 4 Schematic diagram of the influence of atmospheric channel fading on pulse transmission time domain;

[0045] Figure 5 Schematic diagram of the statistical distribution of atmospheric channel fading;

[0046] Figure 6 Schematic diagram of the receiving end dynamic polarization feedback control system based on the local local oscillator architecture. DETAILED DESCRIPTION

[0047] The application will be described in detail below with specific embodiments. The following examples will help those skilled in the art to further understand the application, but do not limit the application in any form. It should be noted that for those skilled in the art, without departing from the concept of the application, a number of changes and improvements can be made. These all belong to the protection scope of the application.

[0048] In an exemplary embodiment, as shown in Figure 1 The dynamic polarization control technology of free space continuous variable quantum key distribution is applied to the continuous variable quantum key distribution classical system of the local local oscillator scheme. In the local local oscillator scheme, the local oscillator light and the signal light can adopt polarization time division multiplexing technology. Referring to Figure 6 The specific steps of the dynamic polarization control technology of free space continuous variable quantum key distribution include:

[0049] Step S1, after the transmission of the local light (or reference light) and the signal light through the free channel, the mixed light is first coupled and received by the receiving telescope at the receiving end. The fixed transmission power of the local light, the reference light and the signal light is P LO , P pilot and P s respectively, and the mixed light power composed of the local light (or reference light) and the signal light is P w =P LO +P s , or P w =P pllot +P s , in units of mW.

[0050] Further, in the free space channel, due to the instability of the channel characteristics, fading phenomenon often occurs, which is manifested as random fluctuations in channel transmittance. We denote the transmittance of the free space channel as T c (t), which represents the random change of the channel transmittance over time.

[0051] Step S2, the mixed light received by the telescope enters the dynamic polarization controller, and the polarization state is adjusted through the received feedback signal.

[0052] Step S3, the feedback signal is obtained by the subtraction operation of the optical power meter.

[0053] Further, in the free space channel, due to the channel fading effect caused by atmospheric turbulence, the light intensity of the mixed light (containing signal light and local light) shows a dramatic random fluctuation in the propagation process. If a photodetector is used for detection, due to its linear response characteristics, noise amplification is easy when the signal light power is lower than the noise power, which leads to distortion of the feedback signal and affects the polarization control accuracy. In contrast, the optical power meter obtains the average power value within a certain time window through logarithmic amplification and other integral operations, which can effectively smooth the noise fluctuations and reduce the influence of instantaneous fluctuations on the detection results. Even if there may be some sampling points with signal light power lower than noise power in a short time, the overall integral result is still dominated by the signal light, avoiding the misjudgment of noise as signal, thereby improving the stability and reliability of the feedback signal and reducing the mutation of the feedback signal.

[0054] Further, the specific steps for generating the feedback signal include:

[0055] Step S3.1: the mixed light passes through the beam splitter 1 and is divided into two paths according to a certain proportion (such as 5:95), where the path with stronger power continues to transmit into the polarization beam splitter for polarization demultiplexing, and the path with weaker power enters the optical power meter for power detection, and the monitoring result is represented as T w T c (t)P w , where T wBob end fixed transmission efficiency of the mixed light from the telescope to the first beam splitter.

[0056] Step S3.2: The polarization beam splitter separates the mixed light into the local light and the signal light with mutually perpendicular polarization states, and the local light is transmitted into the subsequent coherent detection.

[0057] Step S3.3: The quantum signal light continues to transmit into the beam splitter 2. The beam splitter 2 divides the quantum signal light into two paths according to a certain ratio (10:90), the path with stronger power continues to transmit and finally interferes with the local light for signal measurement, and the path with weaker power enters the optical power meter for power measurement, and the measurement result is Wherein Bob end dynamic transmission efficiency of the signal light from the telescope to the second beam splitter, Depends on the calibration polarization state, when the polarization controller changes the polarization direction according to the feedback signal, the dynamic transmission efficiency of the signal light changes accordingly.

[0058] Further, the step S3.3 monitors the quantum signal light with weaker power. In the continuous variable quantum key distribution system, the polarization feedback control often monitors the local light with stronger power. Here, we choose to monitor the weak signal light, mainly considering that many common large dynamic range optical power meters are based on logarithmic amplification principle, and the absolute measurement sensitivity of strong light and weak light is different, that is, the measurement sensitivity of weak light is higher than that of strong light. For example, the logarithmic change is 0.25dB, which represents a relative change of 5.8% in power. Assuming that the optical power meter monitors weak light of 0.01mW and strong light of 10mW respectively, when the polarization controller changes the optical power by 0.25dB, the absolute optical power changes of weak light and strong light are 0.01mW*5.8%=0.58μW and 10mW*5.8%=0.58mW respectively. Under the same 0.25dB logarithmic adjustment, the absolute optical power change corresponding to the strong light is about 1000 times that of the weak light, which means that the adjustment precision of the weak light is much higher than that of the strong light. Even if the adjustment amplitude is the same, the actual power change of the weak light is much smaller than that of the strong light, which is more conducive to fine control.

[0059] Step S3.4: After the power detection of the two light paths in the optical power meter, we perform subtraction operation on the output results of the optical power meter.

[0060] Further, according to the step S3.1 and the step S3.3, the output results of the two optical power meters are T w T c (t)P w And Convert the results in dBm unit and subtract them as

[0061]

[0062] wherein, is a fixed value, the channel transmittance T c (t) cancellation, the influence of spatial link transmittance jitter can be removed, and stable signal feedback is realized, and the variable in the formula is only determined by the adjustment result of the polarization state.

[0063] Step S4: according to the formula result as a feedback signal, driving the dynamic polarization controller to adjust.

[0064] Further, when the polarization demultiplexing is completed. Before the polarization demultiplexing is completed, part of the local oscillator light is coupled to the signal light path by the polarization beam splitter, so that the power of the path is higher than the actual signal light power. The polarization control adjusts the polarization state to minimize the detected signal light power, so as to minimize the transmittance of the signal light polarization demultiplexing is completed.

[0065] As an optional implementation, the dynamic polarization control technology of free space continuous variable quantum key distribution can be applied to a continuous variable quantum key distribution system of a local oscillator scheme,

[0066] Optionally, in the local oscillator scheme, the signal light and the local oscillator light can adopt polarization time division multiplexing technology, such as Figure 2 or time division multiplexing technology with the polarization state, such as Figure 3 .

[0067] The application also discloses a free space continuous variable quantum key distribution dynamic polarization control system, referring to Figure 6 , comprising:

[0068] Telescope: coupled to receive mixed light transmitted by a free space channel;

[0069] Dynamic polarization controller: for adjusting the polarization state of the output light according to the feedback signal, so as to reach the expected polarization state;

[0070] First beam splitter: for splitting the received optical signal into two or more paths in a certain proportion, wherein the power of the weaker light is used for power monitoring;

[0071] Polarization beam splitter: for splitting light beams with different polarization states;

[0072] Second beam splitter: for splitting the signal light after polarization splitting into two beams in a certain proportion, wherein the power of the stronger one is used for signal measurement, and the power of the weaker one is used for power measurement;

[0073] The first optical power meter is used for measuring the split optical power generated by the first beam splitter.

[0074] The second optical power meter is used for measuring the split optical power generated by the second beam splitter.

[0075] The free-space continuous-variable quantum key distribution dynamic polarization control system adopts the free-space continuous-variable quantum key distribution dynamic polarization control system method for dynamic polarization control.

[0076] The specific embodiments of the present application are described above. It needs to be understood that the present application is not limited to the specific embodiments described above, and various changes or modifications can be made by those skilled in the art within the scope of the claims, which does not affect the essential content of the present application. The embodiments of the present application and the features in the embodiments can be combined with each other arbitrarily without conflict.

Claims

1. A method for dynamic polarization control in free-space continuous-variable quantum key distribution, characterized in that, The method comprises the following steps: Step S1: receiving mixed light from free channel transmission through a telescope; Step S2: inputting the mixed light received by the telescope into a dynamic polarization controller, adjusting the mixed light through the dynamic polarization controller, and outputting the light through a beam splitter for beam splitting; Step S3: obtaining the power parameter of the output light through an optical power meter, generating a feedback signal according to the power parameter, and inputting the feedback signal into the dynamic polarization controller to adjust the polarization state of the output light.

2. The free-space continuous-variable quantum key distribution dynamic polarization control method according to claim 1, wherein, The mixed light comprises local light and signal light. Alternatively, the mixed light comprises reference light and signal light.

3. The free-space continuous-variable quantum key distribution dynamic polarization control method of claim 1, wherein, The optical power meter obtains the average power value in a certain time window through logarithmic amplification integration operation when sampling.

4. The free-space continuous-variable quantum key distribution dynamic polarization control method of claim 1, wherein, The method for generating the feedback signal comprises the following steps: Step S3.1: separating the output light through a first beam splitter to obtain a first light path and a second light path; the first light path is subjected to power detection through a first optical power meter, and the second light path is input into a polarization beam splitter; Step S3.2: separating the second light path into local light and signal light with mutually perpendicular polarization states through the polarization beam splitter; the local light is transmitted into subsequent coherent detection; Step S3.3: separating the signal light through a second beam splitter to obtain a third light path and a fourth light path; the third light path is subjected to power detection through a second optical power meter, and the fourth light path continues to be transmitted and finally interferes with the local light for signal measurement; Step S3.4: subtracting the detection results of the first optical power meter and the second optical power meter to obtain the feedback signal.

5. The free-space continuous-variable quantum key distribution dynamic polarization control method according to claim 4, characterized in that, The separation ratio of the first beam splitter is 5:95, wherein the light path with lower power is the first light path, and the light path with higher power is the second light path.

6. The free-space continuous-variable quantum key distribution dynamic polarization control method according to claim 4, wherein, The separation ratio of the second beam splitter is 10:90, wherein the light path with lower power is the third light path, and the light path with higher power is the fourth light path.

7. The free-space continuous-variable quantum key distribution dynamic polarization control method according to claim 4, characterized in that, The detection result of the first optical power meter is T w T c (t)P w , wherein T w is a fixed transmission efficiency of the mixed light from the telescope to the first beam splitter at Bob's end, T c (t) is the transmittance of the free space channel, which changes randomly with time t, and P w is the mixed light power; the detection result of the second optical power meter is the dynamic transmission efficiency of the signal light from the telescope to the second beam splitter for Bob, P depends on the calibrated polarization state s is the fixed launch optical power of the signal light; The detection results of the two optical power meters are converted into results in units of dBm and subtracted as follows: wherein is a constant value.

8. The free-space continuous-variable quantum key distribution dynamic polarization control method according to claim 7, characterized in that, The formula result As a feedback signal, the dynamic polarization controller is driven to adjust when The polarization demultiplexing of the dynamic polarization controller is completed when the value is lower than the preset value.

9. The free-space continuous-variable quantum key distribution dynamic polarization control method according to claim 2, wherein, The local light and the signal light are generated by using polarization time division multiplexing technology or same polarization time division multiplexing technology.

10. A free-space continuous-variable quantum key distribution dynamic polarization control system, comprising: a polarization controller; a polarization analyzer; a polarization rotator; and a polarization modulator. The method comprises the following steps: Telescope: coupling to receive mixed light transmitted in a free space channel; Dynamic polarization controller: used for adjusting the polarization state of the output light according to the feedback signal to make it reach the expected polarization state; First beam splitter: used for separating the received optical signal into two or more paths according to a certain ratio, wherein the light with lower power is used for power monitoring; Polarization beam splitter: used for separating light beams with different polarization states; Second beam splitter: used for separating the signal light after polarization into two beams according to a certain ratio, wherein the light path with higher power is used for signal measurement, and the light path with lower power is used for power measurement; First optical power meter: used for measuring the power of the light separated by the first beam splitter; Second optical power meter: used for measuring the power of the light separated by the second beam splitter; The dynamic polarization control system for free space continuous variable quantum key distribution adopts the method for dynamic polarization control of the free space continuous variable quantum key distribution dynamic polarization control system according to any one of claims 1 to 9.

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