Research On The Effect Of Atmospheric Turbulence On Spatial Quantum Information Technology

Since the birth of quantum informatics in the 1990s,quantum information technology has been developing rapidly based on its unique advantages and potential applications,which has received more and more attention from researchers and scholars.However,when these emerging quantum information technologies are implemented in the realistic free space environment,they will inevitably interact with atmospheric molecules,aerosols,and atmospheric turbulence and be affected by the atmospheric environment,leading to the non-unitary evolution,decoupling,and decoherence of the quantum system,a reduced quantum advantage,which significantly limits the utilizing of quantum information technology.In order to give a detailed investigation of the negative effect of atmospheric turbulence on spatial quantum information technologies,we focus our efforts primarily on the three specific spatial quantum information technologies:quantum key distribution,quantum parameter estimation,and quantum target detection.Quantum key distribution mainly focuses on how to employ the superposition principle of quantum states and their entanglement effect to realize the secret distribution of the encoded information with the assistance of classical communication;quantum parameter estimation focuses primarily on enhancing the measurement accuracy of parameter estimation in the dynamical evolution of the quantum probe system by using the features of quantum mechanics,while quantum target detection aims to choose a suitable entangled source and measurement strategy to reduce the error probability in detecting the existence of a target.Therefore,the central issue in studying the effect of atmospheric turbulence on the above three quantum information technologies is to find out how a general quantum state is affected by atmospheric turbulence.In order to essentially address these aforementioned questions,this thesis presents more detailed research on some problems of space quantum information technology itself,the atmospheric transmission model of quantum light and the effect of atmospheric turbulence on space quantum information technology,the main contents and results of which can be summarized into three aspects as follows:1、We investigate the possible factors that determine the accuracy limit of quantum parameter estimation and the discrimination performance of quantum target detection.Firstly,we argue that the phase-dependent error distribution of locally unentangled quantum states is the one of factors determining quantum parameter estimation accuracy through the use of a displaced squeezed vacuum state,which is a special single-mode Gaussian state with a phase-dependent error distribution.we find that the optimal estimation accuracy can beat the shot-noise limit and constantly approach the Heisenberg scaling only in the regime of suitable phase-matching conditions,which is determined by the phase-sensitive parameter of the displaced squeezed vacuum state.Secondly,we study the target detection performance of quantum target detection based on three different types of entangled coherent states and employ the two-mode squeezed vacuum state and the coherent state as benchmarks to analyze the relationship between the entanglement strength of the three types of entangled coherent states and their overall performance of quantum target detection under different illumination conditions.We find that under general illumination conditions,there is no evidence to demonstrate the entanglement strength of entangled coherent states being associated with their target detection performance.2、We elaborate the physical meaning of the deviation scale between the result of the infinitesimal propagation equation prediction and that of the single phase screen approximation.Since the critical point,known as the so-called deviation scale,bridges the connection between the result of the single phase screen approximation and that of the infinitesimal propagation equation prediction,we first reexamine whether the minimal set of parameters obtained by the infinitesimal propagation equation remains valid to completely determine the crosstalk evolution considering the radial-mode scrambling based on the split-step beam propagation method.Secondly,we elucidate the physical meaning of the deviation scale through calculating the vortex-splitting ratio and the average orbital angular momentum of the received optical field.We demonstrate that the deviation between these two results is induced by the spatial accumulation effects of vortex splitting and vortex-antivortex pairs regeneration due to the intensity modulation of the input orbital angular momentum beam.3、We analyze the overall performance of the orbital angular momentum encoding scheme in free-space high-dimensional quantum key distribution and the quality enhancement of adaptive optics correction on orbital angular momentum quantum key distribution.Firstly,we propose a modified infinitely long phase screen method to simulate the variation of atmospheric turbulence,and study the effect of atmospheric turbulence on orbital angular momentum quantum key distribution based on this method and the atmospheric crosstalk model of a single photonic orbital angular momentum state.Secondly,considering the failure of adaptive optics while encountering phase cuts,we propose an enhanced adaptive optics correction scheme based on the phase unwrapping algorithm,which implements the wrapped cuts elimination on the basis of the conventional adaptive optics system.We observe that in the weak scintillation regime,the employment of the enhanced adaptive optics scheme for correcting the turbulenceinduced aberrations can lead to a significant performance enhancement compared to the conventional correction scheme.At last,we quantitatively evaluate the effect of time delay effect caused by our enhanced adaptive optics scheme on orbital angular momentum quantum key distribution,and conclude that this scheme is still able to mitigate the impact of atmospheric turbulence on orbital angular momentum quantum key distribution,even in the large wind velocity regime.4、We study the phase fluctuation mechanism of a coherent state while propagating through atmospheric turbulence and its performance in the phase estimation of quantum interferometric radar.Under the assumption that the spatial structure characteristic of the input coherent state is ignored and the beam width w0 is much smaller than the atmospheric coherence length r0,we first calculate the effect of the phase fluctuation caused by atmospheric turbulence on the coherent states using the so-called phase diffusion master equation,then evaluate the influence of phase fluctuation on the phase estimation resolution and optimal sensitivity of the coherent state quantum radar on the basis of this theoretical model.Finally,we demonstrate that in the regime of weak phase fluctuation,parity detection is a quasi-optimal detection through the comparison with the Cramer-Rao limit of parameter estimation.