| Quantum information is an emerging science that combines the various coherence properties of quantum systems into contemporary scientific research and realizes the combination of quantum theory and information science.Quantum-associated imaging technology has high sensitivity and detection accuracy,and has stronger anti-interference and anti-spoofing ability.Quantum communication technology has the advantages of high speed,large capacity and unconditional security,and has great development potential and broad application prospects.Since surface free space quantum information technology,such as Orbital Angular Momentum（OAM）,needs to work in a natural medium channel,random fluctuation of refractive index caused by turbulence which results from temperature,pressure,density and other factors change will lead to wavefront distortion,amplitude fluctuation and phase irregularity of transmitted light wave.This will result in quantum decoherence of entangled photons and quantum polarization fluctuation of polarization state and finally quantum error emerges.This paper will study the effects of turbulent medium on the vortex beam and orbital angular momentum transfer characteristics.Firstly,the relationship between the vortex beam and the orbital angular momentum is introduced to illustrate the necessity of studying the vortex beam,and several methods for generating the vortex beam are introduced.The basic theory of the turbulent atmosphere involved is explained,and three common atmospheric turbulence power spectrums are introduced.The process of obtaining the turbulent phase screen using the power spectrum inversion method is described in detail.Based on this,the transmission distance can be dispersed by the multi-layer phase screen method to establish the beam propagation model in the turbulent medium.Secondly,the phase structure function is introduced,and the subharmonic compensation method is used to compensate the low frequency.At the same time,the correct phase of the simulated phase screen is verified.The numerical simulation is used to study the distribution of intensity,phase and topological charges of a vortex beam after passing through a turbulent medium.The flicker characteristics,drift characteristics,OAM detection probability of vortex beam and OAM transmission in turbulent medium are studied by the distribution of intensity and phase.Finally,the complete experimental link is designed and the test bench is built.The OAM is carried by the spiral phase plate to make the Gaussian beam into a vortex beam.The experiment uses 3.5%Na Cl solution instead of seawater,and the motor is agitated to obtain different turbulence intensity.Add some masses of Al2O3 particles to enhance the disturbance of the beam;use corroded glass sheets to verify the theoretical correctness of the turbulent phase screen.Analyze the experimental results and reliability of the two experimental methods.Complete the the experimental study of the effect of turbulent medium on the vortex beam and it can provide a simulated and experimental data basis for the accurate deciphering of the information carried by the vortex beam and the orbital angular momentum. |
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