Propagation And Protection Of Photons With Spatial Modes In Atmospheric Turbulence

Based on the principles of quantum mechanics,quantum communication provides a novel way for information transmission and is theoretically proven to be secure.Since confidentiality of information is calling more and more attention,quantum communication has become one of the most attractive research fields which leads to some practical applications.Photons serve as the most common information carrier in quantum communication,with the advantages of weak interaction with the environment,easy preparation and manipulation,and fast propagation speed.Quantum communication channels consist of optical fiber channel and free space channel.Due to the inherent loss in optical fiber which limits the transmission distance,the distance of fiber based quantum communication could hardly be extended since the quantum repeater is still far from optimal.With lower losses,free space serves as a good alternative and supplement that lays the foundation for long distance quantum communication network.Among the multiple degrees of freedom of photons,spatial modes allow a Hilbert space with infinite dimensions,which could improve the channel capacity and be more tolerable to the noise.However,the atmospheric turbulence in free space would disturb the spatial modes and lead to a distortion of the wavefront,which increases the error rate as well as the energy loss.Research on this issue will help advance the practical progress of quantum communication.The present thesis is mainly considering the propagation of spatially-structured photons through the atmospheric turbulence and ways to mitigate the influence of the atmospheric turbulence.The main achievements are summarized as follows:(1)The evolution of entangled states in the atmospheric turbulence is studied,where the spatial mode is encoded in the Ince-Gauss modes that constitute a complete family of exact and orthogonal modes.We therefore find the effects of atmospheric turbulence on an entangled state are related to the chosen mode.(2)Adaptive optics is widely employed to protect the light beam from turbulenceinduced degradation.Such a compensation is applied to the propagation of entangled Ince-Gauss states,which significantly improves the concurrence and the trace of the density matrix.The dependence of the evolution of different modes on the mode parameter is opposite to that without adaptive optics,while the trace is not affected by the mode parameter.Since adaptive optics only compensates for the phase distortion,the results reveal the influence of scintillation on different modes.(3)The quantum error correction is applied to protect the spatial mode encoded photonic qubits from the atmospheric turbulence.The method is based on an optimization problem,which varies with different forms of noise and is employed to find the encoding and recovery that maximize the state fidelity.This method does not contradict adaptive optics owing to their different implementation processes.By combining the error correction with adaptive optics,the reliability of transmissions can be further improved.