Effects Of Atmospheric Turbulence On Orbital Angular Momentum Entanglement And Quantum State Polarization Properties

Quantum communication is a combination of quantum mechanics and communication theory.It has been transformed from theory to practical application since 30 years.Quantum communication technology has a unique development potential and application prospect in improving the safety of information transmission and the efficiency of information transmission,which is becoming a hot spot of current research and development.Quantum communication channels can be divided into optical fiber channel and free space channel.Due to the inherent transmission loss in optical fiber,the transmission distance of quantum communication is hard to be extended.Free space quantum communication is a feasible scheme of quantum communication.Free space quantum channel has low losses,and in atmospheric turbulence essentially without birefringence effect,which lays a foundation for global quantum communication.However,free space quantum communication works between the ground and atmospheric channel.When light propagates across turbulence,the atmospheric refractive index fluctuation causes light wavefront distortion and the amplitude fluctuantion,which will change the relative phase of quantum superposition state,lead the relative phase to be random,reduce the signal energy,and make a phase-bit error.The present dissertation is mainly about the effects of atmospheric turbulence on orbital angular momentum(OAM)entangled photon through turbulence.In addition,non-classical polarization properties of two-orthogonally-polarized mode field of quantum state propagating through non-Kolmogorov turbulence are also studied.The three main works in this thesis are summarized as follows:1.Using the multiphase screen model,we numerically investigate the entanglement evolution of OAM photons propagating in non-Kolmogorov turbulence,especially focusing on the influences of the azimuthal mode and turbulence parameters on entanglement evolution in the weak scintillation regime.The case in which both photons pass through turbulence is considered.Our numerical results show that the azimuthal mode,the generalized exponent,and the outer scale of turbulence have obvious influences on OAM entanglement.However,the influence of the turbulence inner scale on OAM entanglement can be ignored.It is believed that these findings may be useful in applications in higher Hilbert free-space optical communications.2.In this paper,we theoretically investigate the entanglement evolution of OAM three-qubit state propagating in Kolmogorov and non-Kolmogorov weak turbulence.Here,the case in which all photons are sent through turbulence is considered.The entanglement evolution of OAM three-qubit state is quantified by the squared concurrence of each qubit with the rest of the system.Additionally,the channel fidelity of the output state relative to the input state is also studied.The results show that OAM entanglement decays through turbulence,but the entangled OAM three-qubit state with larger azimuthal modes is more robust against atmospheric decoherence.Moreover,it is found that the entangled OAM three-qubit state with small beam waist has a higher overall transmission in turbulent atmosphere,which agrees well with the case of two photons.3.In this paper,we propose theoretical model to describe the polarization degree of the two orthogonally polarized modes quantization dark hollow beams(DHB)propagating through non-Kolmogorov turbulence.The analytical expressions for the polarization degree of the two orthogonally polarized modes partially coherent DHB of photon number state and coherence state are obtained.The polarization degree of the two orthogonally polarized modes partially coherent DHB of photon number state and coherence state have been investigated for the turbulence factors and beam parameters with the detection photon numbers.An investigation of the changes in the on-axis propagation point and off-axis propagation points show that the polarization degree of quantization partially coherent DHB exists oscillation for a short propagation distance and gradually returns to zero for a sufficiently long distance.That is to say the light polarization state will become to completely unpolarized as the beam propagation.