Theoretal And Numerical Study On Transmission And Manipulation Of Polarized Photons In Free Space Quantum Communication

Traditional cryptography is based on complicated mathematical algorithms. However, with the development of quantum computers, the traditional cryptography could be deciphered, so communications security constitutes a conspicuous problem. Based on basic principles of quantum mechanics, quantum cryptography is "can not decipher" and "absolute security", therefore quantum cryptography can achieve a real sense of secure communications and an important research field of national security as well as the bright future for the development of information technology.Because free-space quantum key transmission is of great significance to future quantum key distribution using satellites as well as to establishing a global network of quantum secure communication, free-space quantum key distribution has aroused widespread concern. This thesis studies the theory of free-space quantum communication and establishes the appropriate theoretical model of photon propagation in the atmosphere, analyzes the photon polarization vector by the atmospheric scattering effects. The completion of this thesis will provide necessary theoretical support and valuable technical program for free-space quantum communication, which will be of great importance for the eventual realization of satellite-based global quantum communication. Main conclusions of this thesis include:We have analyzed and compared Rayleigh scattering and Mie scattering. It is shown that Rayleigh scattering and Mie scattering are in good agreement when the scale parameter is small. But when the scale parameter is large, dependence of Mie scattering intensity on wavelength is not obvious. We have analyzed and compared of Born approximation and Mie scattering of spherical particle with refractive index near 1 and present a criterion that can be used to replace Mie scattering theory.A new physical model of forward scattering of transmission of a polarized light beam in the atmosphere is established. Based on the incoherent light assumption, the double and multiple forward scattering theories are derived. To the symmetrical system, the effective Mueller matrix of the forward scattering of polarized light is obtained,.It is shown that only seven matrix elements are independent, the nine matrix elements are obtained by rotating independent elements. To validate the scattering model, the effective Mueller matrix of the forward scattering of polarized light of the transmission 10km in the atmosphere are simulated with Monte Carlo method, it is showed theatrical and numerical results are in good agreement.On this basis the scattering model of polarized photons in the multilayered atmosphere is established. The Monte Carlo simulation of the propagation of polarized photons in the multilayered atmosphere is made based on the model.We propose a scheme for optical realization of deterministic entanglement concentration of polarized photons. To overcome the difficulty due to the lack of sufficiently strong interactions between photons, teleportation is employed to transfer the polarization states of two photons onto the path and polarization states of a third photon, which is made possible by the recent experimental realization of the deterministic and complete Bell state measurement. Then the required positive operator-valued measurement and further operations can be implemented deterministically by using a linear optical setup.