The Research On Characters Of Photon Polarization Qubits And OAM Propagating Through Free Space

It is well known that a photon can carry two kinds of angular momentum:spin angular momentum (SAM) and orbital angular momentum (OAM). The two SAMs of photon are associated with left-handed circular polarization and right-handed circular polarization of light respectively, which is described by a two-dimensional Hilbert space thus can be used as the basis of a quantum binary digit (i.e. qubit). Qubits-based free space quantum optical communication, in principle, is capable of providing an absolute security. Further, it is promising for realizing a global quantum optical communication through satellite. Accordingly, it has a significant practical significance. While, the OAM of photon is associated with the azimuthal phase of the optical field. Its eigenmode contains a helical phase factor exp(ilθ) where l can take any integer. As a result, there are infinite eigenmodes of OAM. These modes form an infinite-dimensional Hilbert space, thus can be employed to encode more information, and increase the information capacity of the communication system significantly. On the other hand, by comparison with the traditional optical fiber channel, OAM modes are able to propagate through the free-space channel better. Consequently, OAM-based free-space optical communication has attracted considerable attention, and has become a hot topic of the current research.Free-space quantum optical communication adopts the polarization-encoding scheme, which utilizes the orthogonal photon polarization states to realize qubits. Therefore, the polarization preservation of the transmitted single photons is a critical issue during the process of propagating through free-space. OAM modes suffer the wavefront phase distortions induced by the deleterious atmospheric turbulence after they propagate through free-space, which thus gives rise to crosstalk between the OAM mode channels and the performance degradation of OAM-based free-space optical communication system. The present thesis focuses on the characters of photon polarization qubits and OAM modes propagating through free-space. The main achievements are as follows:(1). We have presented a general model of satellite-to-ground quantum optical communication. The eccentricity of satellite orbit in the general model ranges from 0 to 1. Furthermore, the geocentric-equatorial coordinate system is used as the reference frame, which can directly utilize the two-line satellite elements provided by the U.S. National Aeronautics and Space Administration. Accordingly, our general model is extremely suitable to calculate the polarization rotations of photon qubits resulting from the relative movement between an arbitrary existing satellite or launching satellite in the future and a ground receiving station. Our research could offer some certain guidance for the engineering practice in the near future.(2). We have theoretically studied the influence of atmospheric particle scattering on photon polarization qubits for the first time. We proposed a new vector Monte-Carlo method, which successfully solved the calculation of the photon free path between two successive scattering events in an inhomogeneous atmosphere medium by the use of transmittance. We have showed that the polarization qubits are well preserved in the both downlink and uplink, which agrees well with the measured results in the previous experiments. However, the number of received single photons is less than half of the total transmitted single photons for both links.(3). We have first studied the effect of the atmospheric turbulence following the most realistic spectrum model on OAM-based free-space optical communication system. Specifically, we evaluated the channel capacity of free-space optical links employing an inconsecutive OAM-modes encoding scheme under the impact of this kind of atmospheric turbulence. The results showed that the inconsecutive OAM-modes encoding can significantly reduce crosstalk between OAM mode channels so that the channel capacities converging to an ideal case.(4). We have specifically demonstrated the real-time correction process of OAM mode wavefornt phase distortions by employing a properly designed closed-loop adaptive optics system model under a strong atmospheric turbulence regime. A significant increase in the free-space optical link channel capacity has been confirmed when the adaptive optics system is deployed. Furthermore, we considered a dynamically evolving atmospheric turbulence with wind. In such a case, we evaluated the temporal properties of OAM-based free-space optical communication with adaptive optics compensation. The results showed that the real-time correction of a properly designed adaptive optics system can not only substantially enhance the Strehl ratio value and the channel capacity, but also effectively suppress fluctuation of both the Strehl ratio and the channel capacity.