Research On Some Issues Of Quantum Communication In Free Space

Quantum communication is a new and developing subject of study, which is born in the fusion of communication technology and quantum physics. It is a new field of science in the near future. Quantum communication can break through the physical limit of classical communication in the aspects of ensuring the security of information, enlarging the capacity of information and improving the precision of detection, therefore, it can accomplish many unimaginable tasks for classical communication.The main forms of quantum communication are quantum cryptography, quantum dense coding and quantum teleportation. Quantum secret code communication relies on the laws of quantum physics. It provides perfectly secure key distribution. It relies on the Heisenberg uncertainty principle and a single quantum cannot be cloned of quantum mechanics. Quantum secret code communication will provide with new principles and methods for the development of information theory in the future.Because Quantum secret code communication can realize the unconditional security of information-interchange, it has great strategic significance and tremendous potential applications. Quantum secret code communication will play an important role in information security.This dissertation is concerned with quantum communication technology in free space centered on the research focus of quantum secret code communication. The dissertation mainly involves four areas of quantum communication in free space, i.e., the experimental system of quantum communication in free space, the propagation characteristics of quantum channel of free space, the quantum repeating technology and the quantum swapping technology used for free space. The main research works and results are listed as follows:1. Analysis to the basic principle and structure of quantum communication system based on entanglement, we presented a scheme of experimental system of quantum communication that operated for free space. This system is composed of two subsystems, which use attenuated laser or entangled photon pairs as photon source, respectively. It can be used for experimental investigation in atmosphere propagation characteristics of system using a single photon source or entangled photon pairs, respectively, going along with a comparison of the propagation characteristics in free space between the two systems. This method studies the characteristics of quantum communication system based on entanglement, by comparing of a single photon source quantum communication system which is more maturely than former. Its merit is that the principle and characteristics of the system can be predominated more quickly by using comparison study. The data of the experiment correlatively can be got effectively hold of as soon as scheme of the system is corrected. It have simulated the distribution of quantum key and entangled photon pairs in free space. A new scheme of quantum communication in earth-space has been presented. It can improve the efficiency of the quantum key distribution in free space with the method of multicast of entangled photon pairs in satellite-earth link by use of stratospheric platform or synchronous satellites.2. Analysis has been made for the propagation characteristics of quantum channel in free space. It has been researched that the influences of atmosphere on propagation of photons in different areas (city, desert, and sea). Take into account the attenuation and depolarization effects on Earth-space paths, a model for propagation of polarized photon is obtained theoretically. A method of calculating the quantum bit transmission rate and quantum bit error rate in free space has been presented. Based on the reciprocity between the polarized photon and the free space channel, the influence of the channel characteristics on propagation of polarized photons has been studied. The effect of atmospheric environment on quantum communication has been analyzing numerically on Earth-space paths. It is shown that under clear sky atmospheric,attenuation are obvious on quantum bit error rate, especially, under rainfall, depolarization effects on quantum bit error rate must also be considered.3. We have studied the quantum repeating technology used for free space. A scheme of quantum repeating communication system used for free space was proposed, which utilize entangled photon pairs as its optical resource. The method of quantum repeating was based on entanglement preparation, entanglement distribution, entanglement purification and entanglement swapping. Based on the entanglement characteristics of the quantum states and the control relationship of the entanglement quantum communication system, the scheme extends the distance of communication by reiteration of the entanglement swapping and entanglement purification. The throughput of the quantum repeater with the probabilities of quantum teleportation and entanglement of the quantum channel has been calculated numerically. A new method of quantum repeating communication is presented. The quantum repeaters were made with ground station, stratospheric platform and synchronous satellites, therefore the transmission efficiency of the quantum repeater can be increased effectively.4. We have studied the quantum swapping technology used in quantum communication networks for free space. A scheme of quantum switch based on entangled photon pairs is presented, which utilize entanglement as its essential resource. The quantum switch is composed of swapping unit, controlling unit and in-out unit. The entanglement of photons between sender and receiver is built by entanglement distribution, and it is the quantum channel of them. The quantum signal is transferred by the principle of quantum teleportation. The quantum switch can finish the quantum swapping by using entanglement purification and entanglement swapping, expeditiously. The function of its switching is quicker than the quantum switch based on photon switch. A scheme of quantum communication network is suggested for earth-space. The nodes of the quantum switch are made with ground station, stratospheric platform and synchronous satellites, and its quantum channel in earth-space link is made by use of entangled photon pairs. It has good ability of quantum communication in earth space link.