Quantum communication and security

This dissertation is composed of the results of our investigation in quantum cryptography - a new topic in computer security. The goal of this study is to explore useful schemes for quantum cryptographic communications. The focus of our study is an investigation into quantum cryptographic protocols based on quantum-mechanical uncertainty principle and Bell's inequality. First, we use the theory of quadrature phase amplitudes and quantum homodyne detection to establish a new quantum key distribution protocol which relies on an optical coupler. In this work we propose, for the first time, the use of this theory in quantum cryptography. Second, we investigate Ekert's protocol using Bell's theorem. The contents in this investigation is not new, but we use our calculation to prove that Ekert's protocol is true for both photon-based and spin- |-particle-bcised systems. Third, we study a quantum coherence key distribution constructed using Bennett and Brassard's protocol. The information theory of the system is used to study the scenario where eavesdropping occurs. Finally, we explore the theory of quantum nondemolition detection (QND) to investigate the results when eavesdropping occurs. A calculation for the QND, using parametric frequency conversion, is given for two different quantum cryptographic systems. Our study is undertaken with the aid of some knowledge of physics, therefore, before our main results are presented, we introduce some useful physics which may help readers understand the contents of our work.