Quantum Information Transmission Via Noisy Channel

Quantum information theory extends information processing into quantum field, em-ploying a quantum state in micro systems to represent information directly, and makes the manipulation and transmission of information enter into a faster, securer and more efficient stage. However, the interaction between quantum systems and environment brings great diffi-culties to the transmission of quantum information, therefore quantum information transmis-sion via noisy channels becomes an important direction of the current international research. Our research mainly studies noises and losses of quantum channels in quantum communi-cation, and discusses several quantum information processes in the major noise models of open systems, such as quantum state transmission, entanglement distribution and quantum key distribution. The dissertation is divided into six chapters, while our works are mainly included in Chapter3to Chapter6.In Chapter1, we introduce the research background, significance and development of quantum communication via noisy channels in detail.In Chapter2, the fundamental concept and theories of quantum information are in-terpreted with emphasis, including qubit, quantum state transformation, pure state, mixed state, density operator, reduced density operator, coherent state, entangled coherent state, and quantum measurement.In Chapter3, considering quantum bit-flip noise, we propose two error-tolerance proto-cols of single-photon state transmission. Compared with the previous error-rejection protocols of quantum state transmission via the quantum bit-flip noise channel, the error rate of the present protocol can be decreased to zero, because two cases of two-photon bit-flip errors can be distinguished and no error occurred on two transmitted photons.In Chapter4, an entanglement distribution protocol of a χ-type entangled state under a collective noise channel is presented. By applying the polarization beam splitters and half-wave plates, the χ-type entangled state is changed between the polarization and spatial modes. The χ-type polarization entangled state is transformed to the χ-type spatial entangled state in the sending process. After photons pass through the noisy channel, the polarization of the entangled state is affected by the channel noise, but it is not so for the spatial mode. So the polarization mode is changed and the spatial mode is unchanged. In the receiving process, nondemolition measurements based on cross-Kerr nonlinearity are performed to measure the polarization mode of the entangled states, thereby eliminating the change of the polarization mode of the entangled state. Exploiting the combination of polarization beam splitters and half-wave plates again, the χ-type spatial entangled state is transformed back to the χ-tyPe polarization entangled state. As its generalization, an entanglement distribution protocol of an arbitrary multi-photon entangled state is presented.In Chapter5, taking into account two main collective noises, collective-dephasing noise and collective-rotation noise, we propose two quantum key distribution protocols and two controlled quantum key distribution protocols respectively, which can realize encoding one-bit secret key per three photons. Communication security is analysed by considering different eavesdropping attacks for these quantum key distribution protocols, and the corresponding countermeasures are proposed.In Chapter6, based on entanglement swapping of entangled coherent states, a quantum key distribution protocol with continuous variable is proposed. It can be concluded that, in an ideal channel, the greater the intensity of coherent state, the greater the success probability of quantum key distribution. But in lossy channels, the decoherence will cause errors of secret keys. The error rate is related to the decoherence coefficient that the decoherence coefficient is smaller, the error rate is also smaller. In addition, the error rate is also pertinent to the intensity of coherent states, that is, the error rate increases rapidly with the increase of the intensity of coherent states. So the intensity of coherent states should be chosen properly for realizing the quantum key distribution protocol with entangled coherent states in the lossy channel.Finally, the summary and prospect are presented.