Quantum Computation And Quantum Communication Based On Cavity QED

A combination of quantum mechanics and information science yields a new subject, that is. quantum information. In recent year, much progress with quantum information in both theory and experiment has been made, which is attracting much attention of the government, the scientist, and the information industry. Quantum information includes quantum computation, quantum communication, quantum cryptography, and so on. Quantum state is the carrier of quantum information. The preparation, manipulation, transmission, and storage for quantum state are called quantum information process. Up to now, different physical systems, including cavity quantum electrodynamics (QED), linear optics, trapped ions, nuclear magnetic resonance, quantum dot, and so on, are explored for quantum information process. Among these systems, cavity QED is one of the promising candidates for acting as the hardware of quantum information process. The main idea of cavity QED is that the captured atoms are hold in a high quality cavity. The atomic states with long lifetime are used as storage qubits while the photons with good transmission capacity are used as flying qubits. Therefore, the realization of quantum computation and quantum communication based on cavity QED has been widely studied. However, it is still a challenging pursuit in the experiment for quantum computation and quantum communication. Because of these reasons, I devoted myself into this field during my graduate study. The main work is listed below:1. Generation of the singlet state for three atoms. In this scheme, three three-level atoms simultaneously pass through two cavity. The present scheme does not involve cavity-photon population during the process of generating entanglement, and thus is insensitive to the cavity decay. Compared to prevous proposal, our method doesn't need the cavity field act as memories, which store the information of atomic systems and transfer them back to the atomic systems. In addition, our scheme does not need to detect the states of cavity field in order to make the state of three atoms collapse into the corresponding entangled state.2. Preparation of the entangled states and realization of quantum communication based on the input-output process of the cavity. First, we propose a scheme for generating multi-atom and multi-photon cluster state, respectively. The results of the numerical simulation show that the entangled states have high fidelity under the influence of some practical noises. Second, we propose a method for the analysis of Bell states and Greenberger-Horne-Zeilinger (GHZ) states for the photons. In the scheme, all of Bell states and Greenberger-Horne-Zeilinger (GHZ) states can be completely indentified without being destroyed.3. The implementation of tunable quantum phase gate and effective preparation of gragh-state entanglement. First, an alternative scheme for tunable quantum phase gate based on the cavity QED or ion trap system is proposed. In the scheme, two logical states of a qubit are encoded on two stable low-energy states, the conditional phase shifts are obtained without any real transitions of atomic internal states. In addition, the accumulated conditional phase shiftφcan vary between 0 and 2πby controlling the total effective interaction time. Secondly, we show the method for effectively prepare graph states with the multiple-qubit entangling gates.4. Frequency-up conversion and quantum swap gate in an optical cavity with atomic ensemble. First, a method is presented for realizing a nonlinear interaction of the cavity field—frequency-up conversion. In the scheme, a V-type atomic ensemble prepared in their ground states collectively mediates the interaction between the two cavity modes. Under certain conditions, the state of atoms is always in their ground states, the cavity-field degree of freedom is decoupled from the atomic degrees of freedom, and the effective coupling strength between the two cavity modes scales up with N^1/2 (N is the number of atoms). Secondly, we further propose a scheme for a two-qubit quantum swap gate for intracavity fields. The numerical simulation shows that the quantum swap gate has still a high fidelity under the influence of the atomic spontaneous emission and the decay of the cavity.