Study Of The Realization Of Quantum Phase Gate Via Cavity QED System

The physical realization of quantum computer is the key to quantum computationscience, So it is very important to find a physical system suitable for the realization ofquantum computer. T. Sleator et al have proved that any quantum unitary operation canbe decomposed into a serious of single qubit rotational operation and control-not(CNOT)gate operation; while a CNOT gate operation can also be decomposed into the operationof a single qubit rotations and a conditional quantum phase gate operation. As a result,the task looking for the physical system to realize a quantum computer is reduced tofinding a physical system suitable for the realization of a conditional quantum phase gate.The general idea of the conditional quantum phase gate for cavity QED is to trapatoms in a high quality cavity. Quantum information is stored on the internal states of theatoms, and the trapped atoms again provide quantum register and the cavity is utilizedboth to perform quantum phase gate and to transfer quantum information; But in thisthesis, I focus my study on the realization of quantum phase gate via cavity QED system inanother way, in which an atom is injected into the cavity, the quantum information is storedonly on the photonic qubit, or both on the photonic qubit and the atomic qubit. After mystudy, several schemes of conditional quantum phase gate have been proposed, which aresimple in operation,easy in technical realization and have high fidelity. Additionally, twoschemes of conditional quantum phase gate via trapped ion in cavity are also proposed,which are not limited by the Lamb-Dicke limit. All my studies can be refereed by theexperimentalists of quantum information processing and quantum computation.The main contents of this thesis are including:1. Brief introduction to the basic theories of quantum computer.2. Brief introduction to the basic theories of cavity QED. 3. The programme of conditional quantum phase gate for cavity QED via a A-typethree-level atom and a bimodal-cavity is proposed, where the techniques of "bimodal nearresonant" and "bimodal balanced coupling" are used. The experimental feasibility arediscussed, and the fidelity of the quantum phase gate is calculated numerically, which ishigh enough to realize experimentally.4. The scheme of quantum phase gate for cavity QED via a bimodal cavity and aladder-type three-level atom is given, three techniques, "bimodal far-off resonant","bimodal nearly balanced coupling" and "adiabatic approximation", has been utilized.The utilization of these techniques makes the scheme has higher fidelity than similarscheme.5. The realization of quantum phase gate for .dissipative cavity QED system is studied, where a single ladder type four-level atom and a single mode cavity are used. theatomic qubit and photonic qubit act as control qubit and target qubit, respectively, audthe quantum information is transferred through the coupling between the atom and theclassical light field and quantum cavity field.6. At the end, the realization of quantum phase gate for trapped ion in cavity isstudied, where the ion trap is not limited by the Lamb-Dicke limit, two kinds of CQPGprogramme are given. The main merit of the schemes is the fast gate speed and simpleoperational process.