Cavity Qed In Atomic Entanglement And Quantum Logic Gate Design Of The Study

The cavity QED provides an effective method to deal with the interaction of light and atom, which is also one of the promising candidates for the physical realization of quantum information processing. The technique of stimulated Roman adiabatic passage (STIRAP) is a method of using partially overlapping pulses (from pump and Stokes lasers) to produce complete population transfer between two quantum states of an atom or molecule. This technique is a valid tool to manipulate atom and molecule. The paper studies on the entanglement between two atoms in a double-mode cavity and a protocol for constructing quantum gates with stimulated Roman adiabatic passage respectively. The main results are as follows:1. We investigate the Raman coupled model of two entangled∧-type three-level atoms interacting with a two-mode quantized cavity field. Using the method of negative eigenvalues of the partial transposition matrix we obtain the time evolution of entanglement between the two atoms. The results show that the entanglement of the two atoms displays periodicity; the evolution regular of entanglement is influenced by the two atom initial state and initial photon number of the cavity. For special initial state, the two-atom quantum state will stay in the maximum entangled state forever and has no relation with the initial photon number. Compared with Tavis-Cummings model, in this model the entanglement of the two atoms can live for much longer time.2. A simple protocol to realize the Toffoli gate and the Fredkin gate by adiabatic passage in an optical cavity is presented. In our protocol, the multi-qubit gates can be generated without using the composition of elementary gates from a universal set, so it involves much smaller numbers of laser pulses and shorter operating time. Moreover, under the adiabatic condition, both the atomic emission and cavity damp are avoided, so the protocol is decoherence-free in theory. It is also pointed out that this protocol can be used to realize multi-qubit gates on photonic qubits.