Preparation Of Mulitqubit Controlled Phase Flip Gate And Concatenated GHZ State Based On Cavity Input-Output Process

Quantum information science is an emerging discipline which has been attract-ed lots of attention of scientists in recent years, and great efforts has been made in theoretically and experimentally so far. In order to perform quantum information processing (QIP) tasks, it is necessary to carry out corresponding unitary operation after encoding the quantum state, which means it require large quantum logic gates, The quantum entanglement plays an important role in quantum information science, which is the basic resources to realize quantum information processing such as quan-tum teleportation, quantum desen coding, quantum key distribution and so on. The GHZ state is a class of common multiparticle entangled state. The decoherence will be induced by noise in the process of preparation of the entangled state, thus will lead to the decrease of the entanglement of the GHZ state. The concatenated GHZ (C-GHZ) state as a new type of entangled state has been proposed to protect com-mon GHZ-type entanglement. Presently, there are many systems can be used to implement quantum entanglement and quantum logic gates such as cavity quantum electrodynamics (QED), linear optical system, nitrogen-vacancy (NV) centers and so on.In this dissertation, we have realized the multi-target-qubit controlled phase flip gate and the C-GHZ state based on the input-output process of the cavity. Firstly, a scheme for multi-target-qubit controlled phase flip gate is proposed with the help of the entangled gates. The entangled gates are performed using the selective rules of two identical atoms encoded in decohence-free subspace (DFS) and single photon. The scheme is immune to collective dephasing caused by the environmental noise because all operations are performed in DFS, thus greatly reduces the complexity of the experiment. Secondly, the Bell state and the GHZ state are realized based on the controlled phase gate between single photon pulse and an atom trapped in the cavity. On this basis, the C-GHZ state is performed using the Bell state, the GHZ state and the multi-controlled-NOT (CNOT) gate of two qubits, which not only retains the advantages of conventional GHZ states but also is more robust in noisy environment. The influence of cavity decay on the fidelity of the C-GHZ state is analyzed.