Schemes For Generation Of Entangled States And Implementation Of Remote Quantum Logical Gates Based On Atom-Cavity-Fibre Systems

As for the cavity quantum electrodynamics, with the increase in the number of the atomic qubits, the manipulations not only on qubit-cavity interaction but also on qubits themselves become difficult. Because some unexpected crossing interactions from qubits which are temporarily out of consideration, might arise and thus influence or spoil the useful interactions. Therefore, there are physical limitations on the number of atomic qubits in a quantum cavity. However, a practical quantum computer involves a large number of qubits, inevitably. And the power of the quantum computer increases as the mumber of qubits increases. Therefore, distributed quantum computing is introduced. It is thought of as a network of spatially separated local processors that contain only a few qubits and are connected via quantum transmission lines. Two of the key problems in the realization of distributed quantum computing are to implement the remote quantum logical gates and prepare the entanglement among the distant nodes. On the other hand, the control of single atom trapped in optical cavities and the perfect fiber-cavity coupling via microfabrication have been realized in experiment. The system, consisting of atoms separately trapped in distant cavities coupled by optical fibers, is very promising for the generation of entanglement shared by distant subsystems and implementing the remote quantum logical gates. And many schemes based on this system for the implementation of quantum information process have been proposed. In this paper, we consider this system and propose some schemes for the generation of entanglement and implementation of the quantum logical gates. Significant new results are shown as following.1. A scheme, based on the system composed of three atoms separately trapped in three cavities coupled by optical fibres, for entangling two distant atoms via adiabatic passage is proposed. It is found that the multi-particle W entangled state can also be generated. Moreover, the quantum information sharing can be implemented using this system. These results may be helpful for the implementation of quantum network and useful in quantum cryptography. And this scheme is convenient to operate since only the laser fields applied to the atoms need to be adjusted to accomplish the processes.2. A system composed of a single-atom-trapped cavity and a remote two-atom-trapped cavity connected by the optical fibre is considered. It is shown that a shared Greenberger-Horne-Zeilinger (GHZ) state of the three atoms can be deterministically generated by controlling the time of interaction or via the adiabatic passage based on this system. The influence of various decoherence processes such as spontaneous emission and photon loss on the fidelity is also investigated. It is found that these schemes can be realized with high fidelity even when these decoherence processes are considered.3. A scheme, based on the two two-level atoms resonantly driven by the classical field separately trapped in two cavities coupled by an optical fibre, for the implementation of remote two-qubit gates is investigated. It is found that the quantum controlled-phase and swap gates can be achieved with the assistance of classical field when there are detunings of the coupling quantum fields. Moreover, the influence of the dissipation of the cavities and the optical fibre is analyzed while the spontaneous emission of the atoms can be effectively suppressed by introducing the∧-type atoms.4. A scheme for implementing the three-qubit controlled-Z gate with three atoms separately trapped in three distant cavities coupled by the optical fibres is proposed. The influence of various decoherence processes, such as spontaneous emission of the atoms and photon leakage of the cavities and the optical fibres, on the fidelity is also investigated. It is found that the gate can be implemented with high fidelity even when these decoherence processes are considered. This scheme can be extended to the implementation of remote N-qubit controlled-Z gate with N atoms separately trapped in N distant cavities coupled by the optical fibres.