Realization Of Quantum Entanglement And Quantum Cloning In Cavity Quantum Electrodynamics System

Quantum information science, combining the superposition, non-locality, no-cloning theorem and other features of quantum mechanics, is the product of application of quantum mechanics to information science. With the development of society, quantum communication defined as transmitting quantum entangled states from one place to another place has been paid much more attention. It also includes quantum teleportation, quantum entanglement swapping, quantum key distribution, ruantum cloning and so on. So generation of quantum entanglement is a basic problem of quantum communication, and quantum cloning is also an important part of quantum communication.At present, researchers have paid much attention on cavity quantum electrodynamics, linear optics, quantum dot and other systems about the theoretical and experimental study. In cavity quantum electrodynamics system, qubits trapped in the cavity are considered as storing quantum information, and photons in optics system are used as flying qubits to carry quantum information. As their respective advantages existed, it has been attracted by many researches doing deeply research. In this paper, we pay much attention on quantum entanglement and quantum cloning by employing cavity quantum electrodynamics and linear optics. The results we obtained as follows:1. Generation of quantum entanglement in cavity QED system(1) We propose a scheme for generating maximally GHZ state for four atoms trapped in a two-mode optical cavity via combination of cavity QED and linear optics system. The GHZ state can be not only generated deterministically with a single resonant interaction in cavity QED, but also can be prepared probabilistically based on cavity QED and linear optics elements. The fidelity of the entangled states is not affected by the atomic spontaneous, cavity decay, and imperfection of the photon-detectors. Finally, we briefly analyze and discuss the experimental feasibility of the proposed scheme.(2) We propose a scheme for generating maximally entangled states for three atoms trapped in three distant cavities connected by two identical single-mode fibers. During the operation, neither the atomic system nor the fibers are excited, which is important in view of decoherence. Under certain conditions, the probability of the excited cavities can be negligible. Taking advantage of adiabatic passage, the GHZ state can be generated deterministically, and the fidelity of entanglement is insensitive to fluctuation of experimental parameters. Compared to the previous schemes, the significant advantage of the proposed scheme is that each cavity can interact with the other two directly, which can avoid the effect of indirect interaction brought about using only local quantum operator and nonlocal resources.(3) Based on the interference effect of indistinguishable polarized photons leaking out of separated cavities with each atom trapped in separate cavity, using quantum nondemolition detection, we propose the robust schemes for the generation of N-atom GHZ state, three-atom W state and four-atom Cluster state with a certain success probability. In Lamb-Dicke limit, the schemes do not require the simultaneous click of the photon-detectors. These made the schemes more realizable in experiments. Meanwhile, the advantage of the scheme is that the fidelity of the entangled states is not affected by the atomic spontaneous, cavity decay, and imperfection of the photon detectors. The schemes would be useful steps towards long-distance quantum communication.2. Realization of quantum cloning in cavity QED system(1) We propose a scheme to realize a special quantum cloning machine in cavity QED system. The quantum cloning machine can copy the information from one atom to another two distant atoms trapped in cavities with the help of a single-photon pulse. Choosing the different parameters, we can perform optimal symmetry1→2real state quantum cloning machine, and the optimal symmetry1→3economical real state quantum cloning machine.(2) We propose a scheme to realize a tunable optimal quantum cloning machines with trapped atoms. Through selecting pairing of Raman transitions and choosing suitable parameters of the external fields, we can not only perform optimal symmetric (asymmetric) universal quantum cloning machine, phase-covariant cloning machine, but also realize optimal symmetric economical phase-covariant cloning machine. Additionally, the atomic excited states and photonic states can be adiabatically eliminated. In our scheme, the quantum cloning machine can copy the information from one trapped atom to arbitrary two distant trapped atoms, which is significant to quantum communication and quantum computation.