Entanglement dynamics and decoherence of an atom coupled to a dissipative cavity field

In this paper, we investigate the entanglement dynamics and decoherence in the interacting system of a strongly driven two-level atom and a single mode vacuum field in the presence of dissipation for the cavity field. Starting with an initial product state with the atom in a general pure state and the field in a vacuum state, we show that the final density matrix is supported on ${\mathbb C}^2\otimes{\mathbb C}^2$ space, and therefore, the concurrence can be used as a measure of entanglement between the atom and the field. The influences of the cavity decay on the quantum entanglement of the system are also discussed. We also examine the Bell-CHSH violation between the atom and the field and show that there are entangled states for which the Bell-BCSH inequality is not violated. Using the above system as a quantum channel, we also investigate the quantum teleportation of a generic qubit state and also a two-qubit entangled state, and show that in both cases the atom-field entangled state can be useful to teleport an unknown state with fidelity better than any classical channel.Comment: 17 pages, 6 figure

On the concurrence of superpositions of many states

In this paper we use the \textit{concurrence vector}, as a measure of entanglement, and investigate lower and upper bounds on the concurrence of a superposition of bipartite states as a function of the concurrence of the superposed states. We show that the amount of entanglement quantified by the concurrence vector is exactly the same as that quantified by \textit{I-concurrence}, so that our results can be compared to those given in [Phys. Rev. A {\bf 76}, 042328 (2007)]. We obtain a tighter lower bound in the case that two superposed states are orthogonal. We also show that when the two superposed states are not necessarily orthogonal, both lower and bounds are, in general, tighter than the bounds given in terms of the I-concurrence. An extension of the results to the case with more than two states in the superpositions is also given.Comment: 1 figure, 8 page