Disentanglement-free state of two pairs of two-level atoms

In this paper, a system composed of two pairs of two-level atoms and a multimode-vacuum field is considered. In our model, either pair contains two atoms that are assumed to be set close. We propose a disentanglement-free state of the two pairs of two-level atoms. It is remarkable that when the two pairs, initially in such a state, fly apart an arbitrary distance, the state will undergo no decoherence. By means of such a state, we can prepare the perfect Einstein-Podolsky-Rosen pair shared by two ‘‘distant’’ parties and realize long-distance entanglement swapping. Finally, a method for preparing such a disentanglement-free state is presented. @S1050-2947~99!00806-9#