Motional entanglement for two trapped ions in cascaded optical cavities

The time-dependent characteristic function for the vibrational quanta of two ions separately trapped in two cascaded optical cavities is obtained analytically beyond the bad cavity limit, in which the motional mode of the ion in the first cavity is coupled to the cavity field via an effective parametric interaction and that of the ion in the second cavity via a linear-mixing interaction as proposed by Peng and Parkins [Phys. Rev. A 65, 062323 (2002)]. It is found that the two ions develop in a two-mode mixed Gaussian entangled state if there exists a unidirectional coupling from the first cavity to the second one.