  The dynamical properties of quantum entanglement in two three-level trapped
ions interacting with a laser field are studied in terms of the negative
eigenvalues of the partial transposition of the density operator. In contrast
to the usual belief that destroying the entanglement can be observed due to the
environment, it is found that the Stark shift can also produce sudden death of
entanglement and long-lived entanglement between the qubits that are prepared
initially in separable states or mixed states.
