Generation of a genuine four-particle entangled state of trap ions

We present a scheme for the generation of a genuine four-qubit entangled state in an ion trap. This state has many interesting entanglement properties and possible applications in quantum information processing and fundamental tests of quantum physics. In our scheme, the ion is driven by a standing-wave field, whose frequency is resonant with the ion carrier transition. By adjusting the phase of the field, both the vibration mode population and the ionic carrier excitation can be avoided. So our scheme is insensitive to the vibration states, which is important in view of decoherence.