  A procedure is discussed for creating coherent superpositions of motional
states of ion strings. The motional states are across the structural transition
linear-zigzag, and their coherent superposition is achieved by means of
spin-dependent forces, such that a coherent superposition of the electronic
states of one ion evolves into an entangled state between the chain's internal
and external degrees of freedom. It is shown that the creation of such an
entangled state can be revealed by performing Ramsey interferometry with one
ion of the chain.
