  Entangled states of two ions are realized by using an adiabatic process.
Based on the proposal by Linington and Vitanov, we have generated Dicke states
in optical qubits of two $^{40}$Ca$^+$ ions by applying frequency-chirped
optical pulses with time-dependent envelopes to perform rapid adiabatic passage
on sideband transitions. One of the biggest advantages of adiabatic approaches
is their robustness against variations in experimental parameters, which is
verified by performing experiments for different pulse widths or peak Rabi
frequencies. Fidelities exceeding 0.5, which is the threshold for inseparable
states, are obtained over wide ranges of parameter values.
