  A two-qubit quantum gate is realized using electronic excited states in a
single ion with an energy separation on the order of a terahertz times the
Planck constant as a qubit. Two phase locked lasers are used to excite a
stimulated Raman transition between two metastable states $D_{3/2}$ and
$D_{5/2}$ separated by 1.82 THz in a single trapped $^{40}$Ca$^+$ ion to
construct a qubit, which is used as the target bit for the Cirac-Zoller
two-qubit controlled NOT gate. Quantum dynamics conditioned on a motional qubit
is clearly observed as a fringe reversal in Ramsey interferometry.
