  We analyze the optical selection rules of the microwave-assisted transitions
in a flux qubit superconducting quantum circuit (SQC). We show that the
parities of the states relevant to the superconducting phase in the SQC are
well-defined when the external magnetic flux $\Phi_{e}=\Phi_{0}/2$, then the
selection rules are same as the ones for the electric-dipole transitions in
usual atoms. When $\Phi_{e}\neq \Phi_{0}/2$, the symmetry of the potential of
the artificial "atom'' is broken, a so-called $\Delta$-type "cyclic"
three-level atom is formed, where one- and two-photon processes can coexist. We
study how the population of these three states can be selectively transferred
by adiabatically controlling the electromagnetic field pulses. Different from
$\Lambda$-type atoms, the adiabatic population transfer in our three-level
$\Delta$-atom can be controlled not only by the amplitudes but also by the
phases of the pulses.
