  We report results of spectroscopic measurements and time-domain measurements
of a superconducting flux qubit. The dc superconducting quantum interference
device (SQUID), used for readout of the qubit, and a shunt capacitor formed an
LC resonator generating a SQUID plasma mode. Higher-order red and blue
sidebands were observed in a simple measurement scheme because the resonant
energy of the resonator, 600 MHz, was comparable to the thermal energy. We also
observed Rabi oscillations on the carrier transition and the first-order
sideband transitions. Because the qubit was coupled to a single arm of the dc
SQUID, the qubit-SQUID coupling was significant at zero bias current, where
these phenomena were observed. The ratios between the Rabi periods for the
carrier transition and the sideband transitions are compared with those
estimated from the coupling constant, which was separately determined. The
result may be explained by assuming initial excitation of the resonator.
