  Under resonant irradiation, a quantum system can undergo coherent (Rabi)
oscillations in time. We report evidence for such oscillations in a
_continuously_ observed three-Josephson-junction flux qubit, coupled to a
high-quality tank circuit tuned to the Rabi frequency. In addition to
simplicity, this method of_Rabi spectroscopy_ enabled a long coherence time of
about 2.5 microseconds, corresponding to an effective qubit quality factor
\~7000.
