  We have observed Rabi-like oscillations in a current-biased dc SQUID
presenting enhanced coherence times compared to our previous realization
\cite{Claudon_PRL04}. This Josephson device behaves as an anharmonic oscillator
which can be driven into a coherent superposition of quantum states by resonant
microwave flux pulses. Increasing the microwave amplitude, we study the
evolution of the Rabi frequency from the 2-level regime to the regime of
multilevel dynamics. When up to 3 levels are involved, the Rabi frequency is a
clear signature of quantum behavior. At higher excitation amplitude, classical
and quantum predictions for the Rabi frequency converge. This result is
discussed in the light of a calculation of the Wigner function. In particular,
our analysis shows that pronounced quantum interferences always appear in the
course of the Rabi-like oscillations.
