  We infer the high-frequency flux noise spectrum in a superconducting flux
qubit by studying the decay of Rabi oscillations under strong driving
conditions. The large anharmonicity of the qubit and its strong inductive
coupling to a microwave line enabled high-amplitude driving without causing
significant additional decoherence. Rabi frequencies up to 1.7 GHz were
achieved, approaching the qubit's level splitting of 4.8 GHz, a regime where
the rotating-wave approximation breaks down as a model for the driven dynamics.
The spectral density of flux noise observed in the wide frequency range
decreases with increasing frequency up to 300 MHz, where the spectral density
is not very far from the extrapolation of the 1/f spectrum obtained from the
free-induction-decay measurements. We discuss a possible origin of the flux
noise due to surface electron spins.
