  The coherent dynamics of relaxing spin qubits driven by a classical
bichromatic field comprising a strong resonant component and a weaker component
with a frequency close to the strong-field Rabi frequency is studied. The
double dressing by the bichromatic field modifies dephasing and dissipation
processes. We demonstrate that detuning of the weaker-field frequency from the
strong-field Rabi frequency prolongs the decay of Rabi oscillations between
some doubly dressed states. The sensitivity of Rabi oscillations to the
modified detuning-dependent relaxation is illustrated for nitrogen-vacancy
qubits in diamond. We discuss a steady-state population inversion of the doubly
dressed-state levels.
