  While the advanced coherent control of qubits is now routinely carried out in
low frequency (GHz) systems like single spins, it is far more challenging to
achieve for two-level systems in the optical domain. This is because the latter
evolve typically in the THz range, calling for tools of ultrafast, coherent,
nonlinear optics. Using four-wave mixing micro-spectroscopy, we here measure
the optically driven dynamics of a single exciton quantum state confined in a
semiconductor quantum dot. In a combined experimental and theoretical approach,
we reveal the intrinsic Rabi oscillation dynamics by monitoring both central
exciton quantities, i.e., its occupation and the microscopic coherence, as
resolved by the four-wave mixing technique. In the frequency domain this
oscillation generates the Autler-Townes splitting of the light-exciton dressed
states, directly seen in the four-wave mixing spectra. We further demonstrate
that the coupling to acoustic phonons strongly influences the FWM dynamics on
the picosecond timescale, because it leads to transitions between the dressed
states.
