  We show that resonance fluorescence, i.e. the resonant emission of a
coherently driven two-level system, can be realized with a semiconductor
quantum dot. The dot is embedded in a planar optical micro-cavity and excited
in a wave-guide mode so as to discriminate its emission from residual laser
scattering. The transition from the weak to the strong excitation regime is
characterized by the emergence of oscillations in the first-order correlation
function of the fluorescence, g(t), as measured by interferometry. The
measurements correspond to a Mollow triplet with a Rabi splitting of up to 13.3
micro eV. Second-order-correlation measurements further confirm non-classical
light emission.
