  We report the observation of dressed states of a quantum dot. The optically
excited exciton and biexciton states of the quantum dot are coupled by a strong
laser field and the resulting spectral signatures are measured using
differential transmission of a probe field. We demonstrate that the anisotropic
electron-hole exchange interaction induced splitting between the x- and
y-polarized excitonic states can be completely erased by using the AC-Stark
effect induced by the coupling field, without causing any appreciable
broadening of the spectral lines. We also show that by varying the polarization
and strength of a resonant coupling field, we can effectively change the
polarization-axis of the quantum dot.
