  We report the first experimental demonstration of interference-induced
spectral line elimination predicted by Zhu and Scully [Phys. Rev. Lett. 76, 388
(1996)] and Ficek and Rudolph [Phys. Rev. A 60, 4245 (1999)]. We drive an
exciton transition of a self-assembled quantum dot in order to realize a
two-level system exposed to bichromatic laser field and observe nearly complete
elimination of the resonance fluorescence spectral line at the driving laser
frequency. This is caused by quantum interference between coupled transitions
among the doubly dressed excitonic states, without population trapping. We also
demonstrate multiphoton ac Stark effect with shifted subharmonic resonances and
dynamical modifications of resonance fluorescence spectra by using double
dressing.
