  We observe dressed states and quantum interference effects in a strongly
driven three-level quantum dot ladder system. The effect of a strong coupling
field on one dipole transition is measured by a weak probe field on the second
dipole transition using differential reflection. When the coupling energy is
much larger than both the homoge-neous and inhomogeneous linewidths an
Autler-Townes splitting is observed. Striking differences are observed when the
transitions resonant with the strong and weak fields are swapped, particularly
when the coupling energy is nearly equal to the measured linewidth. This result
is attributed to quantum interference: a modest destructive or constructive
interference is observed depending on the pump / probe geometry. The data
demonstrate that coher-ence of both the bi-exciton and the exciton is
maintained in this solid-state system, even under intense illumina-tion, which
is crucial for prospects in quantum information processing and non-linear
optical devices.
