  We present an experimental investigation of ultrafast-laser driven Rabi
oscillations of atomic rubidium. Since the broadband spectrum of an ultrafast
laser pulse simultaneously couples all the electronic hyperfine transitions
between the excited and ground states, the complex excitation linkages involved
with the D1 or D2 transition are energy degenerate. Here, by applying the
Morris-Shore transformation, it is shown that this multi-state system is
reduced to a set of independent two-state systems and dark states. In
experiments performed by ultrafast laser interactions of atomic rubidium in the
strong interaction regime, we demonstrate that the ultrafast dynamics of the
considered multi-state system is governed by a sum of at most two decoupled
Rabi oscillations when this system interacts with ultrafast laser pulses of any
polarization state. We further show the implication of this result to possible
controls of photo-electron polarizations.
