  We demonstrate fast control of the interatomic interactions in a
Bose-Einstein condensate by coherently coupling two atomic states with intra-
and inter-state scattering lengths of opposite signs. We measure the elastic
and inelastic scattering properties of the system and find good agreement with
a theoretical model describing the interactions between dressed states. In the
attractive regime, we observe the formation of bright solitons formed by
dressed-state atoms. Finally, we study the response of the system to an
interaction quench from repulsive to attractive values, and observe how the
resulting modulational instability develops into a bright soliton train.
