Atom-molecule dark states in a Bose-Einstein condensate.

We have created a dark quantum superposition state of a Rb Bose-Einstein condensate and a degenerate gas of Rb2 ground-state molecules in a specific rovibrational state using two-color photo-association. As a signature for the decoupling of this coherent atom-molecule gas from the light field, we observe a striking suppression of photo-association loss. In our experiment the maximal molecule population in the dark state is limited to about 100 Rb2 molecules due to laser induced decay. The experimental findings can be well described by a simple three mode model.