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 Rb-2 ground-state molecules in a specific rovibrational state using two-color photoassociation. As a signature for the decoupling of this coherent atom-molecule gas from the light field, we observe a striking suppression of photoassociation loss. In our experiment the maximal molecule population in the dark state is limited to about 100 Rb-2 molecules due to laser induced decay. The experimental findings can be well described by a simple three mode model.