Quantum Gas of Deeply Bound Ground State Molecules

We create an ultracold dense quantum gas of ground state molecules bound by more than 1000 wavenumbers by stimulated two-photon transfer of molecules associated on a Feshbach resonance from a Bose-Einstein condensate of cesium atoms. The transfer efficiency exceeds 80%. In the process, the initial loose, long-range electrostatic bond of the Feshbach molecule is coherently transformed into a tight chemical bond. We demonstrate coherence of the transfer in a Ramsey-type experiment and show that the molecular sample is not heated during the transfer. Our results show that the preparation of a quantum gas of molecules in arbitrary rovibrational states is possible and that the creation of a Bose-Einstein condensate of molecules in their rovibronic ground state is within reach.