  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.
