Ultracold ground state molecules in an optical lattice

We produce ultracold and dense samples of rovibrational ground state (RGS) molecules near quantum degeneracy. We first associate dimer molecules out of a lattice-based Mott-insulator state loaded from an atomic BEC and then coherently transfer the molecules to the RGS by a multiphoton STIRAP process. With an overall efficiency of 50\%, we prepare a molecular quantum gas state in which every second site of an optical lattice is occupied with a RGS molecule.\footnote{An ultracold, high-density sample of rovibronic ground-state molecules in an optical lattice, J.G. Danzl et al., arXiv:0909.4700 (2009), to appear in Nature Physics.} We expect that, with further optimization of the transfer procedure, a BEC of RGS molecules is possible.