Efficient state transfer in an ultracold dense gas of heteronuclear molecules

Polar molecules have bright prospects for novel quantum gases with long-range and anisotropic interactions1, and could find uses in quantum information science2 and in precision measurements3, 4, 5. However, high-density clouds of ultracold polar molecules have so far not been produced. Here, we report a key step towards this goal. We start from an ultracold dense gas of loosely bound 40K87Rb Feshbach molecules6, 7 with typical binding energies of a few hundred kilohertz, and coherently transfer these molecules in a single transfer step into a vibrational level of the ground-state molecular potential bound by more than 10 GHz. Starting with a single initial state prepared with Feshbach association8, we achieve a transfer efficiency of 84%. Given favourable Franck–Condon factors9, 10, the presented technique can be extended to access much more deeply bound vibrational levels and those exhibiting a significant dipole moment.