  A quantum gas of ultracold polar molecules, with long-range and anisotropic
interactions, would not only enable explorations of a large class of many-body
physics phenomena, but could also be used for quantum information processing.
We report on the creation of an ultracold dense gas of 40K87Rb polar molecules.
Using a single step of STIRAP (STImulated Raman Adiabatic Passage) via
two-frequency laser irradiation, we coherently transfer extremely weakly bound
KRb molecules to the rovibrational ground state of either the triplet or the
singlet electronic ground molecular potential. The polar molecular gas has a
peak density of 10^12 cm^-3, and an expansion-determined translational
temperature of 350 nK. The polar molecules have a permanent electric dipole
moment, which we measure via Stark spectroscopy to be 0.052(2) Debye for the
triplet rovibrational ground state and 0.566(17) Debye for the singlet
rovibrational ground state. (1 Debye= 3.336*10^-30 C m)
