  We produce ultracold dense trapped samples of 87Rb133Cs molecules in their
rovibrational ground state, with full nuclear hyperfine state control, by
stimulated Raman adiabatic passage (STIRAP) with efficiencies of 90%. We
observe the onset of hyperfine-changing collisions when the magnetic field is
ramped so that the molecules are no longer in the hyperfine ground state. A
strong quadratic shift of the transition frequencies as a function of applied
electric field shows the strongly dipolar character of the RbCs ground-state
molecule. Our results open up the prospect of realizing stable bosonic dipolar
quantum gases with ultracold molecules.
