  Ultracold polar molecules possess long-range, anisotropic, and tunable
dipolar interactions, providing the opportunities to probe quantum phenomena
inaccessible with existing cold gas platforms. However, experimental progress
has been hindered by the dominance of two-body loss over elastic interactions,
which prevents efficient evaporative cooling. Though recent work has
demonstrated controlled interactions by confining molecules to a
two-dimensional geometry, a general approach for tuning molecular interactions
in a three-dimensional (3D), stable system has been lacking. Here, we
demonstrate tunable elastic dipolar interactions in a bulk gas of ultracold
40K87Rb molecules in 3D, facilitated by an electric field-induced shielding
resonance which suppresses the reactive loss by a factor of thirty. This
improvement in the ratio of elastic to inelastic collisions enables direct
thermalization. The thermalization rate depends on the angle between the
collisional axis and the dipole orientation controlled by an external electric
field, a direct manifestation of the anisotropic dipolar interaction. We
achieve evaporative cooling mediated by the dipolar interactions in three
dimensions. This work demonstrates full control of a long-lived bulk quantum
gas system with tunable long-range interactions, paving the way for the study
of collective quantum many-body physics.
