  Ultracold polar molecules possess long-range, anisotropic, and tunable
dipolar interactions, providing unique opportunities to probe novel quantum
phenomena. However, experimental progress has been hindered by excessive
two-body loss, which also limits further cooling via evaporation. Recent work
shows the loss can be mitigated by confining molecules in a two-dimensional
geometry. However, a general approach for tuning molecular interactions in a
full three-dimensional (3D) stable system has been lacking. Here, we
demonstrate the use of an electric field-induced shielding resonance to
suppress the reactive loss by a factor of 30 while preserving elastic,
long-range dipolar interactions in a bulk gas of ultracold 40K87Rb molecules in
3D. The favorable ratio of elastic to inelastic collisions enables direct
thermalization, the rate of which depends on the angle between the collisional
axis and the dipole orientation controlled by an external electric field. This
is a direct manifestation of the anisotropic dipolar interaction. We further
achieve dipolar-interaction-mediated evaporative cooling in 3D. This work
demonstrates 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.
