Evaporation of microwave-shielded polar molecules to quantum degeneracy

Ultracold polar molecules offer strong electric dipole moments and rich internal structure, making them ideal platforms to explore exotic quantum matter, implement quantum information schemes and test fundamental symmetries. Realizing their full potential requires cooling interacting molecular gases deeply into the quantum-degenerate regime, but intrinsically unstable short-range collisions have so far prevented direct cooling via elastic collisions to quantum degeneracy in three dimensions. Here we demonstrate evaporative cooling of a three-dimensional gas of fermionic sodium–potassium molecules to well below the Fermi temperature using microwave shielding. A blue-detuned circularly polarized microwave couples rotational states and creates a repulsive barrier that protects molecules from reaching short range, while inducing strong, tunable dipolar interactions that yield high elastic collision rates exceeding inelastic ones by at least a factor of 460. This enables cooling to 21 nanokelvin, corresponding to 0.36 times the Fermi temperature. Such cold and dense samples of polar molecules open a path to exploring many-body phenomena with strong dipolar interactions.