Evaporative cooling of polar molecules to Fermi degeneracy in 3D and 2D confinement

Production of low-entropy samples of ultracold dipolar molecules would enable observations of novel phases and dynamics predicted by spin-motion models [1, 2], such as the generalized t-J model [3]. Here, we report evaporative cooling of ultracold KRb molecules confined in either 3D or 2D trap geometries into deep Fermi degeneracy. Efficient evaporation is initiated by providing a favorable ratio of elastic to inelastic dipolar collisions, enabled by resonant electric field shielding for 3D samples [4, 5] or by purely repulsive dipolar interactions for quasi-2D samples [6] predominantly residing in a single layer. We observed that the evaporation efficiency into deep degeneracy is limited by the onset of Pauli blocking that suppresses the ratio of elastic to inelastic collisions. This work explores the influence of dimensional confinement on the interplay of the density of states of degenerate fermions and dipolar scattering, setting the stage for future studies of novel many-body dynamics with deeply degenerate polar molecules.

[1]   J. Li, et al, Nature 614, 70–74 (2023)

[2]   C. Miller, et al., Nature 633, 332–337 (2024)

[3]   A. N. Carroll, et al., Science 388, 381-386 (2025)

[4]   K. Matsuda, et al., Science 370, 1324-1327 (2020)

[5]   J. Li, et al, Nat. Phys. 17, 1144–1148 (2021)

[6]   G. Valtolina, et al., Nature 588, 239–243 (2020)