Evaporative cooling of shielded polar molecules to degeneracy in 3D and 2D confinements

Ultracold polar molecules offer an ideal platform for exploring spin-motion models [1-3], such as the generalized t-J model, with well-developed tools for controlling their rich internal structure and their strong, long-range dipole-dipole interactions. To observe novel phases and dynamics predicted by these models, a low-entropy sample of molecules is essential. Here, we report recent progress toward realizing deeply degenerate KRb molecules confined in 3D and 2D trap geometries with evaporative cooling. By applying electric field-induced shielding [4], efficient 3D evaporation is initiated, and KRb molecules are cooled to 0.3 times the Fermi temperature (T_F). Further evaporation is found to be inefficient, as Pauli blockade modifies the ratio of elastic to inelastic collisions which is confirmed by a direct measurement of the ratio. Our previous work on electric-field-assisted evaporative cooling [5] showed a higher ratio of elastic to inelastic collisions can be achieved by confining KRb molecules in 2D optical traps, configured as a stack of layers formed by a 1D optical lattice. However, the record T/T_F = 0.6 was limited by the number of molecules per layer of optical lattice. To achieve a lower-entropy sample, instead of having molecules distributed between 5 layers of optical lattices [5], we create a single-layer quasi-2D molecular sample for evaporation. A preliminary evaporative cooling experiment shows we can achieve a much larger number of molecules in a single layer of lattice with a temperature below T_F, further optimizations are under way.

This work provides a deeper understanding of evaporative cooling of Fermionic polar molecules in 3D and 2D confinements, with the interplay of the density of states and elastic to inelastic collision ratio. setting the stage for studying novel many-body dynamics with deeply degenerate polar molecules.

Reference:

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

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

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

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

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