Evaporation of microwave-shielded polar molecules to quantum degeneracy

Abstract

Ultracold polar molecules offer strong electric dipole moments and rich internal structure, which makes them ideal building blocks to explore exotic quantum matter [1-8], implement novel quantum information schemes [9][10][11], or test fundamental symmetries of nature [12]. Realizing their full potential requires cooling interacting molecular gases deeply into the quantum degenerate regime. However, the complexity of molecules which makes their collisions intrinsically unstable at the short range, even for nonreactive molecules [13][14][15][16][17][18], has so far prevented the cooling 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. The molecules are protected from reaching short range with a repulsive barrier engineered by coupling rotational states with a blue-detuned circularly polarized microwave. The microwave dressing induces strong tunable dipolar interactions between the molecules, leading to high elastic collision rates that can exceed the inelastic ones by at least a factor of 460. This large elastic-toinelastic collision ratio allows us to cool the molecular gas down to 21 nanokelvin, corresponding to 0.36 times the Fermi temperature. Such unprecedentedly cold and dense samples of polar molecules open the path to the exploration of novel many-body phenomena, such as the long-sought topological p-wave superfluid states of ultracold matter.

An Electro-Optical Trap for Polar Molecules

“…At the same time, optical traps are compatible with laser 13 and sideband 14 cooling of the trapped molecules as well as with control of the molecules' mutual interactions 15,16 -both critical to achieving quantum degeneracy in molecular systems 17 . The compatibility of optical traps also extends to optical cavities 18 .…”

Long-lived fermionic Feshbach molecules with tunable $p$-wave interactions

“…The surprisingly strong suppression of dimer-dimer loss in the non-halo regime suggests that after molecule formation, dimer loss can be strongly reduced by quenching the magnetic field into the regime of large binding energies. This has, for example, been essential in our recent work on the generation of degenerate Fermi gases of NaK molecules [29,30].…”

A High Phase-Space Density Gas of NaCs Feshbach Molecules

“…The data demonstrates excellent starting conditions for the preparation of ultracold NaCs ground state molecules. Recently, effective evaporative cooling has been shown for ground state molecules using resonant collisional shielding [54,55] and microwave shielding [56,57]. For NaCs, resonant shielding is predicted to lead to a superb ratio of elastic to inelastic collisions of 10 6 [55,58] and microwave shielding should be highly effective thanks to weak hyperfine interactions [59].…”

“…In conclusion, we have demonstrated the creation of high phase-space density gases of NaCs Feshbach molecules, which is an ideal stepping-stone for the preparation of ultracold gases of NaCs ground state molecules. In the ground state, evaporative cooling supported by microwave [56,57,59] or resonant shielding [54,55] may open the door to a BEC of dipolar NaCs molecules. In addition, mixtures of Na and Cs have favorable properties for the creation of dual Mott insulators in an optical lattice.…”