Evaporation of microwave-shielded polar molecules to quantum degeneracy

Ultracold polar molecules, possessing strong electric dipole moments and rich internal structure, are ideal for exploring exotic quantum matter, implementing quantum information schemes, and testing fundamental symmetries. However, achieving quantum degeneracy has been hindered by unstable short-range molecular collisions, preventing direct cooling via elastic collisions in three dimensions. Previous attempts have used 2D confinement or specific DC electric fields to suppress loss, but producing a degenerate 3D gas through collisional cooling has remained unsuccessful due to low elastic-to-inelastic collision ratios and low initial phase-space density. This research aims to overcome these challenges by employing microwave shielding to control molecular interactions and achieve evaporative cooling to quantum degeneracy in three dimensions.

The field of ultracold polar molecules is rapidly advancing, with potential applications in quantum simulation (p-wave superfluids, supersolids, Wigner crystals, novel spin systems, extended Hubbard models). While non-interacting quantum-degenerate gases have been produced by assembling degenerate atomic mixtures, active cooling to degeneracy remains challenging. The primary obstacle is rapid collisional loss due to attractive head-to-tail collisions and inelastic collisions at short range, even for molecules nominally stable against chemical reactions. Existing strategies to mitigate collisional loss include engineering repulsive interactions using external fields (DC electric fields, blue-detuned circularly polarized microwaves), with some success in 2D but limited success in 3D.

This study utilizes microwave shielding to induce fast elastic dipolar collisions while suppressing inelastic collisions in a 3D gas of fermionic 23Na40K molecules. A circularly polarized microwave couples the lowest rotational states, creating a dressed state with an induced rotating dipole moment. The interaction potential between molecules in this dressed state features a repulsive barrier at intermediate range, preventing short-range inelastic collisions. The microwave field is generated by a helical antenna. The elastic and inelastic collision rate coefficients are characterized by measuring two-body decay in a thermal gas and applying parametric heating to measure cross-dimensional thermalization. Evaporative cooling is achieved by exponentially lowering the power of the optical dipole traps, allowing the hottest molecules to escape. The remaining molecules rethermalize through elastic collisions, leading to a reduction in temperature and increase in phase-space density. The temperature and phase-space density are determined from polylogarithmic fits to the momentum distribution after time of flight.

The researchers achieved a remarkable elastic-to-inelastic collision ratio (γ) of at least 460, significantly higher than previous 3D experiments. This high ratio enabled efficient evaporative cooling of the 3D molecular gas to 21(5) nK, corresponding to 0.36(9) times the Fermi temperature (TF). This is a significant achievement, representing deep quantum degeneracy. At these low temperatures, the inelastic collision rate becomes negligible, resulting in a lifetime of the degenerate molecular sample of up to 0.6 s. The elastic collision rate increases dramatically with the effective dipole moment, tunable via microwave power and detuning. The hydrodynamic regime is reached at high collision rates, limiting the maximum measured γ to 460(110), although calculations suggest γ exceeding 1000 is possible with optimized microwave polarization. Forced evaporative cooling, followed by plain evaporative cooling, resulted in the coldest samples. The temperature was consistent with both Fermi-Dirac and Gaussian fits to the momentum distribution. The optical density in a small region of the cloud center was higher than the Fermi-Dirac fit for the coldest sample, potentially hinting at dipolar interaction effects, although further investigation is needed.

The results demonstrate a highly effective method for evaporative cooling of polar molecules to quantum degeneracy in 3D, surpassing previous achievements by a significant margin. The large elastic-to-inelastic collision ratio achieved via microwave shielding is key to this success. The achieved low temperatures and long lifetimes of the degenerate molecular gas open up exciting opportunities for studying many-body phenomena governed by strong, tunable dipolar interactions. The simplicity of the experimental setup makes the method widely applicable to various ultracold molecule experiments. The observed dipolar interaction strength is significantly higher than previously achieved in degenerate Fermi gases of magnetic atoms, promising observations of various dipolar many-body phenomena in future experiments.

This work presents a highly efficient and broadly applicable technique for evaporatively cooling ultracold polar molecules to deep quantum degeneracy in three dimensions. Microwave shielding produces a large elastic-to-inelastic collision ratio, enabling cooling to 21 nK (0.36 TF). Future improvements, such as optimizing STIRAP transfer, reducing microwave phase noise and improving polarization purity, are anticipated to reach temperatures below 0.1 TF, facilitating exploration of predicted quantum phases like p-wave superfluids and Bose-Einstein condensates of tetramers.

The study notes that the maximum measured elastic-to-inelastic collision ratio is limited by the hydrodynamic regime reached at high collision rates. While calculations predict even higher ratios are possible with optimal microwave polarization, achieving these ratios experimentally remains a challenge. The relatively low signal-to-noise ratio in the coldest sample prevents conclusive determination of the origin of increased optical density in the cloud center.