From a strongly interacting Bose-Fermi mixture to a dipolar quantum gas of molecules

The interplay of quantum statistics and interactions in atomic Bose–Fermi mixtures leads to a phase diagram markedly different from pure fermionic or bosonic systems. However, investigating this phase diagram remains challenging due to detrimental collisional losses when bosons condense. Here, by density-matching a degenerate Fermi gas with a decompressed Bose–Einstein condensate, we mitigate the atomic loss and observe evidence for a quantum phase transition from a polaronic to a molecular phase. By driving through the transition, we produce a large and long-lived degenerate Fermi gas of Feshbach molecule. This provides a good starting point to create a low-entropy sample of ground-state molecules for evaporative cooling. Finally, using an intense, circularly-polarized microwave field emitted from a single helical antenna, we induce fast dipolar elastic collisions in microwave-dressed molecules while strongly suppressing collisional loss in all three dimensions. This enables efficient evaporation of NaK molecules, reaching 21 nK, corresponding to 0.36 times the Fermi temperature [2]. Our results point to an exciting future of ultracold polar molecules.

[1]. Marcel Duda et. al., Transition from a polaronic condensate to a degenerate Fermi gas of heteronuclear molecules, arXiv:2111.04301 (2021).

[2]. Andreas Schindewolf et. al., Evaporation of microwave-shielded polar molecules to quantum degeneracy, arXiv:2201.05143 (2022).