Microwave-shielded polar molecules: from evaporation to tetratomic molecules

Thanks to their strong electric dipole moments and rich internal structure, ultracold

polar molecules are a promising platform for realizing exotic quantum matter, for

implementing quantum information schemes and for performing precision

measurements.

In many of these applications, samples of interacting molecular need to be prepared

in the quantum-degenerate regime. For a long time, employing direct evaporative

cooling via elastic collisions has been prevented by intrinsically unstable two-body

collisions of molecules at short range. Protecting molecules against such detrimental processes can be

achieved by engineering a repulsive barrier using a blue-detuned, circularly polarized

microwave field which couples the two lowest rotational states.

Here, I demonstrate how microwave shielding can be employed to evaporatively cool

a fermionic, three-dimensional gas of ²³Na⁴⁰K well below the Fermi temperature.

Furthermore, I will show how the microwave field can be tuned to shape the

intermolecular potential, independently tuning dipolar and contact interaction,

allowing us to observe of a novel kind of scattering resonance. These universal fieldlinked

resonances arise due to the existence of stable, tetratomic bound states. I will

also present our results regarding the creation and observation of these bound

states, whose properties agree very well with parameter-free theory calculations.

Lastly, I will lay out our pathway towards achieving ever colder samples, that

will allow us to explore quantum many-body phenomena like p-wave superfluidity or

the extended Fermi-Hubbard-models. Besides the physical aspect of microwave

shielding, I will also discuss the technical challenges associated with

building and controlling the requisite high-power microwave setups.