E4: Few- to many-body physics with ground state bosonic NaK polar molecules

Ultracold dipolar quantum gases pose new challenges for the experimental physics of ultracold quantum gases. There has been a tremendous amount of theoretical work to explore the rich physics of quantum systems with dipolar interactions. For example the existence of rotons for systems with weak dipolar interactions, or vortex lattices of rotating Bose condensates have been shown to be dramatically altered by dipolar coupling and can lead to new stripe and bubble phases. In the strongly interacting limit (where dipolar interaction dominates over contact interaction), the roton mode that can already be seen in weakly interacting systems can lead to an instability towards a so called dipolar crystal - a self-organized 'solid' of dipoles. When the dipolar interaction is cut off at short distances by e.g. dressing the dipoles with radio frequency, supersolid phases have also been predicted. In a periodic potential, fractional Mott insulators as well as supersolids may also form.

While there has been tremendous progress in the experimental realization of dipolar gases with weak and intermediate interaction strength, the realization of a strongly interacting dipolar system has proven difficult. Candidates for such systems are Rydberg gases, which for the moment either don't allow large particle number or don't allow the investigation of dynamical effects due to the short lifetime (frozen regime). The reason for this is that by choosing a large principal quantum number to achieve a long lifetime, the blockade radius and therefore the Rydberg density becomes low. Current realizations of dipolar molecules are plagued by the fact that two-body collisions are inelastic and hence evaporative cooling is inhibited.

The goal of this project is to create a near degenerate sample of ground state polar molecules made from sodium and potassium (²³Na³⁹K) along the lines of what was done with ⁴⁰K⁸⁷Rb. The inelastic two-body decay channel that exists for KRb is not present in this combination. Simultaneously the molecular dipole moment of 2.7 Debye in the ground state is more than five times larger than in KRb leading to a dipole strength of 25 times the magnitude found in ⁴⁰K⁸⁷Rb. With these parameters one can expect to directly observe the formation of dipolar crystals in 2D using high resolution imaging as the structure size of such a crystal can be expected to be on the order of 1 μm in typical trap geometries when applying moderate electric fields in the range of 5-10 kV/cm.

After creating ground state molecules, the scattering properties of the ground state molecules will first have to be explored: the question whether the dipolar interaction is dominating depends on the ratio between dipolar length and scattering length but no reliable predictions on the latter are available. In fact, there exists a conjecture that the scattering spectrum of diatomic molecules might be dense (i.e. the scattering is always resonant)so that a small scattering length is actually very unlikely and three body loss is always strong. If this is the case, possibilities to reduce such losses need to be explored. Ideas in this direction include confining potentials to reduce dimensionality, polarizing the system as well as modifying the interaction potential using radio frequency dressing. Once the system has been stabilized, the plan is to proceed with evaporatively cooling the molecules towards quantum degeneracy and to explore transitions from superfluid to crystalline phase as well as fractional Mott insulators and supersolids in optical lattices.