Dipolar quantum gases of strongly magnetic atoms

Ultracold quantum gases, made of bosonic or fermionic particles, have constituted a fruitful platform opening new frontiers in the understanding of modern quantum physics. Our project aims at investigating the impact of long-range and anisotropic interparticle interactions on the few- and many-body physics in ultracold quantum gases. Such special type of interactions, named dipole-dipole interactions, arises from the large (magnetic or electric) dipole moment of the particles. Gases with dipolar interactions have become one of the most successful and competitive areas within the quantum gases research community. Our project is part of a DFG-FWF Collaborative Research Unit (FOR2247), to which we contribute by performing experimental studies using quantum gases of atomic erbium, among the most magnetic species of the periodic table. By renewing our Collaborative Unit, we will build on the knowledge and control gained on such systems and on the collaborative efforts within our Collaborative Research Unit to unveil new aspects of dipolar quantum phenomena. We have two major goals. Our first goal focuses on dipolar quantum Bose gases. In those gases, recent works within our Collaborative Research Unit, also triggering an international research effort, have revealed the existence of exotic states of matter whose properties intrinsically relates to the many-body correlated behavior. This includes droplet assemblies, macro-droplets, stripe states and roton excitations. The possibility of such exotic states brings the hope of realizing a highly paradoxical phase with supersolid properties arising from the mere interparticle interactions. In this project, we will build on our collaborative power on this topic, both theoretical and experimental, to further explore such a possibility. Our second goal centers on dipolar quantum Fermi gases, made of two spins, similar to the case of electrons in solids. We have recently shown our ability to produce such spin mixtures and to tune their interactions. Our aim will now be to realize and study the possibility for a superfluid behavior via a pairing of the spins. By tuning the interaction, one could cross from a superfluid of delocalized pairs similar to superconductor to a superfluid of bound fermions, behaving like bosons. While such a crossover has been studied for years in alkali gases, lanthanides bring an exceptional scattering scenario, including dipole-dipole interactions, anisotropic short-range interactions, and finite-range effects. In a recent study benefiting from collaboration within the Collaborative Research Unit, we have deepened our understanding of the impact of the dipolar interactions on the behavior of the Fermi gas. Here we want to expand this collaboration and work in the aim to reveal how these few- body features modify the behavior of the fermionic assemblies at a many-body level.

The goal of the project "Dipolar quantum gases of strongly magnetic atoms", part of the DFG-FWF Forschergruppe (FOR2247), was the investigation of the influence of anisotropic and long-range interactions onto the few- and many-body physics. Our experimental studies were performed using ultracold atoms of lanthanides with exceptionally strong magnetic moment, erbium and dysprosium. Previously we already found the roton mode excitations in dipolar gases, and we have investigated the newly found beyond mean-field correction coming from quantum fluctuations, which is capable of stabilizing a collapsing dipolar gas. This led to exciting predictions of a possible realization of a supersolid state. This exotic state of matter features solid as well as superfluid properties at the same time. Within this project we were not only able to prepare and investigate dipolar supersolids and their excitations for the first time, but also to expand supersolidity to two dimensions. Our ability to create such supersolid states via evaporation allowed us to investigate in detail how such a supersolid state comes into existence, and how it vanishes. We were also able to bring such a supersolid state out-of-equilibrium and to monitor how the common phase gets reestablished with time. In a second path we expanded and improved our knowledge and our tools to control and manipulate ultracold lanthanide atoms to an unprecedented level. We did a first spectroscopy of a clock-like transition in erbium that can be used for precision spectroscopy, efficient spin-preparation of all isotopes, and the manipulation of spin-exchange and spin-relaxation processes. We also studied the contact interaction properties for all isotopes of erbium alone, and also for an erbium dysprosium dipolar mixture, where we got first hints on species-induced supersolid transitions. We also investigated a strongly dipolar gas in reduced dimensions i.e. in an array of two-dimensional pancakes. Using Bloch oscillations we were able to determine the importance of beyond-mean-field contributions on the system and observed a self-localized state on a single lattice site. A third direction of investigation was our investigation of many-body dynamics under the influence of rotation. Here, we prepared for the first time quantum vortices in a strongly dipolar gas through a novel scheme using rotating magnetic fields, therefore called magnetostirring. Controlling the relative strength of the dipolar interactions, we observed the arrangement of the vortices along stripes, a peculiar consequence of the anisotropy of the dipolar interactions. We additionally looked in more detail onto the creation process of vortices in dipolar quantum gases. Finally, in a theory work, we were able to bring the physics of rotating supersolids in connection to fast rotating neutron stars, possibly explaining the physics behind the mysterious glitches of those objects.