Supersolid Phases in Dipolar Quantum Gases

This project aims at exploring and understanding the physical properties of a novel phase of matter, which has remained elusive for 50 years: the supersolid phase. Supersolidity is a paradoxical phase where the matter is both crystallized and superfluid. This means that, in this phase, the atoms are at the same localized around some specific positions of space - the crystal sites - and delocalized over the full system, allowing a frictionless flow. In year 2019, supersolid state were first observed in dilute gases, cooled down very close to the absolute zero temperature of -273C, and made of lanthanide atoms. Atoms from the lanthanides family have the specificity to interact via long-range and anisotropic dipole-dipole interactions. It is via a fine tuning of the interplay between the dipolar interactions, the isotropic contact interactions arising in the collision between the atoms, and the gas shape that supersolid states could be created and observed in experiments. In the following years, this project will aim at answering questions that are puzzling the scientific community regarding this novel phase of matter. In particular, we will question the interplay between the two broken symmetries (i.e. the translational symmetry broken by the crystral and the gauge symmetry broken by the superfluid): how does the crystalline structure impact the supersolid flow? As well as the interplay of these broken symmetry with and the system geometry: Can supersolids exist in two directions, and in lower dimensional space? Are rotational and translational superfluid flows allowed in these states? The nature itself of the phase transition, yielding to these two spontaneous symmetry breakings, as well as its persistence, behavior, and scaling when the size of the systems is increased toward infinity, will also be questioned and studied. The investigation will be based on two existing experimental platforms, producing ultracold quantum gases of erbium and dysprosium, two of the most magnetic lanthanides species, with which supersolid states have been observed. To undertake our study, we will exploit and adapt the rich toolbox developed over the last two decades by the ultracold-gas community, as well as extend concepts and analysis schemes developed in other fields. The success of these methods in other contexts promises interesting outcomes to our project. Thanks to our research, we hope to shed new lights and deeper insights into the very general topics of the unintuitive properties of quantum matter, many-body phenomena, and phase transitions, as well as open up new prospects for the ultracold-gas platforms.

Quantum mechanics predicts behaviors that are counter-intuitive and difficult to conceive for most of us, even among scientists. Supersolidity is a paradigm example of this fact at a many-body level. Indeed, supersolidity is a phase of (quantum) matter, where both crystallization and flow of particles (without friction) occur. The possibility of these phases was debated even among physicists. It was predicted by several theorists back in the late 50's but remained long unexplored before being first observed in ultracold gases of magnetic atoms in 2019, by three groups among which our group in Innsbruck. In the project "Supersolid Phases in Dipolar Quantum Gases", we proposed ourselves to deepen the study of this newly discovered phase and extend it to new regimes. Despite its premature end, the project achieved important results. In particular, we better understood the dynamical behavior of supersolids and obtained several evidences of their superfluidity via detailed studies of their dynamics after quenches, in particular of their phase dynamics. By experimentally and theoretically probing the dynamical or thermodynamical transition crossing to supersolid, we gained deeper insights into the mechanisms and critical behavior that allows for supersolidity. In further studies, we also brought up new possibilities for manipulating or addressing gases of magnetic atoms which can be used in the future for many-body studies. All these studies have significantly contributed to extending our understanding of supersolidity, and realized important first steps to broaden their realizations.