The five-year duration of the RARE project has been divided in three main parts: (1) The construction phase of a new experimental apparatus (M1-18); (2) the intermediate result phase (M18-36); and (3) the main result phase (M36-60).
In the first phase of the project, a fully functional experimental setup being able to produce dipolar quantum mixtures of erbium and dysprosium was build up. The experimental setup consists of a complete vacuum apparatus and dedicated laser and magnetic field setups. The demonstration of simultaneous laser cooling of erbium and dysprosium, and dual magneto-optical trapping have been reported in a manuscript published in Phys. Rev. A 2018.
In the second phase of the project, we have produced the first double-degenerate dipolar quantum gas mixture using evaporative cooling in an optical dipole trap. We were able to prepare several isotope combinations to realize Bose-Bose and Bose-Fermi mixtures. These results have been published in Phys. Rev. Lett. (featured as Synopsis in Physics and selected as Editors’ Suggestion).
Simultaneously, we were able to create the first strongly-interacting Fermi-Fermi mixture of two spin states in erbium (published in Phys. Rev. Lett.). The large number of fermionic isotopes can be used for quantum simulation and lattice spin models, where we performed first experiments using erbium in a three-dimensional lattice and realized the so-called Heisenberg XXZ-model (published in Phys. Rev. Research).
Finally, we were able to observe rotonic excitations in our dipolar gas, which were predicted in theory about 20 years ago. This work has been published in Nature Physics and has attracted a high interest across many communities and led to the speculation that such systems could realize a new state of matter, the so-called supersolid. We were able to prepare this fascinating state in both species, with dysprosium exhibiting a very long lifetime, allowing us to directly cool into this state. This work has been published in Physical Review X (The paper has been featured as Viewpoint in Physics).
In the third phase of the project, we further investigated dipolar quantum gas mixtures of Er and Dy and have been able to observe broad interspecies Feshbach resonances in a variety of isotope combinations. We also experimentally probed interactions using the in-situ repulsion between the two species (both published in Phys. Rev. A).
In parallel we have performed high-resolution EIT Rydberg spectroscopy using an atomic beam of Er on a newly build separate experimental setup. With this, we could observe a multitude of Rydberg lines which, together with a multi-channel theoretical analysis in collaboration with theorists (published in Phys. Rev. Research).
We also further continued our survey on the supersolid state of matter. In particular, we experimentally probed the high-energy Bragg-scattering response and worked on a theory model for the experimental conditions (published in Phys. Rev. A and Phys. Rev. Research). For the long-lived dipolar supersolid in Dy, we were able to probe phase coherence in an out of equilibrium situation which has been published in Nature Physics and investigated the birth, life and death of the supersolid, showing that within the evaporation process the crystalline structure seems to appear before global phase coherence (published in Phys. Rev. Research).
In a major breakthrough we were able to observe supersolidity in two-dimensional systems, showing spontaneous symmetry breaking along two spatial dimensions, which has been published in Nature 2021 (associated News&Views article in Nature). Following this important step, we further investigated experimentally and theoretically the 2D systems in a series of works (published in Phys. Rev. A and Phys. Rev. Lett.).
Finally, we investigated rotating dipolar gases to directly create and detect quantized vortices. We successfully implemented a genuine scheme to introduce angular momentum by a rotating magnetic field and were able to produce and detect vortices and their special properties in a dipolar gas. Our results got recently published in Nature Physics 2022 (associated News&Views article in Nature Physics).
