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Dipolar Physics and Rydberg Atoms with Rare-Earth Elements

Periodic Reporting for period 3 - RARE (Dipolar Physics and Rydberg Atoms with Rare-Earth Elements)

Reporting period: 2019-07-01 to 2020-12-31

The overall objective of the RARE project is the investigation of dipolar quantum gases and quantum mixtures of the strongly magnetic rare-earth atomic species erbium and dysprosium. Those species feature a strong anisotropic and long-range dipole-dipole-interaction (DDI), which makes them ideal candidates to study few- and many-body dipolar quantum physics, opening the way to explore exotic quantum phases of matter otherwise not accessible. They will also allow to realize unique quantum simulators, where a well controllable quantum system is used to simulate very complicated or not easy accessible systems like condensed-matter materials, allowing an in-depth investigation of e.g. quantum magnetism.
The project follows three main lines of research: (A) to realize strongly magnetic dipolar quantum mixtures with the aim to experimentally produce and study exotic states of matter; (B) to investigate dipolar phenomena in very low or even zero polarizing magnetic fields, where new quantum phases related to the spin-orbit coupled nature of dipolar gases or quantum nematic phases are expected to exist; (C) to open new horizons in the field of Rydberg physics by exploiting the multi-electron nature of erbium which adds novel degrees of freedom to those systems like simultaneous trapping, manipulating, detecting and cooling of Rydberg excited atoms.
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, including a dual-species high-temperature oven creating an atomic vapour beam, a transversal cooling chamber, a dual-species Zeeman-slower, and a main experimental chamber. Dedicated laser and magnetic field setups have been developed and installed to cool, trap, and manipulate the atoms. The demonstration of simultaneous laser cooling of erbium and dysprosium, and dual magneto-optical trapping have been reported in a manuscript published in Physical Review A 2018.
In the second phase of the project, we worked on the next step on cooling of the erbium-dysprosium mixture to reach quantum degeneracy. We have produced the first double-degenerate dipolar quantum gas mixture using evaporative cooling in an optical dipole trap. Cooling efficiency and the magnetic field dependence of the interspecies thermalization rates have been carefully investigated. As both species come in a variety of isotopes with both, bosonic and fermionic nature, we were able to prepare several isotope combinations to realize Bose-Bose and Bose-Fermi mixtures. These results have been published in Physical Review Letter 2018 (The paper has been 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, which will be the base on future investigations on BEC-BCS crossover with the unexplored addition of dipole-dipole interaction. The corresponding result has been published in Physical Review Letters 2018. The fermionic isotope in erbium and dysprosium feature an extremely large number of spin states, which can be used for quantum simulation and lattice spin models. We have performed first experiments using erbium in a three-dimensional lattice and realized the so-called Heisenberg XXZ-model. These results have been published in Physical Review Research in 2020.
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 2018 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. This counterintuitive state combines two opposing properties, a crystal-like structure and superfluid behaviour, at the same time. Very recently we were able to prepare this fascinating state in both species, erbium and dysprosium, with the latter exhibiting a very long lifetime in this state, allowing us to directly cool into this state. This work has been published in Physical Review X 2019 (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. This study has been conducted using a variety of isotope combinations and the corresponding work has been published in Physical Review 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. A manuscript summarizing our results is currently in preparation.
We also further continued our survey on the properties of the recently discovered supersolid state of matter in dipolar quantum gases. In particular, we experimentally probed the high-energy Bragg-scattering response (manuscript under evaluation) and worked on a theory model for the experimental conditions (published in Physical Review 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 the high-impact journal 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 (manuscript under evaluation).
We have been able to produce for the first time a dipolar quantum mixture. This is a completely novel system and very little is known about its quantum behaviour, ground-state properties, and out-of equilibrium dynamics.