Among all known isotopes, Thorium-229 has the lowest nuclear excited state, only 8.4 eV above the ground state. This so-called “isomer” is accessible to VUV laser excitation and a multitude of applications at the interface of atomic and nuclear physics have been proposed, including a nuclear clock, a gamma laser and a sensitive detector for variations of fundamental constants. After decades of attempts to determine the exact isomer energy and other nuclear properties, we report on two experiments which resonantly excite the isomer by lasers and spectroscopically resolve the nuclear quadrupole splitting in a single crystal environment. This allows us to determine the sensitivity of the nuclear clock transition to variations of the fine structure constant, which exceeds all schemes involving electron shell transitions by 3 orders of magnitude.

Ultracold mixtures combine atomic species with fundamentally different properties. Bosons can meet fermions, heavy particles can meet light ones, interparticle interactions can be controlled precisely by external fields, and novel quantum systems emerge with intriguing properties. Over the past 25 years, great progress to prepare various types of mixtures in the laboratory has opened up numerous excited research avenues, related to the few- and many-body physics of atomic and molecular quantum systems.

I will start with a brief review of early experiments carried out in the late 1990’s at the MPI for Nuclear Physics in Heidelberg on mixtures of lithium and cesium. These activities triggered many exciting further developments, including experiments in Heidelberg and Innsbruck on cooling and trapping techniques, thermodynamics, heteronuclear molecules, Efimov quantum states, and on the physics of quasi-particles.

In the main part of my talk, I will focus on the specific topic of quasi-particles called “Fermi polarons”, which we have extensively studied in Innsbruck with potassium atoms (bosonic or fermionic) immersed as impurities in a Fermi sea of lithium atoms. In a series of experiments over the past decade, we have explored the energy spectrum, the formation dynamics, decay and coherence properties, and mediated interactions between polarons. Ongoing experiments are dedicated to the motion of impurities after controlled kicks by photon momentum transfer. Most strikingly, we observe a motion-induced breakdown of the polaron above a critical momentum, where the particle gets “undressed” and becomes a bare particle.