Collège de France Jean Dalibard Chaire Atomes et Rayonnement Année 2023-2024 Séminaire - Rudolf Grimm : Ultracold Fermion Mixtures with Tunable Interactions: Polarons and the Quest for Novel Superfluids Rudolf Grimm University of Innsbruck and IQOQI, Austrian Academy of Sciences, Autriche Résumé The possibility to tune interatomic interactions by means of magnetically controlled Feshbach resonances has opened up unprecedented opportunities for experiments concerning the intriguing many-body physics of ultracold matter in the strongly interacting regime. After a brief general introduction, I will report on two main research lines pursued in our laboratories based on fermionic quantum mixtures with tunable interactions. (1) A Fermi sea of Li-6 atoms is doped with impurity atoms of fermionic K-40 or bosonic K-41 atoms, which under strong interactions form quasiparticles. We explore the various properties of these "Fermi polarons" by means of radio-frequency spectroscopy. In our most recent experiments [1], we have observed mediated interactions between the polarons, with the surprising finding that the sign of the interaction depends on the quantum statistics of the impurity atoms. (2) In a Fermi-Fermi mixture of Dy-161 and K-40, we are proceeding towards the creation of novel imbalanced fermion superfluids. After exploring the complex spectrum of Feshbach resonances, we have studied the spectrum of collective modes in the deep hydrodynamic regime and we have demonstrated the magneto-association of ultracold bosonic molecules under conditions near quantum degeneracy [2]. [1] C. Baroni, I. Fritsche, E. Dobler, G. Anich, E. Kirilov, R. Grimm, M. A. Bastarrachea-Magnani, P. Massignan, G. Bruun, Nat. Phys. (2023). [2] E. Soave, A. Canali, Z.-X. Ye, M. Kreyer, E. Kirilov, R. Grimm, Phys. Rev. Research 5, 033117 (2023).