The heavy Fermi polaron I: the Lithium-Cesium experiment

This paper introduces a new experimental platform designed to investigate the physics of Fermi polarons in a system with extreme mass imbalance. A Fermi polaron is a quasiparticle formed when an impurity atom is dressed by excitations of a surrounding quantum bath—in this case, a deeply degenerate Fermi gas of lithium-6 atoms. By immersing cesium-133 impurities into this lithium sea, the researchers achieve a mass ratio of approximately 22, allowing them to explore regimes of polaron physics that were previously inaccessible. The experiment utilizes magnetic Feshbach resonances to tune the interspecies interactions and optical Raman spectroscopy to probe the energy spectrum of the impurities.

Creating this mixture requires a sophisticated preparation sequence because the two species have different polarizabilities and cooling requirements. The researchers prepare the lithium Fermi gas and cesium impurities separately before combining them in a final optical dipole trap. A key feature of the setup is the use of a second, effectively non-interacting cesium hyperfine state as a reference for spectroscopy. This allows for precise measurements of the polaron energy shifts relative to a bare impurity state. The team also implements state-resolved absorption imaging to measure the transferred fraction of atoms between states, which significantly reduces measurement noise compared to single-state imaging.

Using injection spectroscopy, the researchers successfully recorded the energy spectra of the heavy Fermi polaron. They observed the emergence of attractive polaron peaks at negative interaction strengths and repulsive polaron peaks at positive interaction strengths. As the system approaches the unitary limit, the spectra exhibit complex structures, providing a clear experimental benchmark for theoretical models of heavy impurities in Fermi gases. The platform demonstrates high stability and control, with spectroscopic resolution sufficient to resolve features on the order of the Fermi energy.