The strongly driven Fermi polaron

Quasiparticles are emergent excitations of matter that underlie much of our understanding of quantum many-body systems. Therefore, the prospect of controlling their properties has both fundamental and practical implications. However, in solid-state materials, it is often challenging to understand how quasiparticles are modified by external fields due to their complex interplay with other collective excitations. Here we demonstrate the manipulation of Fermi polarons—quasiparticles formed by impurities interacting with a Fermi gas—in a homogeneous atomic gas using fast radio-frequency control. Exploiting two internal states of the impurity species, we develop a steady-state spectroscopy, from which we extract the energy of the driven polaron. By varying the drive Rabi frequency, we measure the decay rate and the quasiparticle residue of the polaron in the weak-drive limit. At large Rabi frequencies, we observe signs that the drive causes a hybridization of the driven polaron with an incoherent background, leading to the breakdown of a description in terms of textbook quasiparticles. Our experiment establishes the driven Fermi polaron as a promising platform for studying controllable quasiparticles in strongly driven quantum matter and calls for a controlled theoretical framework to describe the dynamics of this strongly interacting quantum system. Fermi polarons are quasiparticles formed by impurities immersed in a Fermi gas. An experiment in an ultracold fermionic gas now shows how to control their properties with a tunable radio-frequency field.