Energy and short-range correlations of Bose Polarons in the strong coupling regime*

The concept of the Bose polaron quasiparticle originates in Landau’s description of a crystal’s conduction electron which deforms the surrounding ionic lattice. Despite the conceptual simplicity of the Bose polaron – i.e. an impurity dressed by a surrounding bosonic bath – this quasiparticle poses remarkable challenges for both theoretical and experimental physicists. These challenges arise at strong interaction strengths where a multitude of bosons can simultaneously interact with the impurity thereby forming a complicated many-body state. We here report on the first creation of an ensemble of Bose polarons – comprised of fermionic ⁴⁰K impurities immersed in a ²³Na Bose Einstein condensate – in local equilibrium and in the strongly interacting regime. Using radio frequency ejection spectroscopy, we measure the Bose polaron ground-state energy and short-range correlations as a function of temperature across a variety of interaction strengths by tuning an interspecies magnetic Feshbach resonance. We further study collective oscillations between the BEC and the impurities, demonstrating strong dissipation of the impurities’ momentum at the transition from collisionless to hydrodynamic regime.