Speaker
Description
We experimentally study the paradigmatic many-body problem of mobile impurities interacting with a homogeneous Bose-Einstein condensate (BEC). We use a combination of injection spectroscopy and many-body interferometry to access the injection spectrum (frequency domain) and the impurity-coherence function (time domain). Our experiments start with a spin-polarized BEC confined in an optical box trap and we use rf pulses to transfer a fraction of atoms into the target spin state. We map out the polaron energy and spectral response from weak to strong impurity-bath interactions (characterized by the interstate scattering length $a$) and study the effects of varying condensate properties, changing both the condensate (intrastate) interactions ($a_{\rm b}$) and density $n$. While we observe that most of the physics is universally set by $n$ and $a$, we find a clear dependence on $a_{\rm b}$ for the attractive polaron branch. For strong repulsive interactions, we observe two distinct spectral features, corresponding to a repulsive polaron and a many-body state related to the Feshbach dimer, whose energy shifts with density. Curiously, despite the significantly reduced inhomogeneous-density broadening in our system, we observe that even at weak interactions the spectra have significant widths proportional to their shifts. Finally, our many-body interferometry provides new insights into the formation dynamics of the Bose polaron.
