Strongly Interacting Mixtures of 23-Na and 40-K

Mixtures of quantum fluids lie at the forefront of research into strongly-correlated quantum matter. We explore the rich phase diagram of the Bose-Fermi mixture in the impurity limit by immersing fermionic impurities in a Bose-Einstein condensate (BEC) with near-resonant interactions. We create Bose polarons near quantum criticality and probe their energy, spectral width, and short-range correlations as a function of temperature. We observe their inverse lifetime, determined via spectral width, to increase linearly with temperature at the Planckian scale, a hallmark of quantum critical behavior.

We further measure collective oscillation spectra of the Bose-Fermi mixture as a function of interspecies coupling in the fermionic impurity limit. We observe a dissipationless flow of the fermionic impurities, even as the mixture enters the strong-coupling regime where the impurities inherit the hydrodynamic modes of the BEC.