Elusive quasiparticles observed

Metastable states in Fermi gases with strong repulsive interactions are of fundamental interest, but the realization of such systems is challenging because they are intrinsically unstable against decay. Two groups have overcome this obstacle and report the detection of the theoretically predicted repulsive Fermi polarons. Kohstall et al. study a three-dimensional system of ultracold potassium impurities resonantly interacting with a Fermi sea of lithium atoms. The character of the interaction stabilizes the repulsive regime, enabling the authors to detect long-lived, metastable repulsive polarons. Koschorreck et al. study both attractive and repulsive Fermi polarons in a two-dimensional, spin-imbalanced Fermi gas of potassium atoms, and find evidence for a pairing transition. The results from these two studies hold promise for the creation of exotic states with ultracold fermionic atoms, such as ferromagnetic quantum phases.