Observation of Feshbach resonances between alkali and closed-shell atoms

Magnetic Feshbach resonances allow control of the interactions between ultracold atoms(1). They are an invaluable tool in studies of few-body and many-body physics(2,3), and can be used to convert pairs of atoms into molecules(4,5) by ramping an applied magnetic field across a resonance. Molecules formed from pairs of alkali atoms have been transferred to low-lying states, producing dipolar quantum gases(6). There is great interest in making molecules formed from an alkali atom and a closed-shell atom such as ground-state Sr or Yb. Such molecules have both a strong electric dipole and an electron spin; they will open up new possibilities for designing quantum many-body systems(7,8), and for tests of fundamental symmetries(9). The crucial first step is to observe Feshbach resonances in the corresponding atomic mixtures. Very narrow resonances have been predicted theoretically(10,11,12), but until now have eluded observation. Here we present the observation of magnetic Feshbach resonances of this type, for an alkali atom, Rb, interacting with ground-state Sr.