  Magnetic Feshbach resonances are an invaluable tool for controlling ultracold
atoms and molecules. They can be used to tune atomic interactions and have been
used extensively to explore few- and many-body phenomena. They can also be used
for magnetoassociation, in which pairs of atoms are converted into molecules by
ramping an applied magnetic field across a resonance. Pairs of open-shell
atoms, such as the alkalis, chromium, and some lanthanides, exhibit broad
resonances because the corresponding molecule has multiple electronic states.
However, molecules formed between alkali and closed-shell atoms have only one
electronic state and no broad resonances. Narrow resonances have been predicted
in such systems, but until now have eluded observation. Here we present the
first observation of magnetic Feshbach resonances in a system containing a
closed-shell atom, Sr, interacting with an alkali atom, Rb. These resonances
pave the way to creating an ultracold gas of strongly polar, open-shell
molecules, which will open up new possibilities for designing quantum many-body
systems and for tests of fundamental symmetries.
