  Feshbach resonances are among the essential control tools used in ultracold
atom experiments. However, for complex atomic species the theoretical
characterization of resonances becomes challenging. For closely spaced
resonances, the measurement of three-body losses does not provide sufficient
resolution to discriminate them. For this reason, resonance spectroscopy of
trapped isolated atoms is becoming the state of the art. Here we show that
trapping the atoms in a double well potential such as an optical lattice or a
pair of optical tweezers enables precise characterization of not only the
resonance position and width, but also its pole strength, giving valuable
information about the atomic structure relevant for subsequent many-body
studies.
