  In this study, we achieved magnetic control of reactive scattering in an
ultracold mixture of $^{23}$Na atoms and $^{23}$Na$^{6}$Li molecules. In most
molecular collisions, particles react or are lost near short range with unity
probability, leading to the so-called universal rate. By contrast, the
Na{+}NaLi system was shown to have only $\sim4\%$ loss probability in a fully
spin-polarized state. By controlling the phase of the scattering wave function
via a Feshbach resonance, we modified the loss rate by more than a factor of
$100$, from far below to far above the universal limit. The results are
explained in analogy with an optical Fabry-Perot resonator by interference of
reflections at short and long range. Our work demonstrates quantum control of
chemistry by magnetic fields with the full dynamic range predicted by our
models.
