  Mixtures of ultracold gases with long-range interactions are expected to open
new avenues in the study of quantum matter. Natural candidates for this
research are spin mixtures of atomic species with large magnetic moments.
However, the lifetime of such assemblies can be strongly affected by the
dipolar relaxation that occurs in spin-flip collisions. Here we present
experimental results for a mixture composed of the two lowest Zeeman states of
$^{162}$Dy atoms, that act as dark states with respect to a light-induced
quadratic Zeeman effect. We show that, due to an interference phenomenon, the
rate for such inelastic processes is dramatically reduced with respect to the
Wigner threshold law. Additionally, we determine the scattering lengths
characterizing the s-wave interaction between these states, providing all
necessary data to predict the miscibility range of the mixture, depending on
its dimensionality.
