  Recently achieved two-component dipolar Bose-Einstein condensates open
exciting possibilities for the study of mixtures of ultra-dilute quantum
liquids. While non-dipolar self-bound mixtures are necessarily miscible with an
approximately fixed ratio between the two densities, the density ratio for the
dipolar case is free. As a result, self-bound dipolar mixtures present
qualitatively novel and much richer physics, characterized by three possible
ground-state phases: miscible, symmetric immiscible and asymmetric immiscible,
which may in principle occur at any population imbalance. Self-bound immiscible
droplets are possible due to mutual non-local inter-component attraction, which
results in the formation of a droplet molecule. Moreover, our analysis of the
impurity regime, shows that quantum fluctuations in the majority component
crucially modify the miscibility of impurities. Our work opens intriguing
perspectives for the exploration of spinor physics in ultra-dilute liquids,
which should resemble to some extent that of 4He-3He droplets and
impurity-doped helium droplets.
