  We report on the formation of heteronuclear quantum droplets in an attractive
bosonic mixture of 41K and 87Rb. We observe long-lived self-bound states, both
in free space and in an optical waveguide. In the latter case, the dynamics
under the effect of a species-dependent force confirms their bound nature. By
tuning the interactions from the weakly to the strongly attractive regime, we
study the transition from expanding to localized states, in both geometries. We
compare the experimental results with beyond mean-field theory and we find a
good agreement in the full range of explored interactions. Our findings open up
the production of long-lived droplets with important implications for further
research.
