Observation of quantum droplets in a heteronuclear bosonic mixture

We report on the formation of heteronuclear quantum droplets in an attractive bosonic mixture of K-41 and Rb-87. 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 good agreement in the full range of explored interactions. Our findings provide access to the production of long-lived droplets with important implications for further research.