Quantum droplets of dipolar mixtures

Self-bound quantum droplets can emerge in either two-component or dipolar Bose-Einstein condensates (BECs), where stability is crucially provided by quantum fluctuations. Motivated by recent experiments producing BEC mixtures of highly-magnetic erbium and dysprosium atoms [PRL 121, 213601 (2018)], we develop a theoretical description of two-component dipolar BECs that includes Lee-Huang-Yang quantum fluctuations [PRL 126, 025301 (2021)]. While non-dipolar self-bound mixtures are necessarily miscible with an approximately fixed ratio between the two densities, the density ratio for dipolar mixtures is free, presenting novel and significantly richer physics. We predict that a self-bound immiscible droplet should be possible due to mutual non-local attraction, resulting in the formation of a droplet molecule. Moreover, our analysis of strongly imbalanced populations 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 4He-3He droplets and impurity-doped helium droplets.