Nature 539, 259–262 (2016)

A drop of water is held together by the interplay between attractive and repulsive forces stemming from inter-particle interactions. But what would happen if you could play around with these interactions? Matthias Schmitt and colleagues did just that, but instead of water they used a dilute magnetic quantum liquid: a condensate of dysprosium atoms.

This is no ordinary condensate. Dysprosium has a very large magnetic dipole moment so the atoms interact strongly through the long-range dipole force. They also experience a repulsive short-range contact interaction, which is tunable by a magnetic field using Feshbach resonances. Schmitt et al. were able to adjust it such that the balance between the two forces allowed droplets to form. The droplets remained stable when the optical trapping of the condensate was switched off.

Interestingly, the droplets can only form if they contain enough atoms: below a critical number of atoms they simply evaporate. These self-bound droplets are quite dilute, but they still bear intriguing similarities with helium droplets or atomic nuclei.