Observation of self-bound droplets of ultracold dipolar molecules

Ultracold gases of dipolar molecules have long been envisioned as a platform for the realization of novel quantum phases1,2,3,4,5,6,7,8. Recent advances in collisional shielding9,10,11,12, protecting molecules from inelastic losses, have enabled the creation of degenerate Fermi gases13,14,15 and, more recently, Bose–Einstein condensation of dipolar molecules16. However, the observation of quantum phases in ultracold molecular gases that are driven by dipole–dipole interactions has so far remained elusive. Here we report the formation of self-bound droplets and droplet arrays in an ultracold gas of strongly dipolar sodium–caesium molecules. Starting from a molecular Bose–Einstein condensate, microwave dressing fields are used to induce dipole–dipole interactions with controllable strength and anisotropy. By varying the speed at which interactions are induced, covering a dynamic range of four orders of magnitude, we prepare droplets under equilibrium and non-equilibrium conditions, observing a transition from robust one-dimensional arrays to fluctuating two-dimensional structures. The droplets show densities up to 100 times higher than the initial Bose–Einstein condensate, reaching the strongly interacting regime and suggesting the possibility of a quantum-liquid or crystalline state9,17. This work establishes ultracold molecules as a system for the exploration of strongly dipolar quantum matter and opens the door to the realization of self-organized crystal phases3,9,18 and dipolar spin liquids in optical lattices19.