Observation of self-bound droplets of ultracold dipolar molecules.

Abstract

Ultracold gases of dipolar molecules have long been envisioned as a platform for the realization of novel quantum phases^1-8. Recent advances in collisional shielding^9-12, protecting molecules from inelastic losses, have enabled the creation of degenerate Fermi gases^13-15 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 state^9,17. 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 phases^3,9,18 and dipolar spin liquids in optical lattices¹⁹.