Quantum liquid droplets in a mixture of Bose-Einstein condensates

Making dilute quantum droplets In recent years, quantum fluids have been studied largely in gaseous form, such as the Bose-Einstein condensates (BECs) of alkali atoms and related species. Quantum liquids, other than liquid helium, have been comparatively more difficult to come by. Cabrera et al. combined two BECs and manipulated the atomic interactions to create droplets of a quantum liquid (see the Perspective by Ferrier-Barbut and Pfau). Because the interactions were not directional, the droplets had a roughly round shape. The simplicity of this dilute system makes it amenable to theoretical modeling, enabling a better understanding of quantum fluids. Science, this issue p. 301; see also p. 274 Tuning interatomic interactions in two ultracold gases of potassium atoms creates quantum liquid droplets. Quantum droplets are small clusters of atoms self-bound by the balance of attractive and repulsive forces. Here, we report on the observation of droplets solely stabilized by contact interactions in a mixture of two Bose-Einstein condensates. We demonstrate that they are several orders of magnitude more dilute than liquid helium by directly measuring their size and density via in situ imaging. We show that the droplets are stablized against collapse by quantum fluctuations and that they require a minimum atom number to be stable. Below that number, quantum pressure drives a liquid-to-gas transition that we map out as a function of interaction strength. These ultradilute isotropic liquids remain weakly interacting and constitute an ideal platform to benchmark quantum many-body theories.