A water droplet has structural integrity because of attractive forces between water molecules. If the molecules get close enough, however, the intermolecular forces are repulsive; water molecules can pack only so tightly. In quantum systems, intrinsically quantum fluctuations can stabilize a cluster of atoms against collapse. As a result, quantum droplets can have extraordinarily low densities.

Such so-called ultradilute liquids were first created in 2016, with single-component systems interacting via dipole–dipole forces. Now Leticia Tarruell and her team at the Institute of Photonic Sciences in Barcelona have created an ultradilute liquid in a system of two potassium-39 Bose–Einstein condensates with different magnetic quantum numbers. The new two-component system avoids some of the complications associated with the anisotropic dipole force.

Tarruell and colleagues tuned the interactions in their system with an external magnetic field. In a multiparticle system such as theirs, the forces acting on an individual atom are often summarized by a so-called mean field. Mean-field theory quantitatively predicts the magnetic field that divides the regime in which particles feel attraction from the regime of repulsive force.

After preparing the system in an optical trap, the Barcelona group adjusted the magnetic field and then released the two-component mixture. The figure shows typical results. In the repulsive regime, the ³⁹K mixture diffused, like a gas. In the attractive regime, the mixture acted like a liquid drop, maintaining its size for some 20 ms. If mean-field theory were the whole story, the ³⁹K liquid droplet would have collapsed. The consistency of its radius, according to analyses that go beyond the mean field, is attributable to quantum fluctuations.

With careful study of their drop images, Tarruell and colleagues could estimate the density of their liquids. Their result, about 10¹⁴ atoms per cubic centimeter, is five orders of magnitude below the density of an ideal gas at room temperature and pressure. (C. R. Cabrera et al., Science, in press, doi:10.1126/science.aao5686.)