Scientists observe the first ‘quantum rain’

In the Quantum Mixtures Lab of the National Institute of Optics (Cnr-Ino), a team of researchers from Cnr, the University of Florence and the European Laboratory for Non-linear Spectroscopy (LENS) observed the phenomenon of capillary instability in an unconventional liquid: an ultradilute quantum gas. This result has important implications for the understanding and manipulation of new forms of matter.

The research, published in Physical Review Letters, also involved researchers from the Universities of Bologna, Padua, and the Basque Country (UPV/EHU).

In physics, it is known that the surface tension of a liquid, caused by intermolecular cohesive forces, tends to minimize the surface area. This mechanism is responsible for macroscopic phenomena such as the formation of raindrops or soap bubbles.

Surface tension is at the origin of capillary instability, also known as Plateau-Rayleigh instability, whereby a thin liquid jet breaks, forming a sequence of drops. The study and understanding of this phenomenon has important implications in the industrial, biomedical and nanotechnology fields.

In atomic gases cooled to temperatures close to absolute zero, atoms lose their individuality and obey the laws of quantum mechanics. These systems under certain conditions behave like liquids, although they remain in the gaseous phase.

Indeed, for some years now, scientists have been able, by precisely controlling the interatomic interactions, to create self-bound, liquid-like droplets from ultracold gases. These small clusters of atoms, stabilized by quantum effects, share properties with classical liquid drops.

By means of imaging and optical manipulation techniques, the experimental team, led by Alessia Burchianti (Cnr-Ino researcher), studied the dynamical evolution of a single quantum droplet created from an ultracold mixture of potassium and rubidium atoms.

The droplet released in an optical waveguide elongates, forming a filament, which, above a critical length, breaks up into smaller droplets. The number of sub-droplets is proportional to the length of the filament at the breaking time.

“By combining experiments and numerical simulations, it was possible to describe the breakup dynamics of a quantum droplet in terms of capillary instability. The Plateau–Rayleigh instability is a common phenomenon in classical liquids, also observed in superfluid helium, but not yet in atomic gases,” says Chiara Fort (UNIFI researcher), who contributed to the research.

“The measurements conducted in our laboratory provide a deep understanding of this peculiar liquid phase and open a pathway for creating arrays of quantum droplets for future applications in quantum technologies,” adds Luca Cavicchioli (Cnr-Ino researcher), first author of the article.