Quantum Rain Falls: Ultracold Atoms Unleash Liquid Secrets

In a groundbreaking experiment, physicists observed a classic liquid phenomenon—capillary instability—in a quantum gas for the first time.

By cooling a mix of potassium and rubidium atoms near absolute zero, researchers created tiny self-bound droplets that behave like liquid despite remaining in a gas phase. When stretched, these quantum droplets split into smaller ones, mimicking how a stream of water breaks into droplets.

Quantum Droplets and Capillary Instability Observed

In the Quantum Mixtures Lab at the National Institute of Optics (CNR-INO), researchers from CNR, the University of Florence, and the European Laboratory for Non-linear Spectroscopy (LENS) observed a well-known fluid phenomenon, capillary instability, within an unusual medium: an ultradilute quantum gas.

This discovery offers new insight into how matter behaves in extreme conditions and could lead to novel ways of manipulating quantum fluids. The study, published in Physical Review Letters, also included contributions from scientists at the Universities of Bologna, Padua, and the Basque Country (UPV/EHU).

The Physics Behind Capillary Instability

In classical physics, surface tension arises from cohesive forces between molecules in a liquid, causing the liquid to minimize its surface area. This effect is responsible for everyday occurrences like the formation of raindrops and soap bubbles. Surface tension also drives capillary instability (also known as Plateau-Rayleigh instability), in which a thin stream of liquid breaks up into droplets. Understanding this process is important in fields ranging from industrial design to biomedicine and nanotechnology.

Ultracold Gases and Liquid-Like Behavior

When atomic gases are cooled to temperatures near absolute zero, they begin to behave according to the rules of quantum mechanics. Under certain conditions, these ultracold gases can act like liquids, even though they technically remain in the gaseous phase. In recent years, scientists have learned how to precisely tune the interactions between atoms to create self-bound, liquid-like droplets. These droplets, stabilized by quantum effects, share several properties with classical liquid drops.

Breakup Dynamics in a Quantum Filament

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.

Classical Phenomenon, Quantum Frontier

“By combining experiment 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 depth understanding of this peculiar liquid phase and opens 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.

Reference: “Dynamical Formation of Multiple Quantum Droplets in a Bose-Bose Mixture” by L. Cavicchioli, C. Fort, F. Ancilotto, M. Modugno, F. Minardi and A. Burchianti, 7 March 2025, Physical Review Letters.
DOI: 10.1103/PhysRevLett.134.093401

This research was also possible thanks to the support of three initiatives financed by the Ministry of University and Research with European Union funds as part of the #NextGenerationEU program (PNRR – National Recovery and Resilience Plan): the “QUANTAMI” – PRIN 2022 project, the “National Quantum Science and Technology Institute” (NQSTI) partnership and the “Integrated Infrastructure Initiative in Photonic and Quantum Science” (IPHOQS) infrastructure.

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