Step inside the strange world of a superfluid, a liquid that can flow endlessly without friction, defying the common-sense rules we experience every day, where water pours, syrup sticks and coffee swirls and slows under the effect of viscosity. In these extraordinary fluids, motion often organizes itself into quantized vortices: tiny, long-lived whirlpools that act as the fundamental building blocks of superfluid flow.
An international study conducted at the European Laboratory for Non-Linear Spectroscopy (LENS), involving researchers from CNR-INO, the Universities of Florence, Bologna, Trieste, Augsburg, and the Warsaw University of Technology, has embarked on this journey by investigating the dynamics of vortices within strongly interacting superfluids, uncovering the fundamental mechanisms that govern their behavior. Using ultracold atomic gases, the scientists open a unique window into this exotic realm, recreating conditions similar to those found in superfluid helium-3, the interiors of neutron stars, and superconductors.
"In a superfluid, vortices are stable objects because decay is suppressed," says Nicola Grani, PhD in Physics and Astronomy at the University of Florence and first author of the publication. "However, dissipation of the superfluid flow can still arise from internal microscopic forces acting directly on the vortices. These forces arise from the interplay between the superfluid and normal components in finite-temperature superfluids, giving rise to the so-called mutual friction. Vortices therefore play a crucial role in determining the efficiency of current transport, and their dynamics can be used as a sensitive probe of the microscopic mechanisms governing mutual friction.
The researchers have studied vortices in a strongly interacting fermionic superfluid gas of lithium atoms cooled to just ten billionths of a degree above absolute zero. In conventional systems, such as superfluid liquid helium or solid-state superconductors,
experiments typically involve large numbers of interacting vortices arranged in complex and hard-to-control configurations. Ultracold atomic gases, by contrast, provide an exceptionally clean and highly programmable platform, enabling unprecedented control over vortex behavior.
"For this study, we used laser light to precisely excite quantized vortices by moving an optical potential, allowing us to engineer arbitrary vortex configurations," explains Diego Hernández-Rajkov, a researcher at CNR-INO at LENS. "This level of control allows us to study the dynamics of a single vortex." Using this approach, the researchers observed the spiraling motion of a vortex orbiting around another vortex pinned at the center of a disk-shaped superfluid. "By analyzing the vortex trajectories, we reconstructed the microscopic processes that regulate vortex motion and we directly access to its internal structure," adds Diego Hernández-Rajkov. "The analysis revealed that, in the explored regime, vortex dynamics are influenced by quasiparticles trapped within the vortex core occupaying the so-called Caroli-de Gennes Matricon states, providing the first indirect experimental evidence of their presence in this regime."
The study, published in Nature Communications, opens new perspectives for understanding vortex dynamics in superfluids and superconductors. According to Giacomo Roati, LENS member, CNR-INO Director of Research, and head of the research group,
"Ultracold atomic gases provide a unique platform to explore the exotic behaviors of superfluids. By precisely controlling these gases in the laboratory, we can recreate and study their fundamental properties in ways that are difficult or impossible in other systems. Understanding how these vortices move is essential for controlling energy dissipation and designing highly efficient new quantum devices. Our platform can also be extended to study systems with many interacting vortices, opening the door to controlled investigations of superfluid turbulence, a phenomenon with implications across physics, from fundamental research to advanced technology."
