For the first time, a study conducted by researchers at the CNR-INO has revealed the hidden structure of ultracold atomic Fermi gases using a technique based on the Doppler effect of sound waves, the same effect that produces two-tone ambulance sirens. The experiment, conducted at the LENS in Sesto Fiorentino, also involved researchers from the Universities of Florence, Trieste, and Trento.

Published in the journal Nature Physics, the study presents an innovative technique for investigating quantum matter by exploiting the Doppler effect, a common phenomenon whereby we perceive an ambulance siren as higher-pitched when approaching, and as lower-pitched when moving away.

“Similarly, the sound propagating in a rotating superfluid changes depending on whether it moves in the same or opposite direction to the superfluid,” explain Marcia Fròmeta Fernandez and Diego Hernandez-Rajkov, CNR-INO researchers and lead authors of the paper. Thanks to these tiny variations, the team was able to reveal the internal structure of superfluid Fermi gases, systems composed of atoms cooled to temperatures close to absolute zero. Under these conditions, Fermi particles pair, giving rise to superfluidity, a state of matter in which friction is eliminated. We created a ring-shaped Fermi superfluid, in which sound waves propagate simultaneously in opposite directions. When rotated, the sound waves are affected differently by the flow, accumulating a very small difference in frequency: the Doppler effect,” the researchers conclude.

“Our observation,” adds Giacomo Roati, CNR-INO research director and leader of the experimental group, “demonstrates that the superfluid is composed of fermions that pair up in pairs, forming composite quantum units of double mass, so-called Cooper pairs, similar to what happens in superconductors.”

The work opens new perspectives for investigating the mechanisms underlying superfluidity and superconductivity, offering new tools to explore the dynamics of quantum matter, still mysterious and fundamental to the development of modern quantum technologies.

This result comes in a particularly significant year: the centenary of the formulation of statistics devised by Enrico Fermi in 1926 on the hills of Arcetri, at the University of Florence, to describe the behavior of particles such as electrons, protons, and neutrons.