A research team involving the European Laboratory for Non-Linear Spectroscopy (LENS), the CNR National Institute of Optics in Sesto Fiorentino, and the Universities of Florence, Trieste and Trento has developed an innovative technique based on the propagation of sound waves to study matter under extreme conditions. The results have been published in Nature Physics.

The researchers developed an innovative technique to investigate the behaviour of quantum matter by exploiting a familiar phenomenon: the Doppler effect. When an ambulance passes us, its siren sounds higher-pitched as it approaches and lower-pitched as it moves away.

“In the same way, sound travelling through a rotating superfluid changes depending on whether it moves in the same direction as the superfluid or in the opposite direction. By detecting these minute variations, the team was able to reveal the internal structure of superfluid Fermi gases, systems made up of atoms cooled to temperatures close to absolute zero. Under these conditions, Fermi particles pair up, giving rise to superfluidity, a state of matter in which friction disappears,” explain Marcia Fròmeta Fernandez and Diego Hernandez-Rajkov, CNR-INO researchers and lead authors of the study.

“We created a ring-shaped Fermi superfluid in which sound waves travel simultaneously in opposite directions. When the superfluid is set in rotation, the two sound waves are affected differently by the flow and accumulate a tiny difference in frequency: the Doppler effect.”

By measuring this effect with extreme precision, “we demonstrated that the circulation of the superfluid around the ring cannot take arbitrary values, but only quantised, or discrete, values, determined by the ratio between Planck’s constant and the mass of the particles. In this case, these values are half those expected for individual particles,” explains Francesco Scazza, Professor of Condensed Matter Physics at the University of Trieste.

“Our observation shows that the superfluid consists of fermions that pair up, forming composite quantum units with twice the mass, known as Cooper pairs, similarly to what occurs in superconductors. The result confirms that, even in these ultracold gaseous systems, superfluidity originates from the formation of fermion pairs,” concludes Giacomo Roati, Research Director at CNR-INO and head of the experimental group.

The study confirms the potential of atom interferometry, a technique that exploits the wave-like properties of atoms to perform extremely precise measurements. In this study, the technique was implemented using sound waves in the superfluid, proving to be an effective tool for investigating strongly correlated quantum systems.

One of the first results obtained was the observation that superfluidity progressively decreases as temperature rises. The work opens up a new avenue for studying the collective behaviour of particles and gaining a deeper understanding of the mechanisms underlying superfluidity and superconductivity, as well as other still-mysterious dynamics of quantum matter that are of fundamental importance for modern quantum technologies,” conclude Massimo Inguscio of LENS and Sandro Stringari of the Pitaevskii BEC Center at the University of Trento.

Their contributions to the development of atomic physics and their scientific collaboration laid the foundations for the work carried out at the LENS laboratory. The study comes in the year marking the centenary of the statistics formulated by Enrico Fermi in 1926 on the Arcetri hills, at the University of Florence, to describe the behaviour of particles such as electrons, protons and neutrons.