Angular momentum of rotating fermionic superfluids by Sagnac phonon interferometry

Year: 2026

Authors: Fernandez M.F., Hernandez-Rajkov D., Del Pace G., Grani N., Inguscio M., Scazza F., Stringari S., Roati G.

Autors Affiliation: Univ Florence, European Lab Nonlinear Spect, Sesto Fiorentino, Italy; CNR, INO, LENS, Sesto Fiorentino, Italy; INFN, Sez Firenze, Sesto Fiorentino, Italy; Univ Florence, Dept Phys, Sesto Fiorentino, Italy; Univ Trieste, Dept Phys, Trieste, Italy; CNR, INO, Trieste, Italy; Univ Trento, Pitaevskii BEC Ctr, CNR INO, Trento, Italy; Univ Trento, Dipartimento Fis, Trento, Italy; INFN, Trento Inst Fundamental Phys & Applicat, Trento, Italy.

Abstract: Fermionic many-body systems provide a setting to investigate how interactions drive collective quantum behaviour, including macroscopic coherence and superfluidity. Central to these phenomena is the formation of Cooper pairs, correlated states of two fermions that behave as composite bosons and condense below a critical temperature. Unlike elementary bosons, these pairs retain an internal structure determined by the underlying fermionic correlations, which is essential for understanding superfluid properties across the crossover from Bose-Einstein condensation to Bardeen-Cooper-Schrieffer crossover. Here we use a sonic analogue of the optical Sagnac effect to probe the composite nature of fermionic condensates across this crossover. We realize an in situ loop interferometer by coherently exciting two counter-propagating long-wavelength phonons of an annular fermionic superfluid with tuneable interactions. By injecting a quantized supercurrent into the superfluid ring, we lift the frequency degeneracy between clockwise and anticlockwise sound modes. The resulting Doppler shift allows us to probe the elementary quantum of circulation and the angular momentum per particle in the fermionic fluid. Our observations reveal that superflow circulation is quantized in units determined by fermion pairs, providing access to the superfluid fraction of the unitary Fermi gas in the low-temperature regime. Our results establish phonon interferometry as a probe of strongly correlated quantum systems.

Journal/Review: NATURE PHYSICS

More Information: G.R. and G.D.P. acknowledge financial support from the PNRR MUR project PE0000023-NQSTI. G.R. acknowledges funding from the Italian Ministry of University and Research under the PRIN2017 project CEnTraL and project CNR-FOE-LENS-2024. S.S. acknowledges support from the Provincia Autonoma di Trento. F.S. acknowledges support from the EU under the Horizon 2020 research and innovation programme (project OrbiDynaMIQs, GA no. 949438). We acknowledge support from the European Union – NextGenerationEU within the ’Integrated Infrastructure Initiative in Photonics and Quantum Sciences’ (I-PHOQS). We acknowledge funding from INFN through the RELAQS project. This publication has received funding under the Horizon Europe programme HORIZON-CL4-2022-QUANTUM-02-SGA (project PASQuanS2.1, GA no. 101113690) and Horizon 2020 research and innovation programme (GA no. 871124).
KeyWords: Quantized Circulation; 2nd Sound; Helium-ii; Vortices; Propagation; Transition; Flux; Gas
DOI: 10.1038/s41567-026-03349-6