Universality of non-equilibrium fluctuations in strongly correlated quantum liquids

Quantum liquids at equilibrium follow Fermi liquid theory, but less is known about non-equilibrium conditions. Carbon nanotubes, which exhibit universal scaling behaviour, provide a testbed for many-body physics beyond equilibrium. Interacting quantum many-body systems constitute a fascinating research field because they form quantum liquids with remarkable properties and universal behaviour1. In fermionic systems, such quantum liquids are realized in helium-3 liquid, heavy fermion systems1, neutron stars and cold gases2. Their properties in the linear-response regime have been successfully described by the theory of Fermi liquids1. The idea is that they behave as an ensemble of non-interacting ‘quasi-particles’. However, non-equilibrium properties have still to be established and remain a key issue of many-body physics. Here, we show a precise experimental demonstration of Landau Fermi liquid theory extended to the non-equilibrium regime in a zero-dimensional system. Combining transport and ultra-sensitive current noise measurements, we have unambiguously identified the SU(2) (ref. 3) and SU(4) (refs 4,5,6,7,8) symmetries of a quantum liquid in a carbon nanotube tuned in the universal Kondo regime. Whereas the free quasi-particle picture is found valid around equilibrium9, an enhancement of the current fluctuations is detected out of equilibrium and perfectly explained by an effective charge induced by the residual interaction between quasi-particles8,10,11,12,13,14,15,16,17. Moreover, an as-yet-unknown scaling law for the effective charge is discovered, suggesting a new non-equilibrium universality. Our method paves a new way to explore the exotic nature of quantum liquids out of equilibrium through their fluctuations in a wide variety of physical systems18.