Universality of non-equilibrium fluctuations in strongly correlated quantum liquids

Abstract

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-particles^{8,10,11,12,13,14,15,16,17}. 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.