QUROPE Quantum Information Processing and Communication in Europe

The conductance of a quantum point contact is quantized in units of 1/h, with h being Planck's constant, which is the universal upper bound to transport set by Heisenberg's and Pauli's principles. Can interactions cause a breakdown of this quantization? Here we answer this question using a cold atom quantum simulation. Our simulation yields the spin and particle conductance of a Fermi gas flowing through a single mode quantum point contact as a function of the strength of attractive interactions. Spin conductance exhibits a broad maximum when varying the chemical potential at moderate interactions. In contrast, the particle conductance is unexpectedly enhanced. It shows non-universal quantization with values of the conductance plateaux ranging from 1/h to 4/h, as the interaction strength is increased from weak to intermediate. For strong interactions, the particle conductance plateaux disappear and the spin conductance gets gradually suppressed. In that regime we interpret our data as demonstrating the emergence of a superfluid with a spin-insulating character. Our observations document the breakdown of universal conductance quantization as many-body correlations appear. The anomalous quantization is incompatible with a Fermi liquid description, shedding new light on the nature of the strongly attractive Fermi gas in the normal phase.