Shot noise does not always provide the quasiparticle charge

The fractional charge of quasiparticles is a fundamental feature of quantum Hall effect states. The charge—important in characterizing the state and in interference experiments—has long been measured via shot noise at moderate temperatures, with the Fano factor revealing the charge of the quasiparticles. However, at sufficiently low temperatures of ~10 mK, we previously found that the Fano factor is instead equal to the bulk filling factor. Noise with this pattern was also observed on intermediate conductance plateaux in the transmission of the quantum point contact, where shot noise is not expected. Here, we extend this low-temperature behaviour of the Fano factor to a situation where the edge modes do not sit at the physical edge of the device but instead reside in an artificially constructed interface at the boundary between two adjoining quantum Hall effect states: the tested state and a different state. We attribute the unexpected shot noise behaviour to upstream neutral modes that proliferate at the lowest spinless Landau level. We present a theoretical approach based on an interplay between charge and neutral modes that hints at the origin of the universal Fano factor.