Observation of ballistic upstream modes at fractional quantum Hall edges of graphene

The structure of edge modes at the boundary of quantum Hall (QH) phases forms the basis for understanding low energy transport properties. In particular, the presence of upstream modes, moving against the direction of charge current flow, is critical for the emergence of renormalized modes with exotic quantum statistics. Here we discuss noise measurements performed at the edges of fractional QH (FQH) phases realized in dual graphite-gated bilayer graphene devices. A noiseless dc current is injected at one of the edge contacts, and the noise generated at contacts at L=4µm or 10µm away along the upstream direction is studied. For integer and particle-like FQH states, no detectable noise is measured. By contrast, for hole-conjugate FQH states, we detect a strong noise proportional to the injected current, unambiguously proving the existence of upstream modes. The noise magnitude remaining independent of length together with a remarkable agreement with our theoretical analysis demonstrates the ballistic nature of upstream energy transport, quite distinct from the diffusive propagation reported earlier in GaAs-based systems. Our investigation opens the door to the study of upstream transport in more complex geometries and in edges of non-Abelian phases in graphene.