▎ 摘　　要

The realization of p-n junctions in graphene, combined with the gapless and chiral nature of its massless Dirac fermions has led to the observation of many intriguing phenomena such as the quantum Hall effect in the bipolar regime, Klein tunnelling and Fabry-Perot interferences, all of which involve electronic transport across p-n junctions. Ballistic snake states propagating along the p-n junctions have been predicted to induce conductance oscillations, manifesting their twisting nature. However, transport studies along p-n junctions have so far only been performed in low mobility devices. Here, we report the observation of conductance oscillations due to ballistic snake states along a p-n interface in high-quality graphene encapsulated by hexagonal boron nitride. These snake states are exceptionally robust as they can propagate over 12 mu m, limited only by the size of our sample, and survive up to at least 120 K. The ability to guide carriers over a long distance provide a crucial building block for graphene-based electron optics.