  Owing to a linear and gapless band structure and a tunability of the charge
carrier type, graphene offers a unique system to investigate transport of Dirac
Fermions at p-n junctions (PNJs). In a magnetic field, combination of quantum
Hall physics and the characteristic transport across PNJs leads to a
fractionally quantized conductance associated with the mixing of electron-like
and hole-like modes and their subsequent partitioning. The mixing and
partitioning suggest that a PNJ could be used as an electronic beam-splitter.
Here we report the shot noise study of the mode mixing process and demonstrate
the crucial role of the PNJ length. For short PNJs, the amplitude of the noise
is consistent with an electronic beam-splitter behavior, whereas, for longer
PNJs, it is reduced by the energy relaxation. Remarkably, the relaxation length
is much larger than typical size of mesoscopic devices, encouraging using
graphene for electron quantum optics and quantum information processing.
