  Graphene is a very promising test-bed for the field of electron quantum
optics. However, a fully tunable and coherent electronic beam splitter is still
missing. We report the demonstration of electronic beam splitters in graphene
that couple quantum Hall edge channels having opposite valley polarizations.
The electronic transmission of our beam splitters can be tuned from zero to
near unity. By independently setting the beam splitters at the two corners of a
graphene PN junction to intermediate transmissions, we realize a fully tunable
electronic Mach-Zehnder interferometer. This tunability allows us to
unambiguously identify the quantum interferences due to the Mach-Zehnder
interferometer, and to study their dependence with the beam-splitter
transmission and the interferometer bias voltage. The comparison with
conventional semiconductor interferometers points towards universal processes
driving the quantum decoherence in those two different 2D systems, with
graphene being much more robust to their effect.
