  Confined to a two-dimensional plane, electrons in a strong magnetic field
travel along the edge in one-dimensional quantum Hall channels that are
protected against backscattering. These channels can be used as solid-state
analogues of monochromatic beams of light, providing a unique platform for
studying electron interference. Electron interferometry is regarded as one of
the most promising routes for studying fractional and non-Abelian statistics
and quantum entanglement via two-particle interference. However, creating an
edge-channel interferometer in which electron-electron interactions play an
important role requires a clean system and long phase coherence lengths. Here
we realize electronic Mach-Zehnder interferometers with record visibilities of
up to 98% using spin- and valley-polarized edge channels that co-propagate
along a PN junction in graphene. We find that inter-channel scattering between
same-spin edge channels along the physical graphene edge can be used to form
beamsplitters, while the absence of inter-channel scattering along gate-defined
interfaces can be used to form isolated interferometer arms. Surprisingly, our
interferometer is robust to dephasing effects at energies an order of magnitude
larger than observed in pioneering experiments on GaAs/AlGaAs quantum wells.
Our results shed light on the nature of edge-channel equilibration and open up
new possibilities for studying exotic electron statistics and quantum
phenomena.
