Micron scale ballistic Josephson junctions in edge-contacted graphene

Despite recent improvements in the electronic quality of graphene, it has remained challenging to make superconducting contacts to it while preserving its high quality. Here, we integrate monolayer graphene encapsulated in hexagonal Boron Nitride with a type-II superconductor (Molybdenum Rhenium - MoRe) via one-dimensional contacts along the edge of the graphene. We observe gate-tunable supercurrents over distances as long as 1.5~$\mu m$. Ballistic, phase coherent transport in these devices causes the switching current to oscillate periodically with the Fermi wave number, thus providing clear evidence of a ballistic Josephson junction. Furthermore, the large critical field of MoRe allows us to resolve several broken symmetry states in the quantum Hall regime, while the MoRe remains superconducting.