  Hybridizing superconductivity with the quantum Hall (QH) effects has major
potential for designing novel circuits capable of inducing and manipulating
non-Abelian states for topological quantum computation. However, despite recent
experimental progress towards this hybridization, concrete evidence for a
chiral QH Josephson junction -- the elemental building block for coherent
superconducting-QH circuits -- is still lacking. Its expected signature is an
unusual chiral supercurrent flowing in QH edge channels, which oscillates with
a specific $2\phi_0$ magnetic flux periodicity ($\phi_0=h/2e$ is the
superconducting flux quantum, $h$ the Planck constant and $e$ the electron
charge). Here, we show that ultra-narrow Josephson junctions defined in
encapsulated graphene nanoribbons exhibit such a chiral supercurrent, visible
up to 8 teslas, and carried by the spin-degenerate edge channel of the QH
plateau of resistance $h/2e^2\simeq 12.9$ k$\Omega$. We observe reproducible
$2\phi_0$-periodic oscillation of the supercurrent, which emerges at constant
filling factor when the area of the loop formed by the QH edge channel is
constant, within a magnetic-length correction that we resolve in the data.
Furthermore, by varying the junction geometry, we show that reducing the
superconductor/normal interface length is pivotal to obtain a measurable
supercurrent on QH plateaus, in agreement with theories predicting dephasing
along the superconducting interface. Our findings mark a critical milestone
along the path to explore correlated and fractional QH-based superconducting
devices that should host non-Abelian Majorana and parafermion zero modes.
