  When a Josephson junction is exposed to microwave radiation, it undergoes the
inverse AC Josephson effect - the phase of the junction locks to the drive
frequency. As a result, the I-V curves of the junction acquire "Shapiro steps"
of quantized voltage. If the junction has three or more superconducting
contacts, coupling between different pairs of terminals must be taken into
account and the state of the junction evolves in a phase space of higher
dimensionality. Here, we study the multi-terminal inverse AC Josephson effect
in a graphene sample with three superconducting terminals. We observe robust
fractional Shapiro steps and correlated switching events, which can only be
explained by considering the device as a completely connected Josephson
network. We successfully simulate the observed behaviors using a modified
two-dimensional RCSJ model. Our results suggest multi-terminal Josephson
junctions are a playground to study highly-connected nonlinear networks with
novel topologies.
