  Aharonov-Bohm oscillations are observed in a graphene quantum ring with a top
gate covering one arm of the ring. As graphene is a gapless semiconductor this
geometry allows to study not only the quantum interference of electrons with
electrons or holes with holes but also the unique situation of quantum
interference between electrons and holes. The period and amplitude of the
observed Aharonov-Bohm oscillations are independent of the sign of the applied
gate voltage showing the equivalence between unipolar and dipolar interference.
