  We have studied quantum interference between electrons and holes in a
split-ring gold interferometer with graphene arms, one of which contained a pn
junction. The carrier type, the pn junction and the phase of the oscillations
in a magnetic field were controlled by a top gate placed over one of the arms.
We observe clear Aharonov-Bohm oscillations at the Dirac point and away from
it, regardless of the carrier type in each arm. We also find clear oscillations
when one arm of the interferometer contains a single pn junction, allowing us
to study the interplay of Aharonov-Bohm effect and Klein tunneling.
