  Interference and tunneling are two signature quantum effects that are often
perceived as the yin and yang of quantum mechanics: particle simultaneously
propagating along several distinct classical paths versus particle penetrating
through a classically inaccessible region via a single least-action path. Here
we demonstrate that the Dirac quasiparticles in graphene provide a dramatic
departure from this paradigm. We show that Zener tunneling in gapped bilayer
graphene (BLG), which governs transport through p-n heterojunctions, exhibits
common-path interference that takes place under the tunnel barrier. Due to a
symmetry peculiar to the BLG bandstructure, interfering tunneling paths form
`conjugate pairs', giving rise to high-contrast oscillations in transmission as
a function of the gate-tunable bandgap and other control parameters of the
junction. The common-path interference is solely due to forward-propagating
waves; in contrast to Fabry-Perot-type interference in resonant tunneling
structures it does not rely on multiple backscattering. The oscillations
manifest themselves in the junction I-V characteristic as N-shaped branches
with negative differential conductivity, enabling new high-speed active-circuit
devices with architectures which are not available in electronic semiconductor
devices.
