  Interference of standing waves in electromagnetic resonators forms the basis
of many technologies, from telecommunications and spectroscopy to detection of
gravitational waves. However, unlike the confinement of light waves in vacuum,
the interference of electronic waves in solids is complicated by boundary
properties of the crystal, notably leading to electron guiding by atomic-scale
potentials at the edges. Understanding the microscopic role of boundaries on
coherent wave interference is an unresolved question due to the challenge of
detecting charge flow with submicron resolution. Here we employ Fraunhofer
interferometry to achieve real-space imaging of cavity modes in a graphene
Fabry-Perot resonator, embedded between two superconductors to form a Josephson
junction. By directly visualizing current flow using Fourier methods, our
measurements reveal surprising redistribution of current on and off resonance.
These findings provide direct evidence of separate interference conditions for
edge and bulk currents and reveal the ballistic nature of guided edge states.
Beyond equilibrium, our measurements show strong modulation of the multiple
Andreev reflection amplitude on an off resonance, a direct measure of the
gate-tunable change of cavity transparency. These results demonstrate that,
contrary to the common belief, electron interactions with realistic disordered
edges facilitate electron wave interference and ballistic transport.
