  We present an electron interferometer defined purely by electrostatic gating
in encapsulated bilayer graphene. This minimizes possible sample degradation
introduced by conventional etching methods when preparing quantum devices. The
device quality is demonstrated by observing Aharonov-Bohm (AB) oscillations
with a period of h/e, h/2e, h/3e, and h/4e, witnessing a coherence length of
many microns. The AB oscillations as well as the type of carriers (electrons or
holes) are seamlessly tunable with gating. The coherence length longer than the
ring perimeter and semiclassical trajectory of the carrier are established from
the analysis of the temperature and magnetic field dependence of the
oscillations. Our gate-defined ring geometry has the potential to evolve into a
platform for exploring correlated quantum states such as superconductivity in
interferometers in twisted bilayer graphene.
