  The advent of few-layer graphenes has given rise to a new family of
two-dimensional systems with emergent electronic properties governed by
relativistic quantum mechanics. The multiple carbon sublattices endow the
electronic wavefunctions with pseudospin, a lattice analog of the relativistic
electron spin, while the multilayer structure leads to electric field effect
tunable electronic bands. Here we use these properties to realize giant
conductance oscillations in ballistic trilayer graphene Fabry-Perot
interferometers, which result from phase coherent transport through resonant
bound states beneath an electrostatic barrier. We cloak these states by
selectively decoupling them from the leads, resulting in transport via
non-resonant states and suppression of the giant oscillations. Cloaking is
achieved both classically, by manipulating quasiparticle momenta with a
magnetic field, and quantum mechanically, by locally varying the pseudospin
character of the carrier wavefunctions. Our results illustrate the unique
potential of trilayer graphene as a versatile platform for electron optics and
pseudospintronics.
