  We show that free electrons can efficiently excite plasmons in doped graphene
with probabilities of order one per electron. More precisely, we predict
multiple excitations of a single confined plasmon mode in graphene
nanostructures. These unprecedentedly large electron-plasmon couplings are
explained using a simple scaling law and further investigated through a general
quantum description of the electron-plasmon interaction. From a fundamental
viewpoint, multiple plasmon excitations by a single electron provides a unique
tool for exploring the bosonic quantum nature of these collective modes. Our
study does not only open a viable path towards multiple excitation of a single
plasmon mode by single electrons, but it also reveals graphene nanostructures
as ideal systems for producing, detecting, and manipulating plasmons using
electron probes.
