  We report on absolute magneto-transmission experiments on highly-doped
quasi-free-standing epitaxial graphene targeting the classical-to-quantum
crossover of the cyclotron resonance. This study allows us to directly extract
the carrier density and also other relevant quantities such as the
quasiparticle velocity and the Drude weight, which is precisely measured from
the strength of the cyclotron resonance. We find that the Drude weight is
renormalized with respect to its non-interacting (or
random-phase-approximation) value and that the renormalization is tied to the
quasiparticle velocity enhancement. This finding is in agreement with recent
theoretical predictions, which attribute the renormalization of the Drude
weight in graphene to the interplay between broken Galilean invariance and
electron-electron interactions.
