  We study the resonant optical transitions of a single nitrogen-vacancy (NV)
center that is coherently dressed by a strong mechanical drive. Using a
gigahertz-frequency diamond mechanical resonator that is strain-coupled to an
NV center's orbital states, we demonstrate coherent Raman sidebands out to the
ninth order and orbital-phonon interactions that mix the two excited-state
orbital branches. These interactions are spectroscopically revealed through a
multi-phonon Rabi splitting of the orbital branches which scales as a function
of resonator driving amplitude, and is successfully reproduced in a quantum
model. Finally, we discuss the application of mechanical driving to engineering
NV center orbital states.
