  Controlling the energy flow processes and the associated energy relaxation
rates of a light emitter is of high fundamental interest, and has many
applications in the fields of quantum optics, photovoltaics, photodetection,
biosensing and light emission. While advanced dielectric and metallic systems
have been developed to tailor the interaction between an emitter and its
environment, active control of the energy flow has remained challenging. Here,
we demonstrate in-situ electrical control of the relaxation pathways of excited
erbium ions, which emit light at the technologically relevant telecommunication
wavelength of 1.5 $\mu$m. By placing the erbium at a few nanometres distance
from graphene, we modify the relaxation rate by more than a factor of three,
and control whether the emitter decays into either electron-hole pairs, emitted
photons or graphene near-infrared plasmons, confined to $<$15 nm to the sheet.
These capabilities to dictate optical energy transfer processes through
electrical control of the local density of optical states constitute a new
paradigm for active (quantum) photonics.
