  Optical nonlinearities offer unique possibilities for the control of light
with light. A prominent example is electromagnetically induced transparency
(EIT) where the transmission of a probe beam through an optically dense medium
is manipulated by means of a control beam. Scaling such experiments into the
quantum domain with one, or just a few particles of both light and matter will
allow for the implementation of quantum computing protocols with atoms and
photons or the realisation of strongly interacting photon gases exhibiting
quantum phase transitions of light. Reaching these aims is challenging and
requires an enhanced matter-light interaction as provided by cavity quantum
electrodynamics (QED). Here we demonstrate EIT with a single atom
quasi-permanently trapped inside a high-finesse optical cavity. The atom acts
as a quantum-optical transistor with the ability to coherently control the
transmission of light through the cavity. We furthermore investigate the
scaling of EIT when the atom number is increased one by one. The measured
spectra are in excellent agreement with a theoretical model. Merging EIT with
cavity QED and single quanta of matter is likely to become the cornerstone for
novel applications, e.g. the dynamic control of the photon statistics of
propagating light fields or the engineering of Fock-state superpositions of
flying light pulses.
