  Photonic quantum technologies$^1$, with applications in quantum
communication, sensing as well as quantum simulation and computing, are on the
verge of becoming commercially available. One crucial building block are
tailored nanoscale integratable quantum light sources, matching the specific
needs of use-cases. Several different approaches to realize solid-state quantum
emitters$^2$ with high performance$^3$ have been pursued. However, the
properties of the emitted single photons are always defined by the individual
quantum light source and despite numerous quantum emitter tuning
techniques$^{4-7}$, scalability is still a major challenge. Here we show an
emitter-independent method to tailor and control the properties of the single
photon emission. We demonstrate a laser-controlled down-conversion process from
an excited state of a quantum three-level system$^8$. Starting from a biexciton
state, a tunable control laser field defines a virtual state in a stimulated
process. From there, spontaneous emission to the ground state leads to
optically controlled single photon emission. Based on this concept, we
demonstrate energy tuning of the single photon emission with a control laser
field. The nature of the involved quantum states furthermore provides a unique
basis for the future control of polarization and bandwidth, as predicted by
theory$^{9,10}$. Our demonstration marks an important step towards tailored
single photon emission from a photonic quantum system based on quantum optical
principles.
