  Sources of single photons are key elements in the study of basic quantum
optical concepts and applications in quantum information science. Among the
different sources available, semiconductor quantum dots excel with their
straight forward integrability in semiconductor based on-chip solutions and the
potential that photon emission can be triggered on demand. Usually, the photon
emission event is part of a cascaded biexciton-exciton emission scheme.
Important properties of the emitted photon such as polarization and time of
emission are either probabilistic in nature or pre-determined by electronic
properties of the system. In this work, we study the direct two-photon emission
from the biexciton. We show that emission through this higher-order transition
provides a much more versatile approach to generate a single photon. In the
scheme we propose, the two-photon emission from the biexciton is enabled by a
laser field (or laser pulse) driving the system into a virtual state inside the
band gap. From this intermediate virtual state, the single photon of interest
is then spontaneously emitted. Its properties are determined by the driving
laser pulse, enabling all-optical on-the-fly control of polarization state,
frequency, and time of emission of the photon.
