  The prospect of realizing building blocks for long-distance quantum
communication is a major driving force for the development of advanced
nanophotonic devices. Significant progress has been achieved in this field with
respect to the fabrication of efficient quantum-dot-based single-photon
sources. More recently, even spin-photon entanglement and quantum teleportation
have been demonstrated in semiconductor systems. These results are considered
as crucial steps towards the realization of a quantum repeater. The related
work has almost exclusively been performed on self-assembled quantum dots (QDs)
and random device technology. At this point it is clear that further progress
in this field towards real applications will rely crucially on deterministic
device technologies which will, for instance, enable the processing of bright
quantum light sources with pre-defined emission energy. Here we report on
enhanced photon-extraction efficiency from monolithically integrated
microlenses which are coupled deterministically to single QDs. The microlenses
with diameters down to 800 nm were aligned to single QDs by in-situ
electron-beam lithography using a low-temperature cathodoluminescence setup.
This deterministic device technology allowed us to obtain an enhancement of
photon extraction efficiency for QDs integrated into microlenses as compared to
QDs in unstructured surfaces. The excellent optical quality of the structures
is demonstrated by cathodoluminescence and micro-photoluminescence
spectroscopy. A Hong-Ou-Mandel experiment states the emission of single
indistinguishable photons.
