  Quantum dots tuned to atomic resonances represent an emerging field of hybrid
quantum systems where the advantages of quantum dots and natural atoms can be
combined. Embedding quantum dots in nanowires boosts these systems with a set
of powerful possibilities, such as precise positioning of the emitters,
excellent photon extraction efficiency and direct electrical contacting of
quantum dots. Notably, nanowire structures can be grown on silicon substrates,
allowing for a straightforward integration with silicon-based photonic devices.
In this work we show controlled growth of nanowire-quantum-dot structures on
silicon, frequency tuned to atomic transitions. We grow GaAs quantum dots in
AlGaAs nanowires with a nearly pure crystal structure and excellent optical
properties. We precisely control the dimensions of quantum dots and their
position inside nanowires, and demonstrate that the emission wavelength can be
engineered over the range of at least $30\,nm$ around $765\,nm$. By applying an
external magnetic field we are able to fine tune the emission frequency of our
nanowire quantum dots to the $D_{2}$ transition of $^{87}$Rb. We use the Rb
transitions to precisely measure the actual spectral linewidth of the photons
emitted from a nanowire quantum dot to be $9.4 \pm 0.7 \mu eV$, under
non-resonant excitation. Our work brings highly-desirable functionalities to
quantum technologies, enabling, for instance, a realization of a quantum
network, based on an arbitrary number of nanowire single-photon sources, all
operating at the same frequency of an atomic transition.
