  Single quantum emitters embedded in solid-state hosts are an ideal platform
for realizing quantum information processors and quantum network nodes. Among
the currently-investigated candidates, Er$^{3+}$ ions are particularly
appealing due to their 1.5 $\mu$m optical transition in the telecom band as
well as their long spin coherence times. However, the long lifetimes of the
excited state -- generally in excess of 1 ms -- along with the inhomogeneous
broadening of the optical transition result in significant challenges. Photon
emission rates are prohibitively small, and different emitters generally create
photons with distinct spectra, thereby preventing multi-photon interference --
a requirement for building large-scale, multi-node quantum networks. Here we
solve this challenge by demonstrating for the first time linear Stark tuning of
the emission frequency of a single Er$^{3+}$ ion. Our ions are embedded in a
lithium niobate crystal and couple evanescently to a silicon nano-photonic
crystal cavity that provides an up to 143 increase of the measured decay rate.
By applying an electric field along the crystal c-axis, we achieve a Stark
tuning greater than the ion's linewidth without changing the single-photon
emission statistics of the ion. These results are a key step towards rare earth
ion-based quantum networks.
