  Atomic and atom-like defects in the solid-state are widely explored for
quantum computers, networks and sensors. Rare earth ions are an attractive
class of atomic defects that feature narrow spin and optical transitions that
are isolated from the host crystal, allowing incorporation into a wide range of
materials. However, the realization of long electronic spin coherence times is
hampered by magnetic noise from abundant nuclear spins in the most widely
studied host crystals. Here, we demonstrate that Er$^{3+}$ ions can be
introduced via ion implantation into TiO$_2$, a host crystal that has not been
studied extensively for rare earth ions and has a low natural abundance of
nuclear spins. We observe efficient incorporation of the implanted Er$^{3+}$
into the Ti$^{4+}$ site (40% yield), and measure narrow inhomogeneous spin and
optical linewidths (20 and 460 MHz, respectively) that are comparable to
bulk-doped crystalline hosts for Er$^{3+}$. This work demonstrates that ion
implantation is a viable path to studying rare earth ions in new hosts, and is
a significant step towards realizing individually addressed rare earth ions
with long spin coherence times for quantum technologies.
