  Optically active defects in solid-state systems have many applications in
quantum information and sensing. However, unlike free atoms, which have fixed
optical transition frequencies, the inhomogeneous broadening of the transitions
in solid-state environments limit their use as identical scatterers for such
applications. Here we show that crystals of argon and neon prepared in a
closed-cycle cryostat doped with thulium atoms at cryogenic temperatures are an
exception. High resolution absorption and emission spectroscopy show that the
1140 nm magnetic dipole transition is split into multiple components. The
origin of this splitting is likely a combination of different classes of
trapping sites, crystal field effects within each site, and hyperfine
interactions. The individual lines have ensemble widths as small as 0.6 GHz,
which temperature dependence and pump-probe spectroscopy indicate is likely a
homogeneous effect, suggesting inhomogeneity is well below the GHz scale.
