High resolution spectroscopy of thulium atoms implanted in solid noble gas crystals

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 $0.6\phantom{\rule{4.pt}{0ex}}\text{GHz}$, which temperature dependence and pump-probe spectroscopy indicate is likely a homogeneous effect, suggesting inhomogeneity is well below the GHz scale.