We have new results on spectroscopy of thulium atoms implanted in solid argon and neon. This atom has it’s outer electronic shells “closed” – the active electrons are in the “submerged” 4f shell. This leads to unusually narrow optical spectra on the magnetic dipole transition at 1140 nm: we see lines as narrow as 500 MHz. You can read more details here: https://doi.org/10.1103/PhysRevB.108.214101