Rare-earth-doped crystals can be attractive materials for quantum information processing, because
of the long coherence times that can be expected, in particular, from non-Kramers ions. In this paper,
Ho3+-doped yttrium and lutetium vanadate single crystals have been
investigated using linear and coherent optical spectroscopy. For
Ho3+:YVO4, the crystal-field
levels of the 5I8, 5F5, 5F4
and 5S2
multiplets have been determined and compared with crystal-field level
calculations. This allowed us to unambiguously assign most of the observed
transitions, although some results suggest that the site symmetry of the
Ho3+ ion could deviate from
D2d. Similar conclusions
were reached for Ho3+:LuVO4. Hole burning measurements indicate that the coherence time of the
5I8–5F5
optical transitions is rather short in both compounds (around 40 ns). Assuming that the
coherence is limited by spin interactions, this is accounted for by the high nuclear moment
of the nearby vanadium ions, since the large crystal-field level splittings of the
5I8
and 5F5
multiplets do not favour a large enhanced nuclear Zeeman effect.