 Recent proposals for space-borne gravitational wave detectors based on atom interferometry rely on extremely narrow single photon transition lines as featured by alkaline earth metals or atomic species with similar electronic configuration, but differ in key parameters such as abundance of isotopes, atomic flux, density and temperature regimes, achievable expansion rates, density limitations set by interactions, as well as technological and operational requirements. This article was authored by S. Lauriani, D. Schlippert, C. Schubert and others.