We have applied experimental techniques that were developed for use in
atomic frequency standards and clocks to investigations of local Lore
ntz invariance, the linearity of quantum mechanics, and anomalous long
-range spin-dependent forces. These experiments used a hyperfine trans
ition in Be-9+ ions in a Penning trap. Recently, we have studied hyper
fine transitions in Hg-199+ ions in a linear rf trap. Hg+ ions might b
e used for similar investigations in the future.