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