  We report on recent results showing that the geometric phase can be used as a
tool in the analysis of many different physical systems, as mixed boson
systems, CPT and CP violations, Unruh effects and thermal states. We show that
the geometric phases appearing in the time evolution of mixed meson systems
like $B_{s}^{0}-\bar{B}_{s}^{0}$ and the $K^{0}-\bar{K}^{0}$ are linked to the
parameter $z$ describing the $CPT$ violation. A non zero phase difference
between particle and antiparticle arises only in the presence of $CPT$ symmetry
breaking. Then the geometric phase can represent a completely new test for the
$CPT$ invariance. Moreover, we study the geometric phase of systems represented
by mixed state and undergoing a nonunitary evolution and propose the
realization of interferometers which can prove the existence of the Unruh
effect and can allow very precise measurements of temperature. We are glad to
dedicate this paper to Professor Gaetano Vilasi in the occasion of his 70th
birthday.
