  Pseudoscalar-photon interactions were proposed in the study of the relations
among equivalence principles. The interaction of pseudoscalar axion with gluons
was proposed as a way to solve the strong CP problem. Subsequent proposal of
axion as a dark matter candidate has been a focus of search. Motivation from
superstring theories add to its importance. After a brief introduction and
historical review, we present (i) the current status of our optical experiment
using high-finesse Fabry-Perot resonant cavity - Q & A experiment - to detect
pseudoscalar-photon interactions, (ii) the constraints on pseudoscalar-photon
interactions from astrophysical and cosmological observations on cosmic
polarization rotation, and (iii) theoretical models of non-minimal interactions
of gravitational, electromagnetic and pseudoscalar (axion) fields, and their
relevance to cosmology.
