Axion-like particles and circular polarisation of active galactic nuclei
  The measurements of the linear polarisation of visible light from quasars
give strong evidence for large-scale coherent orientations of their
polarisation vectors in some regions of the sky. We show that these
observations can be explained by the mixing of the photons with very light
pseudoscalar (axion-like) particles in extragalactic magnetic fields during
their propagation. We present a new treatment in terms of wave packets and
discuss the circular polarisation.
