  Using the phenomena of linear and circular birefringence we propose a device
that can alter general elliptical polarization of a beam by a predetermined
amount, thereby allowing conversion between linearly-polarized light and
circularly polarized light or changes to the handedness of the polarization.
Based on an analogy with two-state adiabatic following of quantum optics, the
proposed device is insensitive to the frequency of the light -- it serves as an
achromatic polarization retarder.
