  Non-equilibrium quantum rings emit circular polarized subterahertz radiation
with a polarization degree controllable on nano- to picosecond time scales.
This we conclude using a theory developed here for the time-dependent detection
of the circular polarization of polychromatic radiations, valid for time scales
comparable to the reciprocal of characteristic emission frequencies. The theory
is applied to driven quantum rings whereby the influence of radiative and
non-radiative processes on the properties of the emitted light is incorporated.
