  The study of impulsive astrophysical radio emission makes it possible to
probe the intervening plasma between the emission source and the Earth. In cold
electron-ion plasmas, the circular propagating wave modes primarily alter the
linear polarization plane that scales with the inverse-square of the emission
frequency. In relativistic plasmas, the wave modes are elliptically polarized,
and it is possible to convert linearly polarized emission into circular and
vice versa. Fast radio bursts (FRBs) enable the study of not only the
electron-ion plasma of the intergalactic medium but potentially the extreme
magneto-ionic medium in which these intense pulses are produced. Here we report
on the polarimetric analysis of a repeat burst from the FRB 20201124A source.
The burst displayed a significant frequency-dependent circularly polarized
component, unlike other bursts from this source or any other FRB found to date.
We model the frequency dependence of the circular polarization using a
phenomenological generalized Faraday rotation framework. We find that the
observed circular polarization in the burst can be adequately explained by the
propagation of an initially linearly polarised burst signal through a
relativistic plasma along the line of sight from the progenitor.
