  A new method to generate short wavelength Free Electron Laser output with
modulated polarisation at attosecond timescales is presented. Simulations
demonstrate polarisation switching timescales that are four orders of magnitude
faster than the current state of the art and, at X-Ray wavelengths, approaching
the atomic unit of time of approximately $24$~attoseconds. Such polarisation
control has significant potential in the study of ultra-fast atomic and
molecular processes. The output alternates between either orthogonal linear or
circularly polarised light without the need for any polarising optical
elements. This facilitates operation at the high brightness X-ray wavelengths
associated with FELs. As the method uses an afterburner configuration it would
be relatively easy to install at exciting FEL facilities, greatly expanding
their research capability.
