  Quantum electro dynamics (QED) comprises virtual particle production and thus
gives rise to a refractive index of the vacuum larger than unity in the
presence of a magnetic field. This predicted effect has not been measured to
date, even after considerable effort of a number of experiments. It has been
proposed by other authors to possibly use gravitational wave detectors for such
vacuum QED measurements, and we give this proposal some new consideration in
this paper. In particular we look at possible source field magnet designs and
further constraints on the implementation at a gravitational wave detector. We
conclude that such an experiment seems to be feasible with permanent magnets,
yet still challenging in its implementation.
