  Faraday rotation of polarised background sources is a unique probe of
astrophysical magnetic fields in a diverse range of foreground objects.
However, to understand the properties of the polarised sources themselves and
of depolarising phenomena along the line of sight, we need to complement
Faraday rotation data with polarisation observations over very broad
bandwidths. Just as it is impossible to properly image a complex source with
limited u-v coverage, we can only meaningfully understand the magneto-ionic
properties of polarised sources if we have excellent coverage in
$\lambda^2$-space. We here propose a set of broadband polarisation surveys with
the Square Kilometre Array, which will provide a singular set of scientific
insights on the ways in which galaxies and their environments have evolved over
cosmic time.
