  Ultra-relativistic heavy-ion collisions are expected to produce the strongest
electromagnetic fields in the known Universe. These highly-Lorentz contracted
fields can manifest themselves as linearly polarized quasi-real photons that
can interact via the Breit-Wheeler process to produce lepton anti-lepton pairs.
The energy and momentum distribution of the produced dileptons carry
information about the strength and spatial distribution of the colliding
fields. Recently it has been demonstrated that photons from these fields can
interact even in heavy-ion collisions with hadronic overlap, providing a purely
electromagnetic probe of the produced medium. In this review we discuss the
theoretical progress and recently advances in experimental for mapping the
ultra-strong electromagnetic fields produced in heavy-ion collisions via the
Breit-Wheeler process.
