Report on progress in physics: observation of the Breit-Wheeler process and vacuum birefringence in heavy-ion collisions.

This report reviews the effort over several decades to observe the linear Breit-Wheeler process (γγ→e+e-) and vacuum birefringence (VB) in high-energy particle and heavy-ion collider experiment. This report, motivated by the STAR collaboration's recent observations, attempts to summarize the key issues related to the interpretation of polarizedγγ→l+l-measurements in high-energy experiments. To that end, we start by reviewing the historical context and essential theoretical developments, before focusing on the decades of progress made in high-energy collider experiments. Special attention is given to the evolution in experimental approaches in response to various challenges, to the demanding detector capabilities required to unambiguously identify the linear Breit-Wheeler process, and to the connections with VB. We close the report with a discussion, followed by a look at near-future opportunities for utilizing these discoveries and for testing quantum electrodynamics in previously unexplored regimes.