A Laser Heterodyne Polarimeter to Search for VMB with ALPS IIc Hardware

An unconfirmed prediction of QED, dating from 1936, is that light propagates with a polarization dependent speed v ≠ c through a background magnetic field, and the resulting vacuum magnetic birefringence (VMB) scales with B². We describe an experiment for the measurement of VMB and report the early development and testing status of a very sensitive polarimeter. We also outline a full-scale VMB experiment at DESY leveraging the ALPS IIc 100 m high finesse optical cavities and HERA magnet strings.