We introduce a laser heterodyne polarimeter designed for the precision measurement of sub μrad differential phase shifts due to birefringence. The polarimeter will be used in an initial testbed for a potential future vacuum magnetic birefringence experiment at DESY. This experiment would use the 212-m-long ALPS magnet string. The vacuum magnetic birefringence signal will be amplified inside a high finesse optical cavity before it is sensed. This paper describes the polarimeter, initial results, and systematic error sources which still have to be minimized before the vacuum birefringence experiment can be realized.

Detecting vacuum magnetic birefringence (VMB) requires an immensely accurate, precisely calibrated experiment. We are working on a new design to detect the birefringence (BF) of individual optical components without a cavity that can also be used to measure VMB in reflection off a cavity. Our design uses two overlapping, orthogonally polarized laser beams to measure the relative phase difference between a reference path and one with rotating polarizations, which may experience oscillating phase shifts in vacuum in the presence of a magnetic field. To test the design, we developed a small-scale setup without cavities that can analyze different birefringent sources with the same principle. Our early results from testing mirrors show spatial variations in BF due to imperfect mirror coatings and show no correlation between the strength of a magnetic field applied parallel to the mirror’s surface and BF amplitude. In addition to assisting in the selection of suitable components for the final design, our laser heterodyne polarimeter (LHP) promises more sensitive results than previous experiments and may very well be the basis for the very first detection of VMB.

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.

Detecting vacuum magnetic birefringence (VMB) requires an immensely accurate, precisely calibrated experiment. We are working on a new design to detect the birefringence (BF) of individual optical components without a cavity that can also be used to measure VMB in reflection off a cavity. Our design uses two overlapping, orthogonally polarized laser beams to measure the relative phase difference between a reference path and one with rotating polarizations, which may experience oscillating phase shifts in vacuum in the presence of a magnetic field. To test the design, we developed a small-scale setup without cavities that can analyze different birefringent sources with the same principle. Our early results from testing mirrors show spatial variations in BF due to imperfect mirror coatings and show no correlation between the strength of a magnetic field applied parallel to the mirror’s surface and BF amplitude. In addition to assisting in the selection of suitable components for the final design, our laser heterodyne polarimeter (LHP) promises more sensitive results than previous experiments and may very well be the basis for the very first detection of VMB.