A laser heterodyne polarimeter for birefringence measurement.

We introduce a laser heterodyne polarimeter designed for the precision measurement of sub $\mu\mathrm{rad}$ 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.