  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.
