In this work we present data characterizing the sensitivity of the Bir{e}fringence Magnetique du Vide (BMV) instrument. BMV is an experiment attempting to measure vacuum magnetic birefringence (VMB) via the measurement of an ellipticity induced in a linearly polarized laser field propagating through a birefringent region of vacuum in the presence of an external magnetic field. Correlated measurements of laser noise alongside the measurement in the main detection channel allow us to separate measured sensing noise from the inherent birefringence noise of the apparatus. To this end we model different sources of sensing noise for cavity-enhanced polarimetry experiments, such as BMV. Our goal is to determine the main sources of noise, clarifying the limiting factors of such an apparatus. We find our noise models are compatible with the measured sensitivity of BMV. In this context we compare the phase sensitivity of separate-arm interferometers to that of a polarimetry apparatus for the discussion of current and future VMB measurements.
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