Intrinsic mirror noise in Fabry-Perot based polarimeters: the case for the measurement of vacuum magnetic birefringence

TL;DR

This paper investigates the intrinsic mirror noise in Fabry-Perot polarimeters, revealing that mirror birefringence dominates noise levels and impacts the detection of vacuum magnetic birefringence, with implications for future experiments.

Contribution

It demonstrates that mirror birefringence noise significantly affects polarimeter sensitivity, highlighting the need to account for intrinsic mirror effects in vacuum birefringence measurements.

Findings

Mirror birefringence noise dominates ellipticity measurements.

Optical path difference sensitivity is $6 imes 10^{-19}$ m/√Hz at 10 Hz.

Intrinsic thermal noise in mirror coatings likely causes observed noise levels.

Abstract

Although experimental efforts have been active for about 30 years now, a direct laboratory observation of vacuum magnetic birefringence, an effect due to vacuum fluctuations, still needs confirmation. Indeed, the predicted birefringence of vacuum is $\Delta n = 4.0\times 10^{-24}$ @ 1~T. One of the key ingredients when designing a polarimeter capable of detecting such a small birefringence is a long optical path length within the magnetic field and a time dependent effect. To lengthen the optical path within the magnetic field a Fabry-Perot optical cavity is generally used with a finesse ranging from ${\cal F} \approx 10^4$ to ${\cal F} \approx7\times 10^5$. Interestingly, there is a difficulty in reaching the predicted shot noise limit of such polarimeters. We have measured the ellipticity and rotation noises along with a Cotton-Mouton and a Faraday effect as a function of the finesse of the cavity of the PVLAS polarimeter. The observations are consistent with the idea that the cavity mirrors generate a birefringence-dominated noise whose ellipticity is amplified by the cavity itself. The optical path difference sensitivity at $10\;$Hz is $S_{\Delta{\cal D}}=6\times 10^{-19}\;$m$/\sqrt{\rm Hz}$, a value which we believe is consistent with an intrinsic thermal noise in the mirror coatings.