Paper

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

F Della Valle, Ruoso, G, W -T Ni, Gastaldi, U, Milotti, E, Ejlli, A, Pengo, R, Zavattini, G

2018

Overview

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.

• Publisher: Cornell University Library, arXiv.org
• Subject: Birefringence / Cotton / Ellipticity / Fabry-Perot interferometers / Faraday effect / Magnetic fields / Noise / Noise generation / Noise prediction / Polarimeters / Shot noise / Thermal noise / Time dependence / Variation