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

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

doi:10.1140/epjc/s10052-018-6063-y

2023 Impact factor 4.2

Particles and Fields

Open Access

Eur. Phys. J. C (2018) 78: 585
https://doi.org/10.1140/epjc/s10052-018-6063-y

Regular Article - Experimental Physics

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

G. Zavattini¹^*, F. Della Valle², A. Ejlli¹, W.-T. Ni³, U. Gastaldi¹, E. Milotti⁴, R. Pengo⁵ and G. Ruoso⁵

¹ Sez. di Ferrara and Dip. di Fisica e Scienze della Terra, INFN, Università di Ferrara, via G. Saragat 1, Edificio C, 44122, Ferrara, FE, Italy
² Sez. di Pisa, gruppo collegato di Siena and Dip. di Scienze Fisiche, della Terra e dell’Ambiente, INFN, Università di Siena, via Roma 56, 53100, Siena, SI, Italy
³ Department of Physics, National Tsing Hua University, Hsinchu, 30013, Taiwan, Republic of China
⁴ Sez. di Trieste and Dip. di Fisica, INFN, Università di Trieste, via A. Valerio 2, 34127, Trieste, TS, Italy
⁵ Lab. Naz. di Legnaro, INFN, viale dell’Università 2, 35020, Legnaro, PD, Italy

^* e-mail: guido.zavattini@unife.it

Received: 10 May 2018
Accepted: 11 July 2018
Published online: 19 July 2018

Abstract

Although experimental efforts have been active for about 30 years, a direct laboratory observation of vacuum magnetic birefringence, due to vacuum fluctuations, still needs confirmation: the predicted birefringence of vacuum is $\Delta n = 4.0\times 10^{-24}$ @ 1 T. Key ingredients of a polarimeter for detecting such a small birefringence are a long optical path within the magnetic field and a time dependent effect. To lengthen the optical path a Fabry–Perot is generally used with a finesse ranging from ${{\mathscr {F}}} \approx 10^4$ to ${{\mathscr {F}}} \approx 7\times 10^5$ . 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 Cotton-Mouton and Faraday effects 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 $10\;\hbox {Hz}$ is $S_{\Delta {{\mathscr {D}}}}=6\times 10^{-19}\;\hbox {m}/\sqrt{\mathrm{Hz}}$ , a value which we believe is consistent with an intrinsic thermal noise in the mirror coatings. Our findings prove that the continuous efforts to increase the finesse of the cavity to improve the sensitivity has reached a limit.