Historical research guide · 0874 · 2016

The PVLAS experiment: measuring vacuum magnetic birefringence and dichroism with a birefringent Fabry–Perot cavity

Federico Della Valle · Aldo Ejlli · Ugo Gastaldi · Giuseppe Messineo · Edoardo Milotti · Ruggero Pengo · Giuseppe Ruoso · Guido Zavattini

In one minute

Quantum electrodynamics says that a strong magnet turns empty space into something like a crystal: light polarised along the field should travel very slightly slower than light polarised across it. At 2.5 tesla the predicted difference is about 2.5 parts in a hundred thousand million million million, and nobody has yet resolved it directly. Federico Della Valle, Guido Zavattini, Giuseppe Ruoso and their colleagues in Ferrara describe the instrument that came closest — a laser bouncing hundreds of thousands of times between two mirrors, through the bores of two rotating permanent magnets. Their central point here is a practical one that touches every experiment of this kind: the mirrors themselves are slightly birefringent, so the two polarisations resonate at slightly different frequencies, the signal is attenuated by a known factor, and birefringence and dichroism leak into each other. Turn that leak around and it becomes useful — one measurement now yields both quantities, and it gives new laboratory limits on light particles that couple to two photons.

Connection

Chapter 2 says the vacuum is a real, structured medium, and vacuum magnetic birefringence is its cleanest optical signature: put a magnet in empty space and light itself changes speed depending on how it is polarised. This is the paper where the Ferrara group states exactly how well that can be measured and what stands in the way. Chapter 1 gets a model case of the evidence ladder — a published noise budget, a calibration against a gas whose birefringence is known, a specific list of what is being changed next, and limits that hold no matter which theory of light particles you prefer. The collaboration’s twenty-five-year account of the whole programme is on the site at /library/stm-08601764f6.

From the source

The Euler-Heisenberg-Weisskopf Lagrangian makes electrodynamics non-linear even in vacuum. To lowest order the magnetic birefringence is three times the parameter A-sub-e times the square of the external field, and with that parameter equal to 1.32 times ten to the minus twenty-four per tesla squared this gives a birefringence of 2.5 times ten to the minus twenty-three at 2.5 tesla, with the accompanying magnetic dichroism negligible.Sect. 1, Eqs. (1) to (4)
The method is heterodyne polarimetry inside a very long optical path. Over a magnetic field length of 1.64 metres at 2.5 tesla with 1064 nanometre light, the QED ellipticity would be 1.2 times ten to the minus sixteen; a Fabry-Perot cavity of finesse near seven hundred thousand multiplies the optical path by 445,000 and lifts that ellipticity to the order of ten to the minus ten.Sect. 2, Eq. (7); Sect. 2.1, Eqs. (13) to (15); Sect. 3
The paper’s central methodological result is that the dielectric mirrors are themselves birefringent in reflection, which has three consequences for every polarimeter of this type: the two polarisation resonances of the cavity are separated in frequency, the extinguished beam is attenuated by a calculable factor, and ellipticity and rotation mix into one another. The ratio of the spurious rotation to the true ellipticity is minus half the amplification factor times the mirror phase difference, so the mixing itself measures the sum of the two mirrors’ birefringences.Sect. 2.2, Eqs. (18) to (23); Sect. 3.2, Eqs. (24) and (25)
The vacuum results, weighted averages over four determinations each, are a magnetic birefringence of minus 1.5 plus or minus 3.0 times ten to the minus twenty-two and a magnetic dichroism of minus 1.6 plus or minus 3.5 times ten to the minus twenty-two, both at 2.5 tesla. All the numbers are compatible with zero, and the birefringence result is an order of magnitude larger than the QED prediction, so it serves as an upper limit.Sect. 4, Eqs. (26) and (27); Tables 2 and 3
The same measurements exclude new regions in the parameter space of axion-like and milli-charged particles, model-independently: the ellipticity result dominates above one milli-electron-volt of axion-like-particle mass, and the fermion milli-charged-particle plot applies to all types of neutrinos, limiting their charge to less than about three times ten to the minus eight of the electron charge for masses below ten milli-electron-volts.Sect. 4.2; Figs. 14 and 15
The sensitivity, four times better than the 2014 apparatus, is still far from the six times ten to the minus nine per root hertz computed from the known noise sources. The noise comes from the cavity, seismic modulation of the mirror birefringence was excluded by shaking the bench at a known amplitude in all three directions, thermal instability shows up only below about one hertz, and the noise is largely independent of the attenuation factor — which points at diffused light. The named next steps are faster magnet rotation, less scattered light, better mirrors, and cooling the mirrors toward liquid nitrogen temperature.Sect. 4.1; Sect. 5, Conclusions

The way in

https://doi.org/10.1140/epjc/s10052-015-3869-8

Historical source-access record. This guide links to the original; it does not reproduce the paper text.

LICENCE. The statement is printed in the article itself, under the acknowledgments: this article is distributed under the terms of the Creative Commons Attribution 4.0 International License, which permits unrestricted use, distribution and reproduction in any medium provided appropriate credit is given, a link to the licence is provided, and changes are indicated. The article is marked Funded by SCOAP3, and the Crossref licence record for the DOI names the same licence. TEXT. The publisher’s server declines automated retrieval, so the version of record was taken from the SCOAP3 repository copy of the published Springer PDF and converted with pdftotext; it carries the Eur. Phys. J. C (2016) 76:24 running head and the received, accepted and published dates. The complete article is reproduced below — the abstract and sections 1 to 5 in full, together with the acknowledgment. Running heads, page numbers, figure captions and table furniture are dropped, the forty-five-item reference list is omitted and is at the source, and reference-number markers are removed from the prose. The fifteen figures are optical schematics, noise curves, Fourier spectra and exclusion plots that cannot be reproduced as text; the numerical contents of the three tables are given in the prose where the text relies on them. Displayed equations reached the library with Greek letters, subscripts and matrix layout lost in extraction, so they are given as named results in plain words with their original numbers, and inequalities are written in words. Published as F. Della Valle, A. Ejlli, U. Gastaldi, G. Messineo, E. Milotti, R. Pengo, G. Ruoso and G. Zavattini, The European Physical Journal C 76:24 (2016), from INFN Trieste and the University of Trieste, INFN Ferrara and the University of Ferrara, and INFN Laboratori Nazionali di Legnaro; received 29 October 2015, accepted 23 December 2015, published online 20 January 2016. The collaboration’s twenty-five-year review of the whole programme, published in Physics Reports in 2020, is on this site at /library/stm-08601764f6.

How to cite it

Federico Della Valle, Aldo Ejlli, Ugo Gastaldi, Giuseppe Messineo, Edoardo Milotti, Ruggero Pengo, Giuseppe Ruoso, Guido Zavattini (2016) The PVLAS experiment: measuring vacuum magnetic birefringence and dichroism with a birefringent Fabry–Perot cavity doi:10.1140/epjc/s10052-015-3869-8

Where it sits in the curriculum

What the vacuum is