STM-D-0765Paper2014Published and peer-reviewed

First results from the new PVLAS apparatus: A new limit on vacuum magnetic birefringence

F. Della Valle · E. Milotti · A. Ejlli · G. Messineo · L. Piemontese · G. Zavattini · U. Gastaldi · R. Pengo · G. Ruoso

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Quantum electrodynamics says 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. The predicted difference is about four parts in a thousand million million million million per tesla squared, and nobody has yet measured it directly. Federico Della Valle, Guido Zavattini, Giuseppe Ruoso and their INFN colleagues report the first results from the rebuilt PVLAS apparatus in Ferrara: a laser at 1064 nanometres locked into a 3.3-metre cavity of finesse 670,000, so the light crosses the field 430,000 times, passing through the bores of two rotating permanent magnets. Calibrating against the known magnetic birefringence of helium, and running 210 hours, they find a value compatible with zero and set a new limit — which cuts the gap to the predicted quantum effect to a factor of fifty. The same data tighten laboratory bounds on axion-like particles and on hypothetical fractionally charged particles, neutrinos included.

Why it matters hereChapter 2 says the vacuum is a real, structured medium, and vacuum magnetic birefringence is its cleanest optical signature: bend light with a magnet and nothing else, and the medium stops being a figure of speech. This paper is the moment the gap to that measurement came down to a factor of fifty, and it is chapter 1’s evidence ladder in miniature — a calibration against a gas whose birefringence is already known, a cross-check with the two magnets deliberately spun at different rates, and a named limiting noise source. The collaboration’s later account of the same apparatus is on this site at /library/stm-e58fdfd01c, and the twenty-five-year review of the whole programme, with the final limits and the LHC-dipole proposal that would close the gap, is at /library/stm-08601764f6.

What it claims

01Vacuum magnetic birefringence is predicted by quantum electrodynamics and has never yet been observed directly. In the Euler-Heisenberg framework the unitary field magnetic birefringence of vacuum is 3.97 times ten to the minus twenty-four per tesla squared, and observing it would be the first direct detection of light-by-light interaction at low photon energy — the only evidence for zero point quantum fluctuations to date coming from the Casimir effect, which applies to photons alone.Sect. I, Introduction, Eqs. (1) to (3)
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02The new Ferrara apparatus is a heterodyne polarimeter on a 4.5 tonne granite bench: a 1064 nanometre Nd:YAG laser locked to a Fabry-Perot cavity 3.303 metres long with finesse 670,000, the beam crossing the bores of two rotating permanent dipole magnets in Halbach configuration, each contributing a field-squared-times-length integral of 5.12 tesla squared metres. The cavity multiplies the single-pass ellipticity of 1.2 times ten to the minus sixteen by 430,000, giving a signal of about 5 times ten to the minus eleven to be measured at twice the magnet rotation frequency.Sect. II, Experimental method and setup; Tables II and IV
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03The apparatus was first calibrated on the Cotton-Mouton effect of helium at 32 microbar, giving a unitary birefringence of 2.2 plus or minus 0.1 times ten to the minus sixteen per tesla squared per atmosphere, in agreement with previously published values and with no spurious peaks at other harmonics. From 210 hours of vacuum data with the magnets rotating between 2.4 and 3 hertz — 40 of those hours with the two magnets deliberately spun at slightly different frequencies, so that neither could be generating a spurious signal — the measured unitary vacuum magnetic birefringence is 4 plus or minus 20 times ten to the minus twenty-three per tesla squared. It is compatible with zero and stands as a new limit, a factor of 50 from the predicted quantum electrodynamic value.Sect. III, Calibration; Sect. IV, Results; Sect. V.A, Eq. (8)
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04The same limit bounds the elastic photon-photon scattering cross section for unpolarised light at 1064 nanometres to less than 4.6 times ten to the minus sixty-six square metres, against a quantum electrodynamic prediction of 1.84 times ten to the minus sixty-nine square metres.Sect. V.A, Eqs. (9) and (10)
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05The measurement also yields improved model-independent bounds on the coupling of axion-like particles to two photons for masses greater than one millielectronvolt, and a factor of two improvement in the fractional charge limit on millicharged fermions and scalars — including neutrinos, whose charge is limited to about ten to the minus seven of the electron charge for masses below 20 millielectronvolts.Sect. V.B and V.C; Figs. 6 and 7
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06The sensitivity is still far from the shot noise limit, and the excess noise is clearly due to the presence of the Fabry-Perot cavity — without the cavity, shot noise is achieved. The authors suspect variations in the intrinsic birefringence of the reflective coating caused by thermal effects, and say that effort will now go into improving the sensitivity.Sect. V.A, final paragraph
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Abstract

Several groups are carrying out experiments to observe and measure vacuum magnetic birefringence, predicted by Quantum Electrodynamics (QED). We have started running the new PVLAS apparatus installed in Ferrara, Italy, and have measured a noise floor value for the unitary field magnetic birefringence of vacuum Δn_u(vac) = (4 ± 20) × 10⁻²³ T⁻² (the error represents a 1σ deviation). This measurement is compatible with zero and hence represents a new limit on vacuum magnetic birefringence deriving from non linear electrodynamics. This result reduces to a factor 50 the gap to be overcome to measure for the first time the value of Δn_u(vac) predicted by QED: Δn_u(vac,QED) = 4 × 10⁻²⁴ T⁻². These birefringence measurements also yield improved model-independent bounds on the coupling constant of axion-like particles to two photons, for masses greater than 1 meV, along with a factor two improvement of the fractional charge limit on millicharged particles (fermions and scalars), including neutrinos.

The way in

https://doi.org/10.1103/PhysRevD.90.092003LICENCE CHECK. The version of record is Physical Review D volume 90, article 092003 (2014), under the APS default licence, and the author copy arXiv:1406.6518v3, dated 15 September 2014, carries the arXiv non-exclusive distribution licence version 1.0. Neither is a Creative Commons grant, so the sheet stays abstract-only and no full text is reproduced. The abstract below is the authors’ own, transcribed from the arXiv posting with the superscripts restored where extraction lost them. The work is by INFN Trieste and the University of Trieste, INFN Ferrara and the University of Ferrara, and INFN Laboratori Nazionali di Legnaro.

How to cite it

F. Della Valle, E. Milotti, A. Ejlli, G. Messineo, L. Piemontese, G. Zavattini, U. Gastaldi, R. Pengo, G. Ruoso (2014) First results from the new PVLAS apparatus: A new limit on vacuum magnetic birefringence. doi:10.1103/PhysRevD.90.092003

Where it sits in the curriculum

What the vacuum is The evidence ladder