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Influence of a magnetic field on the speed of light: Precise measurements in gases and in vacuum

Since the XIXth century, magneto-optics is one of the most powerful ways to investigate the microscopic properties of matter. This domain of physics corresponds to the study of light propagation through a medium in the presence of a magnetic field. The latter modifies in particular the speed of light (about 3x108 m/s for propagation in a vacuum without magnetic field). However, the change in the speed of light is extremely small. In the presence of a 1 T magnetic field (20 000 times higher than the earth’s magnetic field), the speed of light propagating in a 1 atmosphere pressure of helium gas varies by 7x10-8 m/s, less than 100 nanometers per second. The measurement of such a small variation is the experimental challenge taken up by physicists at the LNCMI in Toulouse with the BMV project (Biréfringence Magnétique du Vide). In particular, they are interested in the influence of the magnetic field on the speed of light as a function of the light polarisation (i.e. the direction of the vibration of the electric field associated with the light wave). The experiment, specially designed for this measurement, includes a pulsed magnet that can deliver up to 10 T. In order to increase the effect to be measured, light is trapped between two high reflectivity mirrors, constituting an optical cavity. Light is trapped during 1 millisecond and thus travels in a transverse magnetic field over an equivalent distance of 40 km, for a real length of the experimental setup of only 3 m.

This apparatus has attained an unprecedented sensitivity regarding the measurement of the variation of the light velocity in a magnetic field, with a sensitivity of a few nanometers per second in helium gas (which has the smallest effect in nature). The measurements, compatible with theory, show that this apparatus can now be used for any other gases, allowing a precise comparison between experimental measurements and theoretical predictions. The speed of light should also decrease when light propagates through vacuum in a magnetic field. Vacuum can therefore be considered as a standard medium regarding its magneto-optical properties. This effect, predicted by the most fundamental theory of light – matter interaction (quantum electrodynamics), has not yet been observed. The BMV project has demonstrated a sensitivity never achieved before; about 1 picometer per second. This first measurement validates the experimental setup. A new version currently under development will allow us to reach a sensitivity of 1 femtometer per second in a magnetic field of 1 T needed to see the effect for the first time.

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and the two most recent related publications:

Vacuum magnetic linear birefringence using pulsed fields : status of the BMV experiment, A. Cadène, P. Berceau, M. Fouché, R. Battesti and C. Rizzo, Eur. Phys. J. D 68, 16 (2014).

Faraday and Cotton-Mouton effects of helium at λ=1064 nm, A. Cadène, D. Sordes, P. Berceau, M. Fouché, R. Battesti and C. Rizzo, Phys. Rev. A 88, 043815 (2013).