Observation of magneto-electric non-reciprocity in molecular nitrogen gas

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We report the direct observation of the non-reciprocity of the velocity of light, induced in a gas by electric and magnetic fields. This bilinear magneto-electro-optical phenomenon appears in the presence of crossed electric and magnetic fields perpendicular to the light wavevector, as a refractive index difference between two counterpropagating directions. Using a high finesse ring cavity, we have measured this magneto-electric non-reciprocity in molecular Nitrogen at ambient temperature and atmospheric pressure; for light polarized parallel to the magnetic field it is 2η∥ exp(N₂)= (4.7± 1)× 10⁻²³m.V⁻¹.T⁻¹ for λ=1064nm, in agreement with the expected order of magnitude. Our measurement opens the way to a deeper insight into light-matter interaction, since bilinear magneto-electric effects correspond to a coupling beyond the electric dipole approximation. We were able to measure a magneto-electric non-reciprocity as small as Δn = (5± 2) × 10⁻¹⁸, which makes its observation in quantum vacuum a conceivable challenge.