We report ultrahigh magnetic field Faraday rotation results on the chiral helimagnet ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$, the first Mott insulator showing skyrmion lattice phases and a linear magnetoelectric effect. Between 180 and 300 T, we find signatures of a Bose-Einstein condensation of magnons, which can be described as a canted XY ferrimagnet. Because of the magnetoelectric coupling, the transverse magnetic order of the individual ${\mathrm{Cu}}^{2+}$ spins is accompanied by a characteristic domelike electric polarization which is crucial for the observation of the condensate via the Faraday rotation effect.

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• Faraday effect
• High magnetic fields
• Magnetism
• Mean-field & cluster methods
• Quantum Monte Carlo