It was proposed in 2008 and 2009 that topological insulators are best understood not as surface conductors per se, but as bulk 3D magnetoelectrics with a quantized magnetoelectric effect. This can be revealed by placing topological insulators in magnetic field. The effect can be described in language similar to that of the hypothetical axion particle of particle physics. The effect was reported by researchers at Johns Hopkins University and Rutgers University using THz spectroscopy who showed that the Faraday rotation was quantized by the fine structure constant.