Direct measurement of the quantized topological magnetoelectric effect

For an axion insulator with $\theta = \pi$ (magnetic topological insulator sandwich or even-layer $\mathrm{MnBi_2Te_4}$), measure the static (dc) magnetoelectric response $P = (\theta \alpha / 4\pi^2) B$ in Gaussian units with $\alpha = e^2/(\hbar c)$, i.e., $\alpha/(4\pi)$ per unit field at $\theta = \pi$, or the equivalent magnetization induced by an electric field. An answer is the measured coefficient in units of $\alpha/(4\pi)$ with accuracy better than 10%.

Capacitive or SQUID detection of field-induced polarization, or electric-field-induced magnetization, in an insulating sample free of dissipative edge or side-wall conduction.