Schematic representation of ferroelectric topological insulators (a) with two polar states and (b) with two polar TI states and an antipolar normal insulator (NI) state. (c) Proposed switching device corresponding to the two polar TI states with ferroelectric polarization. (d) Switching boundary conductivity on/off of an antipolar NI and two polar TI states. The chirality of electronic current is labeled by red and blue lines respectively. The up and down spins are indicated by short black arrows on the lines.