  A profound manifestation of topologically non-trivial states of matter is the
occurrence of fractionally charged elementary excitations. The quantum spin
Hall insulator state is a fundamentally novel quantum state of matter that
exists at zero external magnetic field. In this work, we show that a magnetic
domain wall at the edge of the quantum spin Hall insulator carries one half of
the unit of electron charge, and we propose an experiment to directly measure
this fractional charge on an individual basis. We also show that as an
additional consequence, a rotating magnetic field can induce a quantized dc
electric current, and vice versa.
