  We show how a chiral Dirac fermion (a massless electron or hole) can be
converted into a pair of neutral chiral Majorana fermions (a particle equal to
its own antiparticle). These two types of fermions exist on the metallic
surface of a topological insulator, respectively, at a magnetic domain wall and
at a magnet-superconductor interface. Interferometry of Majorana fermions is a
key operation in topological quantum computation, but the detection is
problematic since these particles have no charge. The Dirac-Majorana converter
enables electrical detection of the interferometric signal.
