  We consider a voltage-biased Josephson junction between two nanowires hosting
Majorana zero modes which occur as topological protected zero-energy
excitations at the junction. We show that two Majorana fermions localized at
the junction, even though being neutral particles, interact with the
electromagnetic field and generate coherent radiation similar to the
conventional Josephson radiation. Within a semiclassical analysis of the
radiation field, we find that the optical phase gets locked to the
superconducting phase difference and that the radiation is emitted at half the
Josephson frequency. In order to confirm the coherence of the radiation, we
study correlations of the radiation emitted by two spatially-separated
junctions in a d.c.-SQUID geometry taking into account decoherence due to
spontaneous state-switches as well as due to quasi-particle poisoning.
