Coherence of Josephson soliton oscillators in the millimeter-wave range

We report on the observation of phase-locking and radiation enhancement from two dc series-biased Josephson fluxon oscillators ac-coupled by a thin film-integrated capacitor. The radiation is detected by a small area Josephson junction displaying, under the effect of the radiation, critical current suppression and Shapiro steps at a voltage of 220?V. The phenomena that we observe are the higher frequency counterpart of the superradiant effects measured by means of X-band room temperature receivers detecting the radiation emitted by series arrays of long Josephson junctions.