Regenerative feedback narrowing of Josephson radiation linewidth

The linewidth of radiation emitted from resonantly coupled indium microbridges has been systematically studied as a function of the loaded Q of the resonator and of the temperature. For high Q values, the radiation linewidth (10 MHz-20 KHz) is much narrower than the resonator bandwidth (14 MHz), and the time-averaged differential resistance, Rsub(D), at the resonance-induced step approaches zero in a highly nonlinear manner. These observations provide strong evidence of regenerative feedback effects inside the resonator-junction system. (orig.)