Collective cavity mode excitations in arrays of Josephson junctions

We report on oscillations of a Josephson-junction system revealing a close analogy with fundamental effects known in laser physics. The experiments are performed on series arrays of junctions whose IV curves show evidence of a mode in which all the junctions oscillate in synchronism on resonances appearing, in zero external magnetic field, at multiples of the first cavity mode. Evidence is provided that the mode is generated by collective oscillations which develop spontaneously because the frequencies of the modes are equal within a 1% uncertainty. Numerical simulations are employed to elucidate the reasons leading to the synchronous collective excitation and to explain why it is characterized by a low emission of electromagnetic radiation.