When a Josephson junction is biased at a constant voltage V, it emits radiation at a frequency nu = 2eV/h, where h is the Planck constant and e is the electronic charge. The quantum-mechanical explanation of this effect, first given by Josephson, is that a tunneling Cooper pair emits a photon with energy h nu. If the photons emitted by one junction can then cause Cooper pairs to tunnel in different junctions via stimulated emission, an array of junctions could act as a maser, with the role of different atoms or molecules being played by Cooper pairs in different junctions.
The analogy between Josephson junctions and atoms had been developed
in a few theoretical works in the early 70s, but never before confirmed
by experiments. Our arrays show all the signatures of laser systems: coupling
of the oscillators to a resonant cavity providing feedback, coherent emission
above a pumping threshold, and synchronization up to sizes larger than
the radiation wavelength.