Condensed Matter Physics Seminar

Thursday, March 16, 2000, 2 p.m.
Plant Sciences Building, Room 1130

Josephson-Junction Array Masers

Paola Barbara

(Center for Superconductivity Research, University of Maryland)

Abstract:  Our recent experiments on the high-frequency properties of Josephson junction arrays show that micron-scale non-identical objects, such as Josephson junctions, can synchronize through stimulated emission in the same manner as atoms in a laser.

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.

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