Low noise switching of a superconducting circuit by a laser induced weak link

The authors have designed and begun testing a high-speed superconducting switch. The switches are lines of Nb about 400-A thick, 2-μm wide, and 2-mm long on a sapphire substrate. The switching is done by illuminating the line with a laser through an optical fiber. The photons break Cooper pairs mostly by scattering, since the film thickness is on the order of the superconductor´s coherence length and the optical penetration depth. The order parameter and critical current are depressed to zero through nonequilibrium dynamics, with the film remaining below its thermodynamic transition temperature. Thus, in principle, the switching rates are limited by the quasiparticle recombination time, rather than by the slower time of phonon escape from the film. The noise seen in the normal state is comparable to that in the superconducting state, and switching rates up to 300 kHz were used