Trapped vortices in a superconducting microbridge

Laser light pulsed onto a Nb microbridge drives it momentarily normal and changes the quantum flux state of a superconducting inductive loop. The flux state is measured by a SQUID coupled to the loop. With a Nd:YAG laser, vortices are never trapped in the microbridge; with a diode laser, vortices are sometimes trapped. The spatial distribution of the trapped flux was studied. The effect of the optical pulse fall time on the frequency of flux trapping was found to be unimportant from 200 ns to 8 ms. Noise spectrum analysis indicates that the laser diode is 5 to 25 times noisier than the Nd:YAG laser at the characteristic frequency of the loop. This noise is believed to be responsible for flux trapping in the microbridge