Current noise measured in the dc SQUID

M. R. Condron, G.M. Gutt, B. Muhlfelder +5 more

Kenneth R. Carroll, Ho Jung Paik

John M. Martinis, John L. Clarke

The current noise circulating around the loop of a tunnel junction dc SQUID has been measured as a function of applied flux. The flux noise generated by this current was coupled via a superconducting transformer to a second dc SQUID that was used to measure the noise in a null-balancing configuration. The voltage noise across the SQUID was also measured, using a cooled LC-resonant circuit, and the correlation between the voltage and current noises was determined. The measured values of the noises are in good agreement with values obtained from an analog simulator.

M. Pullano, P. Carelli, Vittorio Foglietti +2 more

R. H. Koch, John L. Clarke, John M. Martinis +3 more

We describe two methods of measuring the 1/f noise in a dc SQUID. One is sensitive only to 1/f noise in the critical currents of the junctions, and the other is sensitive only to 1/f flux noise that is not associated with critical current fluctuations. From measurements on a planar thin-film dc SQUID incorporating Josephson tunnel junctions we conclude that the predominant source of 1/f noise is not noise in the crititcal currents, but rather an apparent flux noise of unknown origin.

Pasquale Carelli, Maria Gabriella Castellano