John Clarke Abstract

John Clarke
UC Berkeley and Lawrence Berkeley National Laboratory

"The SQUID: Principles, Practice and Noise"

I briefly outline the principles of the de SQUID (Superconducting QUantum Interference Device), an exquisitely sensitive detector of magnetic flux. I describe the fabrication of SQUIDs, and discuss how they are operated. For many applications, the magnetic field sensitivity is enhanced by means of a superconducting input circuit know as a flux transformer, resulting in a noise of typically 1 fT Hz-1/2. Configured as a voltmeter, the SQUID achieves a typical noise of 1 fV Hz-1/2. I discuss the underlying sources of noise in the classical and quantum regimes, and show how the resulting magnetic field noise scales with circuit parameters. At millikelvin temperatures, SQUID amplifiers are close to the quantum limit at frequencies around 1 GHz.