John Clark Abstract

John Clarke
University of California, Berkeley

"Everything You Need to Know About SQUIDS"

ABSTRACT

I briefly outline the principles of the dc 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 known as a flux transformer, resulting in a noise of typiucally 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 curcuit paramenters At millikelvin temperatures, SQUID amplifiers are close to the quantum limit at frequencies around 1 GHz.