HMC Physics Colloquium

Tuesdays at 16:30 in Shanahan Center for Teaching and Learning, Room B460

John Clarke

University of California at Berkeley

The Ubiquitous SQUID: From Cosmology to Medicine

March 7, 2006

Many materials become superconducting at sufficiently low temperatures. In the superconducting state, electrons of opposite momentum and spin form Cooper pairs. These pairs constitute a single, macroscopic quantum state that is described by a quantum mechanical wave function with well defined amplitude and phase. This quantum state is responsible for the zero-electrical resistance of superconductors and for such properties as flux quantization-the fact that the magnetic flux in a closed superconducting loop is quantized in units of the flux quantum Φ0 = h/2e = 2.07 x 10-15 T m2, and Josephson tunneling-the ability of Cooper pairs to tunnel coherently through an insulating barrier. The SQUID (Superconducting QUantum Interference Device) combines the phenomena of flux quantization and Josephson tunneling to make an ultra-sensitive detector of magnetic flux. The principles, fabrication, and operation of the SQUID are outlined. With the aid of a superconducting flux transformer, the SQUID achieves a magnetic field noise of 10-15 T Hz-1/2; it can also be configured as a voltmeter with a noise of 10-15 V Hz-1/2.

Applications of SQUIDs-ranging from geophysics to quantum computing-are briefly reviewed. Two other applications are discussed in more detail. The first is for cosmology, in a search for galaxy clusters, looking for tiny deviations of the cosmic background radiation from the Planck distribution. The electromagnetic radiation is detected by a bolometer consisting of a superconducting transition-edge sensor coupled to a SQUID amplifier. An array of 330 such detectors is about to be fielded on a telescope at Atacama, Chile. The second application is concerned with nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) in magnetic fields of about 10-4 T, comparable to the Earth’s field and four orders of magnitude lower than that used in conventional MRI. Images of phantoms and of the human forearm are presented, enhanced contrast due to weighting by the longitudinal relaxation time is illustrated, and the ability to obtain undistorted images in the presence of metals is demonstrated. Potential clinical applications are discussed.