The Ubiquitous SQUID: From Cosmology to Medicine
Department of Physics, University of California, Berkeley
Materials Sciences Division, Lawrence Berkeley National Laboratory
The SQUID (Superconducting QUantum Interference Device) is an ultrasensitive detector of magnetic flux that combines the phenomena of Josephson tunneling and flux quantization. I briefly describe the principles of the SQUID and its practical implementation. With the aid of a superconducting flux transformer, the SQUID achieves a magnetic field noise below 10-15 THz-1/2. SQUIDs are having significant impact on cosmology. It is now accepted that the major constituents of the universe are dark energy and cold dark matter, both of which are of unknown origin. Several telescopes around the world are imaging galaxy clusters at far infrared frequencies to investigate dark energy. These telescopes contain many hundreds of superconducting sensors coupled to SQUIDs. A candidate particle for cold dark matter is the axion which, in the presence of a magnetic field, is predicted to decay into a photon with energy given by the axion mass. A search for the axion is underway at the Lawrence Livermore National Laboratory. A near-quantum limited SQUID amplifier will decrease the search time by three orders of magnitude compared with that achieved with semiconductor amplifiers. In the area of medical physics, we use a SQUID to obtain magnetic resonance images (MRI) in a magnetic field of 132 mT, corresponding to a proton Larmor frequency of 5.6 kHz—four orders of magnitude lower than in conventional MRI. Low fields yield much greater differences in the relaxation time (T1) of different tissue types compared with high fields. In preliminary experiments, we have measured T1 in ex vivo specimens of surgically removed healthy and malignant prostate tissue. The values of T1 in healthy tissue are typically 60% higher than in malignant tissue, suggesting that ultralow field MRI may have clinical applications to imaging cancer. John Clarke received his B.A. (1964), Ph.D. (1968), and Sc.D. (2005) from Cambridge University. After a postdoctoral fellowship at UC Berkeley, he joined the Physics faculty in 1969. He received the Distinguished Teaching Award in 1983. He is a fellow of the Royal Society of London, the American Physical Society, the American Association for the Advancement of Science, and the Institute of Physics (UK). He has been a Sloan Foundation Fellow, a Guggenheim Fellow and a Miller Professor. In 1987, he was named California Scientist of the Year and was awarded the Fritz London Prize for research in low-temperature physics. In 1998, he received the American Physical Society's Joseph F. Keithley Award for Advances in Measurement Science and, in 1999, the National Academy of Sciences' Comstock Prize for Physics. He was awarded the Hughes Medal of the Royal Society in 2004, and was the UC Berkeley Faculty Research Lecturer in 2005.