by Lynn Yarris
Magnetic fields are everywhere. They arise from the beating of a human heart and the fiery motion of the Earth's molten core. They issue from creatures large and small, and every star and planet in the heavens. Anywhere there is an electrical current flowing, there is a magnetic field surrounding it. Measuring the magnitude of a magnetic field or detecting unusual fluctuations can yield a wealth of valuable, even life-saving, revelations -- irregularities in the heartbeat of an unborn child; flaws within the steel supports of a highway overpass or bridge; the migration patterns of bacteria and microbes through the environment; faults in the crust of the earth, deep below the planet's surface; possibly even the identity of the invisible "dark" matter that seems to comprise the bulk of our universe.
The most sensitive instruments used to measure magnetic fields feature circular or square-shaped devices called squids that are less than a millimeter across (about the size of the period at the end of this sentence). Not to be confused with the tentacled sea-creatures served raw in Japanese restaurants, this squid stands for Superconducting QUantum Interference Device and it is the most sensitive type of detector known to science. How sensitive? John Clarke, a physicist in Berkeley Lab's Materials Sciences Division and a professor in the Physics Department on the University of California's Berkeley campus, says a convenient criterion for squid sensitivity is the smallest change in magnetic energy the device can detect in one second. For a typical squid, this energy is about 10-32 joule.
"This incredibly tiny amount is roughly equal to the mechanical energy required to raise a single electron one millimeter in the earth's gravitational field," Clarke says. "In fact, the best squids we have ever manufactured are 100 times more sensitive than that."
Clarke should know more than a thing or two about squids. When he was a graduate student at Cambridge University in the mid-1960s, he built the world's first squid-based instrument for measuring voltages. Since his move to Berkeley in the early 1970s, Clarke has overseen one of the world's premier programs for squids research.
A squid works via a phenomenon of quantum mechanics. It starts with a superconductor, a material in which electrical resistance drops to zero when it is cooled below a critical temperature ( symbolized Tc ). When a material becomes superconducting, it can carry an electrical current forever with no loss of energy. A squid is formed when its superconducting material is separated by an insulating barrier. Conventional wisdom holds that an electrical current should be blocked by an insulator. However, if the insulating barrier is sufficiently slender -- only a few layers of atoms -- electron pairs will "tunnel" through it: the insulator acts like a weak superconductor.
