SQUID Magnetometer Sensitivity

The SQUID magnetometer may be the most sensitive measurement device known to man, according to John Clarke, one of the developers of the concept. He evokes the following images to illustrate its sensitivity:

• It can measure magnetic flux on the order of one flux quantum. A flux quantum can be visualized as the magnetic flux of the Earth's magnetic field (0.5 Gauss = 0.5 x 10^-4 Tesla) through a single human red blood cell (diameter about 7 microns).
• It can measure extremely tiny magnetic fields. The energy associated with the smallest detectable change in a second, about 10^-32 Joules, is about equivalent to the work required to raise a single electron 1 millimeter in the Earth's gravitational field!

The sensitivity of the basic SQUID can be increased by attaching it to a flat coil of superconducting wire, such as niobium. Called a "flux transformer", this increases the current induced in the junction and permits the detection of magnetic fields as small as 10^-15 Tesla or one femto-Tesla. This is a resolution of some 10^-11 times the Earth's magnetic field. By comparison, the auroral displays in Earth's polar region produce magnetic field fluctuations on the order of 1% of the Earth's field.

Taking 1 fT as the nominal resolution of the SQUID, it is capable of detecting changes in magnetic fields in the human body:

The SQUID has been used to measure and localize seizure activity in the human brain. Resolutions on the order of 30 fT have been achieved with the YBaCO type high-temperature superconductors with which the SQUID can be operated at liquid nitrogen temperature.

A Bit of SQUID History

John Clarke is a professor at the University of California, Berkeley and a faculty senior scientist at Lawrence Berkeley Laboratory. He earned his M.A. and Ph. D from the University of Cambridge in 1968. Much of his career has been devoted to the study of superconductors and the development of SQUIDs. He has participated in the design of SQUID magnetometers for a number of applications.

He relates having constructed his first Josephson junction after a discussion with a fellow research student at the traditional afternoon tea at the Cavendish Laboratory. He later constructed a voltmeter capable of measuring 10 femtovolts (10^-14 volts), 100,000 times more sensitive than a conventional semiconductor voltmeter.

Brian Josephson, also working at Cambridge, had earlier developed the concept of the junction which bears his name. He predicted the tunneling of Cooper pairs of electrons from superconductors through a thin insulating layer. This tunneling was verified a few months later at Bell Telephone Laboratories by Philip W. Anderson and John M. Rowell.