Measuring small magnetic fields plays an important role in many areas of physics. Mapping small variations in Earth’s magnetic field, tracing the history of geodynamo reversals as recorded in ancient rocks, characterizing new superconductors and other materials, searching for deviations from the standard model of particle physics, mapping the fields and currents of heart or brain activity—all require precise measurements of magnetic fields that are orders of magnitude smaller than Earth’s.
A key benchmark in such applications is a device’s sensitivity—the square root of the mean square field noise per unit bandwidth of the device. The device front-runner in most magnetometry applications is the superconducting quantum interference device (SQUID). Formed from superconducting rings interrupted by Josephson junctions, SQUID magnetometers for applications such as magnetoencephalography have sensitivities of around . Their input coils, which couple magnetic flux to the superconducting ring, can be...
