The Ubiquitous SQUID

The Superconducting QUantum Interference Device (SQUID) is 50 years old this year. Since its beginnings as a primitive device confined to a handful of cryogenics laboratories, the SQUID has evolved into a sensor able to detect exquisitely tiny signals generated by sources in a rich variety of disciplines. Today’s SQUIDs are fabricated on a wafer scale with high reproducibility using photolithographic or electron-beam patterning techniques. Although there are many different SQUID designs, the workhorse dc SQUID consists of a thin-film Nb loop interrupted by two Nb-AlOx-Nb tunnel junctions. When the SQUID is biased in the voltage state, a magnetic flux applied to the loop causes the voltage to oscillate with a period of one flux quantum. Suitable electronics enables one to resolve changes in flux corresponding to a millionth of a flux quantum—or even less—in one second. Coupled to an appropriate input circuit, the SQUID can detect tiny changes in, for example, magnetic field, magnetic field gradient, magnetic susceptibility, voltage, position and temperature. SQUID amplifiers operate at frequencies extending into the microwave regime, with quantum-limited noise performance. SQUIDs find applications in physics, chemistry, biology, medicine, materials science, nondestructive evaluation, geophysics, cosmology, astrophysics and quantum information. The Axion Dark Matter eXperiment (ADMX) at the University of Washington, Seattle is designed to search for the axion, a candidate for cold dark matter. The detector consists of a cooled microwave cavity surrounded by a 7-T superconducting magnet. In the presence of a magnetic field, the axion is predicted to decay into a photon, which, if its frequency is on resonance with a cavity mode, couples energy into an antenna inserted into the cavity. In the prototype detector, the cavity temperature was about 2 K and the signal was amplified by a cooled semiconductor amplifier. In the version to begin operation in 2014, the cavity will be cooled to 0.1 K and the signal will be amplified by a quantum-limited SQUID, increasing the axion search rate by three orders of magnitude. Ultralow field magnetic resonance imaging (ULF-MRI) in magnetic fields of the order of 0.1 mT—four orders of magnitude lower than in conventional MRI systems—is enabled by the combination of pre-polarized proton spins and signal detection with a SQUID. A particular advantage of ULF-MRI is that the longitudinal relaxation time is more sensitive to different tissue types than high field MRI. Furthermore, this tissue contrast can be enhanced by a careful choice of imaging frequency, typically a few kilohertz. The next generation of ULF-MRI systems is expected to reduce the imaging time by an order of magnitude. Potential clinical applications include imaging tumors and traumatic brain injury.