SQUID

For other uses, see Squid (disambiguation).

Superconducting Quantum Interference Devices (SQUID) are very sensitive magnetometers used to measure extremely small magnetic fields, based on superconducting loops containing Josephson junctions. They have noise levels as low as 3 fT·Hz^−½. For comparison, a typical refrigerator magnet produces 0.01 tesla (10⁻² T), and some processes in animals produce very small magnetic fields between 10⁻⁹ T to 10⁻⁶ T. Recently invented SERF atomic magnetometers are more sensitive but are physically huge and power intensive to operate compared to SQUIDs. For decades SQUID sensors were the only way to measure very small magnetic fields.

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History and design

The DC SQUID was invented in 1964 by Robert Jaklevic, John Lambe, Arnold Silver, and James Mercereau of Ford Research Labs after B. D. Josephson postulated the Josephson effect in 1962 and the first Josephson Junction was made by John Rowell and Philip Anderson at Bell Labs in 1963. The RF SQUID was invented in 1965 by James Edward Zimmerman and Arnold Silver at Ford.

There are two main types of SQUID: DC and RF. RF SQUIDs can work with only one Josephson junction, which might make them cheaper to produce, but are less sensitive.

The traditional superconducting materials for SQUIDs are pure niobium or a lead alloy with 10% gold or indium, as pure lead is unstable when its temperature is repeatedly changed. To maintain superconductivity, the entire device needs to operate within a few degrees of absolute zero, cooled with liquid helium.

"High temperature" SQUID sensors are more recent; they are made of high temperature superconductors, particularly YBCO, and are cooled by liquid nitrogen which is cheaper and more easily handled than liquid helium. They are less sensitive than conventional "low temperature" SQUIDs but good enough for many applications.

Uses for SQUIDs

The extreme sensitivity of SQUIDs makes them ideal for studies in biology. Magnetoencephalography (MEG), for example, uses measurements from an array of SQUIDs to make inferences about neural activity inside brains. Because SQUIDs can operate at acquisition rates much higher than the highest temporal frequency of interest in the signals emitted by the brain (kHz), MEG achieves good temporal resolution. Another area where SQUIDs are used is magnetogastrography, which is concerned with recording the weak magnetic fields of the stomach.

Probably the most common use of SQUIDs is in magnetic property measurement systems. These are turn-key systems, made by several manufacturers, that measure the magnetic properties of a material sample. This is typically done over a temperature range from that of liquid helium (~4K), to a couple of hundred degrees above room temperature.

For example, UC Berkeley Physics Professor John Clarke has been using SQUIDs as a detector to perform Magnetic Resonance Imaging. While high field MRI uses precession fields of one to several tesla, SQUID-detected MRI uses measurement fields that lie in the microtesla regime. Since the NMR signal drops off as the square of the magnetic field, a SQUID is used as the detector because of its extreme sensitivity. The SQUID coupled to a second-order gradiometer and input circuit, along with the application of gradients are the fundamental entities which allows his research group to retrieve noninvasive images. SQUID-detected MRI has many advantages such as the low cost required to build such a system, its compactness, the ability to image human extremities, and its application for tumor screening.

Another application is the scanning SQUID microscope, which uses a SQUID immersed in liquid helium as the probe. The use of SQUIDs in oil prospecting, mineral exploration, earthquake prediction and geothermal energy surveying is becoming more widespread as superconductor technology develops; they are also used as precision movement sensors in a variety of scientific applications, such as the detection of gravity waves. Four SQUIDs were employed on Gravity Probe B in order to test the limits of the theory of general relativity.  

SQUIDs in fiction

• The science fiction writer William Gibson made reference to SQUIDs in his 1981 story Johnny Mnemonic, where a genetically engineered ex-military dolphin uses a SQUID implant to read a memory device in the title character's brain.
• In the film Strange Days, SQUIDs are used to record and play back human memories, some of which are exchanged on the black market.
• In Michael Crichton's 1999 novel Timeline, SQUIDs are mentioned as a part of the Quantum Teleportation device developed by ITC.
• Jon Courtenay Grimwood's novel redRobe makes reference to SQUID probes being used to read memories and thoughts as part of a particularly invasive interrogation.

See also

• Josephson Effect
• Spallation Neutron Source Superconducting RF Cavities
• Mineral exploration
• Geophysics
• Electromagnetics