SQUID
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- For the open-source web proxy cache, see Squid cache; also see Squid (disambiguation)
SQUIDs, or Superconducting Quantum Interference Devices, were invented in 1962 when B. D. Josephson developed the Josephson junction. There are two main types of SQUID, DC and RF (or AC). RF SQUIDs have only one Josephson junction whereas DC SQUIDs have two or more junctions. This makes DC SQUIDs more difficult and expensive to produce, but DC SQUIDs are much more sensitive.
Most SQUIDs are fabricated from lead or pure niobium. The lead is usually in the form of an alloy with 10% gold or indium, as pure lead is unstable when its temperature is repeatedly changed. The base electrode of the SQUID is made of a very thin niobium layer, formed by deposition, and the tunnel barrier is oxidised onto this niobium surface. The top electrode is a layer of lead alloy deposited on top of the other two, forming a sandwich arrangement.
The basic principle of operation is closely linked to flux quantisation. This is the phenomenon that the favoured states for a loop of superconductor are those where the flux inside is a multiple of the flux quantum.
SQUIDs are used to measure extremely tiny magnetic fields; they are currently the most sensitive such devices (magnetometers) known, with noise levels as low as 3 fT·Hz−½. Some processes in animals produce very small magnetic fields (typically sized between a nanotesla and a microtesla (1000 nT) — a typical fridge magnet is a tenth of a millitesla), and SQUIDs are well suited to studying these. 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 application is the scanning SQUID microscope, which uses a SQUID immersed in liquid helium as the probe. The use of SQUIDs in oil prospecting, 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 are currently employed on Gravity Probe B in order to test the limits of the theory of general relativity.
The science fiction writer William Gibson made reference to SQUIDs in his 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. SQUIDs are also referenced in the film Strange Days, where they are used to record and play back human memories, which are exchanged on the black market.
External link
- Dr. John Bland’s explanation (http://www.cmp.liv.ac.uk/frink/thesis/thesis/node47.html).da:Squid