Background
SQUID is an acronym for Superconducting Quantum Interference Device[1][2][4]. It is a very sensitive magnetometer which measures small/weak magnetic fields very accurately[2].
Fig 1. Quantum Design MPMS XL SQUID[8]
The sensitivity of the SQUID is linked to measuring the changes in the magnetic field corresponding to one flux quantum[7].
Principle of Working
The principle behind SQUID was discovered by a theoretical physicist, Brian Josephson in 1962 at the University of Cambridge while pursuing his Ph.D.[12].
In a SQUID, two superconductors are separated by thin insulating layers. At the junction, a current will flow without any voltage drop. If there is a voltage drop, the current will oscillate at a frequency in sync to the voltage and this is linked to the magnetic field.
This superconductor junction is called as Josephson junction & effect is called a Josephson effect[2][7].
Fig 2. SQUID working concept[2][7]
Now as the current density through a weak electric contact between the two superconductors depends on the phase difference ‘Δφ’ of the two superconducting wave functions – the time derivative of Δφ is correlated with the voltage and is used for sensing via electronic circuitry as shown below.
Fig 3. SQUID Flux-Voltage Conversion[4][5][6]
The following video gives a better idea regarding SQUID along with the sample loading & movement along the pickup coil which disrupts the magnetic field of the Josephson junction.
Realizing SQUID
- The materials used to realize SQUID’s are normally made from pure niobium or a lead alloy with 10% gold or indium[5][8][14]
- High-temperature SQUID’s are realized using YBCO. In this case, Liquid Nitrogen is used as a coolant instead of helium[5][8][14]
Types of SQUID
Radio frequency (RF) SQUID
- Consists of only one Josephson junction
- Cheaper
- Less Sensitive
Direct Current (DC) SQUID
- Two or more Josephson junction
- Expensive
- Sensitive
Applications
- SPMR for cancer detection[13]
- Voltameter for voltage detection
- Magnetic Tanks in biological labs and food industry
- Quantum Information Processing[11]
- Defect Detection
- Gradiometer for surveying and mineral exploration[10]
- Defense application like a submarine and mine detection
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References
- https://en.wikipedia.org/wiki/SQUID
- http://hyperphysics.phy-astr.gsu.edu/hbase/Solids/Squid.html
- https://www.qdusa.com/products/laboratory-squids.html
- http://squid.iitd.ernet.in/Basic_Literature.htm
- http://www.cmp.liv.ac.uk/frink/thesis/thesis/node48.html
- https://www.nobelprize.org/nobel_prizes/physics/laureates/1973/josephson-facts.html
- http://ffden-2.phys.uaf.edu/212_fall2003.web.dir/T.J_Barry/applications.html
- https://www.iitr.ac.in/hi/centers/IIC/pages/Organisation+SQUID.html
- https://www.slideshare.net/dlinkdiangeorgiev/squids-26824489
- http://www.gemsys.ca/mineral-exploration-applications/
- http://www.npl.co.uk/science-technology/quantum-detection/research/nanomagnetism/nanosquids-for-single-particle-detection
- https://en.wikipedia.org/wiki/Brian_Josephson
- A. Matlashov et.al.,”SQUID instrumentation for early cancer diagnostics”, IEEE 14th International Superconductive Electronics Conference (ISEC), 2013
- https://en.wikipedia.org/wiki/SQUID
- https://www.nature.com/news/superconductivity-record-sparks-wave-of-follow-up-physics-1.18191
Lateral Thinking 
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