Editing SQUID (section)

== Uses ==
[[File:Squid prototype.jpg|thumb|The inner workings of an early SQUID, circa 1990<!-- seems to have been built after the [[National Bureau of Standards]] was renamed [[NIST]] in 1988.  Much more primitive SQUIDS were built at NBS in the first half of the 1970s. -->]]

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 [[neuron|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. A novel application of SQUIDs is the [[magnetic marker monitoring]] method, which is used to trace the path of orally applied drugs. In the clinical environment SQUIDs are used in [[cardiology]] for [[magnetic field imaging]] (MFI), which detects the magnetic field of the heart for diagnosis and risk stratification.

Probably the most common commercial use of SQUIDs is in magnetic property measurement systems (MPMS). These are [[Turnkey|turn-key]] systems, made by several manufacturers, that measure the magnetic properties of a material sample which typically has a temperature between 300 mK and 400 K.<ref>{{Cite journal | last1 = Kleiner | first1 = R. | last2 = Koelle | first2 = D. | last3 = Ludwig | first3 = F. | last4 = Clarke | first4 = J. | title = Superconducting quantum interference devices: State of the art and applications | doi = 10.1109/JPROC.2004.833655 | journal = Proceedings of the IEEE | volume = 92 | issue = 10 | pages = 1534–1548 | year = 2004 | s2cid = 20573644 }}</ref> With the decreasing size of SQUID sensors since the last decade, such sensor can equip the tip of an [[Atomic force microscopy|AFM]] probe. Such device allows simultaneous measurement of roughness of the surface of a sample and the local magnetic flux.<ref>{{cite web|url=http://neel.cnrs.fr/spip.php?article914&lang=en|title=Microscopie à microsquid - Institut NÉEL|website=neel.cnrs.fr}}</ref>

For example, SQUIDs are being used as detectors to perform [[magnetic resonance imaging]] (MRI). While high-field MRI uses precession fields of one to several teslas, SQUID-detected MRI uses measurement fields that lie in the microtesla range. In a conventional MRI system, the signal scales as the square of the measurement frequency (and hence precession field): one power of frequency comes from the thermal polarization of the spins at ambient temperature, while the second power of field comes from the fact that the induced voltage in the pickup coil is proportional to the frequency of the precessing magnetization. In the case of untuned SQUID detection of prepolarized spins, however, the NMR signal strength is independent of precession field, allowing MRI signal detection in extremely weak fields, on the order of Earth's magnetic field. SQUID-detected MRI has advantages over high-field MRI systems, such as the low cost required to build such a system, and its compactness. The principle has been demonstrated by imaging human extremities, and its future application may include tumor screening.<ref>{{cite book|last1=Clarke|first1=J.|last2=Lee|first2=A.T.|last3=Mück|first3=M.|last4=Richards|first4=P.L.
|chapter=Chapter 8.3|title=Nuclear Magnetic and Quadrupole Resonance and Magnetic Resonance Imaging|pages=56–81}} in {{harvnb|Clarke|Braginski|2006}}</ref>

Another application is the [[scanning SQUID microscope]], which uses a SQUID immersed in liquid [[helium]] as the probe. The use of SQUIDs in [[Petroleum|oil]] [[prospecting]], [[mineral exploration]],<ref>{{cite journal|author1=P. Schmidt |author2=D. Clark |author3=K. Leslie |author4=M. Bick  |author5=D. Tilbrook|author6-link=Cathy Foley |author6=C. Foley |s2cid=14994533 |name-list-style=amp |title=GETMAG—A SQUID magnetic tensor gradiometer for mineral and oil exploration|journal=Exploration Geophysics|volume=35|pages=297–305|year=2004|doi=10.1071/eg04297|issue=4|bibcode=2004ExG....35..297S }}</ref> 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 [[gravitational wave]]s.<ref>{{cite book
|last=Paik
|first=Ho J.
|chapter=Chapter 15.2 
|title = "Superconducting Transducer for Gravitational-Wave Detectors" in [volume 2 of] "The SQUID Handbook: Applications of SQUIDs and SQUID Systems"
|pages=548–554
}} in {{harvnb|Clarke|Braginski|2006}}</ref>
A SQUID is the sensor in each of the four gyroscopes employed on [[Gravity Probe B]] in order to test the limits of the theory of [[general relativity]].<ref name=Ran04/>

A modified RF SQUID was used to observe the [[Dynamical Casimir Effect|dynamical Casimir effect]] for the first time.<ref>{{cite magazine|url=http://www.technologyreview.com/blog/arxiv/26813/|title=First Observation of the Dynamical Casimir Effect|magazine=Technology Review}}</ref><ref>{{cite journal|last1=Wilson|first1=C. M.|title=Observation of the Dynamical Casimir Effect in a Superconducting Circuit|journal=Nature|volume=479|pages=376–379|year=2011|doi=10.1038/nature10561|arxiv = 1105.4714 |bibcode = 2011Natur.479..376W|pmid=22094697|issue=7373|s2cid=219735}}</ref>

SQUIDs constructed from super-cooled [[niobium]] wire loops are used as the basis for [[D-Wave Systems]] 2000Q [[quantum computer]].<ref>{{cite web|title=Not Magic Quantum|accessdate=2021-10-26|website=Lanl.gov|date=July 2016|url=http://www.lanl.gov/discover/publications/1663/2016-july/_assets/docs/1663_JULY-2016-Not-Magic-Quantum.pdf |archive-url=https://web.archive.org/web/20160729074720/http://www.lanl.gov/discover/publications/1663/2016-july/_assets/docs/1663_JULY-2016-Not-Magic-Quantum.pdf |archive-date=2016-07-29 |url-status=live}}</ref>

===Transition-edge sensors===
One of the largest uses of SQUIDs is to read out superconducting [[Transition-edge sensor]]s. Hundreds of thousands of multiplexed SQUIDs coupled to transition-edge sensors are presently being deployed to study the [[Cosmic microwave background]], for [[X-ray astronomy]], to search for dark matter made up of [[Weakly interacting massive particles]], and for spectroscopy at [[Synchrotron light sources]].

===Cold dark matter===
Advanced SQUIDS called near quantum-limited SQUID amplifiers form the basis of the [[Axion Dark Matter Experiment]] (ADMX) at the University of Washington. Axions are a prime candidate for [[cold dark matter]].<ref>A Squid-Based Microwave Cavity Search For Axions By ADMX; SJ Sztalos, G Carlos, C Hagman, D Kinion, K van Bibber, M Hotz, L Rosenberg, G Rybka, J Hoskins,  J Hwang, P Sikivie, DB Tanner, R Bradley, J Clarke; Phys.Rev.Lett. 104:041301; 2010</ref>

===Proposed uses===
A potential military application exists for use in [[anti-submarine warfare]] as a [[magnetic anomaly detector]] (MAD) fitted to [[maritime patrol aircraft]].<ref>{{cite web|url=http://www.aip.org/tip/INPHFA/vol-4/iss-2/p20.pdf |title=SQUID Sensors Penetrate New Markets |first=Jennifer |last=Ouellette |page=22 |publisher=The Industrial Physicist |url-status=dead |archive-url=https://web.archive.org/web/20080518032905/http://aip.org/tip/INPHFA/vol-4/iss-2/p20.pdf |archive-date=18 May 2008 }}</ref>

SQUIDs are used in [[superparamagnetic relaxometry]] (SPMR), a technology that utilizes the high magnetic field sensitivity of SQUID sensors and the superparamagnetic properties of magnetite [[nanoparticle]]s.<ref>{{Cite journal|last1=Flynn|first1=E R|last2=Bryant|first2=H C|title=A biomagnetic system for in vivo cancer imaging|journal=Physics in Medicine and Biology|volume=50|issue=6|pages=1273–1293|doi=10.1088/0031-9155/50/6/016|pmc=2041897|pmid=15798322|bibcode=2005PMB....50.1273F|year=2005}}</ref><ref>{{Cite journal|last1=De Haro|first1=Leyma P.|last2=Karaulanov|first2=Todor|last3=Vreeland|first3=Erika C.|last4=Anderson|first4=Bill|last5=Hathaway|first5=Helen J.|last6=Huber|first6=Dale L.|last7=Matlashov|first7=Andrei N.|last8=Nettles|first8=Christopher P.|last9=Price|first9=Andrew D.|date=2015-10-01|title=Magnetic relaxometry as applied to sensitive cancer detection and localization|journal= Biomedical Engineering / Biomedizinische Technik|volume=60|issue=5|pages=445–455|doi=10.1515/bmt-2015-0053|pmid=26035107|osti=1227725|s2cid=13867059|issn=1862-278X|doi-access=free}}</ref> These nanoparticles are paramagnetic; they have no magnetic moment until exposed to an external field where they become ferromagnetic. After removal of the magnetizing field, the nanoparticles decay from a ferromagnetic state to a paramagnetic state, with a time constant that depends upon the particle size and whether they are bound to an external surface. Measurement of the decaying magnetic field by SQUID sensors is used to detect and localize the nanoparticles. Applications for SPMR may include cancer detection.<ref>{{Cite journal|last1=Hathaway|first1=Helen J.|last2=Butler|first2=Kimberly S.|last3=Adolphi|first3=Natalie L.|last4=Lovato|first4=Debbie M.|last5=Belfon|first5=Robert|last6=Fegan|first6=Danielle|last7=Monson|first7=Todd C.|last8=Trujillo|first8=Jason E.|last9=Tessier|first9=Trace E.|date=2011-01-01|title=Detection of breast cancer cells using targeted magnetic nanoparticles and ultra-sensitive magnetic field sensors|journal=Breast Cancer Research|volume=13|issue=5|pages=R108|doi=10.1186/bcr3050|issn=1465-542X|pmc=3262221|pmid=22035507 |doi-access=free }}</ref>