# Superconducting Quantum Interference Devices (SQUIDs)

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```							Superconducting Quantum
Interference Devices (SQUIDs)
By Leyan Lo
Overview
   Background

   SQUID Theory

   SQUID Uses
SQUID Background
   The SQUID is an extremely sensitive
magnetic field detector.

   Can detect fields on the order of 10-15 T
◦ Earth’s magnetic field: 10-4 T
◦ Heart’s magnetic field: 10-11 T
◦ Brain’s magnetic field: 10-15 T
SQUID Background
   Invented in 1964 by Robert Jaklevic, John
Lambe, Arnold Silver, and James
Mercereau of Ford Research Labs.

   Two years after the Josephson effect was
postulated in 1962.

   One year after the first Josephson
Junction was made by John Rowell and
Philip Anderson at Bell Labs in 1963.
SQUID Background
   Two kinds of SQUID: DC and RF.

   RF SQUIDs have only one Josephson and
are cheaper to produce, but are not as
sensitive.

   DC SQUIDs have two or more junctions
and are much more sensitive. (We will
only focus on these types).
Cooper Pairs
   A superconductor is a many body system
made of electrons bound in pairs, called
“Cooper pairs”.

   These pairs can be described by the
wavefunction:
Flux Quantization
   The current density for a
superconducting ring is approximately
zero.
Flux Quantization
   Introduce the vector potential A into the
Schroedinger equation:
Flux Quantization
   Integrate over the curve Γ:

   This is called the
“Flux quantum”
Screening Current
   A superconducting ring will always have
an integer multiple of this 0.

   The ring will generate a screening current
to satisfy this property:
Screening Current
   But how can we measure this screening
current? This is a periodic function:

   V = I R Will not work in this case
because there is no resistance in a
superconductor!
Josephson Junction
   Cooper pairs can tunnel across a thin
barrier separating two superconductors
up until a critical current value:
Bias Current
   Remember, our task was to measure the
screening current!

   Inject a bias current to ride the “knee” of
the curve.
Periodic Relationship
   Periodic relationship between voltage and
flux:

   Introduce Phase Locked Loop for a direct
relationship.

   A two coil system allows the
SQUID to measure the
derivate of the B-field.

   This system ignores plane
waves emitted from distant
sources, and focuses attention
to local sources.
http://www.aston.ac.uk/
A Lot of Loops
   Photograph of a
dc-SQUID

   10 x 10 mm2

Barone, A.
SQUIDS in the Body
   Biomagnetism is one of the most
promising applications of SQUIDs

   Today it is a new field of research where
interdisciplinary collaboration takes place
by physicists, mathematicians, physiologists
and psychologists.
SQUIDs in the Body
   In 1791, Galvani discovered animal
electricity

   In 1887, English physiologist Waller
measured electric potentials in the heart.

   It wasn’t until 1969 when the heart’s
magnetic field could be observed by Baule
and McFee with the SQUID.
SQUIDs in the Body
able to detect fields in
the brain, which are
10,000 weaker than
those from the heart.

   This is called
Magnetoencephalography
(MEG)
Singh, Manbir, et al. 1990
SQUIDs in the Field
   Portable SQUID vector system developed
in Japan

   Could be used
to detect
geological
activity

Machitani, Y., et al. 2003
SQUIDs as accelerometers
   SQUID sensors can
be used to sense
small displacements
in objects under
acceleration.

Hull, John R., and Thomas M. Mulcahy 1999
SQUIDs Searching for Gravitational
Waves
   The search for gravitational waves began
in the 1960’s.

   Two types of detectors:
◦ Michelson interferometers
 light paths altered by GW
◦ Bar detectors
 large “bells” rung by GW
AURIGA
 Resonant bar detector
 3m, 2.3 ton Aluminum
mass
 Q ~ 4x106 @ 100mK
 Resonance is at
920Hz

http://www.auriga.lnl.infn.it/
Conclusion
 SQUIDs are cool (literally!)
 There are many applications for SQUIDs
in various fields:
◦ MEG
◦ Geology
◦ Gravitational Waves
   The field is still young
References
   Barone, Antonio, ed. Principles and Applications of Superconducting Quantum Interference
Devices. Singapore:World Scientific, 1992.
   Hull, John R., and Thomas M. Mulcahy. "Gravimeter Using High-Temperature Superconducting
Bearing." IEEE (1999). 29 Jan. 2007 <http://ieeexplore.ieee.org>.
   Kirtley, J. R., et al. “Design and applications of a scanning SQUID microscope.” Journal of
Research & Development (1995). 29 Jan. 2007 <http://www.neiu.edu>.
   Machitani,Y., et al. “Vector HTS-SQUID System for ULF Magnetic Field Monitoring.” IEEE (2003).
29 Jan. 2007 <http://ieeexplore.ieee.org>.
   Singh, Manbir, et al. "Neuromagnetic Localization Using Magnetic Resonance Images." IEEE
(1990). 29 Jan. 2007 <http://ieeexplore.ieee.org>.

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