Superconducting Quantum Interference Devices (SQUIDs)

							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.
Gradiometer

   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
   Nowadays, SQUIDs are
    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
  near Padova, Italy
 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>.