SQUIDs: From Cosmology to Magnetic Resonance Imaging in Microtesla Fields • Milestones in superconductivity • The SQUID • Applications of SQUIDs: an overview • Searching for cold dark matter with a SQUID • Magnetic resonance imaging with a SQUID The Finnish Academy 8 November 2004
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SQUIDs: From Cosmology to Magnetic Resonance Imaging in Microtesla Fields
• Milestones in superconductivity
• The SQUID
• Applications of SQUIDs: an overview
• Searching for cold dark matter with a SQUID
• Magnetic resonance imaging with a SQUID
The Finnish Academy8 November 2004
Centigrade/Kelvin/Fahrenheit Temperature ScalesoF
32
-459
0
-100
-200
-273
oCRoom temperature
Ice point
Vostok, Antarctica -88 oC8/24/60
B.P. liquid nitrogen 77K
B.P. liquid helium 4.2K
K
273
173
73
0Absolute zero
Milestones in Superconductivity
1911 Kamerlingh Onnes discovers zero resistance
Heike Kamerlingh Onnes
Courtesy Kamerlingh Onnes Laboratory, University of Leiden
The Discovery of Superconductivity
Resistance vanishes below the transition (or critical) temperature Tc
4.0 4.2 4.4
Res
ista
nce
(Ω)
Temperature (K)
Magnetic Fields
The earth Bar magnet
~1 gauss10-4 tesla
~1000 gauss0.1 tesla
Zero Resistance
I
Magnetic field
• Current persists forever
• Resistance at least one billion billiontimes less than copper
Superconducting Flux Transformer:Magnetometer and Gradiometer
Flux-locked SQUID
Flux-locked SQUID
BN ~ 1 fT Hz-1/2
Magnetic Fields
1 femtotesla
10-10
10-8
10-6
10-4
10-12
10-14
10-16
Earth’s field
Urban noise
Car at 50 m
Human heart
Fetal heart
Human brain response
SQUID
tesla
Applications of SQUIDs:
An Overview
Olli Lounasmaa
Olli was did much to bring SQUIDs to Finland, and greatly encouraged their application to Magnetoencephalography (MEG).MEG is the single biggest consumer of SQUIDs, and has important applications in both brain research and clinical diagnosis. Neuromag is a leading supplier of MEG systems around the world. Olli was also deeply involved with the use of SQUIDs to study nuclear ordering in copper and silver at ultralowtemperatures.
Neuromag® 306-Channel SQUID System for Magnetoencephalography
Applications of Magnetoencephalography
Clinical (Reimbursable in the United States)• Presurgical screening of brain tumors (evoked response)
• Location of epileptic foci (spontaneous signals)
Research• Language mapping in the brain
• Identification of patients with schizophrenia
• Identification of patients with dyslexia
• Alzheimer's disease
• Parkinson's disease
• Neurological recovery following stroke or hemorrhage
CardioMag Imaging System for Magnetocardiography
Quantum Design "Evercool"
2G Superconducting Rock Magnetometer
SQUID Surveying for Minerals
Courtesy Cathey Foley (CSIRO)
MAGMA-C1 Scanning SQUID Microscope Neocera, Inc.
Atacama Pathfinder EXperiment
Gravity Probe-BTests of General Relativity
Courtesy Stanford University and NASA
UC Berkeley Flux Qubits
35 µm
Qubit 1
Qubit 2
Searching for Axions: The Microstrip SQUID Amplifier
University of GießenMichael MückJost GailChristoph Heiden†
UCB, LBNL and LLNLMarc-Olivier André Darin KinionJan Kycia
Support: DOE/BESDOE/HEPNSF
Cold Dark Matter
• Recent cosmic microwave background measurements indicate that
~25% of the mass of the universe is cold dark matter (CDM).
• A candidate particle is the axion, proposed in 1978 to explain the
absence of a measurable neutron electric dipole moment.
• The axion is predicted to be a very light particle with no charge or spin.
Pow
erFrequency
610~ −∆νν
Resonant Conversion of Axions into PhotonsPierre Sikivie (1983)
Primakoff Conversion Expected Signal
Axion Detector at Lawrence Livermore National Laboratory
Noise Temperature
-ART
( )[ ]RRTT4kA(f)S NB20
V +⋅=
V0
LLNL Axion Detector
• Current system noise temperature: TS = T + TN ≈ 3.2 K
Cavity temperature: T ≈ 1.5 K
Amplifier noise temperature: TN ≈ 1.7 K
• Time to scan the range of frequencies from f1 to f2:
For f1 = 0.24 GHz, f2 = 2.4 GHz: τ ≈ 45 years
τ(f1, f2) ≈ 4 x 1016(TS/1 K)2(1/f1 – 1/f2) sec
• Note: There is only a factor of 2 to be gained in TS by reducing T unless TN is also reduced.
• Source connected to both ends of coil • Source connected to one end of the coil and SQUID washer; the other end of the coil is left open
Gain vs. Coil Length
600
400
200ν res (M
Hz)
604020Coil Length (mm)
30
25
20
15
10
Gai
n (d
B)
700600500400300200100Frequency (MHz)
71 mm33 mm
15 mm
7 mmGai
n (d
B) ν r
es (M
Hz)
Noise Temperature of Microstrip Amplifier
At 20 mK the noise temperature is 50mK, about 40 times lowerthan that of the current semiconductor amplifier
Microstrip SQUID Amplifier: Impact on Axion Detector
• Current LLNL axion detector: TS ≈ 3.2 K
• For T ≈ TN ≈ 50 mK: TS ≈ T + TN ≈ 100 mK
τ ≈ 45 years x (0.1/3.2)2
≈ 18 days
Summary
• Gain ≥ 20 dB for frequencies ≤ 1 GHz
• Cooled to 20 mK, TN is within a factor of 2 of the quantum limit
• Noise temperature 40 times lower than state-of-the-art cooled semiconductor amplifiers
Future directions
• Implement second-generation axion detector: expected to increase scan rate by three orders of magnitude
• Post-amplifier for radio-frequency single-electron transistor: should enable quantum-limited charge amplifier
Microtesla Nuclear Magnetic Resonance and Magnetic Resonance Imaging
• Nuclear magnetic resonance
• Magnetic resonance imaging
Michael HatridgeNathan KelsoSeungKyun LeeRobert McDermottMichael MössleMichael MückWhit MyersBennie ten HakenAndreas TrabesingerErwin HahnAlex Pines
Ener
gy
B0
E = +µpB
E = -µpBω0= γB0 Magnetic moment (µpB0 << kBT)
B0
Nuclear Magnetic Resonance
z
ν0 = 42.58 MHz/tesla
TkBN
NNNN
NMB
pp
02µ
µ =−−
=↓↑
↓↑
Protons
B
Equilibrium RF pulse Precession
M
B0
M
B1
M
B0
High Field MRI
3T MRI scanner (GE) 1.5T MRI scanner (GE)
TimelineMichael Crichton, 1999
“Most people”, Gordon said, “don’t realize that the ordinary hospital MRI works by changing the quantum state of atoms in your body ... But the ordinary MRI does this with a very powerful magnetic field - say 1.5 tesla, about twenty-five thousand times as strong as the earth’s magnetic field. We don’t need that. We use Superconducting QUantum Interference Devices, or SQUIDs, that are so sensitive they can measure resonance just from the earth’s magnetic field. We don’t have any magnets in there”.
The “Cube”
1 6
5 5
1 23
4 6
1 3
6
2
4
20 mm
Three dimensional images of pepper
T1-weighted Contrast Imaging
• T1 is the relaxation time of the proton spins
• T1 depends strongly on the environment of the protons
• T1-weighted contrast imaging is widely used in conventional MRI to distinguish different types of tissue
• T1 (malignant tissue) > T1 (normal tissue)
• T1-contrast can be much higher in low fields
T
B = 13.2 mTint
agarose
0.5%0.25%
B = 300 mTint
T1 contrast images of agarose gel
Forearm (20 mm slice)
Bp ~ 40 mT B0 = 132 µT
B0 = 4 T
4T image:Courtesy of Ben Inglis,Henry H. Wheeler, Jr.Brain Imaging Center,
UC Berkeley
Future directions for low-field MRI
• Reduce system noise • Increased signal-to-noise ratio• Reduced acquisition time