squids: from cosmology to magnetic resonance imaging in

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

• Basis of superconducting magnets

A Few Other Superconductors

Element Tc

Aluminum 1.2 KIndium 3.4 KTin 3.7 KLead 7.2 KNiobium 9.2 K

• “Type I” superconductors

• Driven normal by magnetic fields less than 2000 gauss



1957 Bardeen, Cooper and Schrieffer develop “BCS” theory

Normal Metals vs. Superconductors

I I

Electron has charge -e

Scattering of electrons produces resistance.

A current generates a voltage, and hence causes dissipation.

Normal Metals Superconductors : BCS Theory

Electrons are paired together : these Cooper pairs have charges -2e

I I

Cooper pairs carry a supercurrent which encounters no resistance.

A supercurrent generates no voltage, and hence causes no dissipation.




1957 Alexei Abrikosov predicts Type II superconductors Large-scale applications are made possible

Type II Superconductors

Alloys: ~2000 known

The secret of their success: Type II materials admit “vortices” of magnetic field, and the supercurrents flow around them.

High field magnets made possible.

Flux Quantization

Φ = nΦ0 (n = 0, ±1, ±2, ...)whereΦ0 ≡ h/2e ≈ 2 x 10-15 Wbis the flux quantum

Φ = n Φ0

J




1957 Alexei Abrikosov predicts Type II superconductors

1960 Ivar Giaever invents tunnel junctions

1962 Brian Josephson invents “Josephson Tunneling”

Large-scale applications are made possible

Superconducting electronics is born

Josephson Tunneling

Brian Josephson 1962• Cooper pairs tunnel through barrier

V

I

I

oxidized Nb film

Nb film

I

superconductor superconductor

~ 20 Å

insulatingbarrier

I

V

VoltageC

urre

nt

Created the field of superconducting electronics




1957 Alexei Abrikosov predicts Type II superconductors

1960 Ivar Giaever invents tunnel junctions

1962 Brian Josephson invents “Josephson Tunneling”

1986 Bednorz and Muller discover high-Tc superconductivity



160

0

40

80

120

1910 1960 1980 2000

liquid N2

liquid He

Year

Tem

pera

ture

(Kel

vin)

Hg Pb

Nb NbN

Nb3Ge

(La/Sr)CuO4

YBa2Cu3O7

Bi2Sr2Ca2Cu3O10

HgBa2Ca2Cu3O8

(1911)

MgB2

-100 °C

SrTiO3

Transition Temperature Over the Years


1911 Kamerlingh Onnes discovers zero resistanceNobel Prize 1913

1957 Bardeen, Cooper and Schrieffer develop “BCS” theoryNobel Prize 1972

1957 Alexei Abrikosov predicts Type II superconductorsNobel Prize 2003

1960 Ivar Giaever invents tunnel junctionsNobel Prize 1973

1962 Brian Josephson invents “Josephson Tunneling”Nobel Prize 1973

1986 Bednorz and Muller discover high-Tc superconductivityNobel Prize 1987



The SQUID

The dc Superconducting Quantum Interference Device

Ib

nΦo

(n+1/2)Φo

∆V

I

V

V

0 1 2 Φo

Φ

δV

Φδ

• Current-voltage (I-V) characteristic modulated by magnetic flux Φ: Period one flux quantum Φo = h/2e = 2 x 10-15 T m2

• dc SQUID Two Josephson junctions on a superconducting ring

I

V

Low-Tc SQUID

Nb-AlOx-Nb SQUIDwith input coil

Josephson junctions

500 µm20 µm

Operating temperature = 4.2 K

Multilayer device: niobium - aluminum oxide - niobium

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.

Microstrip SQUID Amplifier

Conventional SQUID Amplifier Microstrip SQUID Amplifier

• 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

• Multichannel system• Increased signal-to-noise ratio• Improved spatial resolution• Increased coverage

• Combine low-field MRI with existing technologyfor magnetoencephalography (MEG)

• Low-cost “open” MRI system• Screening for tumors (with T1-weighted contrast) • Imaging knee, foot, elbow, wrist ...• Monitoring T1 in bone marrow

squids: from cosmology to magnetic resonance imaging in

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