Recent advances in

atomic magnetometry

Michael Romalis

Princeton University

Magnetic Field Scale

Attotesla magnetometry

SQUID Magnetometers

• Based on Josephson tunneling effect

• Best Field Sensitivity: Low - Tc SQUIDs (4 K) 1 fT/Hz1/2

High- Tc SQUIDs (77 K) 20 fT/Hz1/2

D. Drung, et al.

In superconducting shields

Spin PrecessionB

=B

h

T2

Quantum noise for N atoms:

=1

T2Nt

S = N/2N1/

Noise

BμτS

dtd

FF

T

Quantum uncertainty principle

T2

1

N atoms

Collisions between alkali atoms, with buffer gas and cell walls

• Spin-exchange alkali-alkali collisions

Increasing density of atoms decreases spin relaxation time

Under ideal conditions:

T 2–1 = se v n

se = 2 –cm2

B 1fT cm3

Hz

T2N = sevV

Mechanisms of spin relaxation

Eliminating relaxation due to spin-exchange collisions

W. Happer and H. Tang, PRL 31, 273 (1973)

F=2

F=1

mF

Ground state Zeeman and hyperfine levels

Zeeman transitions +

Zeeman transitions

SE

• High magnetic field:

• Low magnetic field:

Spin-exchange relaxation free regime

S

BChopped pump beam

10 20 30 40 50Chopper Frequency (Hz)

-0.1

0.0

0.1

0.2

Loc

k-in

Sig

nal (

V rm

s

)

in phase

out of phase

High-field linewidth: 3 kHz

Low-field linewidth: 1 Hz

J. C. Allred, R. N. Lyman, T. W. Kornack, and MVR, Phys. Rev. Lett. 89, 130801 (2002)

Linewidth at finite field Linewidth at

zero field

Operate the magnetometer near zero field

Spins are polarized along the pump laser

Measure rotation of spin polarization due to a torque from the magnetic field

Use optical polarization rotation of a probe beam to measure spin response

ProbePump

B

S

1/2-1/2

+

probe beam

+

Cell = (n+ - n-) L /

~ T2

Cartoon picture of atomic magnetometer

Alkali metal vapor in a glass cell

MagnetizationMagnetization

Magnetic Field

Linearly Polarized Probe light

Circularly Polarized Pumping light

Polarization angle rotation

ByT2 x

z

y

Cell contents[K] ~ 1014 cm-3

4 He buffer gas, N2 quenching

Atomic magnetometer review: D. Budker and M. V. R., Nature Physics 3, 227 (2007).

Johnson current noise in -metal magnetic shields

Ferrite Magnetic Shield

• Ferrite is electrically insulating, no Johnson noise

• Single-channel sensitivity 0.75fT/Hz1/2

• Remaining 1/f noise due to hysteresis losses Determined by the imaginary part of magnetic permeability

10 cm

Low intrinsic noise, prospect for 100 aT/Hz1/2 sensitivity

T. W. Kornack, S. J. Smullin, S.-K. Lee, and MVR, Appl. Phys. Lett. 90, 223501 (2007)

SERF Magnetometer Sensitivity

0.2 fT/Hz1/2

Noise due to dissipation in ferrite magnetic shield

Typical SQUID sensitivity

Record low-frequency magnetic field sensitivity

Applications: Paleomagnetism

Single-domain nanoparticle detection

Magnetoencephalography

Auditory response

H. Weinberg, Simon Fraser University

• Low-temperature SQUIDs in liquid helium at 4K

• 100 300 channels, 3-5fT/Hz1/2, 2 3 cm channel spacing

• Cost ~ $1-3m

• Clinical and functional studies

Elekta Neuromag

Magnetoencephalography with atomic magnetometer

SubjectSubject

256 channel detector

Alkali-metal cell

Magnetic shieldsPump and probe beam arrangement

Brain signals from auditory stimulationMagnetic fields from 64 center channels

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6-200

-100

0

100

200

300

400

500

Ma

gn

etic f

ield

(fT

)

Time (s)

N100m peak; averaging 250 epochsSNR~11 for the best channel

Stimulus onsetK cell

Probe beam

Pump beam

Pneumatic earphone

Mu-metal magnetic shield

Kiwoong Kim et al

Detection of Explosives with Nuclear Quadruple Resonance

Similar to NMR but does not require a magnetic field

NQR frequency is determined by the interaction of a nuclear quadrupole moment

with electric field gradient in a polycrystalline material

Most explosives contain 14N which has a large quadrupole moment

Each material has a very specific resonance frequency in the range 0.5-5 MHz

Very low rate of false alarms

Main problem – detection with an inductive coil gives very poor signal/noise ratio

Quantum Magnetics, GE

Reduction of spin-exchange broadening in finite magnetic field

Linewidth dominated by spin-exchange broadening

Linewidth broadened by pumping rate

Optimal pumping rate

= (Rex Rsd /5)1/2/2

I.M. Savukov, S.J. Seltzer, MVR, K. Sauer, PRL 95, 063005(2005)

Detection of NQR signals with atomic magnetometer

Spin-echo sequence

22 g of Ammonium Nitrate4 minutes/point(2048 echoes, 8 repetitions)

Y Y Y YX

Signal/noise is 12 times higher than for an RF coil located equal distance away from the sample!

S.-K. Lee, K. L. Sauer, S. J. Seltzer, O. Alem, M.V.R ,Appl. Phys. Lett. 89, 214106 (2006)

0.2 fT/Hz1/2

At high frequencies conductive materials generate much less thermal magnetic noise

Pump laser

Probe laserB0

Brf S

rf = B0

K-3He Co-magnetometer

1. Use 3He buffer gas in a SERF magnetometer

2. 3He nuclear spin is polarized by spin-exchange collisions with alkali metal

3. Polarized 3He creates a magnetic field felt by K atoms

4. Apply external magnetic field Bz to cancel field BK

K magnetometer operates near zero field

5. In a spherical cell dipolar fields produced by 3He cancel

3He spins experience a uniform field Bz

Suppress relaxation due to field gradients

BK = 83

0MHe

m

m

m

m

B

Magnetic field self-compensation

Magnetic noise level in the shields

0.7fT/Hz1/2

• Rotation creates an effective magnetic field Be

f

f

= /

)(4

~ 2 Heeff

Keff

B BBT

S

Nuclear Spin Gyroscope

deg/hour) fT/(117.0deg/hour) fT/(124

K

He

Beff

Beff

effS

H B SSΩ

Random angle walk: 0.5 mdeg/hour1/2 = 1.510rad/secHz1/2

Long-Range Spin Forces

• Monopole-Monopole:

• Monopole-Dipole:

• Dipole-Dipole:

• Massless propagating spin-1 torsion:

221

~4

fr

eggV

mrss

mm

3222

21

~1

)ˆˆ(8 f

err

mrS

M

ggV mrps

md

4322132

2

2121

211

~1

)ˆˆ(33

)ˆˆ)(ˆˆ(16 f

err

mSS

rr

m

r

mrSrS

MM

ggV mrpp

dd

J. E. Moody and F. Wilczek (1984)

Mediated by light bosons: Axions, other Nambu-Goldstone bosons

Axions:

CP-violatingQCD angle

Torsion:

r

SSmmGVt

2121ˆˆ

Recent phenomenology• Spontaneous Lorentz Violation

Arkani-Hamed, Cheng, Luty, Thaler, hep-ph/0407034 Goldstone bosons mediate long-range forces

Peculiar distance and angular dependence Lorentz-violating effects in a frame moving relative to CMB

• Unparticles (Georgi …)

Spin forces place best constraints on axial coupling of unparticles

• Light Z’ bosons (Dobrescu …)

r

SSrSrS

F

MVSLV

)ˆˆ()ˆˆ)(ˆˆ(

82121

2

2

1222

2

~ ddA

un r

cV d- non-integer, in the range 1…2

Experimental techniques

• Frequency shift

• Acceleration

• Induced magnetization

rS ˆ1̂

21ˆˆ SS

B

S

SSQUID or

S or S

Magnetic shield

Search for long-range spin-dependent forces

Spin Source: 1022 3He spins at 20 atm.

Spin direction reversed every 3 sec with AFP

Uncertainty (1) = 18 pHz or 4.3·10-26 eV 3He energy

2= 0.87

aTaTbb ne 56.005.0

K-3He co-magnetometer

Sensitivity: 0.7 fT/Hz1/2

New limits on neutron spin-dependent forces

• Constraints on pseudo-scalar coupling:

Anomalous spin forces between neutrons are:< 210 of their magnetic interactions< 210 of their gravitational interactions

Present work

Limit from gravitational experiments for Yukawa coupling only

)()()(2 5 xxx

m

gLDer

)()()( 5 xxxigLYuk

Limit on proton nuclear-spin dependent forces

First constraints of sub-gravitational strength!

G. Vasilakis, J. M. Brown, T. W. Kornack, MVR, arXiv:0809.4700v1

Support: ONR, DARPA, NIH, NRL, NSF, Packard Foundation, Princeton University

Collaborators Tom Kornack (G) Iannis Kominis (P) Scott Seltzer (G) Igor Savukov (P) SeungKyun Lee (P) Sylvia Smulin (P) Georgios Vasilakis (G) Andrei Baranga (VF) Rajat Ghosh (G) Hui Xia (P) Dan Hoffman (E) Joel Allred (G) Robert Lyman (G)

Karen Sauer (GMU) Mike Souza – our glassblower