1. Recent advances in atomic magnetometry Michael Romalis Princeton University

2. Attotesla magnetometry
Magnetic Field Scale
Attotesla magnetometry

3. SQUID Magnetometers Based on Josephson tunneling effect
In superconducting shields
Best Field Sensitivity:
Low - Tc SQUIDs (4 K) fT/Hz1/2
High- Tc SQUIDs (77 K) fT/Hz1/2
D. Drung, et al.

4. Quantum uncertainty principle
Spin Precession B m w B μ = τ S dt d w = 2m B h
Quantum uncertainty principle
S = N/2
Noise 1/ N T2
N atoms
FFT
Quantum noise for N atoms: 1 dw = 1 T 2 Nt
pT2

5. Mechanisms of spin relaxation
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 – 1 = s v n 2 se
T2N = ssevV d B  1 fT cm 3 Hz

6. Eliminating relaxation due to spin-exchange collisions
High magnetic field:
Low magnetic field:
Zeeman transitions +w
F=2 SE
F=1
Zeeman transitions -w
mF =
Ground state Zeeman and hyperfine levels
W. Happer and H. Tang, PRL 31, 273 (1973)

7. Spin-exchange relaxation free regimeB
Chopped pump beam
High-field linewidth: 3 kHz
Low-field linewidth: 1 Hz 10 20 30 40 50
Chopper Frequency (Hz)
-0.1
0.0
0.1
0.2
Lock-in Signal (V
rms )
- in phase
- out of phase
Linewidth at finite field
Linewidth at zero field
J. C. Allred, R. N. Lyman, T. W. Kornack, and MVR, Phys. Rev. Lett. 89, (2002)

8. 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
Probe
Pump dB S a
a ~ wT2 
probe beam s+ s-
Cell
1/2
-1/2 + -
 =  (n+ - n-) L /

9. Cartoon picture of atomic magnetometer
Alkali metal vapor in a glass cell
Linearly Polarized
Probe light
Magnetization
Magnetization
Magnetic Field
Circularly Polarized
Pumping light
Cell contents
[K] ~ cm-3
4 He buffer gas, N2 quenching
With optically pumped alkali atom vapor, we can make a sensitive magnetic field sensor without cryogenic maintenance.
Alkali atoms will be polarized by absorbing circularly polarized light due to the selection rule.
And this makes total magnetization.
The magnetization will be tipped around the magnetic field. In this case, the By field going through the screen.
The tipping makes x component of magnetization, this gives the different refraction indexes for the right and the left circularly polarized lights, respectively.
As a result, the cell rotates the polarization angle of the linearly polarized probe light.
So we can measure the By field by detecting this angle rotation. z
Polarization angle rotation
 gByT2 x y
Atomic magnetometer review: D. Budker and M. V. R., Nature Physics 3, 227 (2007).

10. Johnson current noise in m-metal magnetic shields

11. 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
T. W. Kornack, S. J. Smullin, S.-K. Lee, and MVR, Appl. Phys. Lett. 90,  (2007)
Low intrinsic noise, prospect for 100 aT/Hz1/2 sensitivity

12. SERF Magnetometer Sensitivity
Typical SQUID sensitivity
Noise due to dissipation in ferrite magnetic shield
0.2 fT/Hz1/2
Record low-frequency magnetic field sensitivity
Applications: Paleomagnetism
Single-domain nanoparticle detection

13. Magnetoencephalography
Low-temperature SQUIDs in liquid helium at 4K
channels, 3-5fT/Hz1/2, cm channel spacing
Cost ~ $1-3m
Clinical and functional studies
Auditory response
Elekta Neuromag
H. Weinberg, Simon Fraser University

14. Magnetoencephalography with atomic magnetometer
256 channel detector
Alkali-metal cell
Magnetic shields
Pump and probe beam arrangement
Subject

15. Brain signals from auditory stimulation
Magnetic fields from 64 center channels
N100m peak; averaging 250 epochs
SNR~11 for the best channel
Stimulus onset
K cell
Probe beam
Pump beam
Pneumatic earphone
Mu-metal magnetic shield
This is the result. The practical noise level of the center channels shows around 20 fT/root herz.
If we eliminate the common mode field noise between adjacent channels, the noise floor drops to 3 fT/root hertz with 4 mm baseline at the best sensor.
After averaging 250 times, we could get a clear N100m peak.
This peak is a typical AEF response which appears after 100 ms from the stimulus.
At the best channel, The Signal to noise ratio was above 10.
The spatial profile of the magnetic field shows clear gradient depending on the distance between the sensor and the source .
Kiwoong Kim et al

16. 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 MHz
Very low rate of false alarms
Main problem – detection with an inductive coil gives very poor signal/noise ratio
Quantum Magnetics, GE

17. Reduction of spin-exchange broadening in finite magnetic field
Linewidth dominated by spin-exchange broadening
Optimal pumping rate
Dn = (Rex Rsd /5)1/2/2p
Linewidth broadened by pumping rate
I.M. Savukov, S.J. Seltzer, MVR, K. Sauer, PRL 95, (2005)

18. Detection of NQR signals with atomic magnetometer
Spin-echo sequence
Pump laser
Probe laser B0
Brf S w
wrf = gB0 Y X
22 g of Ammonium Nitrate
4 minutes/point
(2048 echoes, 8 repetitions)
Signal/noise is 12 times higher than for an RF coil located equal distance away from the sample!
0.2 fT/Hz1/2
At high frequencies conductive materials generate much less thermal magnetic noise
S.-K. Lee, K. L. Sauer, S. J. Seltzer, O. Alem, M.V.R ,Appl. Phys. Lett. 89,  (2006)

19. K-3He Co-magnetometer B = 8 p 3 k M
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 B K = 8 p 3 k M He m B

20. Magnetic field self-compensation
Magnetic noise level in the shields
0.7fT/Hz1/2

21. Nuclear Spin Gyroscope
Rotation creates an effective magnetic field Beff = W/g
eff S H B Ω × - = m
Beff He = 24
fT/(1
deg/hour)
Beff K = . 17
fT/(1
deg/hour)
Random angle walk: 0.5 mdeg/hour1/2 = 1.510-7rad/secHz1/2

22. Long-Range Spin Forces
Mediated by light bosons: Axions, other Nambu-Goldstone bosons
Axions:
J. E. Moody and F. Wilczek (1984)
CP-violating
QCD angle
Monopole-Monopole:
Monopole-Dipole:
Dipole-Dipole:
Massless propagating spin-1 torsion:
Torsion:

23. Recent phenomenology Spontaneous Lorentz Violation
Arkani-Hamed, Cheng, Luty, Thaler, hep-ph/
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 …)
d- non-integer, in the range 1…2

24. Experimental techniquesˆ 1 × B m w S
Frequency shift
Acceleration
Induced magnetization 2 1 ˆ S × or S S or S
SQUID
Magnetic shield

25. Search for long-range spin-dependent forces
Spin Source:
He spins at 20 atm.
Spin direction reversed every 3 sec with AFP
2= 0.87
K-3He co-magnetometer
Sensitivity: 0.7 fT/Hz1/2
Uncertainty (1) = 18 pHz or 4.3·10-26 eV 3He energy

26. New limits on neutron spin-dependent forces
Constraints on pseudo-scalar coupling:
Limit on proton nuclear-spin dependent forces
Limit from gravitational experiments for Yukawa coupling only
Present work
G. Vasilakis, J. M. Brown, T. W. Kornack, MVR, arXiv: v1
Anomalous spin forces between neutrons are:
< 210-8 of their magnetic interactions
< 210-3 of their gravitational interactions
First constraints of sub-gravitational strength!

27. 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)
Support: ONR, DARPA, NIH, NRL, NSF, Packard Foundation, Princeton University
Karen Sauer (GMU)
Mike Souza – our glassblower