Recent advances in atomic magnetometry Michael Romalis Princeton University
description
Transcript of Recent advances in atomic magnetometry Michael Romalis Princeton University
Recent advances in atomic magnetometry
Michael RomalisPrinceton 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
= Bh
T2
Quantum noise for N atoms:
= 1T2Nt
S = N/2N1/Noise
BμτSdt
d
FFT
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
Lock
-in S
igna
l (V r
ms )
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
Subject
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
Mag
netic
fiel
d (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 magnetometerSpin-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-magnetometer1. 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 cancel3He 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 BBTS
Nuclear Spin Gyroscope
deg/hour) fT/(117.0deg/hour) fT/(124
K
He
Beff
Beff
effSH 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
freggV
mrss
mm
3222
21
~1)ˆˆ(8 f
err
mrSMgg
V mrpsmd
4322132
2
2121
21 1~1)ˆˆ(33)ˆˆ)(ˆˆ(16 f
err
mSSrr
mr
mrSrSMM
ggV mrpp
dd
J. E. Moody and F. Wilczek (1984)
Mediated by light bosons: Axions, other Nambu-Goldstone bosons
Axions: CP-violating
QCD angle
Torsion:
rSSmmGVt
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 …)
rSSrSrS
FMVSLV
)ˆˆ()ˆˆ)(ˆˆ(8
21212
2
1222
2
~ ddA
un rcV 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= 0.87aTaTbb 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
mgLDer
)()()( 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