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Recent advances in atomic magnetometry Michael Romalis Princeton University.

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Presentation on theme: "Recent advances in atomic magnetometry Michael Romalis Princeton University."— Presentation transcript:

1 Recent advances in atomic magnetometry Michael Romalis Princeton University

2 Magnetic Field Scale Attotesla magnetometry

3 SQUID Magnetometers Based on Josephson tunneling effect Best Field Sensitivity:  Low - T c SQUIDs (4 K) 1 fT/Hz 1/2  High- T c SQUIDs (77 K) 20 fT/Hz 1/2 D. Drung, et al. In superconducting shields

4 Spin Precession B    =  B h T2T2 Quantum noise for N atoms:  = 1 T 2 Nt S = N  /2 N 1/ Noise Bμ   τ S  dt d FFT Quantum uncertainty principle T2T2 1 N atoms

5 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 vn  B  1fT cm 3 Hz T 2 N =  se vV Mechanisms of spin relaxation

6 Eliminating relaxation due to spin-exchange collisions W. Happer and H. Tang, PRL 31, 273 (1973) F=2 F=1 m F  Ground state Zeeman and hyperfine levels Zeeman transitions +  Zeeman transitions  SE High magnetic field: Low magnetic field:

7 Spin-exchange relaxation free regime S B Chopped pump beam 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

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 BB S 1/2-1/2 ++   probe beam ++  Cell  =  (n + - n - ) L /  ~  T 2 

9 Cartoon picture of atomic magnetometer Alkali metal vapor in a glass cell Magnetization Magnetic Field Linearly Polarized Probe light Circularly Polarized Pumping light Polarization angle rotation   B y T 2 x z y Cell contents [K] ~ 10 14 cm -3 4 He buffer gas, N 2 quenching Atomic magnetometer review: D. Budker and M. V. R., Nature Physics 3, 227 (2007).

10 Johnson current noise in  -metal magnetic shields

11 Ferrite Magnetic Shield Ferrite is electrically insulating, no Johnson noise Single-channel sensitivity 0.75fT/Hz 1/2 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/Hz 1/2 sensitivity T. W. Kornack, S. J. Smullin, S.-K. Lee, and MVR, Appl. Phys. Lett. 90, 223501 (2007)

12 SERF Magnetometer Sensitivity 0.2 fT/Hz 1/2 Noise due to dissipation in ferrite magnetic shield Typical SQUID sensitivity Record low-frequency magnetic field sensitivity Applications: Paleomagnetism Single-domain nanoparticle detection

13 Magnetoencephalography Auditory response H. Weinberg, Simon Fraser University Low-temperature SQUIDs in liquid helium at 4K 100  300 channels, 3-5fT/Hz 1/2, 2  3 cm channel spacing Cost ~ $1-3m Clinical and functional studies Elekta Neuromag

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

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 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 14 N 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

17 Reduction of spin-exchange broadening in finite magnetic field Linewidth dominated by spin-exchange broadening Linewidth broadened by pumping rate Optimal pumping rate  = (R ex R sd /5) 1/2 /2  I.M. Savukov, S.J. Seltzer, MVR, K. Sauer, PRL 95, 063005(2005)

18 Detection of NQR signals with atomic magnetometer Spin-echo sequence 22 g of Ammonium Nitrate 4 minutes/point (2048 echoes, 8 repetitions) Y Y Y Y X 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/Hz 1/2 At high frequencies conductive materials generate much less thermal magnetic noise Pump laser Probe laser B0B0 B rf S   rf =  B 0

19 K- 3 He Co-magnetometer 1. Use 3 He buffer gas in a SERF magnetometer 2. 3 He nuclear spin is polarized by spin-exchange collisions with alkali metal 3. Polarized 3 He creates a magnetic field felt by K atoms 4. Apply external magnetic field B z to cancel field B K  K magnetometer operates near zero field 5. In a spherical cell dipolar fields produced by 3 He cancel  3 He spins experience a uniform field B z  Suppress relaxation due to field gradients B K = 8  3  0 M He m m m m B

20 Magnetic field self-compensation Magnetic noise level in the shields 0.7fT/Hz 1/2

21 Rotation creates an effective magnetic field B eff =  /  Nuclear Spin Gyroscope deg/hour) fT/(117.0 deg/hour) fT/(124   K He B eff eff S H B SSΩ   Random angle walk: 0.5 mdeg/hour 1/2 = 1.5  10  rad/secHz 1/2

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

23 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 …) d- non-integer, in the range 1…2

24 Experimental techniques Frequency shift Acceleration Induced magnetization rS ˆ ˆ 1  21 ˆˆ SS  B   S S SQUID or S S Magnetic shield

25 Search for long-range spin-dependent forces Spin Source: 10 22 3 He spins at 20 atm. Spin direction reversed every 3 sec with AFP Uncertainty (1  ) = 18 pHz or 4.3·10 -26 eV 3 He energy  2 = 0.87 K- 3 He co- magnetometer Sensitivity: 0.7 fT/Hz 1/2

26 New limits on neutron spin-dependent forces Constraints on pseudo-scalar coupling: Anomalous spin forces between neutrons are: < 2  10  of their magnetic interactions < 2  10  of their gravitational interactions Present work Limit from gravitational experiments for Yukawa coupling only 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

27 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


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