Recent advances in atomic magnetometry Michael Romalis Princeton University

1. Recent advances in atomic magnetometryMichael RomalisPrinceton University

2. Magnetic Field Scale Attotesla magnetometry

3. SQUID Magnetometers • Based on Josephson tunneling effect In superconducting shields • 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.

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

5. 3 cm B  1 fT d Hz 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: – 1 s T v n = se 2 T2N = ssevV

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 = -2 -1 0 1 2 Ground state Zeeman and hyperfine levels W. Happer and H. Tang, PRL 31, 273 (1973)

7. B Chopped pump beam S 0.2 - in phase - out of phase ) rms 0.1 Lock-in Signal (V 0.0 -0.1 10 20 30 40 50 Chopper Frequency (Hz) Spin-exchange relaxation free regime High-field linewidth: 3 kHz Low-field linewidth: 1 Hz Linewidth at finite field Linewidth at zero field J. C. Allred, R. N. Lyman, T. W. Kornack, and MVR, Phys. Rev. Lett. 89, 130801 (2002)

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

9. Magnetic Field Linearly Polarized Probe light Circularly Polarized Pumping light Magnetization Magnetization Cartoon picture of atomic magnetometer Alkali metal vapor in a glass cell • Cell contents • [K] ~ 1014 cm-3 • 4 He buffer gas, N2 quenching 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, 223501 (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 • 100 - 300 channels, 3-5fT/Hz1/2, 2 - 3 cm channel spacing • Cost ~ $1-3m • Clinical and functional studies Auditory response Elekta Neuromag H. Weinberg, Simon Fraser University

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

15. Pneumatic earphone Probe beam K cell Pump beam Mu-metal magnetic shield N100m peak; averaging 250 epochs SNR~11 for the best channel Stimulus onset Brain signals from auditory stimulation Magnetic fields from 64 center channels Kiwoong Kim et al

16. 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 Detection of Explosives with Nuclear Quadruple Resonance 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 Dn = (RexRsd /5)1/2/2p I.M. Savukov, S.J. Seltzer, MVR, K. Sauer, PRL 95, 063005(2005)

18. Brf 22 g of Ammonium Nitrate S 4 minutes/point Pump laser (2048 echoes, 8 repetitions) w B0 Probe laser Y Y Y Y X Detection of NQR signals with atomic magnetometer wrf = gB0 Spin-echo sequence 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, 214106 (2006)

19. p 8 k B = M 3 0 He K m B m m m 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

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

21. m = - × = - × B H Ω S S eff S Nuclear Spin Gyroscope • Rotation creates an effective magnetic field Beff = W/g He = Beff 24 fT/(1 deg/hour) K = Beff 0 . 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 • Monopole-Monopole: • Monopole-Dipole: • Dipole-Dipole: • Massless propagating spin-1 torsion: Axions: J. E. Moody and F. Wilczek (1984) 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. B m S w ˆ ˆ ˆ × × ˆ S S S r 1 2 1 Experimental techniques • Frequency shift • Acceleration • Induced magnetization or S S or S SQUID Magnetic shield

25. Search for long-range spin-dependent forces Spin Source: 1022 3He 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:0809.4700v1 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 Mike Souza – our glassblower Karen Sauer (GMU)