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Recent advances in atomic magnetometry Michael Romalis Princeton University

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Magnetic Field Scale Attotesla magnetometry

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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

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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 T2T2 1 N atoms

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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

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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:

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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

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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 BB S 1/2-1/2 ++ probe beam ++ Cell = (n + - n - ) L / ~ T 2

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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).

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Johnson current noise in -metal magnetic shields

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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)

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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

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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

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

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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

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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

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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)

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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

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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

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

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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

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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:

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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

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

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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

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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

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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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