Recent advances in atomic magnetometry Michael Romalis Princeton University

Attotesla magnetometry Magnetic Field Scale Attotesla magnetometry

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.

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

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

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)

Spin-exchange relaxation free regime B 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, 130801 (2002)

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 /

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] ~ 1014 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).

Johnson current noise in m-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 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

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

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

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

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

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 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, 063005(2005)

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, 214106 (2006)

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

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

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

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:

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

Experimental techniques ˆ 1 × B m w S Frequency shift Acceleration Induced magnetization 2 1 ˆ S × or S S or S SQUID 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 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

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!

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