Ultrasensitive Atomic Magnetometers

Transcription

1 Ultrasensitive Atomic Magnetometers Faculty Thad Walker Ron Wakai Grad Students Bob Wyllie Zhimin Li (University of Texas Houston Health Science Center) University of Wisconsin-Madison Principles and Sensitivity of Atomic Magnetometry Application to Biomagnetism UW Portable Biomagnetometer

2 Biomagnetism Scales Fetal MCG Earth s Field Puts requirements on magnetometer: Sensitivity ~ 10fT/Hz 1/2 Bandwidth ~ 100Hz Field Size (Tesla) Urban Noise Adult MCG Fetal MCG Adult MEG Evoked MEG Fetal evoked MEG 10-15

3 Highly Sensitive Magnetometers SQUID Cryogenics Expensive Can operate in large fields Sensitivity ~1 ft/hz 1/2 High bandwidth (up to microwave frequencies) Atomic Magnetometer Portable Inexpensive Fully sensitive when in fields <10 nt Sensitivity <1 ft/hz 1/2, shot noise limit <10aT/Hz 1/2 Limited bandwidth

4 Optical Pumping 2 P 1/2 795nm 2 S 1/2 m s =-1/2 m s =1/2

5 Optical Pumping 2 P 1/2 795nm Circularly Polarized light carries +1 ang. Momentum in propagation (z) direction 2 S 1/2 m s =-1/2 m s =1/2

6 Optical Pumping 2 P 1/2 795nm Circularly Polarized light carries +1 ang. Momentum in propagation (z) direction 2 S 1/2 m s =-1/2 m s =1/2

7 Optical Pumping 2 P 1/2 795nm Circularly Polarized light carries +1 ang. Momentum in propagation (z) direction 2 S 1/2 m s =-1/2 m s =1/2

8 Optical Pumping 2 P 1/2 795nm Circularly Polarized light carries +1 ang. Momentum in propagation (z) direction 2 S 1/2 m s =-1/2 m s =1/2 Γ

9 Optical Pumping 2 P 1/2 795nm Circularly Polarized light carries +1 ang. Momentum in propagation (z) direction m s =-1/2 m s =1/2 Γ 2 S 1/2 d! F dt " = R$ #! s 2!! S % ' &! (! S

10 Atomic Magnetometer

11 Atomic Magnetometer

12 Atomic Magnetometer

13 Atomic Magnetometer Faraday rotation of probe beam is

14 Atomic Magnetometer Faraday rotation of probe beam is

15 Optically Pumped Magnetometers F = I + S Electron Spin Total Spin Nuclear Spin df dt = Ω! S " #S + R $ & ẑ % 2 " S ' ) ( Magnetic precession Spin-Relaxation DC Response: S x = S z! y " + R Optical Pumping

16 !B " Sensitivity! # g s µ B nvt n = density; V = volume T $ Measurement Time Spin-Exchange Collisions:! = n" SE v Limits on traditional AM sensitivity: Traditional AM s had coherence times that were limited by Alkali-Alkali spin-exchange. When density is turned up, the rate of these collisions increases, and! / n " const. Shot noise limit ~10 ft/rt(hz)

17 Spin-Exchange Collisions Conserves F but redistributes between S and I

18 Spin-Exchange Collisions Spin-Exchange Collisions Conserves F but redistributes between S and I

19 Spin-Exchange Relaxation Free (SERF) Regime Larmor Frequency:!!! F =! I ± 1 2 = ± g µ! B J B (2I + 1)"! >> 1 T SE! " 1 T SE! << 1 T SE (SERF regime) From Happer and Tam, PRA 16,1877 (1977) B SERF << 1 µt Operation in SERF regime allows the magnetometer sensitivity to depend on much weaker collisions with other atoms, hence several orders of magnitude greater sensitivity Depends on alkali density

20 !B " Spin-Exchange Relaxation Free (SERF) Regime! # g s µ B nvt Larmor Frequency:!!! F =! I ± 1 2 = ± g µ! B J B (2I + 1)"! >> 1 T SE! " 1 T SE! << 1 T SE (SERF regime) From Happer and Tam, PRA 16,1877 (1977) B SERF << 1 µt Operation in SERF regime allows the magnetometer sensitivity to depend on much weaker collisions with other atoms, hence several orders of magnitude greater sensitivity Depends on alkali density

21 SERF Magnetometer Rapid spin-exchange compared to the precession frequency leads to the spin-temperature distribution. Links total spin to electron F = q S! spin through slowing down factor. 4<q < 6 q ds dt = Ω! S " #S + R $ & % ẑ 2 " S ' ) ( Spin-exchange does not contribute to Γ! Kominis et. al. Nature 422, 596 (2003)

22 Frequency Response If we apply a small oscillating field!! 1 =! 1 cos"t ŷ in the presence of a larger DC field!! 0 =! 0 ẑ S 1x = S z! 1y 2 " ' 4 +! 0 2 $ q 2 # 2 (" ' 2 + q# 2 ) +! 2 1x! 0 ( ) 2 + 2" ' 2 (! q 2 # 2 ) S 1x = S! 0 "0 z! 1y # ' 2 + q 2 $ 2 resonance at

23 DC Field Cancellation SERF limit is about 1 µt, but to get full sensitivity in our (DC) operation mode, Residual field in our 4-layer shield is ~10s nt, so this requires further cancellation using triaxial Helmholtz coils. Automation of this process should be straightforward. S x =!S z " y # '! " x " z " 2 + # ' 2! ' = R +! SD Shot-noise limit 20 at/ Hz w/ 100 Hz bandwidth Allows easy residual field canceling

24 Pump optics Setup Probe collimation Probe detection tubes 3 Rb cell in ceramic and Teflon oven Rb 87 with N2 (100T) buffer gases Circularly polarized pump at 795nm Linearly polarized probe at ~780nm Cell heated to 180 C All lasers fiber coupled Apparatus inside 4-layer mu-metal shield in lab Clinical use is in 3-layer shielded room

25 Some More Pictures Tri-axial Helmholtz coils 4-layer mu-metal shield

26 Technical Considerations Plastic, ceramic, and Teflon parts reduce Johnson noise Electric heating: dipole/quadrupole suppressed resistive film heater sandwich heats w/ small residual field (~100pT) Insulation allows subject to be 1cm away from Rb cell Simple 1-cm baseline gradiometer with pump tube Commercial 1 cm! 1 cm! 6 cm vapor cell

27 Portability Bob Wyllie

28 Results Frequency Response

29 Heart Signals Average of 4 beats

30 Z-Mode Z Li, R T Wakai, and T G Walker, Appl. Phys. Lett. 89, (2006) For best sensitivity, z-field Larmor frequency~parametric frequency, sets Allows us to run the magnetometer as a 2- component vector magnetometer -Z1, Sx signal oscillates at -Z2, Sx signal oscillates at for B x for B y Future: lock! z to " z to measure changes in z-field

31 Technical Noise Reduction DC Mode Z- Mode

32 Multichannel FMCG Should be easily miniaturized to a few cm/channel

33 Other AM MCG results Shot-noise limited M x non-serf magnetometer Bison et al., OPTICS EXPRESS 11, 904 (2003) Poster Measurement of Biomagnetic Fields in Small Animals by use of an Optical Pumping Atomic Magnetometer S. Taue et. al

34 Evoked MEG XIA et al, APPLIED PHYSICS LETTERS 89, Development of a Wide-coverage Atomic Brain Magnetometer System (K. Kim, H. Xia, S. Lee, M.V. Romalis)

35 Summary Atomic magnetometers are performance-competitive with SQUIDs. Potentially substantially less expensive to build and operate Developed a portable, highly sensitive atomic magnetometer suited for fetal MCG measurements. Sensitivity ~ 40 ft/hz 1/2 Bandwidth ~ 40 Hz Z-mode technical noise suppression and the ability to simultaneously detect two components of the magnetic field simultaneously.