Chip-Scale Atomic Magnetometers: Femtotesla Sensitivity on a Chip without Cryogenics

Transcription

1 Chip-Scale Atomic Magnetometers: Femtotesla Sensitivity on a Chip without Cryogenics John Kitching Time and Frequency Division, NIST Boulder, CO USA Funding: 1

2 Instruments Based on Atomic Spectroscopy NIST F-1 Primary Atomic Frequency Standard Time: most accurately measured physical quantity NIST: most accurate clock in the world Frequency uncertainty ~ 4x10-16 Timing instability < 1 ns over 1 week Telecom Sync GPS System 2

3 Chip-Scale Devices and Applications Tools Devices Applications e N E 1 E 2 h at Precision Spectroscopy Clocks GPS Wireless comms Semiconductor Lasers Micromachining Magnetometers Gyroscopes Gravimeters Remote sensing NMR/MRI Biomagnetics Inertial navigation Geophysics/oil Defense/security 3

4 Laser Technology Low-power spectroscopic light source Vertical-cavity, surface-emitting lasers (VCSELs) Typical i th < 1 ma; P op ~ 4 mw Single-frequency operation (single long. and trans. mode) Wavelengths: Rb D2: 780 nm, D1: 795 nm Cs - D2: 850 nm, D1: 894 nm 1 mm 50 m 4

5 Alkali Vapor Cell Fabrication using MEMS Filling Tube Windows Glass Svenja Knappe Li-Anne Liew Bond Glass tube Silicon ~ 1 mm Hole Glass Bond Advantages: Small, scalable Wafer fabrication Planar structure 1 mm Buffer gas Alkali atoms 1 mm L. Liew, et al., Appl. Phys. Lett., 84, 2694,

6 Cell Fabrication: Process Vacuum Chamber Alkali Silicon Glass Heat HV HV Pump Backfill N 2, Ar 6

7 Integration: Optics Assembly Micro-refractive lens Inkjet deposition of optical epoxy Commercially available in arrays ND filters (opaque glass): Spacers Waveplate Quartz 70 m thick 850 nm Lens VCSEL 250 m Alumina 11 W ND Quartz Si ND Glass 50 m 1.5 mm

8 NIST Chip-Scale Atomic Clock, Feb Cell: Cs+buffer gas 4.2 mm Photodiode Cell Optics Volume: 9.5 mm 3 Cell volume: 0.81 mm 3 Heat power: 75 mw 1.5 mm 1.5 mm Laser 1 mm S. Knappe, et al., Appl. Phys. Lett. 85, 1460, 2004.

9 Chip-Scale Atomic Clocks DARPA MTO MEMS Guided by DARPA PMs: Bill Tang Clark Nguyen Amit Lal Andrei Shkel Gyros Phase I Phase II Phase III Phase IV Cold atoms Concept DARPA/NIST Oscillators CU Integrated electronics 1cc instrument Commercial CSAC available: 16 cm 3, 120 mw 10 1 mo. Cells NIST Physics Package NIST NIST Companies 9

10 Power (W) CSAC Power and Performance Power of (commercial) CSAC beats all other atomic clocks by 10X Performance currently limited by electronics (PP 5X better) 10 1 HP 5065 Chiba FCS 1985 FRK GPS IIR PRS-10 LPRO XPRO FE-5650 SA.35m 0.1 CSAC Year Substantial (10X?) further reduction in power seems feasible

11 Work at NIST since 2005 Adapt CSAC technology to other instrumentation Clocks Magnetometers Applications 11

12 Physics of Magnetic Sensors Lorentz force: induce voltages Hall probes, search coils, flux gates Spin-dependent scattering of e- GMR, CMR + superconducting flux transformers Hard disk drives Phase shift of currents in superconductor Superconducting quantum interference devices (SQUIDs) Gold standard of magnetometers: ~ 1 ft/ Hz Magnetostriction, peizoelectricity, magnetooptics Multi-ferroic sensors Torques on mechanical structures Precession of spins Atomic magnetometers B 0 V R B 0 V B 0 B 0 Candidates for ft sensitivity B 0 12

13 Balabas, et al. (2010) Shah, et al. (2010) Ledbetter, et al. (2008) Budker, Romalis (2008) Wolfgramm, et al. (2010) Bison, et al. (2003) Broad View of Research in Atomic Mags Collisions Spin-exchange Wall coatings Romalis, Budker, Walker Quantum Effects Spin squeezing, QND Polzik, Mitchell, Vuletic, Romalis, Mabuchi Applications Novel devices NMR, Biomagnetics NIST, Weis 13

14 NIST Atomic Magnetometry Program Main goals: High sensitivity: pt ft at? No cryogenics: low system complexity Small size, low power, low cost Arrays for imaging and gradiometry Applications: Low-cost biomagnetic instrumentation Unattended magnetic anomaly detection 14

15 The Beginning of Atomic Magnetometry Magnetic moment Angular momentum ~ 1.4 MHz/G ~ B DB(t) I. I. Rabi (1930s) B 0 w L S, B 2p/w L Other early work by P. Bender, N. F. Ness, R. Slocum, Varian 15

16 Atomic Magnetometers B 0 Photodetector Evacuated glass cell Vapor cell DB RF Coils w RF l/4 Filter Lamp w L w RF Magnetometer output: w RF = w L Density of vapor B0 wrf / phase depends on Alkali metal cell temperature (Cs, Rb, K ) 16

17 Commercial Atomic Magnetometers Specs: db min ~ 1 pt/ Hz Bandwidth ~ 100 Hz Size ~ 300 cm 3 Power ~ 10 W Cost ~ $10,000 Scalar sensor Applications: Geophysical surveying (ground, airborne, underwater) Unexploded ordinance detection Remote monitoring (ships, vehicles) Sensor Electronics 17

18 Can We Make Atomic Mags Smaller? Photodetector Vapor cell l/4 Filter RF Coils Micromachined Cells Lamp Semiconductor Laser 18

19 CSAM Assembly: Laser and Spacer VCSEL Vertical Cavity Surface Emitting Laser (VCSEL) Lower power consumption Vertical emission l = 795 nm Kapton spacer Thermal Isolation Kapton Spacer 1 mm 19

20 4.5 mm CSAM Physics Package Peter Schwindt 1.7 mm Volume: 19 mm 3 Power Consumption: 198 mw 20

21 Magnetic field noise [pt RMS / Hz 1/2 ] M X CSAM Performance Measured field amplitude [pt RMS ] Sensitivity Frequency Response F 3dB = 1 khz 4 6 pt/ Hz 5.9 pt / Hz 1/2 over Hz Frequency [Hz] Frequency [Hz] P. D. D. Schwindt, et al., Appl. Phys. Lett., 90, ,

22 Effects of Atomic Collisions B 0 Higher T cell More atoms Higher signal Atom-atom collisions Faster relaxation Broader linewidth w L / B 0 db min Collision-limited sensitivity ~ 1 ft/ Hz cm 3/2 n at [cm -3 ] or T cell [K] 22

23 Spin-Exchange Relaxation-Free Magnetometry Happer (1973) 1/T 2 Spin-exchange relaxation can be suppressed at high cell temperatures and low B-fields Romalis (2003) Atomic magnetometry can be greatly improved with SERF Sub-fT sensitivity n at [cm -3 ] or T cell [K] db min Caveats: Field atoms see must be small Vector, not scalar, sensor n at [cm -3 ] or T cell [K] 100x 23

24 Chip-Scale SERF Magnetometry in Weak Field Transmitted Optical Power ( W) Our work: SERF operation in a MEMS vapor cell Small size Simple: single optical field 3 mm Cell PD B 0 DB B Heaters T cell = 152 C B 0 = nt Transverse Magnetic Feild, B 0 (nt) 24

25 Sensitivity (ft/hz 1/2 ) SERF Magnetometry - Results Vishal Shah ft/ Hz mm B 0 DB 10 1 A Frequency [Hz] Photon shot noise V. Shah, et al., Nature Photonics, 1, 649,

26 Two-Beam SERF MEMS Magnetometer Clark Griffith Rb cell balanced polarimeter linear, off res. probe beam (VCSEL) B circ. pump beam (ECDL) 7 ft/ Hz 26

27 Thermal Eddy Current Noise Sources t Di eddy a db d B sheet ( f) 1 6p 0 kt t a Wires Si Source Approx. distance (mm) Noise est. (ft/hz 1/2 ) Silicon (ρ = 5 ohm-cm?) 1 3 Rb film 1 nm thick Rb sphere ø100 m Metal in electrical heaters 1 3 Heater connections 12 2 Wires 30 1 Ferrite shield (@ 100 Hz) Mirrors Quadrature sum: 5.7 ft/ Hz 8.5 ft/ Hz Alkali metal film Resistive heaters Need to remove conductive elements 27

28 Magnetic Field Sources in Magnetometers Heaters: Heat cell to operating temperature Coils: Drive atoms or provide modulation to detect resonance via lock-in Laser: DC current generates offset field Metallic components Magnetic noise due to thermal currents Problems for Magnetometer arrays Highly sensitive instruments for biomagnetics Heaters Coils Laser Metal Heaters Coils Heaters PD B 0 DB 28

29 Photonic Magnetometer Design VCSEL (795 nm) VCSEL (795 nm) Heating (915 nm) Heating (915nm) SM fiber SM-fiber BS BS Rb Cell Rb-cell Kapton Kapton Jan Preusser Svenja Knappe Detection Detection BS 1 cm 29

30 Photonic Magnetometer Sensitivity [ft/ Hz] Avoid any metallic elements on sensor Fiber-optic design Preliminary results: ft/ Hz Frequency [Hz] 30

31 Magnetometer Arrays and Instrumentation NIST devices at PTB for brain field measurements Draper Labs advanced engineering for mass production and commercialization 31

32 Sensitivity [ft/rthz] CSAM Sensitivity over ~ 5 Years Measurements Devices 4.5 mm 10 6 MR (a) 6 5 (b) Flux Gate mm Atomic 5 ft/rthz SQUID Frequency [Hz] No cryogenics, small size, low power, low cost PD Cell B 0 DB Heaters 32

33 Energy Resolution Energy resolution of magnetometer DE 2 db V 2 0 Minimum detectable magnetic field energy For db = 5 ft/ Hz, V = 1 mm 3 : DEDt ~ 1x10-32 J/Hz ~ 100 ħ Does ħ delineate a fundamental limit on the sensitivity of magnetometers? Appears to for SQUIDs: Koch, et al Maybe not for atomic mags: Lee, Romalis, DAMOP 2008 Poster 33

34 Sensitivity [ft/ Hz] Survey of Experimental Atomic Magnetometers BEC Evan. wave MEMS cells NIST Niche Glass cells Shield noise DC mags RF mags Squeezed de ~ hbar Volume [cm 3 ] 34

35 Magnetic Field [Tesla] CSAM Sensitivity Comparison Geophysical Magnetic Anomaly Detection Non-Destructive Test and Evaluation Magnetocardiography Magnetoencephalography Transient Electromagnetics 2007 High Tc SQUID mm Adapted from R. L. Fagaly, Rev. Sci. Instrum., 77 (2006). 87 Rb, 1 mm 3 atom shot noise Frequency [Hz] Low Tc SQUID 35

36 Preliminary Biomagnetic Measurement on Mouse 4.5 mm At U. Pittsburgh with Dr. V. Shusterman Mouse anaesthetized, placed near CSAM ECG and MCG signals recorded Brad Lindseth MCG (a) 6 (b) ECG mm 36

37 Biomagnetic Measurements on Humans Measurements at PTB-Berlin with Til Sander, Lutz Trahms PTB magnetically shielded room: 7 high-perm metal layers 1 Al layer 37

38 NMR with a CSAM With M. Ledbetter, A. Pines, D. Budker, UC-Berkeley M. P. Ledbetter, et al., PNAS 105, 2286 (2008). 38

39 J-Coupling in Microfluidic NMR With A. Pines, D. Budker, M. Ledbetter, UC-Berkeley 1 H B 0 12 C 13 C Ethanol 12 C 13 C 13 C 12 C 39

40 Some Future Directions Low-frequency noise Measurements limited by B-field leakage through shields Modulation techniques Earth s field operation for SERF mags Bucking coils Very high cell temperatures (~ 300 C) Improved thermal isolation for low-power operation 10 mw possible Sensitivity vs. cell size Smaller cells worse sensitivity but lower power Elimination of thermal magnetic noise 40

41 Conclusions Miniaturization of atomic magnetometers and clocks using diode lasers and MEMS vapor cells Millimeter-scale, no cryogenics, low power Magnetic sensors based on spin-precession Sensitivity ~ 5 pt/ Hz (earth field); ~ 5 ft/ Hz low field Fundamental limits < 1 ft/ Hz for 1 mm vapor cell Atom shot noise limited detection, suppression of alkalialkali collisions Funding: 41

42 Acknowledgements NIST Team Svenja Knappe Rahul Mhaskar Ethan Pratt Ricardo Jimenez Phoenix Dai Clark Griffith LANL Amber Post Jan Preusser Vishal Shah Symmetricom Leo Hollberg Draper Labs John Leblanc Mark Mescher UC-Berkeley Micah Ledbetter Vik Bajaj Dima Budker Alex Pines PTB-Berlin Til Sander Lutz Trahms 42