Transcript Atomic Physics in the Shadow of the Giant Corpse Flower

Atomic Physics in the Shadow of
the Giant Corpse Flower
Measuring Magnetic Fields in the UC Botanical Garden
Brian Patton, Dmitry Budker
Magnetometry Team: Eric Corsini, James Higbie, Andrew Park
PI: Dmitry Budker
March 17, 2009
UC Berkeley Botanical Garden
Outline
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What is that experiment on the research plateau?
History of magnetometry
Intro to atomic physics
How to make an atomic magnetometer in 4 easy
steps
• Our experiment
• Data and applications
• Future directions
Image credit: Tom Kornack
Early Magnetometers
• The compass was in widespread use by ~1100
AD.
• … followed by the dip needle.
• Measure strength of magnetic field by
observing oscillations.
• Improvements by Gauss
led to higher precision.
• In fact, the Gauss is now
a unit of magnetic field.
Fluxgate Magnetometer
can measure field changes as low as 0.1 mG (5 million × smaller than Earth’s field)
Fluxgate Magnetometer
US Geological Survey Web site
SQUIDs
•
•
•
•
Superconducting
QUantum
Interference
Device
SQUIDs
• Can measure magnetic fields as low as
~4×10-12 Gauss (almost a million times
smaller than fluxgates).
• … but
– Need to be cooled near absolute zero.
– Measure changes in the magnetic field, not the
field itself.
… with credit to Clarke & Pines,
UC Berkeley
Our Friend the Atom
What Is Spin?
Important Physics Concept #1:
• Particles with spin behave like magnetized
gyroscopes.
=
+
http://www.graphcomprints.com/Images-new/gyroscope.gif
There’s no such thing as a free electron...
Why alkali metals?
Atom–Laser Interactions
Important Physics Concept #2:
• Polarized light can affect atomic spins…
Simulations courtesy of
Simon Rochester, UC
Berkeley
Atom–Laser Interactions
Important Physics Concept #2(½):
• Polarized light can affect atomic spins…
and vice versa.
S. Seltzer, Ph.D. Thesis, Princeton Univ. (2008)
Atomic Magnetometers – the Basic Idea
• Use a strong “pump” beam to align all the
atomic spins in the same direction.
• Allow those atomic spins to precess in a
magnetic field.
• Monitor the precession with a weak “probe”
beam; observe the rotation of the probe beam
polarization.
• Count the frequency, and… voilà!
Some atomic magnetometers can measure magnetic fields as
low as 2×10-12 Gauss (~200 billion × smaller than Earth’s field)
Magnetic Resonance
Atomic Magnetometers in Action
Miniature alkali
vapor cell, NIST
9mm
Budker & Pines groups,
UC Berkeley
M. Ledbetter
Atomic Magnetometers in Action
Romalis lab, Princeton Univ.
Atomic Magnetometers in Action
“Chip-scale” atomic
magnetometer, NIST group
Image: Loel Barr
Atomic Magnetometers in Action
Geometrics, Inc.
Our Magnetometer
• All-optical, self-oscillating magnetometer
using nonlinear magneto-optical rotation
• High sensitivity
• Works at Earth’s field (0.5 Gauss)
• Responds very quickly to changes in the
field.
Why the UC Botanical Garden?
Atomic Magnetometers at Earth’s Field
• Physics Concept 2 ½ (½):
– At Earth’s magnetic field, need synchronous
pumping:
How to Make an Atomic Magnetometer
• Start with a glass cell containing alkali
atoms.
Higbie, Corsini, & Budker; Rev. Sci. Instr. 77, 113106 (2006)
How to Make an Atomic Magnetometer
• Add in a strong (polarized) pump beam
that’s pulsed at the correct rate:
Higbie, Corsini, & Budker; Rev. Sci. Instr. 77, 113106 (2006)
How to Make an Atomic Magnetometer
• Send the pump beam through the cell.
Higbie, Corsini, & Budker; Rev. Sci. Instr. 77, 113106 (2006)
How to Make an Atomic Magnetometer
• Meanwhile, send in a polarized probe
beam to watch the spin precession.
Higbie, Corsini, & Budker; Rev. Sci. Instr. 77, 113106 (2006)
How to Make an Atomic Magnetometer
• Use the rotation of the probe beam to set
the pulsing rate of the pump beam:
Higbie, Corsini, & Budker; Rev. Sci. Instr. 77, 113106 (2006)
The Result:
• All-optical, self-oscillating magnetometer using
nonlinear magneto-optical rotation
Eric Corsini
Graduate
Graduate student
student
The Result:
x 10
-4
Fluxgate
Berkeley NMOR magnetometer
0
-2
Field (G)
-4
-6
-8
-10
50
100
150
200
250
300
Time (s)
Solution: Use two magnetometers.
350
400
450
500
The Nuts & Bolts
• Laser and optics:
The Nuts & Bolts
• Electro-optic modulator:
The Nuts & Bolts
• Cell and cell holder:
The Nuts & Bolts
• Sensor head:
The Nuts & Bolts
• Electronics:
The Nuts & Bolts
• Field assembly:
The Damage
• Laser, optics, & modulators: 20 k$
• Optical fibers: 4 k$
• Coated cell w/ enriched 87Rb: priceless
• Custom-designed electronics: 1 k$
– plus many months of design & de-bugging
• Sensor housing: 1 k$
– plus ~ 1 year design, assembly, & machining
• Diagnostic electronics: 20 k$
When it’s working…
• See spontaneous oscillation at a frequency of
~700 kHz in both sensors:
Sensitivity (early data)
• Demonstrated sensitivity to DC fields of < 1 µG
Sensitivity (early data)
• Demonstrated sensitivity to DC fields of < 1 µG
1
x 10
-3
1
Difference in Field Readings (G)
Geometrics G858
Berkeley NMOR Magnetometer
Field (G)
0.5
0
-0.5
-1
-1.5
0
50
100
Time (s)
150
200
x 10
-3
0.5
0
-0.5
-1
0
Difference between
Berkeley & Geometrics magnetometers
Sensors ~ 1 meter apart
50
100
Time
150
200
Source of Magnetic Noise?
24 hour
Field measurement
48500
3D Fluxgate Magnetometer
Total field (nT)
48450
48400
48350
48300
48250
Last BART Train
48200
7pm
12am
First BART Train
5am
Time
10am
3pm
Current Work
• Recently switched from Cs to 87Rb
– Nuclear spin effects important at Earth’s field
• Get 2 atomic magnetometers working
simultaneously
• Improve sensor speed, sensitivity
– Goal is < 1µG at 1 kHz.
• In lab: 3 nG in 1 s demonstrated.
– Experimental noise is limiting factor now
• Reduce size, weight, power consumption
• Recover from gravitational mishap!
Applications
• Magnetic mapping
– Oil & mineral exploration
– Geophysics research
Applications
• Space magnetometry
– Earth’s magnetosphere
– Other planets
– Requires a smaller-size, lower-power sensor!
Applications
• Defense
– e.g., land mine detection
Geometrics, Inc.
• Medical diagnostics at Earth’s field
– NMR, MRI
– Magnetocardiograms
– Biomagnetism
Plant Biomagnetism?
Plant Biomagnetism?
Plant Biomagnetism?
Plant Biomagnetism?
Future Work
• Track down noise; improve sensitivity of device.
• Characterize common-mode noise sources.
• Re-design electronics.
– Smaller package, lower power requirements.
• Weather balloon test flights
• Design satellite magnetometer.
Fin
Thanks to:
• Magnetometer team:
– Eric Corsini, James Higbie, Chris Hovde,
Andrew Park, Simon Rochester, Dima Budker
(& others)
• Botanical Garden
–
–
–
–
Prof. Paul Licht
Holly Forbes
Deepa Natarajan
Everyone else!
More Thanks
• Physics Dept. demo team:
– Cindy Holmes and Roberto Barrueto
• Collaborators:
– Michael Purucker, Stewart Bale
• Funding:
– LLNL: Joe Tringe & Chuck Stevens
– NASA
– ONR MURI Collaboration
Background Slides
Light Polarization
Wikipedia
www.elkadot.com
High Bandwidth (Cs Data)
Bias field
provided by
nearby coil
Self-oscillation
frequency shift
Higbie, Corsini, & Budker; Rev. Sci. Instr. 77, 113106 (2006)
“Equation 1”
• Fundamental sensitivity of atomic
magnetometer:
– gmB  wL
– N = # atoms; t = coherence time; T = measurement
time
→ Increase number of atoms.
→ Maximize coherence time.
• Buffer gases, wall coatings, etc.
NMR, MRI with an Atomic Magnetometer
• Sensitivity of < 80 fT/Hz½ allows:
– Magnetic resonance imaging at low fields
Xu, Crawford, Rochester, Yashchuk, Budker, & Pines;
Phys. Rev. A 78 013404 (2008)
High Bandwidth (Cs data)
87Rb
& the non–Linear Zeeman Effect