Download How to Get Atoms Really, Really Cold Laser Cooling

Chad Orzel
Union College
Physics 100
Laser Cooling
OR
How to Get Atoms
Really, Really Cold
Laser Cooling
Chad Orzel
Union College
Physics 100
Physics 100:
100. Freshman Seminar (Fall). Team-taught course introducing
physics at Union. Topics covered may include astronomy, atomic
and molecular physics, biophysics, chaotic dynamics and fractals,
computational physics, laser physics, solid-state physics, and
statistical physics.
Describe current research
What’s new, what’s cool
My opinion:
Laser Cooling
Nobel Prize in Physics, 1997
Chad Orzel
Union College
Physics 100
Laser Cooling?
NOT about keeping lasers from overheating
Shine laser light on sample of atoms
=> Atoms get cold
A little counter-intuitive...
... but it works:
T ~ 100 µK
(1 µK = 1/1,000,000 ºC above absolute zero)
Chad Orzel
Union College
Physics 100
Laser-Cooled Atoms at NIST
Laser-cooled and magneto-optically trapped sodium (NIST)
T ~ 200 µK
Chad Orzel
Union College
Physics 100
Temperature
What do we mean by temperature?
Average Energy of particles in sample
KE = 3/2 kB T = ½ mv2
Hot Sample = Fast atoms
Room Temperature
Cold Sample = Slow atoms
Liquid Nitrogen:
v ~ 300 m/s
T ~ 70 K
Speed of Sound
Half the speed
Why Bother?
Chad Orzel
Union College
Physics 100
Why do we do laser cooling?
Want to study properties of atoms
Main tool for looking at atoms in detail:
Spectroscopy: What color light do they emit?
Different atoms emit different colors:
Neon:
red light (neon signs)
Sodium:
yellow light (street lights)
Learn about atomic properties by looking at light
Chad Orzel
Union College
Physics 100
Doppler Effect
Change in frequency of waves from moving source
Stationary Source
Moving Toward
Higher Frequency
Moving Away
Lower Frequency
Shift Depends on velocity:
∆ ω = -2 π v / λ = -k v
Chad Orzel
Union College
Physics 100
Color and Frequency
Light behaves like a wave:
Color of light Frequency
of wave
Blue light: high frequency
Red light: low frequency
Fast-moving atoms have their light Doppler shifted
Slow atoms no Doppler shift
Want cold atoms for best measurements
Chad Orzel
Union College
Physics 100
Cold Gases
Want extremely cold samples of atoms to do measurements
Problem:
Anything but Helium is solid
(And He is a liquid)
Solids, liquids have interactions
which mess things up
Solution:
Work at very low density
Too dilute to solidify
Problem with Solution:
Can’t cool dilute samples by usual methods
Laser Cooling
Chad Orzel
Union College
Physics 100
Temperature
What do we mean by temperature?
Average Energy of particles in sample
KE = 3/2 kB T = ½ mv2
Hot Sample = Fast atoms
Room Temperature
Cold Sample = Slow atoms
T = 100 µK
v ~ 300 m/s
v ~ 10 cm/s
Speed of Sound
Speed of bug
Atoms and Photons
Chad Orzel
Union College
Physics 100
|e>
|g>
Atoms have discrete energy levels
ground state, excited state
Change states by absorbing or emitting light
absorption
emission
Photons: “particles” of light
little bundles of energy
Also Carry Momentum
Light Force
Chad Orzel
Union College
Physics 100
Photons carry momentum => absorbing photon exerts force
p
p
Atom gets “kick” in photon direction
Very small velocity
87Rb
λ=780 nm
v=5.8 mm/s
Lots of photons (1015 per second)
What about emission?
Also gives “kick”, but in random direction
Over many absorption/ emission cycles, averages out
Chad Orzel
Union College
Physics 100
Photographic Evidence
Absorb, emit photons a million times....
Shine laser on Bose Einstein Condensate
~ 1,000,000 atoms
As close to absolute zero as you can get
Scattered Atoms
BEC
Light Force II
Chad Orzel
Union College
Physics 100
Light Scattering Exerts force => change atomic motion
Not enough for cooling!
Problem:
p
p
v
v
v
Can make fast atoms slow down
slow atoms speed up
As likely to heat as to cool!
Need clever trick to do laser cooling
v
Chad Orzel
Union College
Physics 100
Doppler Effect
Change in frequency of waves from moving source
Stationary Source
Moving Toward
Higher Frequency
Moving Away
Lower Frequency
Shift Depends on velocity:
∆ ω = -2 π v / λ = -k v
Doppler Cooling
Chad Orzel
Union College
Physics 100
How to use Doppler effect to do cooling?
|e>
ω3
ω1
Transition between states depends on frequency
ω2
Too high, too low => no absorption
ω ~ ωo => light force
|g>
Doppler Effect changes frequency seen by moving atom
Tune laser to lower frequency (red) ω < ωo
Stationary Atom
No Force
p
Moving Toward Laser
p
v
v
Cooling Force!
Chad Orzel
Union College
Physics 100
Optical Molasses
Use two lasers, in opposite directions:
Laser 1
Laser 2
Atoms move left:
Absorb from Laser 1
Slow down
Atoms move right:
Absorb from Laser 2
Slow down
One-dimensional Cooling Force:
Opposes motion in either direction
F=−αv
Like viscous fluid: “Optical Molasses”
3-D Cooling
Chad Orzel
Union College
Physics 100
What about other dimensions?
Use three pairs of beams
Counter-propagating pairs along X, Y, Z axes
Sodium in optical molasses at NIST
Cool in 3-D
Low velocities in all directions
Chad Orzel
Union College
Physics 100
Laser-Cooled Atoms at NIST
Laser-cooled and magneto-optically trapped sodium (NIST)
T ~ 200 µK
Why Bother?
Chad Orzel
Union College
Physics 100
So, what’s this all good for?
Basic Physics
Atomic Spectroscopy
Quantum nature of atoms becomes apparent
Bose Einstein Condensation
Atom Optics
Precision Measurement
Cold-atom devices already most sensitive detectors of:
Rotation, Acceleration, Gravity Gradient
Atomic Clocks – Time Standards
Best clocks in the world
Exotic stuff:
Atom Lithography, Parity Violation
Quantum Computing, ???
Chad Orzel
Union College
Physics 100
NIST-F1
Built by:
Dawn Meekhof
Steve Jefferts
Accurate to 0.1 ns / day
History of NIST clocks
Chad Orzel
Union College
Physics 100
What’s It Good For?
Global Positioning System:
24 Atomic Clocks on satellites
Chad Orzel
Union College
Physics 100
Global Positioning
Works by triangulation:
Satellites broadcast time
Measure travel delay from three satellites
(Use fourth to determine time)
Gives position to ~10 m