Transcript Optical Molasses

Optical Molasses
Colleen Downs
Stephanie Pietromonaco
Sanjay Talluri
Definition
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Laser cooling technique that cools
atoms to temperatures lower than the
Doppler limit
Uses 3 pairs of counterpropagating
circularly polarized laser beams which
intersect where the atoms are present
History
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First demonstrated in 1985 by S. Chu
Laser cooling first became popular in
1970’s
This led to the idea of the Doppler limit
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Doppler limit – theoretical lowest possible
temperature of atoms
Falsified with finding of process of optical
molasses
Optical Trapping
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Momentum imparted by photons
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“light pressure” is the greatest portion
of this effect
Optical Molasses vs. MagnetoOptical Trap (MOT)
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Both use 3 pairs of counterpropagating laser beams
Trap about the same amount of atoms
Detection of atoms is easier in MOT because of
higher density (less spatial extension)
In MOT the magnetic field only acts on atoms as
they fall from trap
Optical molasses uses circularly polarized lasers
Optical molasses breaks Doppler limit
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In sodium: 40 μK in Optical molasses vs 300 μK in MOT
Current Applications
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GPS (Global Positioning System)
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Uses time signals from atomic clocks for
positioning
Atomic clock – use laser cooling for more
precise time signals
Now: use optical molasses for fountain
clocks which are even more precise
This leads to better GPS systems
Current Applications
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Lene Hau slowed speed of light from
186,282 miles/second to 38 mph
Used a combination of Optical
molasses and Bose-Einstein
condensate
Results could improve computer, TV,
and night vision goggles
Bose-Einstein Condensate
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http://www.youtube.com/watch?v=bdzHnAp
HM9A
Not made til 1995 because didn’t have the
appropriate technological “cooking pot”
Needed vacuums hundreds of trillions of
times lower than atmospheric pressure and
temperatures of -459.7˚F (within a few
billionths of degree from absolute zero)
Process of slowing light atoms
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Bose-Einstein condensate slows atoms
down to 100 mph which can then be trapped
in optical molasses
Optical molasses creates a clump of cold
atoms
These cold atoms then undergo evaporative
cooling
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Take out atoms that are still too hot or energetic
from the magnetic field
Slowing of light
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First laser – shot across cloud of
condensate
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controls speed of second because of
quantum interference
Second laser – shot perpendicular to
cloud and interferes with the first
These 2 lasers result in light traveling
at 38 mph
Applications of slow light
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Improve communication
Reduce electronic noise
Cut power requirements (a million
fold!)
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Ranges from telephones to super
computers
Hau currently trying to reduce speed of
light to 1 cm/second