Optical Molasses

Colleen Downs

Stephanie Pietromonaco

Sanjay Talluri

Definition

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

First demonstrated in 1985 by S. Chu Laser cooling first became popular in

1970’s This led to the idea of the Doppler limit

Doppler limit – theoretical lowest possible temperature of atoms

Falsified with finding of process of optical molasses

Optical Trapping

Momentum imparted by photons

“light pressure” is the greatest portion of this effect

Optical Molasses vs. Magneto-Optical Trap (MOT)

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

In sodium: 40 μK in Optical molasses vs 300 μK in MOT

Current Applications

GPS (Global Positioning System) 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

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

http://www.youtube.com/watch?v=bdzHnApHM9A

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

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 Take out atoms that are still too hot or energetic

from the magnetic field

Slowing of light

First laser – shot across cloud of condensate 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

Improve communication Reduce electronic noise Cut power requirements (a million

fold!) Ranges from telephones to super

computers Hau currently trying to reduce speed of

light to 1 cm/second