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The DRAMATIC slowing of light in certain gasses

B

bwanaaa

Senior member
Bose-Einstein condensates were theorized a long time ago but it was recently that the phenomenon of slowing light - almost to a standstill - was observed. What fraction oof the light energy is absorbed by thiis process - most or just a little? What is going on at the quantum level to explain this event? And why did it take so long to observe this - after all, it's just sodium gas. Isnt that the same stuff in those orange sodium lamps?
 
Well maybe some sort of weird process is going on. Like the sodium atom capturing the photon and held temporarily and emitting it again a later time hence slowing it.

Well just a guess. Flame me
 
The slowing of light in gases was actually observed several years ago. The Bose-Einstein condensate you're talking about is not sodium gas, but a gas made of rubidium atoms. "Stopping" light relies on something called electromagnetically induced transparency. Rubidium gas is opaque to the frequency of the laser pulse used in the experiment, but by shining a coupling laser tuned to a slightly different frequency than the first laser pulse the gas is made transparent (since it does not absorb the frequency of the light produced by the two lasers). The combined laser pulse, however, still interacts with the spin of the rubidium atoms and the light in the laser pulse is slowed due to this interaction. Once the first laser pulse enters the rubidium gas chamber the coupling laser is turned off. What occurs then is that the speed of the first laser pulse is reduced up to a point where you don't have any more visible light present and the first laser pulse is effectively stored in the spins of the rubidium atoms. As soon as the coupling laser is turned on, causes the first laser pulse to be reconstituted from the spin of the rubidium gas atoms.

The only problem is that due to the random motion of the affected rubidium atoms (i.e. rubidium atoms with an altered spin) light can only be stored for very short amounts of time.

I hope this makes sense 🙂 I tried to explain as best as I could.
 
^ teh winnar...that's the jist of a discover article i read on it.

i don't think it is physically "slowing" light but absorbing it and storing the momentum in the spins of electrons, the reemitting later. when you shine light through it, it effectivly slows the light
 
<----is part of a research group that does slowing of light in rubidium cells.

What sarotara said is basically right. Right now we're looking at improving storage times by using a combination of wax coating on the inside of the cells along with a buffer gas to reduce rubidium-cell wall interaction.
 
Besides the Rubidium cells, I believe the sodium gas also works, but in a very different way. Using applied electric and magnetic fields, the electronic configuration of the sodium atoms can be arranged carefully so that the sodium atoms absorb light very well in a narrow range of frequencies, but are transparent in a very very narrow window within the absorbtion range (I think this is how it works anyways). When light is shined on the atoms with an appropriate range of frequencies, the multiple frequencies of photons interact with the sodium gas. As there is only a very narrow transmission window, not all the light gets through.

Basically, the net effect is the group velocities of the different frequencies add up to produce a very slow phase velocity, which is the "apparent" speed of light in the medium. It's sort of like when you tune a guitar. If the two notes are fairly far apart, you will get a rapid beat. As the frequencies get closer, the beat becomes extremely slow and the guitar comes in tune. They are basically just tuning sodium atoms with laser strings.
 
Actually there are many possible experiements and uses that could open up due to this research. Here are a few:

1) Ability to simulate what happens to light near the edge of a black hole
2) Many uses in nonlinear optics. One of these is the ability to create very sensitive optical switches.
3) An easy way to convert between qbits in quantum computers.
 
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