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Slowing Down Atoms And Biomolecules With Lasers
Tokyokid writes "In an interview on Berkeley Groks, Nobel Laureate Steve Chu talks about cooling atoms down using lasers. In another words, the atoms or molecules are slowed down in this "optical molasses." Scientists now are using these techniques to study the interactions and forces between biomolecules. These studies may give a better understanding of how life works on the molecular level."
The idea is that you want to reduce the average velocity of these atoms down to a very very low speed and that's what they really mean by cooling the temperatures and measuring the average motional energy of these atoms. You do this by shining light on the atoms.
As the light scatters from the atoms, they cool down. The trick is to arrange the light to preferentially scatter off of photons opposing the motion and this is done simply by tuning the frequency of the light so that when the atoms are moving towards this laser beam, it has a frequency shift called the Doppler shift that actually shifts it more into resonance.
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According to this, about 100 billionths a degree above absolute zero. This just turned up with a quick google and it has most likely gotten lower since then. As for refridgeration, not in the near... or distant future.
Not to troll but... this technique has been used for years! I've seen the setups (I still remember the issue of Scientific American that had a story about this - one of the reasons I went into physics).
I guess it's not a bad interview, but was there something groundbreaking in there I missed? Or is this just one of those "it's not news, but it's still for nerds" things?
Cheers,
Justin
Is this related to Compton scattering (usually between an electron and a photon)?
Nope.
This also sounds a lot like how the bose-einstein condensate (a recently discovered "new state" of matter, and cause of a nobel) was created.
Yep.
It isn't particularly related, except that the ideas of quantum mechanics that are demonstrated in the Compton effect are important for understanding laser cooling.
Compton scattering is an elastic collision of a photon (usually a rather high energy one, like an X-ray) and an electron. The target electron is initially a free stationary particle.
Laser cooling involves the absorption of a photon (probably visible or infra-red) by an atom, which has been brought into resonance with that photon by a Doppler shift (and possible also a Zeeman shift in an magneto-optical trap or MOT). Therefore one of the electrons in that atom is promoted to a higher energy level. The atom stays in such an excited state until spontaneously decaying, releasing another photon in a random direction. The cooling occurs because the momentum of all the photons absorbed act to slow the atom down (or push it to the centre of a trap in a MOT), but the photons emitted can go in any direction so have no net effect.
The energy from the absorbed laser photons is not lost, so there is a heating effect somewhere. The experimental setups are clever enough that the atoms are cooled, but since each atom in the process of being cooled reradiates as many photons in random directions as it absorbs, these are free to go off and heat things up. In a typical experiment, they will hit the stainless steel walls of the vacuum chamber and heat them up very slightly.
In case anyone is interested, thermodynamics isn't violated either. The decrease in entropy (disorder) of the cooled atoms is more than matched by the large increase in entropy of the photons, which go from being in a highly ordered coherent beam with high mode occupancy to being in all sorts of random modes with no coherence.