Atom Trapping

Well, it's been a long time since my last post. So long, in fact, that I have now taken the third-year classical mechanics course to which I referred in my last entry.

Lemme tell you. There have been a few really fascinating and really difficult classes since then. Some of the classes I've taken are:

Statistical Mechanics and Thermodynamics
Modern Physics (basically meaning Quantum Mechanics)
Methods in Theoretical Physics (pretty much a physics-centered math course)
Experimental Techniques in Laser Physics and
Atom Trapping (a direct sequel to the one above)

Atom Trapping was by far the coolest course I've ever taken. We literally captured a bunch of tiny Rubidium atoms inside a vacuum chamber (with little glass view ports) using two lasers, a bunch of mirrors (and other optical devices), and two copper coils. When I say "we caught a bunch of atoms", I mean it. A sealed chamber has no other particles inside but Rubidium atoms, and we captured a small group of those at the center using the above-mentioned equipment. We pointed a camera at one of the view ports and streamed the feed onto a monitor and could see a small cloud (about the size of a tiny dust mote) of trapped atoms.

Screenshot of camera view into vacuum chamber. White dot at center is the group of trapped atoms.
Taken 2 February 2015

Normally atoms are flying all around the room. Right now there are atoms whizzing past your elbow, bouncing off your head, and ricocheting off the walls in your room. The speed at which atoms move around is determined by the internal kinetic energy, or (as we measure it) their temperature. When you remove internal kinetic energy (temperature) from an atom it loses its speed and cools down. For this reason atom trapping is also known as atom cooling. We're stealing away their energy so that they grind to a sluggish halt and stay in one area.

Here's a crude explanation of how that works:

Imagine there is a truck coasting towards you, and he's moving pretty fast. All you have to stop him with is a ping pong ball gun. If you fire one, two, five, or even a thousand ping pong balls at the truck he's not going to stop... but he will slow down (though not enough to tell). But if you shoot a hundred billion balls at him, then he will stop. Atom cooling works on a similar principle.
We point a laser beam at a group of atoms. The laser beam is just billions of photons (particles of light) firing in the direction the laser is pointing. Now, the photons don't simply "bounce off the front" of the atom and slow him down. If the laser light is tuned to the "resonant frequency" of the atom then its energy will be absorbed by the atom. This amounts to "bouncing off the front of it" as far as its effect on the atoms kinetic energy is concerned. After a huge number of these absorptions, the atom will slow down (its temperature will drop) and it will sit still.

Now, you may ask yourself a couple of questions:

1. "But you just said that the atoms are flying all over the place in different directions! So how do you know that your laser is hitting the atom in the 'right direction' to slow it down, how do you know you're not speeding it up away from you?"

An excellent question! And one I'm really glad you asked!

There is an effect, you may have heard of it, known as the Doppler Effect. Without going into too much detail, because of the Doppler Effect (and your choice of laser tuning), the atoms will only absorb photons moving in the direction opposite to its motion. I know that sounds like a convenient solution, but to go into the Doppler Effect would be a post of its own.

2. " But if the atom is absorbing the energy of all these photons wouldn't it end up gaining more energy and moving around faster?"

Another great question. You guys are good.

Well, the atom actually ejects the photon in a random direction after a very (very) short amount of time. This gives the atom a little boost in the direction opposite to the ejected photon, but because the direction is random, over time the atom will shoot photons off in all directions averaging out to zero gained kinetic energy.

3. "What do the copper coils you mentioned earlier do?"

The laser uses the Doppler Effect to slow down atoms based on their velocity. The copper coils, when a current is sent through them, create a magnetic field (this is basic electricity and magnetism) which help slow down the atoms based on their position. By limiting an atom's movement based on its speed and position you can ensure that the cooled atoms end up in the center of the trap. For this reason this type of atom trap is called a MOT, a Magneto-Optical Trap.

There! Now you guys are ready to trap your own atoms! All you need are:
2 780 nm lasers
2 laser controllers
1 Acousto-Optical Modulator
2 absorption spectroscopy setups
30-40 small mirrors
5 cube beam splitters
2 optical irises
4 photodiodes
3 oscilloscopes
1 power meter
1 DC power source
2 copper coils (connected to the above power source)
1 CCD camera
1 infrared viewscope
10-15 half-wave plates
6 quarter-wave plates
1 vacuum chamber filled with Rubidium gas,
Oh!
and a more complete explanation of the theory than that found above.

Have fun!