Trap Day

Day before yesterday, Ken got a trap.

Yesterday he got it again, only bigger and brighter.

Yay! Hooray! You're cheering, right?

No, you're asking "What's a trap, and why does it matter?" That's the problem with achievements in a field like this -- any specialized field, I guess.

A "trap" is a small collection of atoms held suspended in mid-air -- except actually there can't be any air in the system or it won't work. A small collection of atoms held suspended in mid-vacuum. They aren't touching anything at all. They are surrounded by almost perfect nothingness. What holds them up? Laser beams, and a magnetic field.

The technical name for it is a "magneto-optical trap." You can actually see the atoms hovering there, a small glowing spot a few millimeters to a centimeter in diameter. You can see them because the lasers make them glow. In our case, they glow in the infrared, so you have to use a viewer to see the light, but here is a picture of someone's sodium MOT, that looks just like ours, only glowing with yellow light.

How does it work? Light actually does exert a force when it interacts with atoms. It can slow them as they come out of a nozzle. If you have light beams pushing up, down, left, right, forward, and back, it can keep atoms from escaping in any direction - it can trap them. Now the problem is to make sure that it exerts the right amount of force, and more, that the left-pushing beam only operates on atoms that are moving to the right, and not on those already moving leftward, and likewise for the other beams. This is where the magnetic field comes in. It changes the properties of the atoms, so that atoms in one part of the field interact with a different kind of light then atoms in the other part, and atoms further from the center are pushed harder than those near it. How do you calculate these forces and interaction energies? Um... Ask Ken. But give him time to go through his notes first.

The best explanation of laser cooling and trapping for the non-scientist (OK, and for the scientist as well. I personally need help turning the math into a conceptual picture) is at the University of Colorado's Bose-Einstein Condensate page. It's got little java videogames (hit "next" at the bottom of the page to see more), to let you try your hand at some laser cooling of your own. In our lab, we stop short of the "evaporative cooling" stage, and we don't get BECs. But we do get cold atoms.

What's nice about cold atoms is that they don't bump into each other so much, and they all have about the same velocity (close to zero). Any time you want to do an experiment that involves putting atoms into certain quantum states (you do this with lasers) -- you really want to do it in a trap, because if the atom collides with anything, your state is ruined. Also, you have to use different laser frequencies depending on the velocity of your atom, so if you can arrange for all the atoms to have the same velocity, you get better results.

So, Ken's project involves putting atoms into a certain quantum state, called the "dark state". You can control whether atoms in this state absorb light or not, and if they don't absorb it, what kind of "phase shift" they give it. (A phase shift just makes the peaks and troughs of the wave occur in different places.) This is supposed to be analogous to what some parts of a computer do to electronic signals and would supposedly be useful for some hypothetical quantum-computer, someday. Possibly. Maybe. Whatever. It means Ken is one step closer to graduating.

It's been a long, gruelling, frustrating process. The lab used to have a trap, and Ken worked on it, but with the departure of one of our post-docs, the whole shebang became Ken's responsibility. There are so many working parts to this, what with three lasers, at least nine different beams to align, electronics and optics to lock the lasers, frequency modulators, waveplates, beam expanders, magnetic coils, cooling water, and, oh yes, the vacuum. The pressure has to be on the order of 10^(-10) torr. Atmospheric pressure is 760 torr. Three different pumps, each with their own plumbing, their own gaskets, their own impossible-to-reach nuts and bolts. So many things have to work, all at once...

Only three people ever understood this system. The other two are now gone. Our former post-doc was able to consult a little, and I could help do the two-person jobs if he told me what to do (turn a knob at this end of the table to adjust a beam at that end. Turn the gas on and off to blow sealant through the coils, etc.) but Ken ran the operation, knew what had to be done, and succeeded.

So now you have enough information to understand what these pictures mean, scientifically and emotionally.






This is the vacuum chamber, in which the atoms are trapped. You can see a camera on the right-hand side.






This is what the camera saw. That bright dot at the center is the trap.






This is my attempt to take a picture through the viewer. The trap is very clear and bright, if you just look through, but it's hard to take pictures through an eye-piece. Anyway, click on the picture to enlarge it, and you can see it, a bright dot at the center of the small window.






This is what the vacuum gauge reads: 7.1*10^(-10). That's .00000000071 torr. Thank you, vacuum gods!