I was decreasing the local entropy in a small part of my abode and found a shoebox full of photos which happened to contain a few shots of my grad school lab, in all its glory. We were building an interferometer which would use cold atoms, which means relatively large deBroglie wavelengths and a correspondingly small system. But one has to trap the atoms and cool them down first, and then generate the cold beam of atoms to feed into the diffraction gratings that comprise the interferometer, so the system is still quite complex. Since I was the first PhD candidate in the group, it meant I was involved in the construction of most of the components of the lab apparatus pictured here. It also meant a lot of fumbling in the dark, both figuratively and literally, since the only one with any experience with doing this kind of work was the PI, who had other duties (like teaching, writing grant proposals, etc.) It was a big day when the group finally reached the point where we didn’t want him playing with the experiment, because we knew more about the details than he did. That took a couple of years.

Here’s the vacuum system.

At the bottom of the picture is the oven, which was basically a pipe bomb with a hole in it (under vacuum, of course), and when heated sufficiently, would spray Rubidium atoms out. These were collimated with a second hole a few cm away (the extra Rb was collected on some cold metal and sent back into the oven during a refill cycle we ran each night after running the oven). Underneath is a turbo pump and a roughing pump; the small green hose was for adding dry nitrogen if we needed to open the system up. The nitrogen was supposed to keep the Rb from reacting with water in the air. It never worked.

The long pipe leads down to the next part of the vacuum system, and it has a “wobble stick” that blocks the atomic beam, if desired, and a valve to isolate the two parts of the system. You can also see an ion gauge on the left which was normally off so the light from the filament wouldn’t register on any photodiodes or the photomultiplier tube.

The tube is about a meter or so long, and needed to be so we could slow the atoms down. A laser was sent down the tube from the other end, and “chirped” in frequency to account for the changing Doppler shift — you wanted the laser to track the resonance of the atoms as they slowed down, so they would keep scattering photons. Once the atoms were slow enough, the laser turned off, and the atoms drifted into the bigger chamber, where they were trapped in a two-dimensional trap called an “atomic funnel.” They were able to move along the axis of the funnel, and were forced out to the left into the region where the interferometer would be. I made the gratings for the interferometer, but never got to the point where the apparatus was finished.

Here is an early version of the funnel:

It’s a “hairpin” made of copper tubing so we could water-cool it while sending several amps through it, and it creates a quadrupole field in two dimensions, so that atoms would be trapped into a pencil shape along the central axis, left-to-right. It’s supported from below by two insulated standoffs — you can see the ceramic, i.e. insulating, part — to keep it straight. The dark hole straight through it is the tube leading to the oven and the lighter window on the left would be where one of the six trapping lasers would come in. Up top there is a tube with some lenses in it for imaging the trap, and on the right is a target for aligning the lasers. The target is on a linear feedthrough, and the target could be inserted into the center and all of the lasers sent through it to make sure they all overlapped where they should.

The funnel tubes were originally soldered together. What you see here is version 2, because at one point we lost cooling water while the system was energized, and the solder melted. Which meant that lots of water was introduced to the system and it became a giant fish tank, sans fish. Fortunately, we had interlocks in the system so that if the pressure rose too high the pneumatic valves would shut (that’s what the rest of the green tubing is for in the first picture), and the damage was limited. The white blobs at the ends of the little elbows are torr-seal, which we used to repair the trap. This eventually failed, too, and we replaced it with a much better trap fashioned from a single piece of copper tubing.

I graduated after having built much of this and characterizing the atomic beam. We were able to extract atoms going up to 10 m/s — this was adjustable, depending on the laser frequencies of the different beams, but atoms going too slow would miss the interferometer because of gravity, so we didn’t bother trying to generate a beam going below a couple of m/s. The atoms were also somewhat cold — less than a milliKelvin — so that the beam didn’t spread out too much. At the time, one other funnel had been built, but it had on of its lasers along the axis of the funnel, which precluded putting any sort of target there. This was novel enough for a publication (T. B. Swanson, N. J. Silva, S. K. Mayer, J. J. Maki, and D. H. McIntyre, “Rubidium atomic funnel,” J. Opt. Soc. Am. B 13, 1833-1836 (1996)) and more importantly, a degree.