Slowing Atoms and Finding Momentum as a Researcher

“I need you to build a Zeeman slower,” my mentor, Professor Ben Jones, told me one May afternoon. I sat in his office while he excitedly explained how we were to cool tritium, a radioactive isotope of hydrogen, down to near absolute zero as part of a neutrino experiment. I desperately tried to keep up with this whirlwind of information. I was two years into my physics degree, and I left that meeting with little more than “particle slower” and “tritium” written down in my notebook.

And so my quest began. For the next handful of weeks, cloistered in the lab in the basement of the University of Texas at Arlington’s Science Hall, I read web articles and research papers about Zeeman slowers. Early on, I spent most of my time looking up terms and concepts, but as I learned more, I could read and understand the dense research papers themselves.

My goal was to cool 500°C lithium atoms down to only 10 degrees above absolute zero. That way, we could test the neutrino experiment with lithium before using tritium. To cool the lithium atoms, we used a laser whose frequency matched the resonant frequency of the atoms. When we pointed the laser straight at oncoming atoms, they absorbed photons. Each time an atom absorbed a photon, it excited an electron up to a higher energy level and then, when the electron dropped back down, the atom emitted a photon. That gave it a little momentum kick backward, which slowed down the atom.

However, the lithium atoms we were handling were going extremely fast—more than 1,000 m/s. At this speed, they experienced the Doppler effect, which makes everything, including the frequency of the photons, appear faster than they actually are to the atoms. Fortunately, there is a way to correct for this. As I had learned in my modern physics class, a magnetic field can shift the atom’s energy levels via something called the Zeeman effect. By applying a very strong magnetic field at the start of the Zeeman slower and a weak magnetic field at the end, you can counteract the Doppler shift so that the laser always slows down the lithium. All I needed were coils of decreasing size and a powerful laser . . . but making that into a reality was challenging.

After spending the entire summer working with equations and sketching models, I continued to refine my design over the fall. I simulated the combined magnetic field of the coils to match my calculations. Then in January, I flew to Washington to talk with my group’s collaborators at Pacific Northwest National Laboratory—all on my own. I got a real taste of being a physics researcher. I workshopped my design with them and came back to Texas with a good idea of how to build this thing. The design solidified over the spring semester, and we ordered parts in May. It was finally going to become a reality!

Back in the basement of one of the oldest buildings on campus for the summer, I worked in the lab alongside my research group. I spent many afternoons winding coils on a lathe, lathering each layer with thick black epoxy to keep them from springing apart. By the time school started up again, the Zeeman slower was complete. Everything I designed fit together perfectly with our existing system. It was one of the hardest things I’ve ever done, but so rewarding and worth every second.

Right now, I am working on the truly experimental part of the project. The Zeeman slower brings atoms from 1,000 K to 10 K. In the next stage, we will bring atomic tritium from 10 K to millikelvins using magnetic quadrupoles, something