It’s About Time
Alex Aeppli ’14 has created a new atomic clock—the most accurate ever
Story by Nancy Shohet West ’84
It goes without saying that Alex Aeppli ’14 was on time for our interview.
After all, the Ph.D. candidate in atomic physics at the University of Colorado Boulder has earned glowing recognition in the scientific community recently for an unusual advance: He led a team that has developed the most accurate clock ever made. How accurate is it? The researchers say it will lose or gain only one second every 40 billion years.
Aeppli got interested in physics and its myriad applications when he took Amy Kumpel’s physics class in his junior year at CA, followed by a more advanced class with Brian Potter-Racine P’12 as a senior. The summer in between, he did an internship in chemical engineering.
“I thought physics was kind of interesting back then, but because it was difficult for me, I never considered it as a career,” he remembers. But those two classes at CA gave him enough of a foundation that at Carleton College he could skip some prerequisites and jump into the more complex iterations of the discipline.
“When you start taking physics classes, you’re doing experiments that I found kind of boring: drop a ball, time its fall to figure out acceleration,” he says, though he acknowledges that in those courses he learned “a lot of good skills.”
As the classes became more advanced, he found them more intriguing, especially once he began working in the lab of a professor whose research was in atomic physics. “Pen-and-paper physics, or solving equations on a computer, didn’t interest me much,” he says. “I like using my hands and building things. The experiments we were doing involved mirrors, lasers, vacuum chambers. I found it exciting to see how what we were doing in the lab translated into actual measurement.”
“Every gain in stability and accuracy opens new realms of exploration.”
— Alex Aeppli ’14
At the University of Colorado Boulder, Aeppli and his team have built an optical atomic clock, meaning it measures the visible light transitions of strontium atoms rather than the more traditional cesium atoms. The method they used to create it relied on research Aeppli has been conducting in which strontium atoms are supercooled by lasers. “When you hit the electron in a strontium atom with the exact right frequency of light, that electron will jump from one state to another,” he explains. “We use the frequency difference between two states to tell time. That’s what allows us to tie our light source to an atomic sample.”
And other than being sure you’re seated for chapel on time, what are the practical applications? “Telling time is a core part of any sort of measurement, especially in physics,” Aeppli says. “GPS satellites operate with microwave atomic clocks. If you replaced them with optical atomic clocks, you could do better positioning because you’d have a much more accurate and stable time reference. Building a clock like this teaches you a lot about basic atomic physics. How these atoms interact, how they respond to magnetic fields.” Or, as Aeppli and his colleagues wrote in “Clock with 8 × 10 −19 Systematic Uncertainty,” their paper published in Physical Review Letters, the journal of the American Physical Society: “Every gain in stability and accuracy opens new realms of exploration, such as placing bounds on dark matter, probing general relativity, and will ultimately result in the redefinition of the Systeme International second.”
He is credited as the lead author of the paper, but Aeppli is quick to point out the many ways the research reflects collaboration. “A lot was done by mutual agreement,” he says. “When you’re working with three or four other people and you are all tied to one experiment, you have to agree on how to make decisions, what questions to ask, what you’ll build in order to get there.”
Aeppli says leading a research team is in many ways an extension of the skills he developed at CA, where his science classes often involved group lab reports: “I learned to work with other people to write a lab report or complete an experiment. In much of what I do today in the lab, there are an infinite number of possible directions to pursue and only a finite amount of time. Making decisions about what to prioritize requires an ability to work well within a group.”
Though he isn’t sure what he’ll pursue once his Ph.D. is complete—a postdoc or employment in an industrial setting—the world of physics offers endless possibilities for further discovery.
“Our work represents a single step forward. You have to consistently take more steps in research,” he says. “But I would encourage any young person excited by physics to consider pursuing a Ph.D. It’s a lot of fun.”
