Photon Finish

Physics professors teach the teachers in quantum-mechanics workshop

by Michelle Simmons

September 17, 2010

Physics professors David Jackson (left) and Brett Pearson use a single-photon down-conversion apparatus to illustrate the paradox of quantum mechanics.

“Two roads diverged … and sorry I could not travel both,” wrote poet Robert Frost. But what if one could?

For David Jackson, associate professor of physics, and Brett Pearson, assistant professor of physics, the question isn’t merely theoretical. In June, they led a hands-on faculty workshop on single-photon quantum mechanics to illustrate how a single particle can be two places at once.

The workshop, facilitated by the Advanced Laboratory Physics Association (ALPhA) and held at Dickinson, brought colleagues from Pomona College and the universities of Alberta-Calgary and Minnesota, among others, to help other professors learn how to introduce undergraduate students to the highly abstract field of quantum mechanics.

Tracking Complexity 

The difficulty of teaching quantum mechanics, explains Jackson, is that not only does it seem to defy the laws of physics, but the language for describing it hasn’t caught up with the science. “All the language was developed for classical physics, but the quantum world is different,” he says.

With the assistance of a 2007 National Science Foundation grant, Jackson and Pearson built a single-photon down-conversion apparatus that can demonstrate a quantum theory of light and began using it for a 200-level course, Introduction to Relativistic Quantum Physics. They’ve been teaching it since 2008.

The physicists focused on sophomores precisely because of the field’s complexity. “You can’t really learn the more advanced mathematics until your senior year,” Jackson says. “I’m trying to motivate sophomores by doing experiments in class.”

In May, Jackson and Pearson co-published an article in the American Journal of Physics about their experience teaching quantum mechanics at the undergraduate level. It caught the attention of ALPhA organizers, who contacted the two about demonstrating the equipment and experiments they use in their course.

“ALPhA was really helpful in getting the word out and handling the registration,” says Jackson. For two intense days, 12 participants tried out the equipment and discussed teaching strategies.

Seeing the Impossible 

In one of the experiments, a powerful laser beam is aimed at a special crystal, which splits the beam into two identical light streams. “We measure one of those two light streams,” explains Pearson. “We use the measurement of one to tell us what we have in the other. The idea is to isolate a single photon.”

The single photon is directed at a Mach-Zehnder interferometer then reflected off a piezo-controlled mirror, which can be positioned with a resolution of two to three nanometers (not much larger than an atom) at a time.

The result? The photon appears to travel in two directions at once while continuing to exhibit the properties of a single entity. That is, it simultaneously appears to be both a particle and a wave.

Captivated by the paradox before their eyes, students want to learn the theory behind the experiments, say Jackson and Pearson. Jackson’s first cohort of sophomores will be in his senior-level quantum-mechanics seminar this year, and he’s eager to start showing them some of those advanced equations.

“In class, I can draw on the board—this is what quantum mechanics looks like,” he says. “Then we come next door and do the experiment and see what the data is showing us. … They’ll say, ‘That’s not possible.’ The result doesn’t add up, but it does. I can bring home that no one understands quantum mechanics.”

Learn more about the physics & astronomy program or read about Brett Pearson’s research on a multiphoton-imaging system.