Physics students across the world learn about different types of particles, from matter particles to force-carrying particles. But now, these aspiring physicists may have to learn about a new type of particle: fractional excitons.
Earlier this January, a Brown research team published a study proving the existence of fractional excitons — novel particles that defy traditional understanding of quantum mechanics and have not been experimentally observed until now.
“We knew that these (fractional excitons) exist, but we (only) knew them from theoretical calculations,” said Navketan Batra GS, a Brown physics PhD student and the study’s lead theorist.
But first, what is quantum mechanics?
Dima Feldman, physics professor and a researcher in the study, said that if he explains the topic to high schoolers, he uses the metaphor of a piece of construction paper that can be cut until it is infinitely small and no longer retains its original properties.
Feldman would take out a piece of colored construction paper, and show it to the room. “Everyone understands what color it is,” he said. “We understand where it is. We understand it doesn’t conduct electricity.”
But when he cuts the paper in half over and over again, “at some point it will become extremely tiny, and the primal properties would change,” he said. “The color will be different. It will begin conducting electricity, and all sorts of other things will happen.”
The study of these characteristics — the behavior of particles that are the same size or smaller than atoms — is known as quantum mechanics.
One type of particle important in quantum mechanics is the exciton: a combination of a negative electron and the positive hole that it is attracted to.
In this study, the researchers aimed to “push this understanding one step further and try to see if we can get excitons that coexist with the fractional quantum Hall effect,” said Jia Li, associate professor of physics and the study’s principal investigator.
This effect causes excitons to exhibit fractional electronic charges, rather than integer values.
“This is kind of amazing,” Li said. “The charge of an electron is a fundamental unit, and a fraction of this charge should not exist.”
As excitons carry no net charge, they are extremely difficult to detect. And fractional excitons have been impossible to detect or observe until now, according to Batra.
In addition to experimentally observing fractional excitons, the researchers also proved that the particles can be controlled, which was previously thought to be impossible due to their lack of a charge, he explained.
To control the excitons, the researchers used two two-dimensional graphene layers, which are “single layers of carbon atoms arranged in a hexagonal lattice,” according to Ron Nguyen GS, another PhD student who worked on the study.
The researchers gave these layers electrical charges, allowing the layers to bind to the electrically neutral exciton, Feldman said. Now, the exciton can be probed using an electric current.
This research has many practical applications, particularly for quantum computers — machines that harness quantum physics to compute large amounts of information. As they exist today, quantum computers are powerful but get overwhelmed easily, Batra explained.
“Noise can really disrupt the whole system,” he said.
Because fractional excitons have less noise, they can be used to create a computer that will not get overwhelmed as easily.
“We are now able to not only control the motion of electrons, but also directly engineer the internal structures of quantum particles,” Li said. “That itself is quite promising in terms of how we can consider building future electronic devices.”
Leah Koritz is a senior staff writer covering science & research. Leah is from Dover, Massachusetts and studies Public Health and Judaic Studies. In her free time, Leah enjoys hiking, watching the Red Sox and playing with her dog, Boba.
