Researchers from the University of California, Harvard University, University of Manchester, UC Santa Cruz and the National Institute for Materials Science in Tsukuba, Japan have conducted an experiment that confirms a 40 year old theory that electrons confined in quantum space would move along common paths rather than producing a chaotic array of trajectories.
Electrons exhibit both particle and wave-like properties and behave in ways that are often counterintuitive, and under certain conditions, their waves can interfere with each other in a way that concentrates their movement into certain patterns. Physicists call these common paths “unique closed orbits.”
Physicist Jairo Velasco, Jr. achieved this in his lab using an intricate combination of advanced imaging techniques and precise control over electron behavior within graphene, whose unique properties and two-dimensional structure make it ideal for observing quantum effects. In their experiment, Velasco’s team utilized the finely tipped probe of a scanning tunneling microscope to first create a trap for electrons, and then hover close to a graphene surface to detect electron movements without physically disturbing them.
The benefit of electrons following closed orbits within a confined space is that the subatomic particle’s property would be better preserved as it moves from one point to another, according to Velasco. He said this has vast implications for everyday electronics, explaining how information encoded in an electron’s properties could be transferred without loss, conceivably resulting in lower-power, highly efficient transistors.
“One of the most promising aspects of this discovery is its potential use in information processing,” Velasco said. “By slightly disturbing, or ‘nudging’ these orbits, electrons could travel predictably across a device, carrying information from one end to the other.”
In physics, these unique electron orbits are known as “quantum scars.” This was first explained in a 1984 theoretical study by Harvard University physicist Eric Heller, who used computer simulations to reveal that confined electrons would move along high-density orbits if reinforced by their wave motions interfering with each other.
“Quantum scarring is not a curiosity. But rather, it is a window onto the strange quantum world,” said Heller, also a co-author on the paper. “Scarring is a localization around orbits that come back on themselves. These returns have no long-term consequence in our normal classical world—they are soon forgotten. But they are remembered forever in the quantum world.”
With Heller’s theory proven, researchers now have the empirical foundation needed to explore potential applications. Today’s transistors, already at the nanoelectronic scale, could become even more efficient by incorporating quantum scar-based designs, enhancing devices like computers, smartphones, and tablets, which rely on densely packed transistors to boost processing power.
“For future studies, we plan to build on our visualization of quantum scars to develop methods to harness and manipulate scar states,” Velasco said. “The harnessing of chaotic quantum phenomena could enable novel methods for selective and flexible delivery of electrons at the nanoscale—thus, innovating new modes of quantum control.”
Velasco’s team used a visual model often referred to as a "billiard" to illustrate the classical mechanics of linear versus chaotic systems. A billiard is a bounded area that reveals how particles inside move, and a common shape used in physics is called a “stadium,” where the ends are curved and the edges straight. In classical chaos, a particle would bounce around randomly and unpredictably—eventually covering the entire surface.
In this experiment, the team created a stadium billiard on atom-thin graphene that measured roughly 400 nanometers in length. Then, with the scanning tunneling microscope, they were able to observe quantum chaos in action: finally seeing with their own eyes the pattern of electron orbits within the stadium billiard they created in Velasco’s lab.
“I am very excited we successfully imaged quantum scars in a real quantum system,” said first and co-corresponding author Zhehao Ge, a UC Santa Cruz graduate student at the time of this study’s completion. “Hopefully, these studies will help us gain a deeper understanding of chaotic quantum systems.”