Physicists have ‘braided’ strange quasiparticles called anyons

Physicists have captured their first clear glimpse of the tangled web woven by particles called anyons.

The observed effect, known as braiding, is the most striking evidence yet for the existence of anyons — a class of particle that can occur only in two dimensions. When anyons are braided, one anyon is looped around another, altering the anyons’ quantum states. That braiding effect was spotted within a complex layer cake of materials, researchers report in a paper posted June 25 at arXiv.org.

“It’s absolutely convincing,” says theoretical physicist Frank Wilczek of MIT, who coined the term “anyon” in the 1980s. Theoretical physicists have long thought that anyons exist, but “to see it in reality takes it to another level.”

Fundamental particles found in nature fall into one of two classes: fermions or bosons. Electrons, for example, are fermions, whereas photons, particles of light, are bosons. Anyons are a third class, but they wouldn’t appear as fundamental particles in our 3-D universe. “It’s not something you see in standard everyday life,” says physicist Michael Manfra of Purdue University in West Lafayette, Ind., a coauthor of the study. But anyons can show up as disturbances within two-dimensional sheets of material. Technically “quasiparticles,” anyons are the result of collective movements of many electrons, which together behave like one particle.

A key way anyons differ from fermions and bosons is in how they braid. If you were to drag one boson or one fermion around another of its own kind, there would be no record of that looping. But for anyons, such braiding alters the particles’ wave function, the mathematical expression that describes the quantum state of the particles. The process inserts an additional factor, called a phase, into the wave function.

In the new study, the researchers created a device in which anyons traveled within a 2-D layer along a path that split into two. One path looped around other anyons at the device’s center — like a child playing duck, duck, goose with friends — while the other took a direct route. The  two paths were reunited, and the researchers measured the resulting electric current.

The extra phase acquired in the trek around the device would alter how the anyons interfere when the paths reunited and thereby affect the current. So the researchers tweaked the voltage and magnetic field on the device, which changed the number of anyons in the center of the loop — like duck, duck, goose with a larger or smaller group of playmates. As anyons were removed or added, that altered the phase, producing distinct jumps in the current.

Seeing the effect required a finely tuned stack of layered materials to screen out other effects that would overshadow the anyons. “It is definitely one of the more complex and complicated things that have been done in experimental physics,” says theoretical physicist Chetan Nayak of Microsoft Quantum and the University of California, Santa Barbara.

Previous work had already revealed strong signs of anyons. For example, physicist Gwendal Fève and colleagues looked at what happened when quasiparticles collide with one another (SN: 4/9/20). Together, the two studies make “a very, very robust proof of the existence of anyons,” says Fève, of the Laboratoire de Physique de l’Ecole Normale Supérieure in Paris.

Like Fève’s work, the new study focuses on a subclass of quasiparticles called abelian anyons. While those quasiparticles have yet to find practical use, some physicists hope that related non-abelian anyons will be useful for building quantum computers that are more robust than today’s error-prone machines (SN: 6/22/20).