Collisions reveal new evidence of ‘anyon’ quasiparticles’ existence

Sometimes, two dimensions are better than three.

In the three-dimensional world we live in, there are two classes of elementary particles: bosons and fermions. But in two dimensions, theoretical physicists predict, there’s another option: anyons. Now, scientists report new evidence that anyons exist and that they behave unlike any known particle. Using a tiny “collider,” researchers flung presumed anyons at one another to help confirm their identities, physicists report in the April 10 Science.

All known elementary particles can be classified either fermions or bosons. Electrons, for example, are fermions. Bosons include photons, which are particles of light, and the famed Higgs boson, which explains how particles get mass (SN: 7/4/12). The two classes behave differently: Fermions are loners and avoid one another, while bosons can clump together.

Then, about 40 years ago, “theoreticians predicted that in a two-dimensional world, you could have new particles with different behaviors called anyons,” says physicist Gwendal Fève of the Laboratoire de Physique de l’Ecole Normale Supérieure in Paris.

Anyons fall somewhere in between bosons and fermions, not entirely avoiding one another or clumping up. Since we don’t live in two dimensions, Fève and colleagues searched for anyons within a 2-D layer of material. There, anyons could show up as “quasiparticles,” disturbances within a solid material that behave like particles (SN: 10/3/14). Such quasiparticles can form when gangs of electrons emulate another variety of particle, sort of like how a school of fish can move in a coordinated fashion to mimic a strange, shimmery creature, confusing predators.

Scientists have already seen evidence for anyons within 2-D materials in a strong magnetic field. Quasiparticles in these materials have a charge that is a fraction of an electron’s, as predicted for anyons. But scientists hadn’t yet confirmed that the quasiparticles fully qualify as anyons: Researchers hadn’t seen the expected bunching behavior in between that of bosons and fermions.

In the new experiment, anyons traveled within a 2-D plane sandwiched inside a layered material. The researchers created two streams of anyons, directed so that they would collide in the center and then exit along one of two paths.

If the researchers had been colliding antisocial fermions, the particles would have gone their separate ways after the collision. Bosons, on the other hand, would tend to clump at the same exit. In the experiment, the researchers saw clumping, but the amount of clumping, and how it changed as the scientists varied the rate at which anyons were sent into the collider, was consistent with theoretical predictions for anyons.

“It’s quite conclusive. It’s a very carefully performed experiment, and it’s a very hard experiment,” says theoretical physicist Bernd Rosenow of the University of Leipzig in Germany. In 2016, he and colleagues had proposed such an experiment in a study in Physical Review Letters.

When anyons swap places or loop around one another, physicists predict, the quasiparticles’ quantum states are altered. Identifying this process, known as braiding, would more fully clinch the case for the existence of anyons, says physicist Chetan Nayak of Microsoft Quantum and the University of California, Santa Barbara.

Braiding some types of anyons may be a useful technique for building better quantum computers (SN: 6/29/17). Current versions of those computers are highly susceptible to mistakes slipping into calculations. Like a neat plait that keeps unruly hair in line, braided anyons could store information in a manner that is resistant to such errors.

Although the new study hasn’t demonstrated braiding, it gets scientists a step closer to understanding anyons. “It’s a beautiful experiment. It is definitely going beyond what was done in the past,” Nayak says.