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 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.