anyon
/ EN-ee-on /
In our familiar three-dimensional world, all particles fall into two camps. Some are like coins: swap two of them and nothing changes at all — these are bosons. Others are pickier: swap two and the system's quantum description flips its sign in a subtle way — these are fermions, like electrons. For a long time it seemed those were the only two options. But flatten the world down to two dimensions and a third, in-between possibility appears.
An anyon is a particle-like excitation, possible only in two-dimensional systems, whose behavior under swapping lies between a boson and a fermion. When you move one anyon all the way around another and back, the system's quantum state acquires a phase — a geometric twist — that can be any fraction of a full turn, not just the all-or-nothing options of the two ordinary kinds. That is where the name comes from: any phase is allowed. Anyons are not particles you could find in empty space; they are emergent excitations of special two-dimensional quantum fluids.
This matters because anyons are the real inhabitants of the fractional quantum Hall fluids, where excitations carry fractional charge and behave exactly this way, and because certain anyons could store and process quantum information extremely robustly. A common confusion is to expect anyons in everyday three-dimensional materials; the in-between statistics genuinely require an effectively two-dimensional setting, where the path of moving one particle around another cannot be untangled.
In 2020, two independent experiments on fractional quantum Hall systems detected the telltale fractional phase that appears when one anyon is braided around another — the first direct laboratory confirmation that anyons, predicted decades earlier, really behave as the in-between kind of particle.
Braiding one anyon around another leaves a measurable fractional phase — proof of in-between statistics.
Most anyons are 'Abelian' — braiding only multiplies the state by a phase. A rarer, more powerful kind, 'non-Abelian' anyons, do something stronger: braiding rotates the state in a way that depends on the order of the moves, which is what makes them candidates for topological quantum computing.