Any student of particle physics can tell you that all subatomic particles in Nature fall into two distinct classes: fermions (electrons, neutrons, protons, etc.) and bosons (photons and other force-carrying “messenger” particles).
Back in 1982, however, physicist Frank Wilczek and several of his colleagues proposed that — theoretically, at least — there could be particles that exist between those two discrete classes.
Wilczek dubbed them “anyons” because any anyon can be anything between a boson and a fermion. “Wilczek is a funny guy,” Tassilo Keilmann told Symmetry Breaking. Keilmann is a physicist at Munich’s Ludwig Maximilian University, and he’s designed an experiment he thinks will bring anyons into the realm of the observable world, using cold atoms and lasers — and a smidgen of ingenuity.
The whole notion of anyons is bizarre, if you think about it. I mean, quantum field theory — the marriage of quantum mechanics with special relativity — dictates that there are only two discrete possibilities for classes of particles: bosons and fermions. The existence of anyons should be ruled out by the quantum mechanical playbook. So what gives?
Imagine you are a humble electron, and you walk into the Subatomic Cafe for a nice big bowl of quark soup. The room is filled with elementary particles, both bosons and your fellow fermions. Where do you sit?
Electrons are restless particles with serious intimacy issues; they need a lot of personal space. Each electron in the cafe occupies only one seat at a given table, and would shove off a stranger who tried to sit in his or her lap –- although if that person happened to be an attractive member of the opposite sex, the two could share a table.
The tables, each with a certain number of seats, represent specific energy states. Only a certain number of electrons can occupy a given state. For instance, a helium atom has two electrons, and both can occupy the same “seat” provided they are opposite “genders,” determined by a mysterious property called “spin” (i.e., one is spin up, the other is spin down). In this case, two is company, and three is definitely a crowd: if a third electron tries to horn in, it will be bumped up to the next energy level.
As an atom absorbs energy and new electrons are added, the electrons fill up the “seats” one by one, beginning with the most desirable “table” — the ground state — and working outward. This is the Pauli exclusion principle: once an electron occupies a seat, it excludes others like itself from sitting in the same seat. (There is no lap dancing in electron land.)
Bosons, in contrast, have serious boundary issues; lap dances are de rigeur. They love to crowd together in the same space. Why, they’d turn the entire Subatomic Cafe into one giant mosh pit given half a chance! You and the other fermions click your tongues in disapproval at the grotesque lack of disciplined order on display.
But at least everyone knows their place, and sticks with their own kind. The Subatomic Cafe is unapologetically segregated in that respect. Anyons upset the rigid quantum social order; they refuse to choose a side. They can do that because — at least to date — they only “exist” in two dimensions. That’s right, the Subatomic Cafe happens to be part of the proverbial Flatland.
So yes, physicists can create two-dimensional systems in the lab. And two-dimensional worlds play by their own rules. “Fermions and bosons are the only possibility if the dimension of space is three or more,” Wilczek explained back in 1991. “If the dimension of space is two, however, then there is a continuous range of possibilities, and those are the anyons.”
This is where Keilmann comes in. We now have the tools to create “artificial particles” in the lab, and manipulate them individually, albeit at ultracold temperatures (on the order of 10-10 kelvin). This enables scientists to study the strange, small world of quantum physics as if they were looking at it through a magnifying glass.
Keilmann proposes building a grid of lasers known as an optical lattice, and then introducing a cloud of boson rubidium 87 chilled to ultracold temperatures. Those particles should settle into the spaces between the laser boundaries, much like eggs settle into an egg crate. Per Symmetry Breaking:
Keilmann calls this “conditional hopping.” Translation: if the bosons start acting more like fermions, avoiding certain wells in the egg crate much like fermions avoid other fermions with the same spin state, then those rigid class boundaries aren’t as rigid after all. They would technically be anyons, straddling the gap between bosons and fermions, bringing desegregation to the quantum world.