Weird particle that remembers its past discovered by quantum computer

A mysterious and long-sought particle that can remember its past has been created using a quantum computer. The particle, called an anyon, could improve the performance of quantum computers in the future.

The anyon is unlike any other particle we know because it keeps a kind of record of where it has been. Normally, repeatedly swapping particles like an electron or a photon renders them completely exchangeable, making it impossible to tell the swap has taken place.

But in the 1970s, physicists realised this wasn’t the case for certain quasiparticles that can only exist in two dimensions, which were later dubbed anyons. Quasiparticles, as the name suggests, aren’t true particles, but rather collective vibrations that behave as if they are particles.

Unlike other particles, swapping anyons fundamentally changes them, with the number of swaps influencing the way they vibrate. Groups of a particular variety, called a non-Abelian anyon, bear a memory of the order in which they were swapped, just as a braided piece of rope retains the order in which its strands have been crossed over. But where the threads of a rope interact physically, anyons interact through the strange quantum phenomena of entanglement, where particle properties are inextricably linked through space.

This inherent memory, and the quasiparticles’ quantum nature, make non-Abelian anyons an attractive way to do quantum computing, but they had never been found experimentally.

Now, Henrik Dryer at quantum computing firm Quantinuum and his colleagues say they have done just that. The researchers developed a new quantum processor, called H2, which uses ytterbium and barium ions trapped using magnetic fields and lasers to create qubits, or quantum bits, the basic building block of a quantum computer.

They then entangled these qubits in a formation called a Kagome lattice, a pattern of interlocking stars common in traditional woven Japanese baskets. This gave the qubits identical quantum mechanical properties to those predicted for anyons and, when the team adjusted the interactions between the qubits in a way that was equivalent to moving the anyons around, they could test for and confirm the distinctive swap-dependent changes to the anyons’ properties.

“This is the first convincing test that’s been able to do that, so this would be the first case of what you would call non-Abelian topological order,” says Steven Simon at the University of Oxford. The fact that you can play around with the anyons using the quantum computer is also useful for researchers who want to better understand this exotic state of matter, he says.

But not everyone agrees that Quantinuum has actually created non-Abelian anyons, rather than merely simulating them. “I know they’re very excited about their work and they should be excited, but it is still a simulation,” says Jiannis Pachos at the University of Leeds, UK. That means it might lack certain properties present in the real thing, he says.

Dryer takes a different view, saying that the quasiparticle nature of anyons means that a simulation is identical to the real thing. “A counterintuitive property of these anyons is that they are not really physical, they don’t care what they’re made of,” says Dryer. “They’re just about information and entanglement – so if you have any system that can create that kind of entanglement, you can create the same type of anyons.”