Physicists report what may be the first impenetrable evidence for the existence of unusual particle-like objects, called anyons, more than 40 years ago. The latest additions to any growing family of phenomena called quasiparticles, which are not elementary particles, but collective excitations of multiple electrons in solid devices. His discovery – using a 2D electronic device – could represent the first steps towards making anyons the basis of future quantum computers.
“It sounds like a big deal,” says theoretical physicist Steven Simon of Oxford University, UK. The results, which have not yet been peer-reviewed, were posted to the arXiv preprint repository at the end of June 2020.
Known quasiparticles exhibit a range of exotic behaviors. For example, magnetic monopole quasiparticles have only one magnetic pole – unlike all ordinary magnets, which always have one north and one south.
Anyons are more strange. All elementary particles fall into one of two possible categories – bosons and fermions. Anyons are none. The defining property of fermions (which include electrons) is Fermi statistics: when two identical fractions switch the spatial position, their quantum-mechanical wave-wave function – is rotated by 180º. When bosons exchange places, their wave doesn’t change. Two anyons switchings must produce a rotation by some intermediate angle, an effect called fractional statistics that cannot occur in 3D space, but is limited only to moving collective states of electrons in two dimensions.
Fractional statistics are the properties that define anyons property, and the latest work – Michael Manfra, led by an experimental physicist at Purdue University in West Lafayette, Indiana – is the first time it has been decisively measured.
The unusual behavior of quasiparticles when switching locations means that if one rotates in a complete circle around the other – equal to the positions of two particles switching twice – it will retain the memory of that motion in its quantum state. That memory is one of the telltale signs of fractional statistics that experimentalists are looking for.
Manfra and his team built a structure consisting of thin layers of aluminum arsenide and gallium arsenide. This limits the electrons to move in two dimensions while protecting them from stray electric charges in the rest of the device. The researchers cooled it to 10,000 degrees above absolute zero and added a strong magnetic field. This led to a state of a substance in a device called the ‘Fractional Quantum Hall’ (FQH) insulator, the peculiarity of which is that no electric current can move in the interior of a 2D device, but can move along the edge. FQH insulators can host quasiparticles, whose electric charge is not a multiplier of the electronic charge, but a third of it: these quasiparticles have long been suspected of being suspicious.
To prove that they were indeed anyons, the team engraved the device so that it could carry currents from one electrode to another with two possible edges. They changed positions by isolating the magnetic field and adding an electric field. These tweaks were expected to create or destroy any state that is internally stuck, and also for any output moving between electrodes. Since there were two possible paths in any moving path, each producing a different turn in its quantum-mechanical waves, when anyone reached the endpoint, their quantum-mechanical waves produced an interference pattern, called pyjama stripes.
This pattern shows how the relative amount of winding between the two paths varies in response to changes in magnetic-field and voltage strength. But the intervention also showed jumps, which were the smoking gun 2 for the presence or disappearance of anyons in the bulk of the material.
Simon says, As far as I can tell, this is a very concrete observation of anyons – directly observing their defining property: when they travel around another, they accumulate a fractional phase.
This is not the first time that researchers have reported evidence for fractional data. Physicist Robert Willett of NokiaBell Labsin Murray Hill, New Jersey, says his team saw “strong evidence” for partial figures in 2013.
And other teams have investigated a separate asset that makes anyons an intermediate between bosons and fermions. The fermions follow the Pauli exclusion principle: no two firms can occupy the same quantum state. But there is no such restriction on Boson. Are in the middle of anything – they bunch, but not as much as bosons do, as an experiment described Science in April. “It’s different from fermionic behavior that we can also investigate in the same set-up,” says Gwenda Favé, an experimental expert at the University of Sorbonne, Paris, who led that effort.
But some theoretical physicists maintain that the evidence in these and other experiments, although striking, was not conclusive. “In many cases, there are many ways to interpret an experiment,” says Bern Rosenow, a condensed-matter theorist at the University of Leipzig in Germany. But, if the evidence is corroborated, Manfra’s team is uneven, Rosano says. “I don’t know the explanation for this experiment which is plausible and doesn’t include fractional statistics.”
The results potentially form the basis for an application for any individual. Simon and others have developed elaborate theories that use anyone as a platform for quantum computers. Quasiparticle pairs can encode information in their memory to see how they move around each other. And because fractional statistics are ‘topological’ – it depends on the number of times one moves around the other, and not on slight changes in its path – it remains unaffected by small holes. This robustness can make topological quantum computers easier than current quantum-computing techniques, which are error-prone. Microsoft (which employs Manfra as a consultant) has been alone in advancing the topology path for quantum computing, while other large companies, including Intel, Google, IBM, and Honeywell.
Topological quantum computing would require someone more sophisticated than Manfra and colleagues; His team is now reshaping his device to do just that. Still, applications of any kind are closed, researchers warn. “Even with this new result, it is very difficult to see anyons as a strong contender for quantum computing,” Simon says.
But the unique physics of quasiparticles is worth exploring: “To me, as a condensed-matter theorist, they are at least as attractive and exotic as the Higgs particle,” Rosen says.
Watch this video for complete details about Anyons (video is long, be patient)