Scientists have long accepted that every particle that exists has its own antiparticle that can smash it out of existence. But in 1937, the physicist Ettore Majorana posited an even stranger idea: According to the known laws of physics, there should exist particles that eliminate themselves. These supposed self-annihilators remained hypothetical for the next 80 years.
But a recent discovery from Stanford University has changed all that.
On Friday, a team of physicists created the first-ever “angel particle” — a chiral Majorana fermion, which represents both matter and antimatter at the same time. While there have been experiments in the past that have detected what could be Majorana neutrinos in a lab, it wasn’t until this latest result, published Friday in Science, that scientists had any true experimental confirmation of their existence.
According to Majorana’s theory — developed nine years after the physicist Paul Dirac, in 1928, experimentally verified the claim that every particle has its own neutralizing antiparticle — smashing “angel particles” into one another will cause both to annihilate in a hot flash of energy. In theory, an antimatter engine would be one of the most powerful energy sources in the universe because of the immense energy that self-annihilation would release.
In an interview with the Independent, Stanford professor of photon science Tom Devereaux, Ph.D., commented on the discovery, saying: “This research culminates a chase for many years to find chiral Majorana fermions. It will be a landmark in the field.”
The chase has continued for 80 years, because Majorana’s fabled particle has proven to be super difficult to spot in the real world. Some folks suspect that neutrinos, those energetic particles that slip daily through the fabric of your body without leaving a trace, are Majorana neutrinos.
In the last decade, physicists have found that they could create signatures of Majorana fermions by generating certain kinds of very still quasiparticles in superconducting wires. But the results weren’t definite; in order to know for sure that Majoranas had appeared, they needed to get them moving.
The Stanford physicist Shoucheng Zhang, Ph.D. had the idea to create an electron racetrack by laying a magnetic topological insulator — basically a skating rink for electrons — on top of a superconductor, which allowed electrons to zoom around without impediment. Modifying that insulator with a small amount of magnetic material put a one-way sign on the electron racetrack. And, crucially, running a magnet across the surface of the racetrack tugged on the electrons, causing them to slow, stop, and switch directions. That reversal is far from smooth, instead taking place in discrete, highly predictable stutter steps.
Along the way, pairs of Majorana fermions — technically, fermion quasiparticles — emerged in a way researchers could definitely measure. (A quasiparticle is an electron passing through a superconductor in such a way that it’s forced to take on the quantum mechanical properties of another kind of particle; the Stanford researchers suggest that, in this case, it’s a fine representative of the Majorana.)
With every step, one member of the pair would get kicked off the track so the researchers could observe it. And while it looked just like a normal electron, it appeared twice as often as expected. The Majorana fermions — these ones are called “chiral” because they only move in one dimension — take stutter steps half as long as regular electrons. It was proof positive of their existence.
“Our team predicted exactly where to find the Majorana fermion and what to look for as its ‘smoking gun’ experimental signature,” Zhang said in a press release. “This discovery concludes one of the most intensive searches in fundamental physics, which spanned exactly 80 years.”
He suggested the name “angel particles” after the antimatter particles from the film Angels and Demons.
Down the road, Majorana fermions could help build more robust quantum computers. At the moment, any qubit of information in a quantum risks getting annihilated by ambient noise from the universe. But Majoranas are harder to destroy and could theoretically be harnessed for more stable systems.
Plus, verification of 80-year-old predictions from the fringes of weirdness in quantum mechanics is cool as hell.
Abstract: Although Majorana fermions remain elusive as elementary particles, their solid-state analogs have been observed in hybrid semiconductor-superconductor nanowires. In a nanowire setting, the Majorana states are localized at the ends of the wire. He et al. built a two-dimensional heterostructure in which a one-dimensional Majorana mode is predicted to run along the sample edge (see the Perspective by Pribiag). The heterostructure consisted of a quantum anomalous Hall insulator (QAHI) bar contacted by a superconductor. The authors used an external magnetic field as a “knob” to tune into a regime where a Majorana mode was propagating along the edge of the QAHI bar covered by the superconductor. A signature of this propagation—half-quantized conductance—was then observed in transport experiments.