In 1928, physicist Paul Dirac made the stunning prediction that every fundamental particle in the universe has an antiparticle – its identical twin but with opposite charge. In 1937, another brilliant physicist, Ettore Majorana, introduced a new twist: He predicted that in the class of particles known as fermions, which includes the proton, neutron, electron, neutrino and quark, there should be particles that are their own antiparticles.
Stanford scientists have found the first firm evidence of such a Majorana fermion. It was discovered in a series of lab experiments on exotic materials at the University of California in collaboration with Stanford University. The experimental team was led by UCLA Professor Kang Wang, and precise theoretical predictions were made by Stanford Professor Shoucheng Zhang’s group.
Although the search for the famous fermion seems more intellectual than practical, he added, it could have real-life implications for building robust quantum computers, although this is admittedly far in the future.
The particular type of Majorana fermion the research team observed is known as a “chiral” fermion because it moves along a one-dimensional path in just one direction. While the experiments that produced it were extremely difficult to conceive, set up and carry out, the signal they produced was clear and unambiguous, the researchers said.
In the latest experiments at UCLA, UC-Davis and UC-Irvine, the team stacked thin films of two quantum materials – a superconductor and a magnetic topological insulator – and sent an electrical current through them, all inside a chilled vacuum chamber.
The top film was a superconductor. The bottom one was a topological insulator, which conducts current only along its surface or edges but not through its middle. Putting them together created a superconducting topological insulator, where electrons zip along two edges of the material’s surface without resistance, like cars on a superhighway.
It was Zhang’s idea to tweak the topological insulator by adding a small amount of magnetic material to it. This made the electrons flow one way along one edge of the surface and the opposite way along the opposite edge.
Then the researchers swept a magnet over the stack. This made the flow of electrons slow, stop and switch direction. These changes were not smooth, but took place in abrupt steps, like identical stairs in a staircase.
At certain points in this cycle, Majorana quasiparticles emerged, arising in pairs out of the superconducting layer and traveling along the edges of the topological insulator just as the electrons did. One member of each pair was deflected out of the path, allowing the researchers to easily measure the flow of the individual quasiparticles that kept forging ahead. Like the electrons, they slowed, stopped and changed direction – but in steps exactly half as high as the ones the electrons took.
These half-steps were the smoking gun evidence the researchers had been looking for.
Majorana fermions (blue, red, and purple lines) travel through a topological insulator (horizontal bar) with a superconductor layered on top in this illustration of new experiments to detect the fermions. Green lines indicate electrons travelling on the edges of the topological insulator.
The results of these experiments are not likely to have any effect on efforts to determine if the neutrino is its own antiparticle, said Stanford physics Professor Giorgio Gratta, who played a major role in designing and planning EXO-200.
“The quasiparticles they observed are essentially excitations in a material that behave like Majorana particles,” Gratta said. “But they are not elementary particles and they are made in a very artificial way in a very specially prepared material. It’s very unlikely that they occur out in the universe, although who are we to say? On the other hand, neutrinos are everywhere, and if they are found to be Majorana particles we would show that nature not only has made this kind of particles possible but, in fact, has literally filled the universe with them.”
Far in the future, Zhang said, Majorana fermions could be used to construct robust quantum computers that aren’t thrown off by environmental noise, which has been a big obstacle to their development. Since each Majorana is essentially half a subatomic particle, a single qubit of information could be stored in two widely separated Majorana fermions, decreasing the chance that something could perturb them both at once and make them lose the information they carry.
A propagating Majorana mode
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.
Majorana fermion is a hypothetical particle that is its own antiparticle. We report transport measurements that suggest the existence of one-dimensional chiral Majorana fermion modes in the hybrid system of a quantum anomalous Hall insulator thin film coupled with a superconductor. As the external magnetic field is swept, half-integer quantized conductance plateaus are observed at the locations of magnetization reversals, giving a distinct signature of the Majorana fermion modes. This transport signature is reproducible over many magnetic field sweeps and appears at different temperatures. This finding may open up an avenue to control Majorana fermions for implementing robust topological quantum computing.