Most physics students are surprised to learn that all basic particles have properties of both particles and waves, but for the first time physicists have observed something even stranger: a particle that is both matter and its own antimatter counterpart.
Known as the “Majorana fermion” the subatomic speck has been theorised since the 1930s but never definitely seen before. The fermion “is expected to emerge at the edge of certain materials," says Professor Ali Yazdani of Princeton University, and this is indeed where it has been observed, sitting at each end of a single-atom-thick wire.
Antimatter was predicted in 1928, and confirmed in 1932, as particles with the same mass as ordinary matter but opposite in charge. In 1937 Ettore Majorana concluded that a particle could be both matter and antimatter, and even more astonishingly, stable in this state. Mojorana disappeared due to mysterious circumstances the following year, cutting short what was already a remarkable career.
Majorana's theory was staggering because normally when matter and antimatter encounter each other, both are destroyed, releasing a lot of energy in the process. However, the mathematics he used to make the claim appeared unassailable. The combination of matter and antimatter properties make the particle neutral so that, gravity aside, it barely interacts with the matter around it. There has been speculation that neutrinos are actually Majorana fermions.
In Science, Yazdani talked about the production of an iron wire sited on an ultrapure superconducting lead crystal. At 1K (-272°C) spectroscopic imaging techniques revealed the ends of the wire to be in zero energy states. This is what would be expected if Majoranas had formed at these ends. The appearance of Majoranas at the ends of superconducting wires was predicted in 2001.
The authors describe their Majoranas as "clean and removed from any spurious particles," unlike what would occur if they had been made in high energy accelerators. A sighting of possible Majoranas was made in 2012, but Yazdani says his is far more certain. The microscope “shows that this signal lives only at the edge," says Yazdani, which was not necessarily the case for the previous experiment. "That is the key signature. If you don't have that, then this signal can exist for many other reasons."
It is hoped that materials incorporating the Majorana would hold onto quantum information in a more stable manner, bolstering the possibility of quantum computing.
Ilya Dorzdov, Yazdani Lab, Princeton University. A scanning-tunneling microscope creates a magnetic field to map the presence of a neutral signal that indicates the presence of Majorana fermions at the ends of an iron wire on a lead crystal.