Physicists Discover New Type of Particle—Sort Of

Science, by Robert F. Service, 12 April 2012

Nabbed. This oddball transistor with a normal metal electrode (N) and
 a superconducting electrode (S) registered signs of Majorana fermions at
the two ends of a nanowire spanning the electrodes. Credit: V. Mourik et al.

In 1937, after the rise of quantum mechanics, Ettore Majorana, an Italian theoretical physicist, realized that the new physics implied the existence of a novel type of particles, now called Majorana fermions. After a 75-year hunt, researchers have now spotted the first solid evidence of their existence. And their discovery could hold the key to finally creating workable quantum computers .

Prior to Majorana's work, Austrian physicist Erwin Schrödinger came up with an equation that describes how quantum particles behave and interact. Paul Dirac, an English physicist, tweaked that equation to apply it to fermions, such as electrons, moving at near-light speed. That work tied together quantum mechanics and Einstein's special theory of relativity. It also implied the existence of antimatter, where every particle has an antimatter counterpart—such as electrons and positrons—and that the two would annihilate each other if they ever met. Dirac's work suggested that some particles, such as photons, could serve as their own antiparticles. But fermions weren't thought to be among them. It was Majorana's manipulations of Dirac's equations that suggested the possible existence of a new type of fermion that could serve as its own antiparticle.

At the time, Majorana thought a type of neutrino, an electrically neutral particle with a tiny mass, might fit the bill for his proposed particle. And scientists continue to search for evidence that neutrinos are or are not their own antiparticles. But decades after Majorana's proposal, theoretical physicists realized that the coordinated motion of large numbers of electrons in electronic devices might mimic the behavior of Majorana fermions. These collective motions aren't elementary bits of matter the way electrons and neutrinos are. Rather, they are "quasiparticles." But they should behave much as would elementary particles of the same type. It is the signs of these quasiparticles that researchers led by physicist Leo Kouwenhoven and colleagues at Delft University of Technology report online today in Science.

To spot their quarry, Kouwenhoven's group created specially designed transistors. In standard transistors, applying a voltage to a metal electrode called a gate turns on the flow of current through a semiconductor between two other metal electrodes. Previous theoretical predictions suggested that if one of the secondary electrodes was a superconductor, and the current was allowed to flow through a special semiconductor nanowire under a magnetic field, the combination would force electrons in the nanowire to behave collectively as if Majorana fermions were present at opposite ends of the wire. Theory further offered that if researchers tried to send an electric current from the normal electrode to the superconducting electrode without the magnetic field turned on, the electrons trying to make the journey would essentially bounce off the superconductor, so no current would be detected at the superconducting electrode. But if the magnetic field is turned on, this would trigger the presence of Majorana fermions, which would enable electrons to enter the superconductor, and that would produce a jump in the current.

This current spike is what Kouwenhoven's team found. When the researchers then removed any one of the conditions needed to induce Majorana fermions—such as the magnetic field, or replacing the superconducting electrode with another metal electrode—the current spike at the second electrode vanished.

The results don't provide a direct detection of Majorana fermions. But the Dutch team did a "very compelling" job of eliminating all other possible explanations, says Jason Alicea, a theoretical physicist at the University of California, Irvine. However, the study doesn't completely nail the case for the presence of Majorana fermions, he cautions. The current spike is only 5% of what theory predicts. But that may be because the equipment used to chill the experimental setup must be improved to get closer to absolute zero, where the signal for Majoranas should be the strongest.

If Majoranas are confirmed, they are expected to have properties that make them ideal for constructing a quantum computer. When you move two Majorana fermions with respect to one another, they essentially "remember" their former position, a property that could be used to encode data at the quantum level. Kouwenhoven's group hasn't spotted that signature yet, but they're on the hunt now.

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