Signal of elusive Majorana particle emerges in a nanowire

Blips of electric current at the end of an atom-thick wire have brought physicists one step closer to confirming the existence of Majorana fermions, particles proposed 77 years ago that are their own antiparticles.

The new experiment, described October 2 in Science, does not definitively prove that these particles exist. But it provides compelling evidence that complements findings from previous research.

“The level of evidence is enough for an arrest but not for the death penalty,” says Leo Kouwenhoven, a physicist at the Delft University of Technology in the Netherlands, whose team has also seen hints of Majorana particles. If confirmed, these exotic particles could help scientists overcome a major barrier toward creating quantum computers.

In 1937, Italian physicist Ettore Majorana proposed the existence of a particle that is also its own antimatter counterpart. (Other subatomic particles have separate antipartners, for instance electrons and positrons.) Some physicists are hopeful that neutrinos, wispy particles that barely interact with matter, qualify as Majorana fermions.

Around 2000, physicists realized that another type of Majorana particle might also exist — one that could emerge on the surfaces of certain materials. Unlike electrons, neutrinos and other familiar particles that can exist in a vacuum, this particle would be a product of its environment, arising from the collective behavior of the electrons around it.

And, despite being its own antiparticle, this special particle would not be a fermion. In fact, it wouldn’t fit into either category physicists use to classify subatomic particles: fermions (for example, protons, quarks and electrons) or bosons (such as the Higgs). “The Majorana in condensed matter is much more subtle and exotic than a Majorana neutrino,” says Joel Moore, a theoretical physicist at the University of California, Berkeley.

In 2012, Kouwenhoven’s team reported the first measurement of the predicted signature of a Majorana particle: an electric current that surged within a specially designed nanowire at zero voltage (SN: 5/19/12, p. 11). The finding suggested that a pair of Majorana particles formed on the wire, one particle on each end. But the researchers could not show exactly where on the wire the signal was coming from.

The new experiment, led by Princeton physicist Ali Yazdani, used a zigzag-shaped wire of iron atoms embedded on a chilled crystal of lead. At temperatures near absolute zero, this setup acts as a superconductor — it whisks electrons around with no resistance. The researchers used a two-story-tall, very powerful microscope to image the electrons in the wire. Sure enough, when there was zero voltage between the tip of the microscope and the superconductor, the researchers detected a peak in electric current at one end of the wire — presumably the calling card of one of a pair of Majorana particles.

“This is the first time the Majorana particle has been observed,” Yazdani says. Moore won’t go that far but says the Majorana particle is “a very plausible explanation for what they’re seeing. This goes significantly beyond the Delft experiment.”

For Kouwenhoven, the two studies together are equivalent to taking a nice picture of something that looks like the Majorana particle. But to prove that the particle exists, “you need to take its DNA,” he says. “And a DNA test has not been done.” He says that test will require manipulating and moving the particles to demonstrate their particle-antiparticle duality.

The lack of experimental confirmation hasn’t stopped Microsoft and funding agencies from supporting research into incorporating Majorana particles into devices. Many physicists envision Majoranas as the ideal qubit, the basic processing unit for quantum computers. The potential of quantum computers to outperform conventional ones depends on qubits’ ability to maintain a fragile quantum state in which they hold a 1 and a 0 simultaneously (SN: 5/31/14, p. 10). According to theory, the spacing between paired Majorana particles should make the particles’ quantum states extraordinarily stable.

“I’m really convinced that it’s scientifically possible,” Kouwenhoven says. “Now we have to do it.”