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Signal of Majorana particle emerges: evidence backs existence of entity that is its own antiparticle.

A blip of electric current at the end of an atom-thick wire has brought physicists one step closer to confirming the existence of Majorana particles, entities 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 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, Ettore Majorana proposed the existence of a particle that is also its own antimatter counterpart. (Other subatomic particles have separate anti-partners, for instance electrons and positrons.) Neutrinos, wispy particles that barely interact with matter, may 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 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.

Despite being its own antiparticle, this special particle wouldn't be a fermion. In fact, it wouldn't fit into either category of 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 physicist Joel Moore of 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). A pair of Majorana particles seemed to form on the wire, one particle on each end. But the team 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 placed atop a chilled lead crystal. At temperatures near absolute zero, this setup acts as a superconductor, whisking electrons around with no resistance. The researchers used a powerful microscope to image the electrons in the wire. When there was no 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."

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

Lack of confirmation hasn't stopped Microsoft and funding agencies from supporting Majorana research. The particle maybe 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). The theorized spacing between Majorana particles should make the particles' quantum states extraordinarily stable.

Caption: An atom-thick iron nanowire atop a lead crystal, illustrated here, may harbor a pair of Majorana particles. The red spot in the top image indicates that one of the particles is confined to the end of the wire, as theory predicts.


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Title Annotation:MATTER & ENERGY
Author:Grant, Andrew
Publication:Science News
Geographic Code:1USA
Date:Nov 15, 2014
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