Brazilian physicists propose an experimental model for detecting the Majorana fermion

The Journal of Applied Physics features an article that describes a possible experiment for detecting the particle proposed by Italian physicist Ettore Majorana, who Enrico Fermi compared to Isaac Newton.

By José Tadeu Arantes

Agência FAPESP – In 1938, 31-year-old Italian physicist Ettore Majorana disappeared without a trace. His mentor, Enrico Fermi, who won the Nobel Prize in Physics that same year, compared Majorana to Isaac Newton (1643-1727), placing him several rungs above the finest examples among a crowded field of scientific geniuses.

The mathematical abilities of Majorana were legendary. He drafted sophisticated theoretical propositions on packs of cigarettes, which he would then ball up and throw away, deeming the scribbling to be infantile. In March 1932, he proposed a model of the nucleus of an atom made up of protons and neutrons, a few months before the German Werner Heisenberg (1901-1976) had proposed his model. However, despite Fermi’s insistence, Majorana refused to publish any article about it.

When Majorana disappeared, it was suspected that he had been kidnapped by the Fascist regime of Benito Mussolini because he knew too much. Subsequently, it was determined that he had meticulously planned his own disappearance.

Other hypotheses have been presented as well: he disappeared because, knowing the destructive potential of nuclear power, he did not want to be obligated to work for the Fascists to produce the atomic bomb; he disappeared because, driven by intense mystical aspirations, he decided to isolate himself in a monastery or wander about as a vagabond. It is also believed that he may have sought refuge in Argentina, spending his life as an engineer, but there is no conclusive evidence for any of these theories.

Of the few studies Majorana did publish, the most famous was Teoria simmetrica dell’elettrone e del positrone (A Symmetric Theory of Electrons and Positrons), dated 1937. In this study, he presented the hypothesis that a particle could be its own antiparticle. The existence of the neutrino had just been postulated by Fermi and Wolfgang Pauli, and Majorana suggested that the neutrino could be this particle.

This hypothetical particle, which is its own antiparticle, is named the Majorana fermion. Eight decades after it was first proposed, the Majorana fermion continues to arouse great interest among physicists. Current research pertaining to the Majorana fermion focuses not only on the neutrino but also on quasiparticles, which are excitations of a superconducting state.

“In the context of condensed matter physics [in which the number of system constituents (atoms, electrons, etc.) is extremely high and produces very intense interactions among them], Majorana fermions can manifest themselves not as real particles, such as protons or electrons, but as quasiparticles, or apparent particles that describe the superconducting state,” said physicist Antonio Carlos Ferreira Seridonio, professor in the Department of Physics and Chemistry at the São Paulo State University (Unesp), Ilha de Solteira campus (SP), in comments to Agência FAPESP.

Seridonio is co-author of the article, “Probing the antisymmetric Fano interference assisted by a Majorana fermion,” recently published as the cover story of the Journal of Applied Physics.

The article proposes an alternative experimental model for detecting the Majorana fermion. The model was conceived by a group of researchers and doctoral students at the Ilha Solteira and Rio Claro campuses of Unesp and the Federal University of Uberlândia (UFU), led by Seridonio, Valdeci Mariano de Souza (Unesp-Rio Claro) and Fabrício Macedo de Souza (UFU).

The first author of the article, Fernando Augusto Dessotti, is a PhD candidate advised by Seridonio. And the second, master’s degree candidate Luciano Henrique Siliano Ricco, is receiving FAPESP funding for research on the topic of the published article.

One scenario considered to be a strong candidate for giving rise to Majorana fermions as quasiparticles is known as the “Kitaev wire,” proposed by Russian physicist Alexei Kitaev (born 1963), currently a professor at the California Institute of Technology (Caltech).

“In 2001, while working at Microsoft, Kitaev devoted himself to finding a basic unit for quantum computing [the qubit or quantum bit], capable of resisting outside disturbances of the environment, thus allowing construction of a quantum computer. The model he presented consisted of one finite superconductor wire. When this wire finds itself in a specific state, called a topological phase, it is possible to isolate one Majorana fermion at each of its points. And this pair of quasiparticles would make up the quantum bit,” reported Seridonio.

The article published by Seridonio and his group in the Journal of Applied Physics describes an experimental channel for detecting these quasiparticles. “The components of the experimental tool we propose have already been produced experimentally. All that’s needed now is to integrate them. We think that it is just a question of time before this happens. And our work offers a way to do this,” he said.

The tool uses an electron interferometer (used to study the wave behavior of electrons) similar to the Bohm-Aharonov interferometer [designed in the late 1950s by American physicist and naturalized Brazilian David Bohm (1917-1992) and Israeli physicist Yakir Aharonov (1932), Bohm’s student at the time].

“Our idea was to connect this interferometer to a Kitaev wire in the topological phase. The transport of electrons in the interferometer would be affected by the Majorana fermions found on the points of the Kitaev wire. And by means of the alteration produced in the electron wave spectra, it would be possible to characterize the Majoranas,” Seridonio explained.

“In the future, we’ll use the proposed interferometer to explore another type of Majorana that generates a current of quasiparticles along the edges of a superconductor,” the researcher added.

The article, “Probing the antisymmetric Fano interference assisted by a Majorana fermion” (doi: 10.1063/1.4898776),” by F.A. Dessotti and colleagues is available at http://scitation.aip.org/content/aip/journal/jap/116/17/10.1063/1.4898776.