Tag Archives: Majorana fermions

Proposed nanodevice made possible by particle that is its own antiparticle (Majorana particle)

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I’m not sure how much the mystery of Ettore Majorana’s disappearance in 1938 has to do with the latest research from Brazil on Majorana particles but it’s definitely fascinating,. From an April 6, 2018 news item on ScienceDaily,

In March 1938, the young Italian physicist Ettore Majorana disappeared mysteriously, leaving his country’s scientific community shaken. The episode remains unexplained, despite Leonardo Scascia’s attempt to unravel the enigma in his book The Disappearance of Majorana (1975).

Majorana, whom Enrico Fermi called a genius of Isaac Newton’s stature, vanished a year after making his main contribution to science. In 1937, when he was only 30, Majorana hypothesized a particle that is its own anti-particle and suggested that it might be the neutrino, whose existence had recently been predicted by Fermi and Wolfgang Pauli.

Eight decades later, Majorana fermions, or simply majoranas, are among the objects most studied by physicists. In addition to neutrinos — whose nature, whether or not they are majoranas, is one of the investigative goals of the mega-experiment Dune — another class not of fundamental particles but of quasi-particles or apparent particles has been investigated in the field of condensed matter. These Majorana quasi-particles can emerge as excitations in topological superconductors.

An April 6, 2018 Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) press release on EurekAlert, which originated the news item,  reveals more about the Brazilian research (Note: Links have been removed),

A new study by PhD student Luciano Henrique Siliano Ricco with a scholarship from the São Paulo Research Foundation – FAPESP, in collaboration with his supervisor Antonio Carlos Ferreira Seridonio and others, was conducted on the Ilha Solteira campus of São Paulo State University (UNESP) in Brazil and described in an article in Scientific Reports.

“We propose a theoretical device that acts as a thermoelectric tuner – a tuner of heat and charge – assisted by Majorana fermions,” Seridonio said.

The device consists of a quantum dot (QD), represented in the Figure A by the symbol ε1. QDs are often called “artificial atoms.” In this case, the QD is located between two metallic leads at different temperatures.

The temperature difference is fundamental to allowing thermal energy to flow across the QD. A quasi-one-dimensional superconducting wire – called a Kitaev wire after its proponent, Russian physicist Alexei Kitaev, currently a professor at the California Institute of Technology (Caltech) in the US – is connected to the QD.

In this study, the Kitaev wire was ring- or U-shaped and had two majoranas (η1 and η2) at its edges. The majoranas emerge as excitations characterized by zero-energy modes.

“When the QD is coupled to only one side of the wire, the system behaves resonantly with regard to electrical and thermal conductance. In other words, it behaves like a thermoelectric filter,” said the principal investigator for the FAPESP fellowship.

“I should stress that this behavior as a filter for thermal and electrical energy occurs when the two majoranas ‘see’ each other via the wire, but only one of them ‘sees’ the QD in the connection.”

Another possibility investigated by the researchers involved making the QD “see” the two majoranas at the same time by connecting it to both ends of the Kitaev wire.

“By making the QD ‘see’ more of η1 or η2, i.e., by varying the system’s asymmetry, we can use the artificial atom as a tuner, where the thermal or electrical energy that flows through it is redshifted or blueshifted,” Seridonio said (see Figure B for illustrative explanation).

This theoretical paper, he added, is expected to contribute to the development of thermoelectric devices based on Majorana fermions.

Here’s a link to and a citation for the paper,

Tuning of heat and charge transport by Majorana fermions by L. S. Ricco, F. A. Dessotti, I. A. Shelykh, M. S. Figueira & A. C. Seridonio. Scientific Reportsvolume 8, Article number: 2790 (2018) doi:10.1038/s41598-018-21180-9 Published online: 12 February 2018

This paper is open access.

As I prepared to publish this piece I stumbled across a sad Sept. 3, 2018 article about Brazil and its overnight loss of heritage in a fire by Henry Grabar for slate.com (Note: Links have been removed),

On Sunday night, a fire ripped through Brazil’s National Museum in Rio de Janeiro, destroying the country’s most valuable storehouse of natural and anthropological history within hours.

Most of the 20 million items housed inside—including the skull of Luzia, the oldest human remains ever found in the Americas; one of the world’s largest archives of South America’s indigenous cultures; more than 26,000 fossils, 55,000 stuffed birds, and 5 million insect specimens; and a library of more than 500,000 books—are thought to have been destroyed.

…

The loss is a symptom of a larger problem as Grabar notes in his article.

Nanodevices and quantum entanglement

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A May 30, 2016 news item on phys.org introduces a scientist with an intriguing approach to quantum computing,

Creating quantum computers which some people believe will be the next generation of computers, with the ability to outperform machines based on conventional technology—depends upon harnessing the principles of quantum mechanics, or the physics that governs the behavior of particles at the subatomic scale. Entanglement—a concept that Albert Einstein once called “spooky action at a distance”—is integral to quantum computing, as it allows two physically separated particles to store and exchange information.

Stevan Nadj-Perge, assistant professor of applied physics and materials science, is interested in creating a device that could harness the power of entangled particles within a usable technology. However, one barrier to the development of quantum computing is decoherence, or the tendency of outside noise to destroy the quantum properties of a quantum computing device and ruin its ability to store information.

Nadj-Perge, who is originally from Serbia, received his undergraduate degree from Belgrade University and his PhD from Delft University of Technology in the Netherlands. He received a Marie Curie Fellowship in 2011, and joined the Caltech Division of Engineering and Applied Science in January after completing postdoctoral appointments at Princeton and Delft.

He recently talked with us about how his experimental work aims to resolve the problem of decoherence.

A May 27, 2016 California Institute of Technology (CalTech) news release by Jessica Stoller-Conrad, which originated the news item, proceeds with a question and answer format,

What is the overall goal of your research?

A large part of my research is focused on finding ways to store and process quantum information. Typically, if you have a quantum system, it loses its coherent properties—and therefore, its ability to store quantum information—very quickly. Quantum information is very fragile and even the smallest amount of external noise messes up quantum states. This is true for all quantum systems. There are various schemes that tackle this problem and postpone decoherence, but the one that I’m most interested in involves Majorana fermions. These particles were proposed to exist in nature almost eighty years ago but interestingly were never found.

Relatively recently theorists figured out how to engineer these particles in the lab. It turns out that, under certain conditions, when you combine certain materials and apply high magnetic fields at very cold temperatures, electrons will form a state that looks exactly as you would expect from Majorana fermions. Furthermore, such engineered states allow you to store quantum information in a way that postpones decoherence.

How exactly is quantum information stored using these Majorana fermions?

The fascinating property of these particles is that they always come in pairs. If you can store information in a pair of Majorana fermions it will be protected against all of the usual environmental noise that affects quantum states of individual objects. The information is protected because it is not stored in a single particle but in the pair itself. My lab is developing ways to engineer nanodevices which host Majorana fermions. Hopefully one day our devices will find applications in quantum computing.

Why did you want to come to Caltech to do this work?

The concept of engineered Majorana fermions and topological protection was, to a large degree, conceived here at Caltech by Alexei Kiteav [Ronald and Maxine Linde Professor of Theoretical Physics and Mathematics] who is in the physics department. A couple of physicists here at Caltech, Gil Refeal [professor of theoretical physics and executive officer of physics] and Jason Alicea [professor of theoretical physics], are doing theoretical work that is very relevant for my field.

Do you have any collaborations planned here?

Nothing formal, but I’ve been talking a lot with Gil and Jason. A student of mine also uses resources in the lab of Harry Atwater [Howard Hughes Professor of Applied Physics and Materials Science and director of the Joint Center for Artificial Photosynthesis], who has experience with materials that are potentially useful for our research.

How does that project relate to your lab’s work?

There are two-dimensional, or 2-D, materials that are basically very thin sheets of atoms. Graphene [emphasis mine]—a single layer of carbon atoms—is one example, but you can create single layer sheets of atoms with many materials. Harry Atwater’s group is working on solar cells made of a 2-D material. We are thinking of using the same materials and combining them with superconductors—materials that can conduct electricity without releasing heat, sound, or any other form of energy—in order to produce Majorana fermions.

How do you do that?

There are several proposed ways of using 2-D materials to create Majorana fermions. The majority of these materials have a strong spin-orbit coupling—an interaction of a particle’s spin with its motion—which is one of the key ingredients for creating Majoranas. Also some of the 2-D materials can become superconductors at low temperatures. One of the ideas that we are seriously considering is using a 2-D material as a substrate on which we could build atomic chains that will host Majorana fermions.

What got you interested in science when you were young?

I don’t come from a family of scientists; my father is an engineer and my mother is an administrative worker. But my father first got me interested in science. As an engineer, he was always solving something and he brought home some of the problems he was working. I worked with him and picked it up at an early age.

How are you adjusting to life in California?

Well, I like being outdoors, and here we have the mountains and the beach and it’s really amazing. The weather here is so much better than the other places I’ve lived. If you want to get the impression of what the weather in the Netherlands is like, you just replace the number of sunny days here with the number of rainy days there.

I wish Stevan Nadj-Perge good luck!