Tag Archives: Ettore Majorana

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

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.

‘Nano-hashtags’ for Majorana particles?

The ‘nano-hashtags’ are in fact (assuming a minor leap of imagination) nanowires that resemble hashtags.

Scanning electron microscope image of the device wherein clearly a ‘hashtag’ is formed. Credit: Eindhoven University of Technology

An August 23, 2017 news item on ScienceDaily makes the announcement,

In Nature, an international team of researchers from Eindhoven University of Technology [Netherlands], Delft University of Technology [Netherlands] and the University of California — Santa Barbara presents an advanced quantum chip that will be able to provide definitive proof of the mysterious Majorana particles. These particles, first demonstrated in 2012, are their own antiparticle at one and the same time. The chip, which comprises ultrathin networks of nanowires in the shape of ‘hashtags’, has all the qualities to allow Majorana particles to exchange places. This feature is regarded as the smoking gun for proving their existence and is a crucial step towards their use as a building block for future quantum computers.

An August 23, 2017 Eindhoven University press release (also on EurekAlert), which originated the news item, provides some context and information about the work,

In 2012 it was big news: researchers from Delft University of Technology and Eindhoven University of Technology presented the first experimental signatures for the existence of the Majorana fermion. This particle had been predicted in 1937 by the Italian physicist Ettore Majorana and has the distinctive property of also being its own anti-particle. The Majorana particles emerge at the ends of a semiconductor wire, when in contact with a superconductor material.

Smoking gun

While the discovered particles may have properties typical to Majoranas, the most exciting proof could be obtained by allowing two Majorana particles to exchange places, or ‘braid’ as it is scientifically known. “That’s the smoking gun,” suggests Erik Bakkers, one of the researchers from Eindhoven University of Technology. “The behavior we then see could be the most conclusive evidence yet of Majoranas.”

Crossroads

In the Nature paper that is published today [August 23, 2017], Bakkers and his colleagues present a new device that should be able to show this exchanging of Majoranas. In the original experiment in 2012 two Majorana particles were found in a single wire but they were not able to pass each other without immediately destroying the other. Thus the researchers quite literally had to create space. In the presented experiment they formed intersections using the same kinds of nanowire so that four of these intersections form a ‘hashtag’, #, and thus create a closed circuit along which Majoranas are able to move.

Etch and grow

The researchers built their hashtag device starting from scratch. The nanowires are grown from a specially etched substrate such that they form exactly the desired network which they then expose to a stream of aluminium particles, creating layers of aluminium, a superconductor, on specific spots on the wires – the contacts where the Majorana particles emerge. Places that lie ‘in the shadow’ of other wires stay uncovered.

Leap in quality

The entire process happens in a vacuum and at ultra-cold temperature (around -273 degree Celsius). “This ensures very clean, pure contacts,” says Bakkers, “and enables us to make a considerable leap in the quality of this kind of quantum device.” The measurements demonstrate for a number of electronic and magnetic properties that all the ingredients are present for the Majoranas to braid.

Quantum computers

If the researchers succeed in enabling the Majorana particles to braid, they will at once have killed two birds with one stone. Given their robustness, Majoranas are regarded as the ideal building block for future quantum computers that will be able to perform many calculations simultaneously and thus many times faster than current computers. The braiding of two Majorana particles could form the basis for a qubit, the calculation unit of these computers.

Travel around the world

An interesting detail is that the samples have traveled around the world during the fabrication, combining unique and synergetic activities of each research institution. It started in Delft with patterning and etching the substrate, then to Eindhoven for nanowire growth and to Santa Barbara for aluminium contact formation. Finally back to Delft via Eindhoven for the measurements.

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

Epitaxy of advanced nanowire quantum devices by Sasa Gazibegovic, Diana Car, Hao Zhang, Stijn C. Balk, John A. Logan, Michiel W. A. de Moor, Maja C. Cassidy, Rudi Schmits, Di Xu, Guanzhong Wang, Peter Krogstrup, Roy L. M. Op het Veld, Kun Zuo, Yoram Vos, Jie Shen, Daniël Bouman, Borzoyeh Shojaei, Daniel Pennachio, Joon Sue Lee, Petrus J. van Veldhoven, Sebastian Koelling, Marcel A. Verheijen, Leo P. Kouwenhoven, Chris J. Palmstrøm, & Erik P. A. M. Bakkers. Nature 548, 434–438 (24 August 2017) doi:10.1038/nature23468 Published online 23 August 2017

This paper is behind a paywall.

Dexter Johnson has some additional insight (interview with one of the researchers) in an Aug. 29, 2017 posting on his Nanoclast blog (on the IEEE [institute of Electrical and Electronics Engineers] website).

Majorana, matter, anti-matter, and nanowires

This is one of my favourite types of science story and I’m going to start with the quantum physics part of this (from the April 13, 2012 news item on Nanowerk),

Scientists at TU Delft’s Kavli Institute and the Foundation for Fundamental Research on Matter (FOM Foundation) have succeeded for the first time in detecting a Majorana particle. In the 1930s, the brilliant Italian physicist Ettore Majorana deduced from quantum theory the possibility of the existence of a very special particle, a particle that is its own anti-particle: the Majorana fermion. That ‘Majorana’ would be right on the border between matter and anti-matter.

The researchers have made a video about the Majorana fermion and nanowires (from the April 12, news release on the TU Delft website),

Here’s a little more about the Majorana fermion and why the researchers as so excited (from the TU Delft news release),

Majorana fermions are very interesting – not only because their discovery opens up a new and uncharted chapter of fundamental physics; they may also play a role in cosmology. A proposed theory assumes that the mysterious ‘dark matter, which forms the greatest part of the universe, is composed of Majorana fermions. Furthermore, scientists view the particles as fundamental building blocks for the quantum computer. Such a computer is far more powerful than the best supercomputer, but only exists in theory so far. Contrary to an ‘ordinary’ quantum computer, a quantum computer based on Majorana fermions is exceptionally stable and barely sensitive to external influences.

This breakthrough was achieved not with the Large Hadron Collider at CERN (European Particle Physics Laboratory) but with nanowires (from the TU Delft news release),

For the first time, scientists in Leo Kouwenhoven’s research group managed to create a nanoscale electronic device in which a pair of Majorana fermions ‘appear’ at either end of a nanowire. They did this by combining an extremely small nanowire, made by colleagues from Eindhoven University of Technology, with a superconducting material and a strong magnetic field. ‘The measurements of the particle at the ends of the nanowire cannot otherwise be explained than through the presence of a pair of Majorana fermions’, says Leo Kouwenhoven.

The device is made of an Indium Antemonide nanowire, covered with a Gold contact and partially covered with a Superconducting Niobium contact. The Majorana fermions are created at the end of the Nanowire. (from the TU Delft website)

At the end of the TU Delft news release, they mention more about Ettore Majorana and this is where the story gets quite intriguing,

The Italian physicist Ettore Majorana was a brilliant theorist who showed great insight into physics at a young age. He discovered a hitherto unknown solution to the equations from which quantum scientists deduce elementary particles: the Majorana fermion. Practically all theoretic particles that are predicted by quantum theory have been found in the last decades, with just a few exceptions, including the enigmatic Majorana particle and the well-known Higgs boson. But Ettore Majorana the person is every bit as mysterious as the particle. In 1938 he withdrew all his money and disappeared during a boat trip from Palermo to Naples. Whether he killed himself, was murdered or lived on under a different identity is still not known. No trace of Majorana was ever found.

Here’s the citation for the article describing the discovery of the Majorana fermion (from the TU Delft news release),

The article is published in Science Express on 12 April: Signatures of Majorana fermions in hybrid superconductor-semiconductor nanowire devices, V. Mourik, K. Zuo, S.M. Frolov, S.R. Plissard, E.P.A.M. Bakkers, L.P. Kouwenhoven

There’s more information and there are more images with the April 12, 2012 TU Deflt news release.