Fraudulent particles exposed and explained

Particles enter a bar. One of them rocks like the elusive Majorana particle. Everyone is thrilled. The door opens, and everyone leaves, including the supposed rock star. The party is no longer rocking. Did we actually observe a Majorana particle? "Not really," says an international research team.

In their search for the Majorana particle, the team, consisting of researchers from the Institute of Science and Technology Austria (ISTA), the Materials Science Institute in Madrid (Spanish Research Council CSIC), and the Catalan Institute of Nanoscience and Nanotechnology, has demonstrated the existence of very convincing Majorana imitators. Their results, based on new theoretical and experimental methods, were published in the journal Nature in December 2022.

On the trail of Majoranas

The best-known particles are the electron and the photon, which belong to two large families: fermions and bosons. All other particles in nature belong to these two groups. Well, almost all. Another possible category of particles is the so-called anions. It is hypothesized that anions in materials small enough to confine the electronic wavefunction arise from the collective dance of many interacting electrons. An example of this are the so-called Majorana null modes. They are an anionic cousin of Majorana fermions and were first proposed by Ettore Majorana in 1937. Majoranas, as these hypothetical anions are affectionately known, are thought to possess numerous exotic properties: They are identical to their antiparticles, can self-destruct, and have the ability to conceal quantum information by not encoding it locally in space. Scientists agree that this last property, in particular, could enable robust quantum computers.

Since 2010, numerous research groups have been searching for Majoranas. Unlike electrons or photons, which exist in a vacuum, Majorana anions must be created in hybrid materials. One of the most promising platforms for this is based on hybrid superconductor-semiconductor nanodevices. Over the past decade, these instruments have been examined in minute detail in the hope of definitively proving the existence of majoranas. The problem, however, is that majoranas are very complex entities that can easily be overlooked or confused with other quantum states.

The Majorana Rockstar

In their latest publication, the scientists delve into the mysteries of Majorana physics. For the first time, two established techniques were applied simultaneously. Surprisingly, the authors discovered that those states observed with the first technique (Coulomb spectroscopy), which strongly suggest Majoranas, were not visible from the perspective of the second method (tunnel spectroscopy).

The observations can be described with the following metaphorical scenario. Searching for the fabled Majorana Rockstar, you peer through a door ("Source," see graphic) into a bar. A concert appears to be taking place. A rockstar dressed as a Majorana is on stage, singing Majorana songs. However, when you open the large door ("Drain") at the far end of the bar, all the fans hurriedly leave—including the supposed rockstar. A real Majorana would never do such a thing.

“That’s precisely what makes Majoranas so special. Much like true rock stars who don’t simply leave the stage when an exit is available, the Majorana anion remains confined to one side of the nanoparticle due to a profound mathematical principle known as topological protection—even when ordinary electrons can escape through the opposite side,” the researchers explain, using the metaphor. “We wanted to determine whether Majoranas exist or not. In our experimental setup, the doors are tunnel barriers through which electrons are sent in and out. There is an outflow door and a source door. When the particles are observed using both spectroscopic methods simultaneously, our Majorana rockstar imitator turns out to be a different kind of quasiparticle. While they aren’t Majoranas, they are certainly interesting superconducting quasiparticles,” the scientists continue.

These findings underscore the fact that “fake” Majoranas can be found everywhere. They can occur in many different systems and distort various measurement methods. By combining two measurement strategies applied to the same instrument, the "cheater" was exposed through an apparent paradox—an approach that could drastically reduce interpretive uncertainties in future experiments. With their work, the researchers are making an important contribution to one day capturing the elusive Majorana particles and harnessing their properties.

Publication:

M.Valentini, M. Borovkov, E. Prada, S. Martí-Sánchez, M. Botifoll, A. Hofmann, J. Arbiol, R. Aguado, P. San-Jose, G. Katsaros. 2022. Majorana-like Coulomb spectroscopy in the absence of zero-bias peaks. Nature. DOI: 10.1038/s41586-022-05382-w

Project Funding:

The ISTA part of the research was supported by the scientific service units of ISTA with funds from the MIBA workshop and the nanofabrication facility, as well as by the NOMIS Foundation. ISTA Postdoc, A. Hofmann, received funding through H2020-MSCA-IF-2018/844511