Imposter particles revealed and explained

Majoranas are theoretical particles of great interest, as they are anticipated to show different exotic properties, including their applications in quantum computing. However, the jury is still out on whether these particles have been detected experimentally or not. In a joint effort, scientists at the Institute of Science and Technology Austria (ISTA) discovered Majorana imposters by applying two measurement techniques, which will help to reduce interpretation uncertainties in the future search for these elusive particles.

Particles enter a bar, one rocks like the indefinable Majorana particle. Everybody rejoices. As soon as one door opens, everyone leaves, including the alleged rock star. No one rocks no more. Was it really the Majorana particle that we observed? ‘‘Not really,’’ say an international team of researchers.

As the pressure builds around the Majorana particle, the team composed of scientists 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 revealed the existence of a highly convincing kind of imposter Majorana particle. Their findings, based on new theoretical and experimental investigations, are published today in the journal Nature.

Majorana who?

The most common particles are the electron and photon, which are understood to be examples from great families, fermions, and bosons, to which all other particles in nature belong. Well, almost all. There is another possible category of particles: the so-called anyons. Anyons are predicted to arise inside materials small enough to confine the electronic state wave function, as they emerge from the collective dance of many interacting electrons. One of these is named Majorana zero mode, anyonic cousins to the Majorana fermions proposed by Ettore Majorana in 1937. Majoranas, as these hypothetical anyons are affectionally called, are predicted to exhibit numerous exotic properties, such as simultaneously behaving like a particle and antiparticle, allowing self-destruction, and the capability to hide quantum information by encoding it non-locally in space. The latter property specifically holds the promise of resilient quantum computing.

Since 2010 many research groups have raced to find Majoranas. Unlike fundamental particles, such as the electron or the photon which naturally exist in a vacuum, Majorana anyons need to be created inside hybrid materials. One of the most promising platforms for realizing them is based on hybrid superconductor-semiconductor nanodevices. Over the past decade, these devices have been studied with excruciating detail, with the hope of unambiguously proving the existence of Majoranas. However, Majoranas are tricky entities, easily overlooked or mistaken with other quantum states.

Majorana Rockstar

In the new paper, the scientists throw further light into the mystery of Majorana physics. For the first time, two well-established techniques were applied simultaneously to the same device. To their surprise, the authors found that the states observed with one technique (Coulomb spectroscopy) that are highly suggestive of Majoranas at first glance, were not present when looking for them from the different perspective afforded by the second technique (tunneling spectroscopy).

The observations are akin to the following metaphorical scenario. In search of the fabled Majorana rock star, you peek through a (source, see graphic) door to a bar. A concert appears to be taking place. You clearly see a remarkable rock star on the stage, dressed in a Majorana outfit, singing the Majorana song. The bar is full of Majorana fans that watch him in adoration. However, when you open a large (drain) door on the far end of the bar the fans rush to leave the premises – among them the supposed rock star. As a true artist, the real Majorana would never do such a thing.

“That’s precisely what makes Majoranas special. Much like the true rock stars don’t simply leave the stage when an exit is available, the Majorana anyon remains pinned to one side of the nanodevice by virtue of a profound mathematical principle called topological protection, even when regular electrons are allowed to escape through the opposite side,” the researchers state, homing in on the team’s metaphorical explanation. “We aimed to find out how to see if there is a Majorana or not. In our experimental conditions, the doors are nothing but tunnel barriers in which electrons are sent in and out. There’s a drain door and a source door. Seen from the two spectroscopy methodologies together at the same time, our Majorana rock star imposter turns out to be another kind of quasi-particle. Don’t get us wrong, these are interesting superconductor quasi-particles, but not Majoranas,” the scientists continue.

The findings highlight the fact that convincing Majorana impostors are everywhere. They can exist in many different types of devices and can deceive different measurement strategies individually. The combination of two measurement strategies applied to the same device revealed the impostor through an apparent paradox, an approach that could drastically reduce interpretation ambiguities of future experiments. This is a much-needed step so that one day we may trap the elusive Majorana, and finally begin to harness its power.

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

Funding information:

The ISTA project part was supported by the Scientific Service Units of ISTA through resources provided by the MIBA Machine Shop and the nanofabrication facility; the NOMIS Foundation. ISTA Postdoc, A. Hofmann, had support from H2020-MSCA-IF-2018/844511.