A two-technique cross measurement allows identifying impostor Majorana particles

by Virginia Greco

Majorana anyons, theoretically predicted particles which are expected to show exotic properties with great potential for quantum computing applications, are both very elusive and difficult to distinguish from other quantum states. Researchers from ISTA, ICN2 and ICMM-CSIC combined two different measurement techniques to unmask an impostor Majorana particle, as described in a study published in ‘Nature’.

Physicists have predicted the possible existence of a family of particles, called anyons, which would add to the already known ones, i.e. fermions and bosons. One of them is the Majorana zero mode (related to the Majorana fermions proposed by Italian physicist Ettore Majorana in 1937). Majoranas are predicted to exhibit numerous exotic and interesting properties, such as simultaneously behaving like a particle and its antiparticle, allowing reciprocal annihilation, and the capability to hide quantum information by encoding it non-locally in space. The latter is particularly appealing as it holds the promise of quantum computing resilient to errors.

Unfortunately, Majoranas are quite elusive and can be easily mistaken with other quantum states. Therefore, researchers are not only avidly looking for them, but also for reliable techniques that can unambiguously prove their identity. Unlike electrons and photons, which naturally exist in a vacuum, Majorana anyons are expected to emerge in specific conditions inside hybrid materials. One of the most promising platforms for generating them is based on hybrid superconductor-semiconductor nanodevices.

In a paper published yesterday in Nature, a team of researchers from the Institute of Science and Technology Austria (ISTA), the Catalan Institute of Nanoscience and Nanotechnology and the Materials Science Institute of Madrid (ICMM-CSIC) shed light into the mystery of Majorana physics. Members of the ICN2 Advanced Electron Nanoscopy Group, including group leader ICREA Prof. Jordi Arbiol, participated in this work.

For the first time, two well-established techniques –i.e., Coulomb spectroscopy and tunneling spectroscopy— were applied simultaneously to the same device to search for Majorana particles. Thanks to them, the authors found that states observed with the fist technique, which appeared to be Majoranas, were not present when looking for them though the second technique. This proved that what they saw with the first method actually was another kind of quasi-particles.

These findings demonstrated that “impostors” –that is, Majorana-anyon-looking particles— can exist in diverse types of devices and can deceive different measurement strategies individually. The combination of two measurement techniques applied to the same device allowed researchers to expose the impostor. This study is key for future investigations, since provides a method to drastically reduce interpretation ambiguities of experiments.

Researchers of course will keep looking for the elusive Majoranas, hoping to finally find them and harness their potentials. But, from now on, it will be harder to be fooled by similar particles.

Image:

Metaphorical illustration of the two-technique observations. In this metaphor, the Majorana particle is a rock star giving a concert in a bar. To look for it, researchers peek through a (entrance) door to a bar. With the first method, they clearly see a rock star on the stage, dressed in a Majorana outfit, singing the song of a Majorana. The bar is full of Majorana fans that watch in adoration. However, when –with the second method– researchers open a large (exit) door on the far end of the bar, the fans rush to leave the premises and, with them, the supposed rock star. But a true artist would not leave the stage; in the same way, the real Majorana would not leave with the other particles. This shows how the combination of the two techniques allows scientists to uncover Majorana impostors. [Credit: ISTA]

Reference article:

Marco Valentini, Maksim Borovkov, Elsa Prada, Sara Martí-Sánchez, Marc Botifoll, Andrea Hofmann, Jordi Arbiol, Ramón Aguado, Pablo San-Jose & Georgios Katsaros, Majorana-like Coulomb spectroscopy in the absence of zero-bias peaks. Nature (2022) DOI: 10.1038/s41586-022-05382-w