The most common particles are electrons and photons, which are understood as examples of the large families of fermions and bosons, to which all other particles in nature belong. But there is another possible category of particles, the so-called anyons. Anyons are predicted to appear inside materials small enough to confine the electronic state wavefunction, as they emerge from the collective dance of many interacting electrons.
One of them is named Majorana zero mode, anyonic cousins of Majorana fermions proposed by Ettore Majorana in 1937. Majoranas, as these hypothetical anyons are affectionately called, are expected to exhibit many exotic properties, such as behaving simultaneously as a particle and an antiparticle, allowing for mutual annihilation and the ability to hide quantum information by encoding it non-locally in space. This last property specifically holds the promise of resilient quantum computing.
Since 2010, many research groups have rushed to find Majoranas. Unlike fundamental particles, such as the electron or photon, which naturally exist in a vacuum, Majorana’s anyons must be created inside hybrid materials. One of the most promising platforms for realizing them relies on hybrid superconductor-semiconductor nanodevices. Over the past decade, these devices have been studied in excruciating detail, hoping to unambiguously prove the existence of Majoranas. However, Majoranas are delicate entities, easily overlooked or confused with other quantum states.
In a new article published in Nature, scientists have shed more light on the mystery of Majorana physics. For the first time, two well-established techniques have been applied simultaneously to the same device. To their surprise, the researchers found that the states observed with one technique (Coulomb spectroscopy), which are very suggestive of Majoranas at first sight, were not present when they looked for them in the different perspective offered by the second technique. (tunnel spectroscopy).
The observations are similar to the following metaphorical scenario. In search of the legendary Majorana rock star, you peek through a door (source) into a bar. A concert seems to be taking place. You clearly see a remarkable rock star on stage, dressed in a Majorana outfit, singing the Majorana song. The bar is full of Majorana fans watching him adoringly. However, when you open a large (escape) door at the far end of the bar, fans rush to leave, including the supposed rock star. As a true artist, the real Majorana would never do such a thing.
“This is precisely what makes Majoranas special. Just as real 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 deep mathematical principle called protection. topological, even when regular electrons are allowed to escape through the opposite side,” the researchers state.
“We investigated whether there is a Majorana or not. In our experimental conditions, the gates are just tunnel barriers where electrons flow in and out. There is a drain gate and a source gate. Seen from the two spectroscopy methodologies together at the same time, our rock star Majorana impostor turns out to be a different type of quasiparticle. Don’t get me wrong, these are interesting superconducting quasiparticles, but not Majoranas,” continue the scientists.
The results highlight the fact that Majorana’s compelling imposters are everywhere. They can exist in many different types of devices and can fool different measurement strategies individually. Combining two measurement strategies applied to the same device revealed the impostor through an apparent paradox, an approach that could drastically reduce ambiguities in the interpretation of future experiments. This is a necessary step to trap the elusive Majorana and eventually begin to harness its power.
Marco Valentini et al, Majorana-type Coulomb spectroscopy in the absence of zero bias peaks, Nature (2022). DOI: 10.1038/s41586-022-05382-w
Provided by the Spanish National Research Council (CSIC)
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