Quantum computing: the ‘impostor’ particles that bring this type of computer closer

Scientists from the Madrid Institute of Materials Science (ICMM-CSIC), the Catalan Institute of Nanoscience and Nanotechnology (ICN2-CSIC-GENCAT) and the Austrian Institute of Science and Technology (ISTA) have discovered physical particles masquerading as Majorana particles. According to the theory, Majorana particles are the key to a more robust quantum computation than the current onedue to its resilience against external disturbances, which is known as quantum decoherence.

The conclusions of this joint work, which applies two different measurement techniques combined with a theoretical analysis, help to greatly reduce the interpretation uncertainties in the experiments. The work is published in the journal Nature.

What properties do these Majorana imposter particles have?

Among the properties of the Majorana particles, its ability to hide quantum information by encoding it non-locally in space. This property holds the promise of a resilient quantum computing, which is essential in the advancement of this type of technology. However, there is currently no consensus as to whether these theoretical particles have been detected in experiments.

In the new paper, the scientists shed more light on the mystery of the Majorana particles. For the first time, two well-established experimental techniques were applied simultaneously to the same superconducting device. The authors found that the observed states that seem to demonstrate the detection of Majorana particles with one technique (Coulomb spectroscopy) are inconsistent with the data obtained with the second technique (tunneling spectroscopy), in which the expected signal is not observed. This apparent experimental contradiction is explained through theoretical calculations and shows that the observed particles are not Majorana states.

“The observations are similar to the metaphorical scenario of the Majorana Bar”the researchers explain. “In his search for the famous rock star Majorana, a scientist walks into a particle bar and sees a particle on stage that acts like it is Majorana: it dresses like her and sings Majorana’s song perfectly, so that all his fans are convinced of being in front of his star. However, as soon as the back door is opened, all the particles of the place leave, including the supposed rock star, something unthinkable if it had been the real Majorana. So, was he really who he seemed to be?” the scientists say. “Not really,” adds this team of researchers.

“That is precisely what makes the Majorana special. As in the Majorana Bar metaphor, a true rock star would not leave the stage even if the audience started to leave in the middle of the concert, the real Majorana would remain anchored to the nanodevice by virtue of a deep mathematical principle called topological protection. This occurs even if there are circumstances that allow conventional electrons to escape through the tunnel effect,” they say.

The objective of the work was detect the presence of a certain variant of Majorana particles, since since its prediction in 1937 by Ettore Majorana they have not been found in laboratory experiments. “In our experimental conditions, the gates are nothing more than tunneling barriers through which electrons enter and exit. There is a drain gate and a source gate. Seen from the combined perspective of the two methodologies, our impostor turns out to be a different type of quasi-particle. “These are interesting superconducting quasi-particles, but they are not Majorana particles,” the scientists continue.

The findings highlight the fact that these impostors Majorana particles can exist in many different types of devices and they can fool different measurement strategies. It is the combination of two different ways of measuring what has revealed to the impostor through an apparent paradox.

This approach could drastically reduce ambiguities of interpretation in experiments, something that has been debated for nearly a decade. “Although it may seem like a negative result, it is very important to understand the fundamental physics that govern these superconducting devices. Our work greatly narrows the chances of false positives in the search for the elusive Majorana. We have taken a further step towards its detection and the future exploitation of all its power in quantum computing”, conclude the researchers.

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