Microsoft’s quantum breakthrough were they wrong?

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Abstract illustration of particle physics

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Named after the Italian physicist Ettore Majorana, Majorana particles are fascinating entities in the field of particle physics. They are unique because they have the extraordinary property of being their own antiparticles. These elusive particles have the potential to revolutionize quantum computing and contribute to the understanding of fundamental physics, offering exciting possibilities for the future of scientific research.

In March 2022, Microsoft announced research findings regarding the manifestation of a unique particle that could potentially be used to create particularly durable quantum bits. However, researchers from the University of Basel are currently questioning these conclusions regarding the so-called Majorana particles. They made calculations that indicate the results can be explained differently.

In 1938, a brilliant mind mysteriously disappeared: having secured a ferry ticket from Palermo to Naples, the young Italian physicist Ettore Majorana seemed to vanish from the face of the planet. A few months earlier he had proposed a particular type of particle. These particles were supposed to be their antiparticles and carry no electric charge.

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In recent years, there has been a renewed interest among physicists in these mysterious particles, which bear the name of their late inventor (whose disappearance has not yet been explained). The particles, it turns out, could be used as particularly robust quantum bits in quantum computers.

Majorana particles

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Disorder in very fine nanowires can lead to measurement results that could be misinterpreted as evidence of Majorana particles. Credits: University of Basel, Department of Physics

The biggest obstacle in building such computers, which promise incredible computing power, is decoherence, or the fact that disturbances from the environment can very quickly destroy the sensitive quantum states with which quantum computers perform calculations. If, however, Majorana particles could be used as quantum bits, the problem could be solved instantly since they have a built-in immunity against decoherence due to their special properties.

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Damned expectations

In a study published in the scientific journal Advertisements

Characteristic anomalies

Through conductance measurements, the experts at Microsoft had detected an anomaly that is characteristic of such Majorana states and also shown that the superconducting properties of the superconductor-nanowire-combination respond to an applied magnetic field in a way that suggests the presence of a so-called topological phase.

In mathematics, topology can be illustrated by looking, for instance, at a coffee cup with a handle (a hole) that can theoretically be deformed into a doughnut (which also has a hole, so both are topological equal) but not into a sphere (no hole). In Majorana states, by contrast, topology is responsible for their much-coveted immunity to decoherence.

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We have now mathematically modeled the experiments by Microsoft and tried to find out whether the measurements could have other trivial, in scientific jargon explanations, explains Henry Legg, a postdoc in Klinovajas group. In fact, the Basel researchers came to the conclusion that both the current anomaly and the superconducting properties can be reproduced by a small amount of disorder from impurities inside the nanowire.

Our results show clearly that disorder plays an important role in such experiments, says Jelena Klinovaja. To detect Majorana states unambiguously and also put them to use in quantum computers, one will eventually need even purer nanowires. This also means that there will be no lack of experimental challenges in the next few years.

Reference: Trivial Andreev Band Mimicking Topological Bulk Gap Reopening in the Nonlocal Conductance of Long Rashba Nanowires by Richard Hess, Henry F. Legg, Daniel Loss and Jelena Klinovaja, 15 May 2023, Physical Review Letters.
DOI: 10.1103/PhysRevLett.130.207001

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