Researchers demonstrate how to eliminate the main 'false positive' of the Majorana particle, key to scalable quantum computing

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A study by the Madrid Institute of Materials Science (ICMM-CSIC) has found a new system that eliminates Majorana false positives. Majorana particles are very elusive, but key to the advancement of topological quantum computing.

The work, just published in Physical Review Letters, demonstrates that an alternative design of hybrid nanowires (about ten thousand times thinner than a human hair) eliminates one of the main sources of Majorana false positives that have hampered the search for these elusive particles.

For decades, physics experts have been searching for Majorana zero-modes: elusive topological states that form at the ends of semiconductor nanowires—materials that conduct electricity only under certain conditions—coated with superconducting material, i.e., material that conducts electricity without resistance or energy loss as heat.

"Since they could store quantum information in a way that is intrinsically protected against errors, these modes offer a path toward highly robust quantum computers," explains Carlos Payá, ICMM-CSIC researcher and first author of the work.

Typically, Majorana states have been sought through a technique called Tunneling Spectroscopy, in which one end of the wire is electrically probed and a characteristic peak at zero voltage is sought in its conductance. "The problem is that some imposter states, known as quasi-Majoranas, produce exactly the same kind of peak despite being topologically trivial," continues Elsa Prada, also an ICMM-CSIC researcher and principal investigator of the study.

These imposters, whose existence was demonstrated by an ICMM-CSIC work a few years ago, have complicated the confirmation of the existence of Majorana states: the ability to unambiguously distinguish between a genuine Majorana state and an imposter is an indispensable step for the development of topological qubits (quantum bits), which promise to be much more robust against errors than conventional qubits.

Now, this work demonstrates that this ambiguity disappears with an alternative design of the hybrid nanowire: the research team proposes that the superconductor form a complete tube around the semiconductor, instead of coating only some of its faces. "In these full-shell nanowires, the same electric field that generates imposters simultaneously creates an electron layer at the end of the wire, displacing the quasi-Majoranas out of the probe's reach," detail Pablo San-José and César Robles, co-authors of the work.

"Consequently, a stable peak at zero voltage measured in a full-shell geometry cannot come from a quasi-Majorana," celebrates Payá, "The definitive proof of a Majorana is, in any case, to do quantum computing with them and verify that it is topological," he adds.

The results of this work provide a solid theoretical basis to guide future experiments, as they eliminate one of the main sources of uncertainty in experimentation with these systems.

Reference:

C. Payá, C. Robles, P. San-Jose, and E. Prada, Absence of quasi-Majorana false positives in full-shell hybrid nanowires, Physical Review Letters. DOI: https://doi.org/10.1103/w9tp-3bbq