Researchers from the USA and India have proposed that Floquet Majorana fermions may improve quantum computing by controlling superconducting currents, potentially reducing errors and increasing stability.

In a study published in Physical Review Letters that was co-authored by [Babak Seradjeh], a Professor of Physics at Indiana University Bloomington, and theoretical physicists [Rekha Kumari] and [Arijit Kundu], from the Indian Institute of Technology Kanpur, the scientists validate their theory using numerical simulations.

In the absence of room-temperature superconductors — the Holy Grail of superconductivity, everybody put your thinking caps on! — the low temperatures required lead to expense (for cooling) and errors (due to decoherence) which need to be managed. Using the techniques proposed by the study, quantum information may be modeled non-locally and be spread out spatially in a material, making it more stable and less error prone, immune to local noise and fluctuations.

Majorana fermions are named after Italian physicist [Ettore Majorana] who proposed them in 1937. Unlike most particles, Majorana fermions are their own antiparticles. In the year 2000 mathematical physicist [Alexei Kitaev] realized Majorana fermions can exist not only as elementary particles but also as quantum excitations in certain materials known as topological superconductors. Topological superconductors differ from regular superconductors in that they have unique, stable quantum states on their surface or edges that are protected by the material’s underlying topology.

Superconductivity is such an interesting phenomenon, where electrical resistance all but vanishes in certain materials when they are very cold. Usually to induce a current in a material you apply a voltage, or potential difference, in order to create the electrical pressure that results in the current. But in a superconductor currents can flow in the absence of an applied voltage. This is because of a peculiar quantum tunneling process known as the “Josephson effect”. It is hoped that by tuning the Josephson current using a superconductor’s “chemical potential” that we discover a new level of control over quantum materials.

Ettore Majorana picture: Mondadori Collection, Public domain.

As the fundamental flaw of today’s quantum computers, improving qubit stability remains the focus of much research in this field. One such stability attempt involves so-called topological quantum computing with the use of anyons, which are two-dimensional quasiparticles. Such an approach has been claimed by Microsoft in a recent paper in Nature. This comes a few years after an earlier claim by Microsoft for much the same feat, which was found to be based on faulty science and hence retracted.

The claimed creation of anyons here involves Majorana fermions, which differ from the much more typical Dirac fermions. These Majorana fermions are bound with other such fermions as a Majorana zero mode (MZM), forming anyons that are intertwined (braided) to form what are in effect logic gates. In the Nature paper the Microsoft researchers demonstrate a superconducting indium-arsenide (InAs) nanowire-based device featuring a read-out circuit  (quantum dot interferometer) with the capacitance of one of the quantum dots said to vary in a way that suggests that the nanowire device-under-test demonstrates the presence of MZMs at either end of the wire.

Microsoft has a dedicated website to their quantum computing efforts, though it remains essential to stress that this is not a confirmation until their research is replicated by independent researchers. If confirmed, MZMs could provide a way to create more reliable quantum computing circuitry that does not have to lean so heavily on error correction to get any usable output. Other, competing efforts here include such things as hybrid mechanical qubits and antimony-based qubits that should be more stable owing to their eight spin configurations.