Ettore Majorana

Sharper focus on Majorana particles

In Nature this week an international team of researchers, with TU/e professor Erik Bakkers among their number, is presenting the chief clue so far for the existence of the mysterious Majorana particle, a promising building block for quantum computers.

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In 1937 the Italian physicist Ettore Majorana hypothesized a new fundamental particle, which was named after him later: the Majorana particle. The special feature of this particle is that it is its own antiparticle.

Majorana quasiparticles appear in materials under very specific circumstances. When a nanowire made of a semiconductor is connected to a superconducting material, researchers observe a so-called ‘zero bias peak’ at certain electric and magnetic fields. This signal is an important characteristic of the presence of Majorana particles.

In 2012 that ‘zero bias peak’ was first reported by a team led by Leo Kouwenhoven (QuTech Delft, Microsoft) and TU/e Professor of Advanced Nanomaterials & Devices Erik Bakkers. Those original experiments have now been repeated with better materials.

Braiding

Bakker says that the signal which has been found is precisely what the theory for the particle predicts. “Thereby we exclude various other explanations for the occurrence of such a peak”, he explains. Nonetheless, it is not yet definitive evidence of the Majorana particle. “To obtain that, you need to make the particles switch places; braid them, as it were.”

Last August the team presented a new nanostructure in the form of a hashtag (#), which should make that braiding possible. Still, Bakkers expects that it may take a few years yet before the definitive evidence of the Majorana particles has been delivered thereby. “But these measurements show that we have made a significant step in the right direction with our improved quantum chips.”

Interface

The special contribution of his group at TU/e, as Bakkers explains, consists of the high quality of the nanomaterials that can be manufactured in the cleanroom in Eindhoven. “We make semiconducting nanowires of indium antimonide, to which superconducting wires are attached. Formerly this was done in two steps, but we now managed to do it in one, in an ultrahigh vacuum, so that the interface between the two materials remains very clean.”

Indium antimonide is a super-heavy variant of the so-called III-V- semiconductors, for which TU/e possesses extensive expertise and facilities. Bakkers: “Because this material is so heavy, comparable to lead, it has properties that make it eminently suited for the formation of Majorana particles, such as a high mobility of charge carriers and a strong spin-orbit coupling.”

Qubit

The ultimate goal of the Microsoft-sponsored team of scientists from Delft, Eindhoven, Maryland and Santa Barbara is the realization of a qubit, a bit for a quantum computer, on the basis of Majorana particles. In Bakkers’s opinion that would be a huge step towards a serious quantum computer. “Recently Google presented a quantum computer with 72 qubits, so in that sense they have progressed further than we have, but the technique they used can actually not be scaled up further because those qubits are so big: a real computer on the basis of that technology would weigh many tons. By means of Majorana particles, on the other hand, you should in principle be able to build a manageable quantum computer.”

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