In 2012 it was big news: researchers from Delft University of Technology and Eindhoven University of Technology presented the first experimental signatures for the existence of the Majorana fermion. This particle had been predicted in 1937 by the Italian physicist Ettore Majorana and has the distinctive property of also being its own anti-particle. The Majorana particles emerge at the ends of a semiconductor wire, when in contact with a superconductor material.
While the discovered particles may have properties typical to Majoranas, the most exciting proof could be obtained by allowing two Majorana particles to exchange places, or 'braid' as it is scientifically known. "That's the smoking gun", suggested Erik Bakkers, one of the researchers from Eindhoven University of Technology. "The behaviour we then see could be the most conclusive evidence yet of Majoranas."
In theNaturepaper, Erik Bakkers and his colleagues present a new device that should be able to show this braiding of Majoranas. In the original experiment in 2012 two Majorana particles were found in a single wire but they were not able to pass each other without immediately destroying the other. Thus the researchers quite literally had to create space. In the presented experiment they formed intersections using the same kinds of nanowire so that four of these intersections form a 'hashtag', #, and thus create a closed circuit along which Majoranas are able to move.
The researchers built their hashtag device starting from scratch. The nanowires made from indium phosphide (InP) are grown from a specially etched substrate such that they form exactly the desired network which they then expose to a stream of aluminium particles, creating layers of aluminium, a superconductor, on specific spots on the wires - the contacts where the Majorana particles emerge. Places that lie 'in the shadow' of other wires stay uncovered.
The entire process happens in a vacuum and at ultra-low temperature - around -273 degree Celsius. "This ensures very clean, pure contacts", stated Erik Bakkers, "and enables us to make a considerable leap in the quality of this kind of quantum device." The measurements demonstrate for a number of electronic and magnetic properties that all the ingredients are present for the Majoranas to braid.
If the researchers succeed in enabling the Majorana particles to braid, they will at once have killed two birds with one stone. Given their robustness, Majoranas are regarded as the ideal building block for future quantum computers that will be able to perform many calculations simultaneously and thus many times faster than current computers. The braiding of two Majorana particles could form the basis for a qubit, the calculation unit of these computers.
An interesting detail is that the samples have traveled around the world during the fabrication, combining unique and synergetic activities of each research institution. It started in Delft with patterning and etching the substrate, then to Eindhoven for nanowire growth and to Santa Barbara for aluminium contact formation. Finally back to Delft via Eindhoven for the measurements.
The article inNatureis entitled "Epitaxy of Advanced Nanowire Quantum Devices". It has been published on 24 August 2017. The authors are Sasa Gazibegovich; Diana Car; Hao Zhang; Stijn Balk; John Logan; Michiel de Moor; Maja Cassidy; Rudi Schmits; Di Xu; Guanzhong Wang; Peter Krogstrup; Roy Op het Veld; Kun Zuo; Yoram Vos; Jie Shen; Daniël Bouman; Borzoyeh Shojaei; Daniel Pennachio; Joon Sue Lee; Petrus van Veldhoven; Sebastian Koeling; Marcel Verheijen; Leo Kouwenhoven; Chris Palmstrom; and Erik Bakkers.
The research has been made possible in part by subsidies from the Dutch research funding organisations NWO and Stichting FOM, the European Research Council and Microsoft Station-Q.