Quantum computers promise many breakthroughs in many areas, including cryptography and the simulation of protein folding. It is not clear which physical system is best for building the quantum bits. These so-called qubits are not like regular bits on your computer. They can't only accept the numbers 0 and 1, but also combinations of both. They can be very useful but also very unstable.Topological qubits, which encode information in spatial arrangements, are one way to solve this problem. This could be a better and more reliable basis for computation than other arrangements. Problem is, no one has yet found a topological quibit.A team of international researchers from Austria, Copenhagen and Madrid, led by Marco Valentini, from the Nanoelectronics Group at IST Austria, has now examined a setup that was predicted to produce the Majorana zero modes, the key ingredient in a topological qubit. The signal that such modes are valid can be false flag, they discovered.Half of an ElectronThis experimental setup is made up of a very small wire that measures just a few hundred nanometers in length. It was grown by Peter Krogstrup, Microsoft Quantum and University of Copenhagen. These so-called nanowires create a free-floating link between two metal conductors on an electronic chip. The superconducting material is applied to them, which reduces their electrical resistance at extremely low temperatures. The junction is the crucial part of the setup. The entire contraption then is exposed to a magnetic force.Scientists predicted that Majorana Zero modes, which are the basis of the topological qubit they were searching for, would be found in the nanowire. Majorana zero modes, which are unusual phenomena, were created as a mathematical trick to show one electron in the wire is composed of two halves. Although electrons are not usually thought of as something that can split, physicists don't often think of them as such. However, this nanowire setup should have made it possible to separate these "half-electrons", and use them as qubits.Marco Valentini, an intern at IST Austria before becoming a PhD student within the Nanoelectronics team, says that "we were excited to work with this very promising material platform." We expected to see the signal of Majorana zero mode modes in the nanowire. But, we didn't find it. We were initially confused and then frustrated. We finally found the problem with the setup after working closely with Madrid's Theory of Quantum Materials group and Solid State Quantum Technologies group.False FlagThe researchers started to modify the nanowire setup in order to determine if any of its architectural effects were affecting their experiment. Valentini says that they tried several different setups in an attempt to figure out why. It took us some time, but once we doubled the lengths of the uncoated junction, from a hundred to two hundred nanometers, we finally found the culprit.Once the junction was large enough, the following happened. The quantum dot formed when the inner nanowire was exposed. This is a small speck that has special quantum mechanical properties because of its restricted geometry. This quantum dot's electrons could interact with those in the superconductor coating it and mimic the signals of the Majorana zero modes, or the "half-electrons", which scientists were trying to find.Valentini says, "This surprising conclusion was reached after we developed a theoretical model of the quantum dot's interaction with the superconductor within a magnetic field. We also compared experimental data with detailed simulations by Fernando Pearanda (a PhD student in Madrid team)," Valentini.Valentini warns that "Mistaking this mimicking sign for a Majorana Zero mode shows us how cautious we have to be with our experiments and our conclusions." This may seem like a backward step in our search for Majorana Zero modes, but it is actually a significant step forward in our understanding of nanowires and their experimental signals. This discovery shows that scientific advancement is based on the exchange of critical and constructive examinations among international peers.
