Ultra-Thin Designer Materials Unlock Unwanted Quantum Phenomena With Great Impact On Quantum Information

Majorana Zero Energy Modes are located on the edge of 2D topological superconductors. Credit: Alex Tokarev, Ella Maru University Studio Aalto

New research, published in Nature, has measured Majorana’s highly sought after quantum states.

A team of theoretical and experimental physicists have designed a new ultra-thin material that they have used to create elusive quantum states. Called one-dimensional Majorana zero energy modes, these quantum states can have a major impact quantum computing.

At the core of a quantum computer is a dome, which is used to do high-speed calculations. The cubits that Google, for example, in its Sycamore processor discovered last year and others currently using are very sensitive to noise and intrusion from computer environments, which introduces errors in calculations. A new type of dome, called a topological dome, can solve this issue, and zero 1D Majorana power modes may be key to making them.

“A topological quantum computer is based on topological cubes, which are supposed to be much more tolerant of noise than other cubes. However, topological cubes have not yet been produced in the laboratory, ”explains Professor Peter Liljeroth, the lead researcher on the project.

What are MZMs?

MZMs are groups of electrons bound together in a specific way so they behave like a particle called Majorana fermion, a semi-mythical particle first proposed by the semi-mythical physicist Ettore Majorana in the 1930s. If Majorana theoretical particles would connected together, they would function as a topological cube. A catch: no proof of their existence has ever been seen, neither in the laboratory, nor in astronomy. Instead of trying to make a particle that no one has seen anywhere in the universe, researchers instead try to make regular electrons behave like them.

To do MZM, researchers need incredibly small materials, an area in which Professor Liljeroth’s group at Aalto University specializes. MZMs are formed by giving a group of electrons a very specific amount of energy, and then blocking them together so that they do not escape. To achieve this, the materials must be 2-dimensional, and as thin as possible physically. To create the 1M MZM, the team had to make a whole new kind of 2D material: a topological superconductor.

Topological superconductivity is the property that occurs at the boundary of a magnetic electrical insulator and a superconductor. To create the 1M MZM, Professor Liljeroth’s team had to be able to insert electrons together into a topological superconductor, however it is not as simple as attaching any magnet to any superconductor.

“If you place most of the magnets on a superconductor, you stop it from being a superconductor,” explains Dr. Shawulienu Kezilebieke, the first author of the study. “Interactions between materials spoil their properties, but to make an MDZ, you need the materials to interact only slightly. The goal is to use 2D materials: they interact with each other enough to make the properties you need for MDZ, but not so much as to interfere with each other. “

The property in question is rotation. In a magnetic material, the rotation is all set in the same direction, while in a superconductor the rotation is anti-directed with alternating directions. Bringing a magnet and a superconductor together usually destroys the alignment and anti-direction of rotation. However, in 2D layered materials the interactions between materials are quite sufficient to “tilt” the rotations of the atoms so much as to create the specific state of rotation, called the Rashba rotational orbital union, needed to make the MZM.

Finding MZMs

The topological superconductor in this study is made of a layer of chromium bromide, a material which is still magnetic when only one-atom-thick Professor Liljeroth’s team grew chromium bromide islands one atom thick on top of a niobium diesel superconducting crystal and measured their electrical properties using a tunneling scanning microscope. At this point, they turned to computer modeling expertise by Professor Adam Foster at Aalto University and Professor Teemu Ojanen, now at Tampere University, to find out what they had done.

“There was a lot of simulation work needed to prove that the signal we are seeing was caused by MZM, and not by other effects,” says Professor Foster. “We needed to show that all the parts were put together to prove that we had produced MZM.”

Now the team is confident that they can make 1D MZM in 2-dimensional materials, the next step will be to try to make them in topological domes. This step has so far eluded teams that have already done 0-dimensional MZMs and the Aalto team are not willing to speculate if the process will be easier with 1-dimensional MZMs, however they are optimistic about the future of MZMs 1D.

“The most interesting part of this paper is that we did MZM in 2D materials,” said Professor Liljeroth. “In principle these are easier to make and easier to personalize their properties and, finally the latter, become a usable device “.

References: “Topological Superconductivity in a van der Waals Heterostructure” by Shawulienu Kezilebieke, Md Nurul Huda, Viliam Vaňo, Markus Aapro, Somesh C. Ganguli, Orlando J. Silveira, Szczepan Głodzik, Adam S. Foster, Teemu Ojeren 16 and Peter December 2020, Nature.
DOI: 10.1038 / s41586-020-2989-years

The research collaboration included researchers from the University of Tampere in Finland and the M.Curie-Sklodowska University in Poland.

The work was performed using the OtaNano research infrastructure. OtaNano offers the highest level of work environment and equipment for nanoscience and technology, and quantum technology research in Finland. OtaNano is operated by Aalto University and VTT and is available to academic and commercial users internationally.

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