Exploring the practical benefits of exotic materials

Niels Schröter has won the SPS Award 2021 in the field of condensed matter physics for his outstanding research. The award will be presented today at the joint annual conference of the Swiss Physical Society (SPS) and the Austrian Physical Society (ÖPG). “I’m very excited! This is the first major science prize I’ve won,” says Schröter.

His research is particularly important because it could lay the foundation for novel types of quantum bits – qubits for short – which would allow quantum computers of the future to carry out highly complex calculations at ultrafast speeds. Although his work essentially involves basic research, Schröter says that from the outset he has always had his eye on practical applications.

Large machines for atomic structures

When talking about his research over the past years, Schröter skips between all the different disciplines that have to be linked together. He describes the structure of the material itself: how atoms can arrange themselves into crystal lattices; how the electrons of these atoms interact with each other and how they sometimes – if everything falls into place – endow the material with completely novel properties.

Then he changes tack entirely to describe the large-scale experimental set-up: the photoelectron spectroscopy machine at the Swiss Light Source SLS, housed at PSI. This experimental station, a world first, enables researchers to detect and measure new electronic states.

Then he switches to the field of mathematics. Without theoretical predictions, the physicist says he would never have been able to perform the experiments for which he now received the SPG Award, which is sponsored by IBM.

Moving towards quantum computers

To explain the connection between his research and quantum computers – or more specifically, how it could lead to qubits – Schröter has to elaborate the topic slightly and explain some technical jargon. He says that he is receiving the SPG Award in recognition of his discovery of chiral materials with a new kind of electronic quasiparticles displaying an exotic topology.

Electronic quasiparticles are states in a material where the collective behaviour of the electrons is as if there were free elementary particles with totally different properties from free electrons. The term ‘chiral’ here means that the arrangement of certain atoms in the material’s crystal lattice takes the form of a spiral staircase. This spiral staircase can run to the left or to the right, giving the material specific electronic properties. Thanks to preliminary theoretical deliberations, Schröter and his research partners have already discovered two chiral materials: in a first project they studied a specially developed aluminium-platinum crystal; in a second one they focused on a palladium-gallium crystal.

Another distinguishing feature of these crystals is that they are topological materials. Topology is a field of mathematics that deals with structures and forms that are similar to each other in certain properties. The number of holes in the shape is important. For example, a ball of modelling clay can be formed into a sphere, a plate, or a bowl by merely pressing and pulling – none of these shapes have a hole and are thus topologically identical. However, to obtain a donut or an 8-shaped form, you have to make holes in the clay: one for the donut, two holes for the 8.

Scientists have already applied this classification according to the number of holes and further topological properties to other physical properties of materials. In physics, a topological material is when the number of ‘holes’ inside the crystal is different from the number of holes outside it. The key area of interest is therefore at the surface of the material: “A topological phase transition has to occur on the surface of our crystal. As a result, the number of holes is not clearly defined there. And so a kind of new physics is happening here.” What Schröter and his colleagues have observed and measured there, they call "topological Fermi arcs". These are the traces that the special states of the electrons leave on the surface of the material. “Physicists are particularly fascinated by these topological surface or edge states, as they could prove to be especially resilient to disturbances from the external environment. The topology affords them special protection,” Schröter explains.

The maximum Chern number

In his second project, Schröter was able to demonstrate the maximum number of topological holes theoretically possible in a crystal. In physics, this value, in other words the class of topology, is known as the Chern number. Schröter managed to identify a chiral palladium-gallium crystal with a Chern number of four. Its mirror image, where the spiral staircase in the crystal lattice structure ran the other way, had a Chern number of minus four.

However fanciful all this may sound, a skilful combination of these surface effects could pave the way for novel types of quantum bits. “Most of the qubits currently being studied are not very stable. To be able to use them to build practical quantum computers, many additional quantum bits are needed for stabilisation,” Schröter explains. “The type of qubits that we have in mind, however, would consist of topological electronic states and would be much more stable. We could therefore design a quantum computer with just a few topological qubits.” Schröter’s position at the PSI was funded by Microsoft, a cooperation partner. “The purpose of our collaboration with Microsoft is to develop novel types of topological materials that can be used to build topological qubits.”

From Villigen to Halle

In May 2021 Schröter left PSI in Villigen to take up a new position at the Max Planck Institute of Microstructure Physics in Halle, Germany. “The mandate of Max Planck Institutes is actually to conduct basic research. But here in Halle we want to interpret that a little more freely and work towards new technologies as well. We don’t just want to understand the world – we also want to change it.” To this end, Schröter will continue to work closely with PSI in the future.

Text: Paul Scherrer Institute/Laura Hennemann