Towards compact quantum computers thanks to topology

Niels Schröter (left) and Vladimir Strocov at one of the experiment stations of the Swiss Light Source SLS at PSI. Here the researchers used soft X-ray angle-resolved photoelectron spectroscopy to measure the electron distribution below the oxide layer of indium arsenide as well as indium antimonide.
图片来源:Paul Scherrer学院/Mahir Dzambegovic

PSI的研究人员比较了两个半导体的氧化物层以下的电子分布。该研究是开发特别稳定的量子位的一部分,因此,又是特别有效的量子计算机。他们现在已经发表了最新研究,该研究得到了Microsoft的一部分,在《科学杂志》中Advanced Quantum Technologies

By now, the future of computing is inconceivable without quantum computers. For the most part, these are still in the research phase. They hold the promise of speeding up certain calculations and simulations by orders of magnitude compared to classical computers.



“Computer bits that follow the laws of quantum mechanics can be achieved in different ways,” explains Niels Schröter, one of the study’s authors. He was a researcher at PSI until April 2021, when he moved to the Max Planck Institute of Microstructure Physics in Halle, Germany. “Most types of qubits unfortunately lose their information quickly; you could say they are forgetful qubits.” There is a technical solution to this: Each qubit is backed up with a system of additional qubits that correct any errors that occur. But this means that the total number of qubits needed for an operational quantum computer quickly rises into the millions.

“Microsoft’s approach, which we are now collaborating on, is quite different,” Schröter continues. “We want to help create a new kind of qubit that is immune to leakage of information. This would allow us to use just a few qubits to achieve a slim, functioning quantum computer.”


Topological materials became more widely known through the Nobel Prize in Physics in 2016. Topology is originally a field of mathematics that explores, among other things, how geometric objects behave when they are deformed. However, the mathematical language developed for this can also be applied to other physical properties of materials. Quantum bits in topological materials would then be topological qubits.

Quasiparticles in semiconductor nanowires

众所周知,薄膜系统的某些半conductors and superconductors could lead to exotic electron states that would act as such topological qubits. Specifically, ultra-thin, short wires made of a semiconductor material could be considered for this purpose. These have a diameter of only 100 nanometres and are 1,000 nanometres (i.e., 0.0001 centimetres) long. On their outer surface, in the longitudinal direction, the top half of the wires is coated with a thin layer of a superconductor. The rest of the wire is not coated so that a natural oxide layer forms there. Computer simulations for optimising these components predict that the crucial, quantum mechanical electron states are only located at the interface between the semiconductor and the superconductor and not between the semiconductor and its oxide layer.

“The collective, asymmetric distribution of electrons generated in these nanowires can be physically described as so-called quasiparticles,” says Gabriel Aeppli, head of the Photon Science Division at PSI, who was also involved in the current study. “Now, if suitable semiconductor and superconductor materials are chosen, these electrons should give rise to special quasiparticles called Majorana fermions at the ends of the nanowires.”

Majorana fermions are topological states. They could therefore act as information carriers, ergo as quantum bits in a quantum computer. “Over the course of the last decade, recipes to create Majorana fermions have already been studied and refined by research groups around the world,” Aeppli continues. “But to continue with this analogy: we still didn’t know which cooking pot would give us the best results for this recipe.”

Indium antimonide has the advantage


At SLS, the PSI researchers used an investigation method called soft X-ray angle-resolved photoelectron spectroscopy – SX-ARPES for short. A novel computer model developed by Noa Marom’s group at Carnegie Mellon University, USA, together with Vladimir Strocov from PSI, was used to interpret the complex experimental data. “The computer models used up to now led to an unmanageably large number of spurious results. With our new method, we can now look at all the results, automatically filter out the physically relevant ones, and properly interpret the experimental outcome,” explains Strocov.

Through their combination of SX-ARPES experiments and computer models, the researchers have now been able to show that indium antimonide has a particularly low electron density below its oxide layer. This would be advantageous for the formation of topological Majorana fermions in the planned nanowires.

“From the point of view of electron distribution under the oxide layer, indium antimonide is therefore better suited than indium arsenide to serve as a carrier material for topological quantum bits,” concludes Niels Schröter. However, he points out that in the search for the best materials for a topological quantum computer, other advantages and disadvantages must certainly be weighed against each other. “Our advanced spectroscopic methods will certainly be instrumental in the quest for the quantum computing materials,” says Strocov. “PSI is currently taking big steps to expand quantum research and engineering in Switzerland, and SLS is an essential part of that.”

Text: Paul Scherrer Institute/Laura Hennemann

About PSI

Paul Scherrer Institute PSI开发,建立和运营大型,复杂的研究设施,并使国家和国际研究界可用。该研究所自己的主要研究重点是物质,材料,能源,环境与人类健康领域。PSI致力于对子孙后代的培训。因此,我们大约四分之一的员工是毕业后,研究生或学徒。PSI总共雇用了2100名员工,因此是瑞士最大的研究所。年度预算约为4亿瑞士法郎。PSI是ETH领域的一部分,其他成员是瑞士联邦技术研究院,Eth Zurich和EPFL Lausanne,以及EAWAG(瑞士联邦水上科学与技术研究所),EMPA(瑞士联邦联邦材料科学实验室和技术)和WSL(瑞士联邦森林,雪和景观研究所)。

Further information

半导体到达量子世界 - 新闻稿从2021年12月22日开始

Exploring the practical benefits of exotic materials – article from 1 September 2021

New material also reveals new quasiparticles – press release from 7 May 2019


Vladimir N. Strocov博士
Paul Scherrer Institute, Forschungsstrasse 111, 5232 Villigen PSI, Switzerland
电话:+41 56 310 53 11,电子邮件 [英语,法语,俄语]

Dr. Niels Schröter
Max Planck微观结构物理研究所,温伯格2,06120 Halle,德国
Telephone: +49 345 5582 793, e-mail:, [German, English]

Gabriel Aeppli博士教授
Head of the Photon Science Division
Paul Scherrer Institute, Forschungsstrasse 111, 5232 Villigen PSI, Switzerland
and Department of Physics, ETH Zurich
and Topological Matter Laboratory, EPF Lausanne
电话:+41 56 310 42 32,电子邮件 [德语,英语,法语]

Original Publication

Electronic structure of InAs and InSb surfaces: density functional theory and angle-resolved photoemission spectroscopy
Shuyang Yang Niels B. M. Schröter, V. N. Strocov, S. Schuwalow, M. Rajpalk, K. Ohtani, P. Krogstrup, G. W. Winkler, J. Gukelberger, D. Gresch, G. Aeppli, R. M. Lutchyn, N. Marom
Advanced Quantum Technologies20. January 2022

Journal: Advanced Quantum Technologies
Method of Research: Experimental study
Subject of Research: Not applicable
Article Title: Electronic structure of InAs and InSb surfaces: density functional theory and angle-resolved photoemission spectroscopy


Sebastian Jutzi
Paul Scherrer Institute


Sebastian Jutzi
Paul Scherrer Institute

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