Researchers at PSI have compared the electron distribution below the oxide
layer of two semiconductors. The investigation is part of an effort to
develop particularly stable quantum bits—and thus, in turn, particularly
efficient quantum computers. They have now published their latest research,
which is supported in part by Microsoft, in the journal 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.

Quantum bits, or qubits for short, form the basis of quantum computers.
So-called topological quantum bits are a novel type that might prove to be
superior. To find out how these could be created, an international team of
researchers has carried out measurements at the Swiss Light Source SLS at
PSI.

### More stable quantum bits

"Computer bits that follow the laws of quantum mechanics can be achieved in
different ways," explains Niels SchrÃ¶ter, 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," SchrÃ¶ter 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."

The researchers hope to obtain such immunity with so-called topological
quantum bits. These would be something completely new that no research group
has yet been able to create.

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

It is known that thin-film systems of certain semiconductors 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
centimeters) 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 optimizing 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

A central concern of the current research project was therefore the
comparison of two "cooking pots". The researchers investigated two different
semiconductors and their natural oxide layer: on the one hand indium
arsenide and on the other indium antimonide.

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,
U.S., 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
SchrÃ¶ter. 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."

## Reference:

Shuyang Yang et al, Electronic Structure of InAs and InSb Surfaces: Density
Functional Theory and Angle‐Resolved Photoemission Spectroscopy, Advanced
Quantum Technologies (2022).
DOI: 10.1002/qute.202100033

Tags:
Physics