Towards compact quantum computers thanks to… topology

Towards compact quantum computers thanks to… topology

By distributing electrons beneath a semiconductor's oxide layer, PSI targets qubits capable of cleaning up errors themselves

The graphical representation of a quantum bit or qubit
The graphical representation of a quantum bit or qubit

The researchers ofPaul Scherrer Institute in Switzerland compared the distribution of electrons under the oxide layer of two semiconductors.
The survey is part of an effort to develop particularly stable quantum bits, and then, in turn, particularly efficient quantum computers.
Swiss scholars have published their latest research, which is backed in part by the Microsoft, in the specialized scientific journal “Advanced Quantum Technologies".
By now, the future of computing is inconceivable without quantum computers. For the most part, these devices are still objects in the making, still being researched. They hold the promise of speeding up certain calculations and performing mathematical simulations by several orders of magnitude compared to classical computers.
I quantum bits: quibit in short, they form the basis of quantum computers. The so-called Topological quantum bits are a new type of quantum bits, but they may turn out to be superior to the former. To find out how they could be created in reality, an international team of researchers carried out measurements at the Swiss Light Source SLS of the PSI.
La topology (from the Greek “τόπος”, “tópos”, i.e. “place”, and “λόγος”, “lógos”, i.e. “study”, therefore meaning “study of places'”) is a branch of geometry that studies the properties of shapes, and in general of mathematical objects, which do not change when a deformation is carried out without "tearing", "overlapping" or "gluing".
It is one of the most important branches of modern mathematics: fundamental concepts such as convergence, limit, continuity, connection or compactness find their best formalization in topology. It is essentially based on the concepts of topological space, continuous function and homeomorphism.

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Gabriel Aeppli is the head of the Photon Science Division at the Paul Scherrer Institute
Gabriel Aeppli is the head of the Photon Science Division at the Paul Scherrer Institute

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“Computer bits that follow the laws of quantum mechanics can be obtained in several ways”, explains Niels Schröter, one of the authors of the study.
He was a researcher atPaul Scherrer Institute until'April 2021, when he moved to the Max Planck Institute of Microstructure Physics in Hallein Germany.
“Most types of qubits unfortunately lose their information rapidly; one could say that they are 'forgetful qubits'…”.
But there is a technical solution to this problem: every quibit is supported by a system of quibit add-ons that correct any errors that occur. But this means that the total number of quibit required for an operational quantum computer to function properly quickly rises into the millions.
“Microsoft's approach, which we're collaborating on, is very different”, keep it going Schröter.
“We want to help create a new type of qubit that is, shall we say, 'immune' to information leakage. This would allow us to use only a few qubits to get a working thin quantum computer.".
Researchers hope to achieve such immunity with so-called topological quantum bits. The latter would be something completely new in the scientific field, which no research group has yet been able to create.
The materials topological have become more known in the world thanks to the Nobel Prize in Physics in 2016, which was awarded half to the British David James Thouless and for the other half jointly with compatriots Frederick Duncan Michael Haldane and John Michael Kosterlitz, "for theoretical discoveries related to topological phase transitions and for the topological phases of matter".
As said, the 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 area can also be applied to other physical properties of materials. THE quantum bits in topological materials would then be topological qubits.

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The graphical representation of the difference between a bit and a quantum bit or qubit
The graphical representation of the difference between a bit and a quantum bit or qubit

Quasiparticles in computer-simulated semiconductor nanowires

It is known that thin-film systems of certain semiconductors and superconductors could lead to exotic states of electrons that would act like the aforementioned topological qubits.
In particular, ultra-thin and short wires made of a semiconductor material could be considered useful for this purpose. These have a diameter of suns 100 nanometers and are 1.000 nanometers long (i.e. 0,0001 centimetres).
On their outer surface, in the longitudinal direction, the upper half of the wires is coated with a thin layer of superconductor. The rest of the wire is not coated, so a natural oxide layer forms there.
Computer simulations for optimizing these components predict that the crucial electron states, in terms of quantum mechanics, are found only at the interface between the semiconductor and the superconductor, not between the semiconductor and its oxide layer.
“The collective and asymmetric distribution of the electrons generated in these nanowires can be physically described as the so-called quasiparticles”Says Gabriel Aeppli, head of Photon Science Division al 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."

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Majorana's Fermions are the right recipe, but in which pot?

In particle physics a Majorana Fermion o Majorana particle, so called in honor of the Sicilian theoretical physicist Ettore Majorana who theorized it for the first time, is a particle Fermionics which is also your own antiparticle.
I Majorana Fermions were topological. They could then act as carriers of information, ergo come quantum bits into a computer quantum.
“Over the last decade, the recipes for creating Majorana fermions have already been studied and perfected by research groups from all over the world”, keep it going Aeppli.
"But, to continue with this analogy: We didn't yet know which pot would give us the best results for this recipe."

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Indium antimonide will be ideal for low electron density

A central concern of the current research project was therefore the comparison of two "pots".
The researchers studied two different semiconductors and their natural oxide layer: on the one hand theindium arsenide and on the other theindium antimonide.
All projects SLS, the researchers of the Paul Scherrer Institute they used a survey method called X-ray photoelectron spectroscopy “soft”, with angular resolution: la SX-ARPES In short.
A new computer model developed by the group of Noa Marom to the Carnegie Mellon University, In United States, Together with Vladimir Strotsov, active in Switzerland in the offices of villas e Würenlingen, of the institution, was used to interpret the complex experimental data.
“The computer models used so far led to an unmanageable number of spurious results. With our new method, we can now look at all the results, automatically filter the physically relevant ones and correctly interpret the experimental result”, explains Strokhov.
Through the combination of experiments SX-ARPES and computer models, the researchers were able to demonstrate that theindium antimonide it has a particularly low electron density under its oxide layer. This would be beneficial to the training of Majorana Fermions topological in the expected nanowires.
“From the point of view of the distribution of electrons under the oxide layer, indium antimonide is therefore more suitable than indium arsenide to serve as a carrier material for topological quantum bits”he concludes Niels Schröter.
However, he points out that in the search for the best materials for a topological quantum computer, other advantages and disadvantages will certainly have to be weighed.
“Our advanced spectroscopic methods will certainly be instrumental in the search for materials for quantum computing”Says Strokhov.
“PSI is making great strides to expand quantum research and engineering in Switzerland, and SLS is an essential part of that”.

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Niels Schröter, left, and Vladimir Strocov, right, in one of the experimentation stations of the Swiss Light Source SLS at the Paul Scherrer Institute: here the researchers used “soft” X-ray photoelectron spectroscopy, with angular resolution, to measure the distribution of electrons under the oxide layer of indium arsenide and indium antimonide (Photo: Mahir Dzambegovic/Paul Scherrer Institute)
Niels Schröter, left, and Vladimir Strocov, right, in one of the experimentation stations of the Swiss Light Source SLS at the Paul Scherrer Institute: here the researchers used “soft” X-ray photoelectron spectroscopy, with angular resolution, to measure the distribution of electrons under the oxide layer of indium arsenide and indium antimonide
(Photo: Mahir Dzambegovic/Paul Scherrer Institute)