From Swiss Federal Institute of Technology in Zürich [ETH Zürich] [Eidgenössische Technische Hochschule Zürich] (CH): “Simplifying quantum systems” 

From Swiss Federal Institute of Technology in Zürich [ETH Zürich] [Eidgenössische Technische Hochschule Zürich] (CH)

22.09.2021
Felix Würsten

If only it were less prone to error quantum physics might already be giving us instant solutions to seemingly unsolvable problems. ETH researchers are therefore working to develop systems that are more robust.

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Quantum systems require sophisticated control technology, a lot of engineering know-​how and a better understanding of the physical correlations. (Photograph: Heidi Hostettler)

In crude terms our digitally driven information society is based on a simple binary opposition: 0 or 1. But what happens when other alternatives exist alongside these polar opposites? Might this give rise to a whole raft of different states and enable us to process complex information much faster?

It is precisely the prospect of going beyond conventional methods of data processing that has inspired such high hopes in the field of quantum physics – not only on the part of scientists in basic and theoretical research, but also among the CEOs of major corporations. Were this vision to materialise, and computers behave in accordance with the laws of quantum mechanics, it would open the door to a whole new world of applications. For example, such a powerful system would be able to determine the mechanism of proteins at a radically faster rate than a conventional computer could ever hope to achieve. This, in turn, would massively accelerate the development of new medicines.

A rocky road

Given such prospects, it is little wonder that quantum physics should exercise a fascination far beyond its immediate circle. Yet the road that will take us to a quantum computer capable of answering everyday questions is a rocky one – and much longer than many are prepared to admit. “We’re talking about decades, not years, before we reach that point,” says Jonathan Home, Professor of Experimental Quantum Optics and Photonics at ETH Zürich. And Professor Home is one of those working in a field in which quantum research is relatively far along. He uses individual atoms as qubits. These are the basic units of information used by a quantum computer to perform calculations. Home uses beryllium and calcium atoms held in special electrical ion traps. These are then manipulated with a laser according to the laws of quantum mechanics. “Atoms are great systems for information processing because they can be isolated – and because, provided they remain isolated, they can store quantum information for a couple of seconds or even minutes,” he explains.

In order to be able to use this information, however, these fragile quantum objects have to be reconnected with the everyday physical world. During this step, even the slightest anomalies can corrupt the entire system. The question is, therefore, how to reduce this susceptibility to error and, at the same time, increase the number of qubits.

Simpler and more robust

An obvious approach is to equip the systems with a degree of redundancy, i.e. to link several physical qubits to a single logical qubit. But this has a major drawback. Although redundancy renders the system more stable it also makes it exponentially more complex – and in turn much more susceptible to error.

This requires not only sophisticated control technology and a lot of engineering know-​how but also a better understanding of the physical correlations. According to Home, the development of quantum computers has already yielded concrete benefits, even if today’s technology is still far removed from being able to investigate protein structures: “In essence, our experiments pose an endurance test for the physical theories. The results then provide us with new insights as to how the quantum world works.” One of ETH’s big strengths is that researchers here are working on very different approaches. The ion traps used by Home are just one of a number of routes that could deliver a breakthrough. Superconducting circuits are another promising option. “It’s highly unusual for one university to be pursuing so many different approaches,” says Home.

Highly specialised infrastructure

In common with his colleagues, Home has big hopes for the planned physics building on the Hönggerberg campus. Funded by an endowment from Walter Haefner, this will feature highly specialised laboratories that are exceptionally well isolated from outside interference. It is here that scientists will attempt to push back the boundaries of quantum research. In so doing, they will also explore ideas that are still very much in their infancy.

One potential route is the use of free electrons in semiconductor materials. These are able to move freely of the influence of the crystal lattice structure and exhibit quantum mechanical properties that can be used for processing information. “But for this purpose, the semiconductors have to be extremely pure,” explains Werner Wegscheider, who as Professor of Solid State Physics has experience in producing these specialised materials. He uses a vacuum chamber to build customised semiconductors atom by atom. “We make the world’s purest semiconductors,” he says with pride. Such materials can exhibit completely new properties. When cooled to a very low temperature and exposed to a magnetic field, the free electrons condense to form a quasiparticle. In other words, they collectively behave in the manner of a single particle and can therefore be described mathematically. Researchers have good reason to believe that such topological quantum systems are more resistant to perturbation than other quantum objects – which is precisely why they may be less prone to error.

A worthwhile effort

Topological quantum systems offer an especially neat example of how in physics theory and experiment can be mutually enriching. The basic quantum Hall effect underpinning these systems was discovered experimentally. This effect was then described theoretically. The resulting theory subsequently led to the prediction of the topological states about which researchers are currently so excited. It has yet to be experimentally verified whether these theoretically predicted states actually exist in practice. If experimental physicists can demonstrate this, they may soon be returning the problem for additional theoretical elaboration.

Like Home, Wegscheider warns it will take some time before a quantum computer can solve practical problems beyond the realm of quantum physics. “Three years ago, I was still sceptical, but now I’m pretty confident that we’ll get there,” he says.

At present, it is still unclear which of the various approaches will ultimately prevail. The answer may well lie in a mix of different solutions – semiconductors with superconducting circuits, for example. “When these two options are combined, you get quasiparticles known as Majorana fermions, which are thought to be less susceptible to error,” says Wegscheider. Yiwen Chu, Assistant Professor of Hybrid Quantum Systems, is investigating combinations of different quantum systems. “There’s a whole range of quantum objects, such as photons, ions or even superconducting circuits,” she explains. “All have their specific strengths, but also disadvantages. The question is how to bring these elements together in a way that combines their strengths.”

Bridging the gap

Her model is the classic computer, which uses, for example, a silicon chip to process information and optical fibre to transfer the data. By analogy, a quantum system might use superconducting circuits to process data, which would then be transferred by photons. “But it turns out that these two quantum objects are not particularly compatible,” says Chu. What is needed, therefore, is something to bridge the gap. Chu and her research group are currently investigating the use of small crystals for this purpose. As mechanical objects, they are able to communicate with both sides by means of acoustic vibrations.

At the same time, it may well be that these crystals themselves are capable of storing and processing quantum information. “The crystals use acoustic vibrations, which are much slower than light waves, so we could use them to build smaller qubits,” she explains. Yet her chief aim here is not to accommodate as many qubits as possible on a given surface. The advantage is rather that these crystals can be isolated from one another much more easily than, for example, superconducting circuits. The greater degree of isolation prevents an unwanted loss of information, which in turn helps reduce the susceptibility to error. Yet the greatest challenge of all is that as more and more qubits are connected together, the system itself has to become increasingly complex.

Yet it would be wrong, she says, to look upon the quantum computer as purely an engineering problem. “There are also a lot of unanswered questions on the physics side of the equation.” One of these is whether the transition between the worlds of classical and quantum physics is continuous or abrupt. “We don’t yet have a definitive answer to this problem,” says Chu. “But either way, it’s going be an exciting time for us physicists!”

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ETH Zurich campus
Swiss Federal Institute of Technology in Zürich [ETH Zürich] [Eidgenössische Technische Hochschule Zürich] (CH) is a public research university in the city of Zürich, Switzerland. Founded by the Swiss Federal Government in 1854 with the stated mission to educate engineers and scientists, the school focuses exclusively on science, technology, engineering and mathematics. Like its sister institution Swiss Federal Institute of Technology in Lausanne [EPFL-École Polytechnique Fédérale de Lausanne](CH) , it is part of the Swiss Federal Institutes of Technology Domain (ETH Domain)) , part of the Swiss Federal Department of Economic Affairs, Education and Research [EAER][Eidgenössisches Departement für Wirtschaft, Bildung und Forschung] [Département fédéral de l’économie, de la formation et de la recherche] (CH).

The university is an attractive destination for international students thanks to low tuition fees of 809 CHF per semester, PhD and graduate salaries that are amongst the world’s highest, and a world-class reputation in academia and industry. There are currently 22,200 students from over 120 countries, of which 4,180 are pursuing doctoral degrees. In the 2021 edition of the QS World University Rankings ETH Zürich is ranked 6th in the world and 8th by the Times Higher Education World Rankings 2020. In the 2020 QS World University Rankings by subject it is ranked 4th in the world for engineering and technology (2nd in Europe) and 1st for earth & marine science.

As of November 2019, 21 Nobel laureates, 2 Fields Medalists, 2 Pritzker Prize winners, and 1 Turing Award winner have been affiliated with the Institute, including Albert Einstein. Other notable alumni include John von Neumann and Santiago Calatrava. It is a founding member of the IDEA League and the International Alliance of Research Universities (IARU) and a member of the CESAER network.

ETH Zürich was founded on 7 February 1854 by the Swiss Confederation and began giving its first lectures on 16 October 1855 as a polytechnic institute (eidgenössische polytechnische Schule) at various sites throughout the city of Zurich. It was initially composed of six faculties: architecture, civil engineering, mechanical engineering, chemistry, forestry, and an integrated department for the fields of mathematics, natural sciences, literature, and social and political sciences.

It is locally still known as Polytechnikum, or simply as Poly, derived from the original name eidgenössische polytechnische Schule, which translates to “federal polytechnic school”.

ETH Zürich is a federal institute (i.e., under direct administration by the Swiss government), whereas the University of Zürich [Universität Zürich ] (CH) is a cantonal institution. The decision for a new federal university was heavily disputed at the time; the liberals pressed for a “federal university”, while the conservative forces wanted all universities to remain under cantonal control, worried that the liberals would gain more political power than they already had. In the beginning, both universities were co-located in the buildings of the University of Zürich.

From 1905 to 1908, under the presidency of Jérôme Franel, the course program of ETH Zürich was restructured to that of a real university and ETH Zürich was granted the right to award doctorates. In 1909 the first doctorates were awarded. In 1911, it was given its current name, Eidgenössische Technische Hochschule. In 1924, another reorganization structured the university in 12 departments. However, it now has 16 departments.

ETH Zürich, EPFL (Swiss Federal Institute of Technology in Lausanne) [École polytechnique fédérale de Lausanne](CH), and four associated research institutes form the Domain of the Swiss Federal Institutes of Technology (ETH Domain) [ETH-Bereich; Domaine des Écoles polytechniques fédérales] (CH) with the aim of collaborating on scientific projects.

Reputation and ranking

ETH Zürich is ranked among the top universities in the world. Typically, popular rankings place the institution as the best university in continental Europe and ETH Zürich is consistently ranked among the top 1-5 universities in Europe, and among the top 3-10 best universities of the world.

Historically, ETH Zürich has achieved its reputation particularly in the fields of chemistry, mathematics and physics. There are 32 Nobel laureates who are associated with ETH Zürich, the most recent of whom is Richard F. Heck, awarded the Nobel Prize in chemistry in 2010. Albert Einstein is perhaps its most famous alumnus.

In 2018, the QS World University Rankings placed ETH Zürich at 7th overall in the world. In 2015, ETH Zürich was ranked 5th in the world in Engineering, Science and Technology, just behind the Massachusetts Institute of Technology(US), Stanford University(US) and University of Cambridge(UK). In 2015, ETH Zürich also ranked 6th in the world in Natural Sciences, and in 2016 ranked 1st in the world for Earth & Marine Sciences for the second consecutive year.

In 2016, Times Higher Education World University Rankings ranked ETH Zürich 9th overall in the world and 8th in the world in the field of Engineering & Technology, just behind the Massachusetts Institute of Technology(US), Stanford University(US), California Institute of Technology(US), Princeton University(US), University of Cambridge(UK), Imperial College London(UK) and University of Oxford(UK) .

In a comparison of Swiss universities by swissUP Ranking and in rankings published by CHE comparing the universities of German-speaking countries, ETH Zürich traditionally is ranked first in natural sciences, computer science and engineering sciences.

In the survey CHE ExcellenceRanking on the quality of Western European graduate school programs in the fields of biology, chemistry, physics and mathematics, ETH Zürich was assessed as one of the three institutions to have excellent programs in all the considered fields, the other two being Imperial College London(UK) and the University of Cambridge(UK), respectively.