Simplifying quantum systems

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 Zurich. 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 Martin 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!”