“Quantum mechanics upended our thinking”

The Nobel Prize in Physics was officially awarded to three quantum researchers last week. How important was their work to quantum research?
Klaus Ensslin: Experiments conducted by these three researchers forty years ago demonstrated that quantum mechanical tunnelling also works for macroscopic objects, not just for individual particles like electrons or protons. This raised the general question of how large a system can be and still follow the laws of quantum mechanics. And a quantum computer is an extremely large system that will comprise a very large number of qubits in order to perform meaningful calculations. So that was an extremely important discovery.

Not only was a Nobel Prize awarded for quantum research this year, but 2025 also marked 100 years of quantum mechanics. What happened in 1925?
The UN also could also have chosen another year. But 1925 was an eventful year for research in which numerous key studies were published. It¡¯s not as if quantum mechanics didn¡¯t exist before that date, though, and everything was clear afterwards though. Many researchers were involved in quantum mechanics over an extended period of time.

The principles of quantum mechanics still seem foreign to many people, even after all those years of research. Why is that?
Many of the statements made in quantum mechanics are concepts we don¡¯t encounter in everyday life. Take ¡°both/and¡±, for example: human beings cannot be in both Paris and Zurich at the same time. At the microscopic level, however, it is possible for objects like electrons or photons to be in multiple locations at the same time. Incidentally, Albert Einstein also considered it impossible at the time. His criticism greatly advanced quantum research.

Nevertheless, it still took decades for anybody to be able to prove that two or more entangled particles can remain correlated with each other, even over large distances.
Yes. Initial experiments only showed in the late 1970s that this nonlocality really exists. That experiment is now being conducted during student internships. Key nonlocality experiments were also conducted in Switzerland. These led to the first viable quantum cryptography applications from the 2000s onwards, with one example being secure encryption systems. Incidentally, those were first marketed by a Geneva-based company.

Someday, quantum computers are supposed to be able to solve arithmetic problems much more quickly than conventional computers. When do you think that will be the case?
The concept of a quantum computer is nearly 50 years old and the fundamental ideas can be traced back to US physicist Richard Feynman. He already predicted in the late 1970s that certain computers would rely on quantum states in future. Google already has a quantum computer capable of performing calculations not possible using conventional methods. Now, everybody is waiting for it to be able to calculate something that¡¯s important to the world. I assume that we¡¯ll be able to use it ten years from now to solve a big problem that¡¯s currently unsolvable.

What kind of problem do you have in mind?
It could be something like calculating the structure and energy level of a complex molecule that¡¯s essential for fertilizer production, for example. Fertilizer production is responsible for several percent of the world¡¯s carbon emissions so every improvement, regardless of how small, would have enormous consequences.

The unit of data used by quantum computers is called a qubit and qubits can be in both states 0 and 1 simultaneously. While that makes quantum computers extremely efficient at performing certain tasks, creating qubits is highly complex. How many qubits are already possible?
University researchers are currently able to create between 50 and 100 qubits. Teams at larger companies and start-ups are already working with around 1,000 qubits. They say you need around one million qubits to do anything meaningful. At ETH Zurich, colleagues like Jonathan Home and Andreas Wallraff are researching ways to create and control more of these qubits.

How do researchers plan to get from a hundred to one million qubits?
There might be a lot of ideas out there, but we¡¯ve still got a long way to go. Another approach is to try to formulate problems in such a way that lets you find a meaningful answer with just 10,000 qubits. Markus Reiher, for example, is researching ways to break down complex problems in a way that allows them to be solved using the simplest quantum computers possible. Both approaches ¨C creating more qubits and breaking problems down ¨C make sense. And at some point, they¡¯ll meet each other halfway.

But the substrate is also key. It¡¯s the substrate that determines how stable a qubit is, how easily it can be controlled and how well it can be entangled with other qubits. How have the materials evolved over the past few decades?
Gallium arsenide, which I was already working with during my doctoral thesis in the 1980s, used to be considered a material of the future. Even today, it¡¯s the purest material we know. It¡¯s problematic for a quantum computer, though, because both gallium and arsenic has a nuclear spin. That disrupts the spins of the electrons in the shell, making them harder to control.

Then silicon was discovered as a potential substrate. First of all, you can create silicon without nuclear spins. Second, the global semiconductor industry knows how to manufacture billions of silicon-based transistors. The material is in every single laptop, after all. As it turns out, however, the whole thing isn¡¯t quite that simple.

Then graphene arrived on the scene around 20 years ago. Our research group examined graphene-based quantum systems early on because graphene has all the advantages of silicon plus another quantum number that we can understand and control precisely. That¡¯s referred to as the valley quantum number. This number indicates which energy valley the electron is currently in. With graphene, we can manipulate them, activate them, deactivate them, and more. With silicon, on the other hand, the valley quantum number appears randomly so you try to circumvent them and just have little control over the situation. We¡¯re now trying to build innovative graphene-based qubits.

How far away are we from the goal of making the first graphene-based qubit?
Only a couple groups in the world are researching qubits in graphene. It seems relatively straightforward when you explain it. It¡¯s the details, though, that complicate the matter. A doctoral student in 2017 had the breakthrough idea of using an additional graphene electrode to create a single-electron transistor that could be switched on and off. It was this idea, simple from today¡¯s perspective, that got the project rolling. Since then we¡¯ve been able to capture single electrons. We understand the spin state, the charge state and now also the valley state. What we can¡¯t do yet is merge and coherently entangle two well-defined states. And that¡¯s what we have to do for a qubit. The measurement technology is known, the measurement procedure too, and we¡¯ve got the material under control. But realising these ideas in a lab is taking longer than expected.

What¡¯s the problem?
There¡¯s a lot that can go wrong in experimental physics. A wire might suddenly fall off at low temperatures. The cryostat gets a leak. Or somebody applies too much voltage and the sample is destroyed. It¡¯s a whole slew of trivial details that we¡¯ve been struggling with for two years. We know how it works in principle. We have the sample, we can do it. That¡¯s why I can say: It¡¯s coming.

What are other quantum physicists working on at ETH Zurich?
In addition to colleagues like Yiwen Chu, Jonathan Home and Andreas Wallraff, who are all using different techniques to improve qubits, there are also researchers like Tilman Esslinger working at ETH. He¡¯s not making qubits but instead working on something called a quantum simulator that can solve entirely different kinds of problems. Christian Degen uses quantum systems for ultra-sensitive sensors. Lukas Novotny is investigating truly macroscopic objects in their basic quantum state. Renato Renner is studying how the theory of relativity and quantum mechanics are connected. Marina Marinkovic is applying quantum algorithms to problems in particle physics. Atac Imamoglu and Jerome Faist are exploring how semiconductor quantum systems interact with photons. That we cover the entire spectrum of quantum technology at ETH is fascinating: the breadth of research topics in this field at ETH Zurich is globally unique.

How important is collaboration in quantum research?
Luckily, we recognised at an early stage that if we wanted to make quantum-related advances, we needed to join forces. We established a small centre back in 2007. Our very modest budget allowed us to fund two or three doctoral students for the roughly ten professorships involved. But the collaboration, the dialogue with others and a good network are essential ¨C even beyond the university. That¡¯s what I think is so spectacular about modern physics: my colleagues used to work in atomic physics, did research on ion traps or devoted their efforts to semiconductor physics. Now, everybody is talking to everybody else because we¡¯re answering similar physics questions but we¡¯re using entirely different methods to do so. Quantum research has brought us all together.

The Quantum Center at ETH Zurich commenced its operations five years ago ¨C to unite quantum researchers. How is it going?
The collaboration is going well and we exchange information regularly. The Quantum Center not only has many physicists, but also electrical and mechanical engineers. There¡¯s quite a lot of interest in quantum systems in computer science as well. Information science is essential when working on quantum computers ¨C there¡¯s new hardware, so we also need new software. What Switzerland, and actually all of Europe, lacks is an industrial partner willing to invest in quantum computers. So far, that¡¯s something only being done by large companies from China and the US. That¡¯s a pity.

What would you personally still like to find out?
I¡¯m at the end of my career. At the same time, I¡¯m also involved in a large EU project that will continue until mid-2026. Until then, I¡¯d really like to create a graphene-based qubit. I hope it¡¯s going to be an exceptionally good qubit, of course.

Read part two tomorrow to find out why quantum mechanics should be mainstream in just ten years and why Klaus Ensslin secretly wishes that there was something fundamentally wrong with quantum mechanics.