‘The quantum computer will not be our salvation’

Were you at home when you heard the good news?
“Yes. I’m not allowed to be at TU Delft. At least the advice is not to be there. But I don’t need to be anyway as I don’t work in a lab. I’m a theoretician.”

An a very good theoretician.
“Being awarded KNAW (theRoyal Netherlands Academy of Arts and Sciences, ed.) membership is recognition indeed. It makes me very happy. It’s the cherry on the cake. I now belong to a group of wise people. Actually, it only takes effect in autumn when I will be officially installed.”

You were the very first person in the Netherlands to earn a doctorate in research into quantum calculations. After your PhD in 1999 in Amsterdam, you moved to the USA. Was there no interest in quantum calculations in the Netherlands?
“No, people in the Netherlands were not that interested. There were exceptions though. There was Hans Mooij at TU Delft who was interested, but the general feeling in the Netherlands was negative. After my PhD I worked at IBM in the USA for a long time. I came back to Europe in 2010 as I wanted to work in academia again. I first worked at the University of Aachen and joined TU Delft in 2017.”

The negative attitude towards quantum calculations has now disappeared, hasn’t it?
“Yes, there’s a lot happening in the field now. Now it’s a question of putting on the brakes. I enjoy resisting the momentum.”

Why?
“Quantum calculations are being hyped. They involve highly interesting physics. If you look at the processes themselves, how systems work, fantastic. But if you look at the economic value of the quantum computer, I am cautious.”

‘If one bit flips, we must be able to correct it’

But there is enormous potential, isn’t there?
“Is that so? I sometimes hear that we could solve the climate crisis with quantum computers, but I question that. The climate crisis is now, quantum computers are not. We may be able to do complex calculations at some point which super computers are barely able to do now. Think of calculations that show how certain chemicals react together.”

Exactly. Very useful if you want to develop new medicines.
“Yes, but new developments mostly start with good ideas, not with calculations. Let’s just say that the quantum computer will be a new tool in the toolbox. It will not be our salvation.”

You work on quantum error correction. Why is this important?
“In the quantum calculation systems that we have now, we see that the answers that they generate become less reliable the longer we let them calculate. The qubits are unstable and change state. This means that errors are continually being introduced. At a certain point, the answer is entirely random. And, of course, this is exactly the opposite of what you want to achieve. You want to be able to do huge calculations and to do this, you need reliable qubits and reliable logical qubits. We need to include redundancy. If one bit flips, we must be able to correct it. This is the holy grail in research into quantum calculations. We will take the first steps in this direction in the next 10 to 20 years.”

The first steps are only now being made in error correction? But we’ve been hearing that the quantum computer is on the way for years?
“When I started working in this field of research at the end of the 1990s, I had to explain to people that I was doing theoretical research and that the computer did not exist. I am still saying that 25 years later. I’m getting a little impatient. But to be very honest, I would not be surprised if that computer is never built. That’s not a reason to stop my work, though. The research is fascinating.”

‘Conferencing is going perfectly well online’

How do you like working at home?
“I see the advantages. I was supposed to organise a workshop in Berkeley in the USA. It did go ahead, but online. It went very well. The good thing was that many more people could join. People who otherwise, without the corona pandemic, would not have been able to come. I believe that online conferencing is moving extremely fast. We hold a seminar a couple of times a month with research partners in Paris, London and Munich. It’s going perfectly well online. I see that there is sometimes greater depth in the online conferences. The questions are better and the answers better thought through.

Before the crisis, I pledged to travel less. Up to 10,000 kilometres a year or 40,000 in four years to retain some flexibility. We need to stop all that travel. At a certain point, scientists become a travelling circus. This is not the case anymore and I hope that this remains after the crisis.”

• Barbara Terhal is Professor of Quantum Computing at the Faculty of Electrical Engineering, Mathematics and Computer Science. She also leads a research group at the quantum technology institute, QuTech. In 1999 she was the very first person in the Netherlands to earn a doctorate in quantum calculations. She introduced the concept of ‘entanglement witnesses’ whereby an entangled quantum state can be differentiated from a non-entangled quantum state. This idea is now being applied at large scale in both theory and experimentation. Terhal is now an international expert in the area of quantum error correction, an area of research that is crucial for building reliable quantum computers. Her quest into error correction has generated much new understanding and a better grasp of what quantum computers can and cannot do.
• The KNAW selected 18 new members this year, four of whom come from TU Delft. The other three new TU Delft members are Ibo van de Poel, Jack Pronk and Frans van der Helm.