‘The quantum computer needs new breakthroughs’

Prof. Lieven Vandersypen is among the 17 new members appointed by the Royal Dutch Academy of Sciences (KNAW) this year, as the Academy announced on 17 May 2024. A week later, on 24 May, the Dutch research funding body NWO awarded the first-ever Summit Grant to research groups at the world’s top level. QuTech, with Vandersypen as the lead applicant, the Quantum Nanoscience Department of TU Delft’s Faculty of Applied Sciences and Leiden University, received EUR 35.4 million for the Quantum Limits proposal. At the same time, the Evolf proposal, led by Cees Dekker and Gijsje Koenderink (both Faculty of Applied Sciences), received EUR 40 million for developing an artificial living cell.

You have had a special month with the Summit Grant and the KNAW membership. What do these mean to you?

“Yes, it’s an honour. Through the KNAW, you can be part of discussions in a way that has more impact than if you try to do it on your own. I’ve always wanted to join discussions about science and society. I see a good opportunity for this through the KNAW.”

Are there also things expected of you?

“It’s not that you have to sign somewhere and say, I commit so many hours in a week. So you can also just be a dormant member and never do anything. The KNAW does hope that you will contribute though, and it may also call on you to be a jury member for the various science prizes they award. But I have already indicated that as long as I am the Scientific Director of QuTech, they cannot count that much on me. I do look forward to doing more of this later though.”

You also received a EUR 35 million Summit Grant from NWO for TU Delft and Leiden University. Does that make you the man who will bring the quantum computer?

“Gee, I don’t think so, and that’s for others to say, but it has created the idea that EUR 35 million will go to TU Delft and Leiden to develop the quantum computer. But that is not what we actually proposed.”

 ‘Can you create entanglement with gravity?’

So what is Quantum Limits about then?

“In summit research, we proposed to do real research that pushes limits. Quantum theory was born 100 years ago from the limits of classical mechanics. As you moved towards the microscopic world, you saw that classical mechanics was no longer correct. This gave rise to quantum theory. Our Quantum Limits proposal explores the limits of quantum mechanics.”

Can you give an example?

“One of the most interesting properties in quantum mechanics is entanglement . So far, we create entanglement via electromagnetic fields. But the big question is if you can also create entanglement through gravity. No one has yet managed to reconcile gravity theory and quantum theory.”

‘We want to bring gravity and quantum mechanics together’

Unlike electromagnetic fields, you cannot turn gravity off for a moment.

“No, definitely not. But because of advances in nanotechnology, in the next decade we think we will be able to directly measure the attraction between two tiny masses to then see if gravity can entangle them.”

The attraction between them?

“We know that gravity decreases as you make the masses smaller. Mutual attraction can be measured down to particles of a millimetre or so. On the other hand, vibrating objects of about a micrometre have been put into quantum states. But, to put it simply, between a micrometre and a millimetre, there is still a gap in our ability and understanding. On the one hand we will explore that area by bringing ever larger objects into quantum states. And, on the other hand, we will measure the gravity of smaller and smaller objects ever more sensitively. That’s how we want to bring quantum mechanics and gravity together.”

So does the theory come from Leiden and the technology from TU Delft?

“No, it is less black and white than you think. Leiden also has experimenters for some of these measurements with really good nano expertise. And at TU Delft there are excellent theorists who can give direction to the measurements.”

 Is what you describe a deepening research programme in addition to the development of the quantum computer everyone is talking about?

“Yes indeed. Many of the developments in quantum technology have also brought new questions to the surface or led to old questions being sharpened or clarified,”

‘Important parts of the know-how are not shared’

Is quantum computer development still a global collaboration or has it now become a tough competition?

“There is still a lot of collaboration. And I also really believe that that is absolutely essential. It is such a complex task that nobody will have all the good ideas on their own. So the exchange of ideas remains very important. But if you read some publications, you also see that important parts of the know-how that make the measurements possible are not actually shared. And this is obviously more the case for companies than for university groups. There, in general, people do share.”

So what about university groups working with industry, such as QuTech with Intel and Microsoft? The latter two probably don’t want you to share all the details either.

“True, we ran into that too. When we did measurements on Intel’s quantum chips, the measurements were allowed to be published without restrictions. But the processes at Intel to make the chips were not shared.”

What do you see as the main obstacles on the path to a working quantum computer?

“The obstacles stem from what we call ‘decoherence’ . We know that the state of a quantum bit only lasts for a limited time. For useful applications, you need to perform quite a lot of operations in succession on a few hundred qubits, as quantum bits are also called. We are currently still very far from the point where quantum bits are good enough to perform such a computation successfully.”

‘For a few hundred perfect qubits, you need a few million physical qubits’

What can be done about that?

“To do something about that, you have to apply error correction. And that to me is one of the most amazingly beautiful aspects of the whole quantum information theory, that it is possible to correct those most fundamental errors that occur due to decoherence, to still reverse them and keep the qubits stable. You do that through redundancy with multiple quantum bits.”

And then it’s a matter of counting the most votes?

“Simply put, yes. But it’s a bit more subtle. Except that the redundancy you need is very big. That means that for a few hundred perfect qubits, you have to have a few million physical qubits. So the decoherence problem translates to scaling up the number of quantum bits, or improving their quality incredibly. Or a combination of both. But we know we are running into limits with current ideas. And those limits are well before the point where we can do anything useful with qubits. That means new ideas and breakthroughs are needed.”

Is that the background to your Quantum Limits proposal?

“It is not the main goal of the research. We actually mainly want to increase our understanding. But that can also really help quantum technology move forward.”

What actually matters more to you? That there will be a working quantum computer? Or more understanding of that wonderful quantum physics?

“For me, it’s really the combination that counts. Quantum physics continues to fascinate me. But if tomorrow someone proves to me that a quantum computer can’t work, I’ll do something else. I can’t work on a technology that I don’t believe in.”

Quantum computer development is a global field with big stakes and huge investments. What is QuTech’s position in that force field?

“Our position lies in innovative concepts. There is a lot of critical mass here: some 300 very smart people who challenge each other and take each other to the next level. Moreover, since the end of last year, the number of people working at TU Delft quantum startups has exceeded the number of people at QuTech. From the start of QuTech in 2014, the intention was that scientific publications would also find their way into business and jobs. And you can see that happening now. QuTech is still growing a bit, but all those companies together are growing faster. It’s nice to see that the group of quantum start-ups is developing its own momentum.”