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'Even hacking gets boring eventually'

She has worked in Pasadena and in Singapore and has published articles in every major scientific journal. Late last year, she returned to the Netherlands to QuTech, to help build a quantum computer: she is Dr Stephanie Wehner.

Her room in the Applied Physics building is spacious and bare. A black leather armchair and bright red sofa form the setting for the interview. The wall opposite serves as a whiteboard; it is scrawled full of formulas and calculations. This is the office of a theoretical quantum physicist. One who started out as a hacker in computer security.

“I worked for banks, insurance firms and pharmaceutical companies that wanted to check how safe their systems were. We tested them by breaking in.”

Can you still do it?

“I haven't tried for a long time, but I don't think that security has improved that much.”

Why did you start a degree course at the age of 22?

“This might sound strange, because everyone thinks that hacking is terribly exciting, and in some ways it is. But after a while, you feel you've seen it all. I wanted to do more so I decided to take a degree.”

What drew you to computers?

“It's the communication that computers facilitate that interests me. The exchange of information, gaining access to information, sharing information or protecting it. That's what fascinates me. In my field, I try to understand nature itself, at the information level. This goes a step further than simply gaining access to information, which is what a hacker does.”

What did you study in Amsterdam?

“I graduated in Computer Science, but took some courses in physics too.”

And why did you study in Amsterdam rather than in Germany, which is where you come from?

“I was already working in Amsterdam, first for XS4ALL and later for a computer security company called ITSX.”

Your thesis was about quantum cryptography. What drew you to the world of quantum?

“It was a sort of halfway house between where I came from and what I do now. As I said, information exchange intrigues me. So quantum information is the next step. My background as a hacker drew me to cryptography because it's about protecting information, security and cracking security. I've moved on again now. I don't only do quantum cryptography, but quantum communication in the broader sense too.”

Does quantum cryptography work yet?

“Yes, you can just buy it. The devices currently on the market still have a few practical teething problems, but the main problem is that it doesn't work beyond a distance of two hundred kilometres. It's not bad, but I'd like to get rid of this restriction. At the moment, I'm working on a quantum network. A network like this facilitates quantum communication whatever the distance.”

A bit like the internet?

“The quantum internet in this case. This would enable us to realise quantum cryptography over distances greater than two hundred kilometres, but of course quantum communication is much more than just cryptography.”

I've been reading some of your publications. You seem to operate on the interface of computer science, quantum physics and thermodynamics. How do you cope with so much complexity?

“I like to be able to think about things in an abstract mathematical manner. And from an abstract perspective, these fields aren't actually all that different. Let me give you an example. In physics, we consider two particles that interact in a particular way. This interaction can be seen as an exchange of information. If you think about all the ways that particles can react to each other, you could see them as two active players trying to communicate to reach a different state via a specific protocol. We call this the operational perspective. It enables you to apply ideas from the theory of quantum information.”

Does this lead to new insights?

“It certainly does. I'm currently working on a study of machines that are too small to analyse with statistics. Thermodynamics was developed for larger systems, such as steam engines. People came up with rules, which they then verified using experiments. They were later checked using statistical mechanics on the basis of large numbers of particles. But we now have tiny machines consisting of just a few particles, which you just cannot test with statistics. Having said this, the processes can be seen as exchanges of information, so you can apply insight from quantum computing to study the thermodynamics on a quantum scale.”

There's a thought experiment in which a figure selects particles that may and may not pass through an imaginary door, thereby disturbing the thermodynamics. Is it possible to construct something like this?

“You mean Maxwell's demon. Imagine that the person carrying out the experiment only has classical information about the system. Like Maxwell's demon, who decides whether a particle goes to the right or the left. However, now imagine that someone has quantum information about the system. It can make an enormous difference. We're just starting to understand the rules that apply when two systems are initially closely entangled. Strange things happen if their quantum states were already linked before they came together. For example, we always assumed that heat flows from a warm to a cold body. It's what everyone thinks, but it's not always true. If the systems have a quantum connection, the heat can also flow from cold to warm. It's difficult to imagine because it doesn't happen in our macroscopic world. But it can certainly occur in a world on a much smaller scale.”

That sounds pretty bizarre. Can you demonstrate interactions like this in experiments?

“We could demonstrate it in a laboratory, but we haven't yet. So many new things are being discovered in quantum thermodynamics, but most ideas are still fairly mathematical and abstract. We're currently working on a project that we'll be able to test with experiments, but we cannot do it yet.”

Is that your role in the group here? Thinking ahead and designing new experiments?

“I have several roles. I also work with Ronald Hanson's group, which is trying to construct a quantum network. It's two-way traffic. I supply the cool theoretical protocol so that they can do the building work. Or they encounter a problem and want to know whether there's a theory to support it. Cooperation comes from both sides.”

QuTech in Delft and QDev in Denmark recently signed a collaboration agreement with the intention of keeping Europe at the forefront of developing a quantum computer. You've worked in Singapore too. How do you see this panning out?

“Let me explain the opportunities and risks. The thing that drew me to Delft is the fact that the people here can build quantum devices, which is actually quite unique. In Singapore, they are strong on the theory, but less so in experiments. So there was little prospect of building a real quantum network. Europe has built up knowledge and infrastructure, which researchers can use for their experiments. Europe is good at quantum computing, including the theoretical side, so I think Europe holds a strong hand. The other issue is money. Researchers here spend a lot of time trying to acquire funding. The equipment and technicians needed for this research field make it an expensive business. If you have more money, you can develop several technologies at the same time before deciding which is the best. Asia, and particularly China, spends huge amounts on quantum research. Although they are lagging behind in terms of theory and infrastructure, this is just a question of time.”

Perhaps I should point out that QuTech is hardly the poorest department of TU Delft.

“But do you realise just how much time researchers spend on acquiring external funding? The budgets here are a lot smaller than in China, and the researchers have less time for their real work.”

You're halfway through your series of lectures on quantum computing and quantum communication. How is it going?

“It's certainly lively! My lectures prompt a lot of questions and discussion, including afterwards. It means that I'm not getting through as much of the material as I'd intended, but that's okay.”

Have you got an idea of how you think a quantum computer will work?

“From the outside, it will look like any other computer, except that it will solve some tasks a lot faster. In some areas, a quantum computer is exponentially faster.”

If you ask about applications, people always talk about breaking codes and cryptography. Are there any other applications that would be useful to ordinary users?

“Imagine you have a lot of questions, like for this interview. You ask each question in turn. In a quantum network, you could ask all your questions at once and you'd get all my answers at the same time. You can search for information much more quickly in a quantum network. This is also true of search engines and databases. I think that speed will be the most obvious improvement.”


Stephanie Wehner (Würzburg, 1977) came to the Netherlands at eighteen to work for XS4ALL in computer security. Four years later, she started a degree course in computer science at the University of Amsterdam. In 2008, she was awarded a PhD in quantum cryptography by the National Research Institute for Mathematics and Computer Science (CWI). She spent the next two years as a post-doctoral researcher at Caltech, working with the renowned physicist John Preskill before starting as an associate professor in the Center for Quantum Technology at the National University of Singapore in 2010. In 2011, she was one of the initiators of QCrypt, which has since developed into the largest conference on quantum cryptography.

In October 2014, she returned to the Netherlands as a computer scientist at QuTech, based in the Kavli Institute of Nanoscience (faculty of Applied Sciences). She lectures in quantum computing and transport and writes articles with titles such as ‘The second laws of thermodynamics’ (PNAS, 2015); ‘Entanglement sampling and its implications' (CRYPTO, 2013); and ‘Long distance two-party quantum cryptography made simple’ (arxiv.org, 2010).

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