# Frank Verstraete: How a passion for physics launched a stellar scientific career

The theoretical physicist introduces the work on quantum networks that won him the ‘Belgian Nobel prize’

## Qubits, entanglement & super-supercomputers

Despite the hype, “no quantum computer has yet been built”, says theoretical physicist Frank Verstraete, who occupies a chair in physics at Ghent University, and who last year won Belgium’s prestigious Francqui prize. While it will still be some time before the first functional machine appears in a laboratory, the approach has implications for algorithms, information processing and cybersecurity.

About 15 years ago, Verstraete and his colleagues initiated a new mathematical approach that he called quantum tensor networks – a powerful framework that allows scientists to tackle several problems of theoretical physics and has the potential to speed up the realisation of the first real quantum computer. It was for his role in the development of this framework that the award judges chose to honour him.

## Spooky action

After obtaining a degree in civil engineering from the University of Leuven, Verstraete (*pictured*) turned to physics, gaining a PhD in quantum physics from the University of Ghent in 2002. “It had always been clear to me that physics was my passion. And I’m extremely thankful that I could turn my hobby into my profession,” he told *De Tijd *last year, shortly after winning the Francqui prize – known as the Belgian Nobel prize.

His PhD work focused on entanglement, a buzzword in both quantum theory and quantum computing. Termed “spooky action at a distance” by Einstein, entanglement is a phenomenon that makes quantum physics radically different from classical physics and even relativity.

The basic ingredient is the concept of non-locality. If you create two quantum particles, such as photons, in a single event, and the particles fly away in opposite directions, their properties remain linked. For example, if you measure the quantum state of one particle and it is “up”, invariably the other particle will occupy the opposite quantum state, “down”. Both particles behave as a single system, no matter how much they are separated.

It had always been clear to me that physics was my passion

Classical computers work with “bits” that are either 0 or 1, represented for example by different electrical charges in tiny capacitors. Quantum computers, however, operate with quantum bits or qubits. An example of a qubit is a trapped electron that can occupy two quantum states, up and down, but it can also occupy a superposition, meaning it can be up and down at the same time.

Future quantum computers will work with millions of qubits that can be in superposition states and that are entangled. Because of their entanglement, these millions of qubits will perform a complex operation in one go, unlike classical computers that perform operations sequentially, following a series of instructions from the software.

Because of this, quantum computers are viewed as a threat to security codes that are based on the product of two very large prime numbers. A quantum computer will break such a code in a few seconds, a task that might take thousands of years with a classical computer.

## A huge opportunity

However, entanglement and superposition became a problem for researchers looking for continuous miniaturisation of traditional circuitry, operating with just zeros and ones. “One of the big problems in making these circuits smaller is that these quantum effects come into play,” Verstraete tells *Flanders Today*. “You get superpositions, and not just the zeros and ones. But then I understood that instead of fighting this, it was a huge opportunity to actually completely rethink the laws of information processing and what this means in the physical world to process information, to do calculations, come up with algorithms, do things that are new.”

I was extremely lucky to be in the right place at the right time – I had no clue that it could be so useful

In 2002 and 2003, Verstraete was finishing his PhD work in Ghent, collaborating with Spanish physicist Ignacio Cirac at the Max Planck Institute for Quantum Optics in Munich. They had decided to pursue theoretical physicist Paul Dirac’s suggestion that quantum theory would be able to describe any desired property of matter that is the result of many-body interactions.

A simple example of a many-body interaction is a pool table on which a number of billiard balls are placed together. In principle, it would be possible to calculate, using simple laws of mechanics, what the end positions of all the billiard balls would be after kicking one of them in a specific direction.

Verstraete and Cirac realised that quantum entanglement provided a new tool in applying many-body interactions to electron systems present in quantum computing architectures and high-temperature superconductors. The mathematical tools that Verstraete had developed during his PhD work turned out to be a seminal contribution to a new research field that he called quantum tensor networks.

“I was extremely lucky to be in the right place at the right time – I had no clue that it could be so useful. It opened up a new type of research field that allowed us to tackle many-body problems using the tools of entanglement theory,” he says. Besides quantum computing, there are several areas where many-body approaches are now used. They include quantum chemistry, condensed matter physics and even string theory. “This is a novel way of looking at problems people have been struggling with for the last 80 years,” says Verstraete.

## More support needed

Verstraete has received a research grant from the Flemish research foundation FWO and two Horizon 2020 grants from the European Research Council. His current project investigates the possibilities of tensor networks to simulate strongly correlated systems, with applications in quantum chemistry and quantum field theory. “One of the problems is the understanding of high-temperature superconductors, which has a clear potential to make a big splash,” says Verstraete.

However, he would like Belgium to be more supportive of this kind of theoretical physics. “Not just Flanders, but Belgium is running behind most of the other European countries. Its strength has always been in applied physics,” he says. “If you talk to people in Belgium about what you are doing, they ask what is it useful for? How can you make money with it? You would never get these questions in Austria or Germany.”

To verify theories, you also need a certain amount of funding of experimental research. Also here, Verstraete feels this research is somewhat at an impasse. “For this you need lots of money. As far as I can judge, unfortunately, lots and lots of money always goes to these big science projects such as CERN.”

*Photo: Frank Verstraete receives the Francqui prize at a ceremony in Brussels in 2018 © Belga/Laurie Dieffembacq*