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Focus on research: theoretical physicist Jean-Sébastien Caux

‘Theoretical physics is very logical. Now, to my mind psychology is something really difficult. Where do you begin? When are you satisfied? I haven’t a clue.’ Not that my work is that easy, says Jean-Sébastien Caux at the Institute for Theoretical Physics. But it is rewarding. ‘I always look for the simplest model and then add small things. I always use very specific methods. As soon as something adds up in physics, you’re set. That’s what I really enjoy.’

Jean-Sébastien Caux
Photo: Bob Bronshoff

Caux, who hails from Canada, came to Amsterdam in 2003. The city appealed to him but the excellent international reputation of the Institute for Theoretical Physics was the main draw. The Foundation for Fundamental Research on Matter (FOM) offered him a stepping stone, which enabled him to develop his talents to the level of professor within five years. That stepping stone structure, meanwhile, is no more, he says. ‘I was one of the last to take the plunge.’ Caux has been a university lecturer since 2007.

His previous research in England was not directly linked to the lines of research pursued at his new institute, and technically he continues to be the odd man out, acknowledges Caux. ‘But that’s a good thing, in fact. I haven’t published anything with my colleagues to date but there is enough overlap to engender proper discussions.’

Caux’s own research concentrates on the theoretical side of condensed matter – solid and liquid matter. ‘I’m interested in the behaviour of systems made up of many particles. In other words: what is the effect of the interaction between those particles, and what quantum mechanical aspects come into play?’

Flipping mini magnets

Caux studies various types of systems. One of these systems revolves around quantum magnetism. ‘Particles often have a magnetic moment’, he explains, ‘just like the magnets you stick on your fridge at home’. In simple terms, north and south poles generally attract, whereas identical poles repel each other. This causes the atoms in a crystal to interact magnetically with each other. ‘There is a collective order or chaos in a system like this. I create models for these microscopic coupled magnets which I use to describe that particular state.’ One of the things he then can do with such a model, says Caux, is make one tiny magnet ‘flip’ or switch poles. ‘This induces magnetic waves in the crystal. I then want to know how fast the waves move, how the particles interact, and how much energy this involves.’

Photo: Bob Bronshoff

An experiment that Caux can describe theoretically is known as inelastic neutron scattering. The procedure is as follows: ‘you throw a neutron into a system. A neutron does not have an electric charge but it does have a magnetic moment. This induces a magnetic wave, which I in turn can calculate’.

The extraordinary thing is that experimental researchers actually perform this test in reality. The test is carried out in a kind of semi-spherical dome, explains Caux. ‘You send a bundle of neutrons at a certain speed to a sample next to the dome. When they collide with that, they release some of their energy to the material. They change direction and speed and then smash into the dome, which is loaded with detectors. Since you know exactly where and when they reach the detectors on the dome, you can calculate the momentum of the neutrons. At the same time, you know how much energy they released to the material.’

He shows two figures, which explain the predicted values and the tests results. They are virtually identical. ‘The relationship between the experimental values and the values achieved is very good. These types of experiments provide an impetus for theoreticians like me and for experimenters.’

Cold atoms

The density-density correlation function in the Lieb-Liniger model

Another research specialisation revolves around things known as cold atoms. Atoms usually vibrate in place, but cold atoms are cooled down to such a degree as to render them virtually stationary. In physics, they are more or less the new kid on the block, says Caux. The first theoretical model in which cold atoms come into play, however, derives from the early sixties. ‘This is the Lieb-Liniger model, a kind of super ideal theory about how particles interact, described with quantum mechanics. It’s really complicated. We were able to solve the problem on paper but it was useless to experimenters and physicists.’

Yet, a relatively precise experimental structure recently proved to be feasible for carrying out measurements on the model. This is done by positioning lasers opposite each other that emit exactly the same light, says Caux. Light is a kind of a wave motion. Two opposite, colliding waves will start interfering with each other which will induce a stationary wave. Similar to the string of a violin that produces a sound, explains Caux. Such a stationary wave is made up of moving parts and of ‘nodes’, which do not move. Researchers can hold onto the cold atoms on that stationary wave using lasers.

Neutron scattering on KCuF3 (courtesy of Alan Tennant)

Light does not contain any impurities and, as a result, the cold atoms remain in place at very regular distances from each other. ‘By changing the light, you can change the wave and thus the distance between the atoms on the nodes. This method allows you to create a one-dimensional crystal, which of course never occurs in nature. For a theoretician like me, this is really interesting, because strange situations occur in one dimension, as opposed to higher dimensions.’

Redundant experiments

Caux says that his theoretical work cannot be encompassed by a single discipline. ‘I wear two or three hats: not only do I use physics but I use a great deal of applied mathematics as well, and I regularly work with experimenters. I talk to researchers from other disciplines a lot, such as mathematicians. That’s one of the luxuries I enjoy; when I’m tired of physics, I can simply change hats.’

In February 20011, scientific research financier the Netherlands Organisation for Scientific Research (NWO) awarded Caux a EUR 1.5 million Vici grant. ‘My initial aim is to use the funds to advance the theory. If this is successful, I will be able to work on new applications for experimental purposes.’ Caux finds the word ‘application’ an amusing term for someone like himself who is deeply engaged in theory. ‘To me, an image is in itself already a kind of application.’ But for people outside the field, the best way to explain his work is through relevant experiments. ‘Without experiments, my work is simply less interesting to most people.’ Yet, experiments are not always required. ‘We also make calculations on things that will never be measurable. They also form part of the theoretical story, and so you may perhaps stumble across a first example of an undiscovered type of behaviour which, in turn, may lead to new physics at some point in the future.’

Caux’s dream is to develop models and methods that provide all answers to his research area. ‘It’s a kind of platonic ideal. I do believe it is achievable; examples are available in mathematics.’ Physics today, however, is not as far advanced, he says: ‘We’re actually still only in the Middle Ages. Quantum mechanics works well for what we know today but there must be much more. We are collectively seeking a point in time at which the theories for a model uniting quantum mechanics with condensed matter can be binned. That’s what motivates me: discovering new things, and being able to understand them at the very least.’