Hans Peter Büchler is Professor of Theoretical Physics and Head of the eponymous institute (ITP) at the University of Stuttgart. He was able to establish his research group, which researches many- particle quantum-mechanical systems, in 2016 thanks to funding from the European Research Council (ERC). The Swiss professor enjoys the challenges of understanding and explaining precisely how complex systems function.
Light passes through light without hindrance, which sounds so banal that one rarely gives it a second thought in daily life. Yet, this “non-aggression pact” between photons may present a real problem for potential future applications. How could ultra-fast computers use individual photons for computational purposes at some point in the future if they do not interact? Luckily, physicists have now found ways and means of overcoming this problem. It is possible to force photons to interact under laboratory conditions, by sending them into a suitable medium where they excite atoms. An interaction can then be induced based on the quantum nature of the photons in combination with the excited atoms, which the physicist can control in a targeted manner by choosing the appropriate framework conditions for the experiment. The whole process – as one would expect – is extremely complex and neither works perfectly under experimental conditions nor is it completely understood from a theoretical perspective.
“Sometimes it's even a challenge formulating the problem in tractable mathematical terms at all”.
Peter Büchler
First Order of Business: Reduce Complexity
Enter Hans Peter Büchler. The professor heads up the Institute of Theoretical Physics III at the University of Stuttgart, and – nomen est omen – is the very man for the theory. Together with his research group, he attempts to describe the processes involved in many-particle quantum-mechanical systems with as much precision as possible. “To answer the question of what is happening to the photons in such systems” says Büchler, explaining the challenge in more detail, “one has to solve the equations for an enormous number of quantum-mechanical particles, a task that cannot be achieved with precision”. That’s why he is looking into ways to reduce the inherent complexity whilst still being able to correctly describe the physics of many-particle systems – at least for a few special cases. “Sometimes it's even a challenge formulating the problem in tractable mathematical terms at all”.
Büchler uses analytical methods to recast the relevant equations in a form that eventually results in a solution. “Of course”, says Büchler, “we also utilise numeric processes, i.e., we attempt to use the computer to approximate a solution. If we find ourselves struggling to make progress then we turn to specialists in the relevant numeric methods: getting to grips with programming code is never our own central research focus". This is why Büchler's research group frequently reverts, as it were, to pen and paper or the good old blackboard. He himself estimates that some 90 per cent of his “maths work” is of an analytical nature.
Physicist from and with Passion
The 44-year-old has been fascinated by mathematics from an early age: “I always found it a doddle!” Büchler grew up in Wattwil, a scattered community in eastern Switzerland with a population of just a few thousand, 40 kilometres by car from St. Gallen and just 20 kilometres as the crow flies from Mount Säntis. By contrast, he was “appalled” by secondary- school physics, despite the fact that he was just as fascinated by it. “If you asked the teacher something that went beyond what was included in the curriculum, you'd be fobbed off and told that it was all way too complicated”. Some would have been satisfied with this answer, but Büchler saw it as a challenge “to get to grips with, say, the theory of relativity by myself”.
After gaining his Abitur (university entrance qualification), he studied Physics at the ETH Zurich (Swiss Federal Institute of Technology). “At the start of the course there”, says Büchler, “mathematics and physics were closely interrelated, which played to my talents”. He also knew early on that he would gravitate towards theoretical physics: “In that field, one can get a real understanding of things from the bottom up”. He might have gone into string theory, “but during a lecture, a professor convinced me to go into theoretical physics instead, as the relevant equations are amenable to empirical verification at some point in the future”. The professor in question was Gianni Blatter, under whose supervision Büchler later completed his master's dissertation and doctoral thesis, both on topics relating to solid-state physics. In 2004, as post-doctoral researcher, he joined a research group headed up by theoretical physicist Peter Zoller, a professor at the University of Innsbruck. Three years later and just 13 years after beginning his studies, Büchler, now 33 years old, took up his W3-professorship at the Institute of Theoretical Physics III in Stuttgart.
Bright Minds are What Count
He had planned to stay for six years, but it’s already been over a decade. “As the old saying goes”, says Büchler with a wink, “you can take the man out of Switzerland, but you can't take Switzerland out of the man. But, I was immediately made to feel very welcome and found the locals to be really open right from the start”. He refers to Stuttgart as one big village – and means it in a positive way. And, it’s not just the region himself that impresses him but, above all “the excellent, extremely productive collaboration” with the experimental physicists in Stuttgart. “They come up with new ideas and experimental set ups that are also extremely interesting for theorists like me”. Following the untimely death of the former institute director, Professor Alejandro Muramatsu, Büchler officially took over the role last autumn.
In 2016 Büchler received funding from the EU in the form of a so-called ERC Consolidator Grant, a funding facility for up to two million euros. That's a lot of money for a theoretical group working in basic research, which has no need to invest in expensive experimental setups, getting by instead with a computer, paper, blackboard and pencil. So, Büchler only applied for the amount he actually needs, most of which is used to cover personnel costs. For, as is usually the case in theoretical research, it is not the equipment that plays the crucial role, but rather a competent team. The fact that Büchler is also rather frugal in this respect is only partially due to the innate thriftiness of the Swiss. The main reason is that the professor simply loves to get to grips with the mathematics himself rather than spending all his time on managerial tasks.
Michael Vogel