Aarhus University Seal

Quantum simulations of quasi particles

Georg Bruun and Jan Arlt at IFA have received a grant of Dkr kr. 5 842 049 from Danmarks Frie Forskningsfond for further work into this exiting and very promising research area.

[Translate to English:] Et enkelt (blåt) atom i en Bose-Einstein gas får gasatomerne (røde) til at fordele sig anderledes og danne en polaron (lyseblåt område), som har kvasipartikelegenskaber.

A fundamental problem with the equations of quantum mechanics is that generally they are impossible to solve. This causes physicists to use approximations and assumptions, but this again often causes uncertainties making it difficult or impossible to know how close our discriptions are to real nature. One way of solving this problem is by asking nature itself!: If you design af flexible quantum system that mimics the system that we are really interested in, this can be used as a quantum simulator. Atomic quantum gasses have presented themselves as extremely efficient quantum simulators.

                                                                 Georg Bruun. Photo: IFA

In the description of the project the two researchers explain that they wish to use quantum gasses to study the effects of 'impurity'-atoms in a Bose-Einstein condensate. The impurities will attract the bosons around them in the condensate, thereby creating a quasi particle named a Bose polaron. Quasi particles are important because they can be utilized for simplified descriptions in lots of quantum systems. The Bose polaron was observed for the first time ever in 2016 i Jan Arlt's lab at IFA. This international break-through was achieved through a close collaboration between the group of theorists led by Georg Bruun. Their work helped predict where and how to look for the polaron, and the experiments were then successfully completed by the group of experimentalists led by Jan Arlt. The two researchers further developed their successfull collaboration into a systematic study of the Bose polaron and its behaviour in totally new regimes. With the new grant the groups are planning to study how fast polarons are created, how termic effects influence it, how the condensate manage to brake the polaron and if two polarons can be mutually bound into what will be called a bi-polaron.

     

The experiments are conducted in a vacuum and the Bose-Einstein gas is trapped in a glass container like the one shown here by Jan Arlt. Photo: AU/Lars Kruse.

In the lab deep under the Department of Physics and Astronomy the Jan Arlt group creates a socalled Bose-Einstein condensate consisting of 39K atoms in an electromagnetic trap at extremely low temperatures. Into this condensate a single "alien" atom is then introduced - actually it is another 39K atom, but having another spin state. This causes the whole of the condensate to regroup, creating what from the outside seems to be a new type of particle - a quasi particle. This is what has been given the name of a polaron. By changing the physical parametres in the trap, e.g. the temperature, studying the behaviour of this special particle and its reactions to external influences is now possible for the researcers.

In an office higher in the IFA building Georg Bruun and his collaborators are then exitedly awaiting the results of the measurements. They have already made theoretical predictions for the expected behaviour of the polaron, and by comparing with the experiments our understanding of this particle and the physics of quasi particles in general can be further improved. There is a highly productive and constant interaction between theory and experiment, and it has been a great advantage that the two groups of researchers are geographically as close as they are at Aarhus University. When the predictions of a theoretical model has been sufficiently confirmed by experiment, the controls in the lab are given a couple of extra tweaks until the fine agreement is maybe not so fine anymore - sending the theoreticians on another task of refining the models. The result of this continuous exchange of challenges in the end will improve our common understanding of how nature behaves at the quantum levels.

Part of the experimental setup in the laser lab at IFA. The Bose-Einstein condensate is inside of the square block of copper. Photo: AU/Lars Kruse.

 

The next step for the continued common research project is to have two polarons trapped at the same time. The predictions from Georg Bruun and his colleagues is that the two polarons are going to be coupled together into what is called a bi-polaron. The results of the nex experiments are now eagerly awaited. Having more than one polaron close by each other and being able to understand and describe their common behaviour is of enourmeus importance for our understanding of semi conductors and super conductors where lots of quasi particles can be seen interacting at the same time. A better understanding of these quantum mechanical aspects will definitely assist in the development of the faster computers and electronic components of the future.