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Dynamics of impurities in a quantum gas

PhD defense, Thursday, 21 January 2021, Kristian Knakkergaard Nielsen.

2021.01.21 | Karen Konradi

Kristian Knakkergaard Nielsen

Illustration of polaron-polaritons. The impurity (transparent sphere) interacts with the environment (blue) and with light (red). The interplay of these interactions leads to the formation of new quasiparticles, called polaron-polaritons. Kristian demonstrates that these quasiparticles can propagate much faster than the speed limit set by the environment

During his studies, Kristian Knakkergaard Nielsen has investigated the dynamics of impurities in a quantum gas. In such a system, the environmental coupling dresses the impurities, making them heavier and less mobile, forming so-called polarons. Kristian has studied the dynamics leading to the formation of these polarons, and successfully compared his theory to recent experimental observations.

He used these insights to investigate what happens when a quantum gas is strongly coupled to light. He demonstrates that these light-matter interactions enable superfluid flow of impurities well above the speed limit provided by the quantum gas.

The new research findings contribute to the understanding of non-equilibrium dynamics in many-body quantum systems. They are also an example of how the critical properties of a quantum system may be manipulated by coupling the system strongly to light.


The PhD study was completed at Department of Physics and Astronomy, Faculty of Natural Sciences, Aarhus University.

This summary was prepared by the PhD student.

Time: Thursday, 21 January 2021. at 15.00
Place: The PhD defence will be held online. To receive a link to the event, please send an e-mail to Kristian Knakkergaard Nielsen <kristianknakkergaard@phys.au.dk>
Title of PhD thesis: Dynamics of quantum impurities in a Bose-Einstein condensate

Contact information: Kristian Knakkergaard Nielsen, e-mail: kristianknakkergaard@phys.au.dk, tel.: +45 23 64 68 27

Members of the assessment committee:

Professor Martin Zwierlein, Department of Physics, Massachusetts Institute of Technology, USA

Professor Hans Peter Büchler, Institute for Theoretical Physics III, University of Stuttgart, Germany

Associate Professor Alberto Imparato (chairman), Department of Physics and Astronomy, Aarhus University, Denmark

Main supervisor:
Professor Thomas Pohl, Department of Physics and Astronomy, Aarhus University, Denmark

Co-supervisor:
Associate Professor Georg M. Bruun, Department of Physics and Astronomy, Aarhus University, Denmark

Language: The PhD dissertation will be defended in English

The defence is public.
Due to the coronavirus situation, the PhD thesis will not be available for reading at the offices of the PhD administration. Instead, interested parties may send an e-mail to gradschool.nat@au.dk to receive a digital copy of the PhD thesis. Please note, it may in certain cases be necessary to make an individual arrangement to read the PhD thesis.

PhD defence
16882 / i43