Aarhus Universitets segl

Ultracold Quantum Gases Group

Welcome to the ultracold quantum gas research group at Aarhus University!

In our research we investigate the properties of atomic gases at extremely low temperatures. This allows us to get fundamental knowledge of quantum mechanical behaviour in few- and many-particle systems.


We are part of the Center for Complex Quantum Systems (CCQ)


New grant will enable 7 years of research on quantum simulation

Our group has been awareded a "New Exploratory Research and Discovery" (NERD) grant by the Novo Nordisk Foundation!

Over the next 7 years we will contribute to the basic understanding of quantum systems and find out how quantum mechanics can help to define technological development.

The NERD grant aims to provide support for research projects in the natural sciences based on wild and unorthodox ideas that can provide new knowledge in technology or the natural sciences.

Further details are given here.


Fluctuations in Bose-Einstein Condensates - Interactions demystified

The fluctuations of the atom number between a Bose-Einstein condensate and the surrounding thermal gas have been the subject of a long standing theoretical debate. Here we introduce the so-called Fock state sampling method to solve this classic problem. A suppression of the predicted peak fluctuations is observed when using a microcanonical with respect to a canonical ensemble. Moreover, interactions lead to a shift of the temperature of peak fluctuations for harmonically trapped gases. Due to the interplay of these effects, there is no universal suppression or enhancement of fluctuations.

Our paper is available on on arXiv.

The paper is under review at SciPost where the review process can be followed.


Life and death of the Bose polaron

Our latest results on the secret life of the polaron are on arxiv

In this paper we watch the polaron´s birth and death both spectroscopically ind interferometrically! In particular, we investigate the polaron birth by interferometric measurements at strong interactions,  revealing faster quantum dynamics at large repulsive interaction strengths than at unitarity. Moreover, we extract the polaron energy  from interferometric measurements of the observed phase velocity in agreement with previous spectroscopic results from weak to strong attractive interactions. Finally, the phase evolution allows us to measure an energetic equilibration timescale, describing the initial approach of the phase velocity to the polaron energy. In total, our results give a comprehensive picture of the many-body physics governing the Bose polaron and thus validate the quasiparticle framework for further studies.

Our paper is available on on arXiv.


Mediated interactions between ions in quantum degenerate gases

We have explored the effect of mediated interaction between two ions in a BEC

We explored the interaction between two trapped ions mediated by a surrounding quantum degenerate Bose or Fermi gas. Using perturbation theory valid for weak atom-ion interaction, we show analytically that the interaction mediated by a Bose gas has a power-law behaviour for large distances whereas it has a Yukawa form for intermediate distances. In partucular we showed that the mediated interaction can be a significant addition to the bare Coulomb interaction between the ions, when an atom-ion bound state is close to threshold. In view of experiments we show that the induced interaction leads to substantial and observable shifts in the ion phonon frequencies.

Our paper is available on on arXiv.


Presentation of the Lee-Huang-Yang Fluid

Recently presented our work on the Lee-Huang-Yang fluid in the UK Quantum Fluids Network

You can watch the talk below or directly on YouTube: https://www.youtube.com/watch?v=rThvx0Y4rug

Our paper is available on the APS pages or on arXiv.



Farewell to Magnus and Thomas

Both Magnus and Thomas have defended their PhDs very sucessfuly in 2021!

The key results from bith thesis were published this year in Nature Physics and PRL.



Observation of a Lee-Huang-Yang Fluid

Our recent observation of a Lee-Huang-Yang fluid was published in PRL!

In our experiments we created a mixture of Bose-Einstein condensates, governed by the so-called Lee-Huang-Yang (LHY) interaction, which describes the effect of quantum fluctuations. Experimentally we realized this by controlling the atom numbers and interaction strengths in a spin mixture of two states of 39K confined in a spherical trap. We measured the monopole oscillation frequency as a function of the LHY interaction strength and found very good agreement with a complete simulation of the experiment done in our group. This confirms that the system and its collective behavior are dominated by LHY interactions!  

Further details on our work are available on the CCQ pages.

Our paper is available on the APS pages or on arXiv.


Microcanonical Fluctuations in a Bose-Einstein Condensate

Published in PRL:
Quantum systems are typically characterized by the inherent fluctuation of their physical observables. However, fluctuations in interacting quantum systems are not well understood theoretically and have resisted experimental measurement efforts. In our paper we report the characterization of atom number fluctuations in weakly interacting Bose-Einstein condensates. In particular we observe fluctuations reduced by 27% below the canonical expectation, revealing the microcanonical nature of our system!

Read or paper at APS or on arXiv.


Coverage in popular science media

Our paper, elucidating dynamics of quantum impurities, was recently described in two popular science articles. You can find these articles on phys.org (here) and space.com (here).

If you are further interested, you can watch our brief explanatory video (here) or read the publication (here).


Farewell to Mick

Mick has recently left our research group and joined Kamstrup. His PhD and PostDoc were very successful and we will profit from his ideas for future research. We wish you an equally productive and successful time in the new job.


Non-equilibrium dynamics of quantum impurities

Update: Accepted for publication in Nature Physics.

Advancing our understanding of non-equilibrium phenomena in quantum many-body systems remains among the greatest challenges in physics. Here, we report on the experimental observation of a paradigmatic many-body problem, namely the non-equilibrium dynamics of a quantum impurity immersed in a bosonic environment. The impurity is created and monitored using an interferometric technique in a quantum degenerate gas. Thus we are able to trace the complete impurity evolution from its initial generation to the ultimate emergence of quasiparticle properties, forming the Bose polaron. These results offer a first systematic picture of polaron formation from weak to strong impurity interactions. They reveal three distinct regimes of evolution with dynamical transitions that provide a link between few-body processes and many-body dynamics. Our measurements reveal universal dynamical behavior in interacting many-body systems and demonstrate new pathways to study non-equilibrium quantum phenomena.

Read our paper here (Nature Physics) or on arXiv.

(Updated 03/2021)


We are part of the Center for Complex Quantum Systems (CCQ).

We are supported by the Novo Nordisk Foundation within the NERD program.

We are supported by the Independent Research Fund Denmark within the Natural Sciences subject area.