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 have investigated the challenging many-body problem of impurities in a repulsive bosonic environment. Most strikingly, we observe an oscillatory signal that is consistent with a quantum beat between two co-existing coherent quasiparticle states: the attractive and repulsive polarons. The interferometric signal allows us to extract the polaron energies for a wide range of interaction strengths, and we identify several dynamical regimes towards the formation of the Bose polaron. Thus our results improve the understanding of quantum impurities interacting strongly with a bosonic environment.
Our paper is also available here.
In a recent paper we have clarified a number of misconceptions regarding the effect of interactions on fluctuations. This discussion in the field is typically centered around the appropriate thermodynamic ensemble to be used for theoretical predictions and the effect of interactions on the observed fluctuations. We introduce the so-called Fock state sampling method to solve this classic problem of current experimental interest for weakly interacting gases. 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 in the temperature of peak fluctuations for harmonically trapped gases. The absolute size of the fluctuations furthermore depends on the total number of atoms and the aspect ratio of the trapping potential. Due to the interplay of these effect, there is no universal suppression or enhancement of fluctuations.
Our paper is available here.
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 also available on arXiv.
Recently two noew PhD stdents have joined our group!
Malthe has joined the "Lattice" lab where his project aims at investigating impurities coupled to light.
Søren is part of the "MIX" team where our sucessfull investigation of the Bose polaron is continued.
COGRATULATIONS on staring the PhD!
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.
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.
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.
We are supported by the Novo Nordisk Foundation within the NERD program.