In recent years spectacular progress in controlling and manipulating ultracold atomic quantum matter has been made. Hence attention has now turned to more complex molecular quantum systems, and the research on ultracold molecules assembled from quantum gases has developed into a new field of its own.
It now provides novel and exciting opportunities for fundamental studies in a broad range of research areas, ranging from few-body and many-body quantum physics to quantum chemistry and quantum computation. We are particularly interested in multi-component systems for investigating Efimov physics, phase separation, beyond mean-field interactions and impurity physics.
Impurity physics explores how a mobile particle becomes dressed by excitations of its surrounding medium, giving rise to emergent quasiparticles such as polarons. These impurity–environment interactions are central to many physical systems, ranging from condensed-matter materials to organic electronics. Ultracold atomic gases provide a clean and highly controllable platform to study impurity physics across interaction regimes, enabling precise tests of many-body theories. In particular, impurities immersed in a Bose–Einstein condensate allow direct access to quasiparticle formation and correlations from weak coupling to the strongly interacting, unitary regime.
Our work on three-body physics in the impurity limit has been published in Physical Review A
We have investigated three-body loss processes of impurity atoms embedded in a medium of a Bose-Einstein Condensate close to a Feshbach resonance. The time-dependent number of atoms in the BEC is recorded which allow for a measurement of the three-body loss rate coefficient L3 and its scaling with the impurity-medium scattering length. Moreover, the medium atom number is reconstructed from spectroscopic loss measurements. This allows for a comparison of the medium densities based on both the extracted loss rates and the spectroscopically reconstructed atom number. Finally, the number of lost medium atoms per loss event is evaluated and found to exceed 2 at strong interactions, which is attributed to secondary collisions in the medium. These investigations establish the use of a fast loss mechanism as a new tool in the field and provide quantitative measurements of three-body losses at large interaction strengths.
Our paper is also available on Arxiv here
(05/2025)
Our work on the repulsive polaron has recently been published in Physical Review Research.
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 on Arxiv here.
(04/2025)
Our review on polarons in atomic gases and two-dimensional semiconductors is available on arxiv
In this work we provide a comprehensive review of theoretical and experimental studies of the properties of polarons formed by mobile impurities strongly interacting with quantum many-body systems. We present a unified perspective on the universal concepts and theoretical techniques used to characterize polarons in two distinct platforms, ultracold atomic gases and atomically-thin transition metal dichalcogenides, which are linked by many deep parallels. We review polarons in both fermionic and bosonic environments, highlighting their similarities and differences including the intricate interplay between few- and many-body physics. Various kinds of polarons with long-range interactions or in magnetic backgrounds are discussed, and the theoretical and experimental progress towards understanding interactions between polarons is described. We outline how polaron physics, regarded as the low density limit of quantum mixtures, provides fundamental insights regarding the phase diagram of complex condensed matter systems. Furthermore, we describe how polarons may serve as quantum sensors of many-body physics in complex environments. Our work highlights the open problems, identifies new research directions and provides a comprehensive framework for this rapidly evolving research field.
(01/2025)
Our latest results on the secret life of the polaron were published in PRR.
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.
(11/2022)
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.
(6/2021)
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)
Recently, two independent experiments reported the observation of long-lived polarons in a Bose-Einstein condensate, providing an excellent setting to study the generic scenario of a mobile impurity interacting with a quantum reservoir. Here, we expand the experimental analysis by disentangling the effects of trap inhomogeneities and the many-body continuum in one of these experiments. This makes it possible to extract the energy of the polaron at a well-defined density as a function of the interaction strength. Comparisons with quantum Monte-Carlo as well as diagrammatic calculations show good agreement, and provide a more detailed picture of the polaron properties at stronger interactions than previously possible. Moreover, we develop a semi-classical theory for the motional dynamics and three-body loss of the polarons, which partly explains a previously unresolved discrepancy between theory and experimental observations for repulsive interactions. Finally, we utilize quantum Monte-Carlo calculations to demonstrate that the findings reported in the two experiments are consistent with each other.
Read our paper on arXiv.
(12/2018)
Ultracold atomic gases are an important testing ground for understanding few-body physics. In particular, these systems enable a detailed study of the Efimov effect. We use ultracold 39K to investigate the temperature dependence of an Efimov resonance. The shape and position of the observed resonance are analyzed by employing an empirical fit, and universal finite-temperature zero-range theory. Both procedures suggest that the resonance position shifts towards lower absolute scattering lengths when approaching the zero-temperature limit. We extrapolate this shift to obtain an estimate of the three-body parameter at zero temperature. A surprising finding of our study is that the resonance becomes less prominent at lower temperatures, which currently lacks a theoretical description and implies physical effects beyond available models. Finally, we present measurements performed near the Feshbach resonance center and discuss the prospects for observing the second Efimov resonance in 39K.
Read our published paper at PRA or on arXiv.
(11/2018)
Ultracold quantum gases provide a unique setting for studying and understanding the properties of interacting quantum systems. Here, we investigate a multi-component system of 87Rb–39K Bose–Einstein condensates (BECs) with tunable interactions both theoretically and experimentally. Such multi-component systems can be characterized by their miscibility, where miscible components lead to a mixed ground state and immiscible components form a phase-separated state. Here we perform the first full simulation of the dynamical expansion of this system including both BECs and thermal clouds, which allows for a detailed comparison with experimental results. In particular we show that striking features emerge in time-of-flight (TOF) for BECs with strong interspecies repulsion, even for systems which were separated in situ by a large gravitational sag. An analysis of the centre of mass positions of the BECs after expansion yields qualitative agreement with the homogeneous criterion for phase-separation, but reveals no clear transition point between the mixed and the separated phases. Instead one can identify a transition region, for which the presence of a gravitational sag is found to be advantageous. Moreover, we analyse the situation where only one component is condensed and show that the density distribution of the thermal component also shows some distinct features. Our work sheds new light on the analysis of multi-component systems after TOF and will guide future experiments on the detection of miscibility in these systems.
Read our manuscript in New Journal of Physics or on the arXiv.
(05/2018)
After our recent observation of the Bose polaron, we are aiming to understand the quasiparticle in more depth.
Here we consider a mobile impurity immersed in a Bose gas at finite temperature. Using perturbation theory valid for weak coupling between the impurity and the bosons, we derive analytical results for the energy and damping of the impurity for low and high temperatures, as well as for temperatures close to the critical temperature Tc for Bose-Einstein condensation. These results show that the properties of the impurity vary strongly with temperature. The energy exhibits an intriguing non-monotonic behavior close to Tc, and the damping rises sharply close to Tc. We finally discuss how these effects can be detected experimentally.
Read our manuscript in Physical Review A or on the arXiv.
(01/2018)
In previous experiments with ultracold mixtures of potassium and rubidium, an unexpected non-universal behavior of Efimov resonances was observed. We have measured the scattering length dependent three-body recombination coefficient in ultracold heteronuclear mixtures of 39K-87Rb and 41K-87Rb and do not observe any signatures of Efimov resonances. This reestablishes universality of the three-body parameter across isotopic mixtures.
The article is published in Physical Review Letters and available on arXiv.
(10/2016)
A figure from our recent paper on phase separation and dynamics of two-component Bose-Einstein condensates was selected to be on display as part of the Phys. Rev. A Kaleidoscope!
Extra credit goes out to Kean Loon Lee who was main author on the paper.
Read the paper here or on arxiv.
(08/2016)
Mobile impurity particles interacting with a bosonic quantum environment play a central role in our understanding of nature and are fundamental for several important technologies such as organic electronics. It is therefore highly desirable to study impurity physics systematically and from a broad perspective as offered by cold atomic gases.
We present the experimental realization of long-lived impurity atoms in an atomic Bose-Einstein condensate. The energy of the impurity is measured and we find excellent agreement with theories that incorporate three-body correlations, both in the weak-coupling limits and across unitarity. For both strong repulsive and strong attractive interactions, our experimental results demonstrate the existence of a polaron quasiparticle.
The manuscript has been published in Physical Review Letters as an Editors' Suggestion, back-to-back with results from the group of Eric Cornell and Deborah Jin at JILA. Additionally, it can be found on arXiv.
Our results are also featured in several news outlets which appeal to a broader audience:
Physics Viewpoint: Bose Polarons that Strongly Interact
Quasipartikler som klumper i kold kvantesuppe (danish)
(07/2016)
The miscibility of two interacting quantum systems is an important testing ground for the understanding of complex quantum systems. Two-component Bose-Einstein condensates enable the investigation of this scenario in a particularly well controlled setting. In a homogeneous system, the transition between mixed and separated phases is characterised by a 'miscibility parameter'. In this theoretical analysis we have shown that this parameter is no longer the optimal one for trapped gases, for which the location of the phase boundary depends critically on atom numbers.
The manuscript has been published in Physical Review A and is also available on arXiv. (07/2016)
Tunable dual-species Bose-Einstein condensates of 39K and 87Rb
We present the production of dual-species Bose-Einstein condensates of 39K and 87Rb. Preparation of both species in the |F=1,mF=−1> state enabled us to exploit a total of three Fesh\-bach resonances which allows for simultaneous Feshbach tuning of the 39K intraspecies and the 39K-87Rb interspecies scattering length. Thus dual-species Bose-Einstein condensates were produced by sympathetic cooling of 39K with 87Rb. A dark spontaneous force optical trap was used for 87Rb, to reduce the losses in 39K due to light-assisted collisions in the optical trapping phase, which can be of benefit for other dual-species experiments. The tunability of the scattering length was used to perform precision spectroscopy of the interspecies Feshbach resonance located at 117.56(2)G and to determine the width of the resonance to 1.21(5)G by rethermalization measurements. The transition region from miscible to immiscible dual-species condensates was investigated and the interspecies background scattering length was determined to 28.5a0 using an empirical model. This paves the way for dual-species experiments with 39K and 87Rb BECs ranging from molecular physics to precision metrology. (11/2015)
The first heteronuclear 39K-87Rb BEC-Mixtures were produced in the MIX laboratory on the 12th of May. The inter-species tunablility of the scattering length between 39K-87Rb allows for a wide range of exciting experiments including fundamental investigations of interactions in heteronuclear many particle quantum systems, molecular quantum gasses, and the simulation of the impurity problem under changing interactions. Currently, both condensates have around 104 atoms. (05/2014)
On the 15.8 the first 39K BECs were realized in the MIX laboratory. The tunablility of the scattering length in 39K allows for a variety of experiments including strongly-correlated systems in optical lattices, molecular quantum gasses and fundamental investigations of interactions in many particle quantum systems.
So far we have about 104 39K atoms in the condensate! (08/2013)
The first BEC in the MIX laboratory was realized on the 15.2.2012! The magnification of the imaging system is only set to 1 at the moment, and so far it is not focussed very well. Nonetheless the low expansion of the cloud provides clear evidence for BEC. It took us roughly 5 months to make a BEC after moving the apparatus from Hannover to Århus. The number of atoms and the temperature seems to be the same as in our previous work! Now we can start to optimise! (02/2012)
The first MOT was realized in the new MIX laboratory on the 13.9.2011 only 10 working DAYS after moving the experiment from Hannover to Århus! The atom number looks good and we can now start setting up the atom transport and detection. (09/2011)
The MIX experiment has finally arrived in Århus and successfully moved into its new laboratory! The experiment was partially disassembled in Hannover and moved to Århus by truck. Only the vacuum system (including glass cells) remained in one piece and under vacuum. The delicate vacuum system remained intact which should allow us to rebuild the experiment very quickly! Have a look at more images from the move! (8/2010)
Optically trapped atoms are very importnat for frequency measurements and quantum memories, but generally suffer from strong dephasing due to inhomogeneous density and light shifts. We have demonstrated a drastic increase of the coherence time to 21 s on the magnetic field insensitive clock transition of 87Rb by applying the recently discovered spin self-rephasing mechanism. The presented frequency standard provide high stability in a potentially very compact setup. arXiv:1103.2283 and Phys. Rev. Lett. 106, 240801 (2011) (06/2011)
We report on a slow guided atom laser beam outcoupled from a Bose-Einstein condensate of 87Rb atoms in a hybrid trap. The acceleration of the atom laser beam can be controlled by compensating the gravitational acceleration and we reach residual accelerations as low as 0.0027 g. arXiv:1005.3964 and Applied Physics B (2010)
