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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)


News


Microcanonical Fluctuations in a Bose-Einstein Condensate

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 our paper on arXiv.

(11/2020)


Observation of a Lee-Huang-Yang Fluid

In our experiments we have 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!  

Read our paper on arXiv.

(11/2020)


Welcome Claus and Andreas

We welcome Claus and Andreas, who just started as PhD students in the Lattice and MIX lab, respectively. Claus has a theoretical background from his Master's degree, while Andreas did his bachelor project within the group and is now enrolled in a combined Master and PhD. They are both from our institute! Best of luck to Claus and Andreas!

(11/2020)


Congratulations and farewell to Mikkel

Mikkel recently finished his PhD in the group and is now ready to move on to new challenges. We wish you good luck in the future.

(9/2020)


Magnus G. Skou
Thomas G. Skov

Young Researchers Workshop on Quantum Fluctuations in Ultra-cold Gases

Magnus G. Skou and Thomas G. Skov will both give a talk at the virtual workshop, Young Researchers Workshop on Quantum Fluctuations in Ultra-cold Gases.

It is a workshop for young researchers working in the areas of ultra-cold quantum gases and liquids and the main focus  is to address the phenomenon of quantum fluctuations in different systems such as polarons, droplets, superfluids, supersolids, etc.

More information is found on the workshop home page.

(09/2020)


Spatial tomography of individual atoms in a quantum gas microscope

In our most recent paper we show a method to determine the position of single atoms in a three-dimensional optical lattice. Typically atoms are sparsely loaded from into a few vertical planes of a cubic optical lattice positioned near a high-resolution microscope objective. In a single realization of the experiment, we pin the atoms in deep lattices and then acquire multiple fluorescence images with single-site resolution. The objective is translated between images, bringing different lattice planes of the lattice into focus. This opens up the possibility of extending the domain of quantum simulation using quantum gas microscopes from two to three dimensions.

Accepted for publication in Phys. Rev. A. - read our paper on arXiv.

(09/2020)


Dynamical regimes of impurity evolution

Non-equilibrium dynamics of quantum impurities

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 on arXiv.

(05/2020)


Oscillating single-species BECs

New Grant: Quantum Fluids beyond the Mean-Field Paradigm

Jan Arlt has received a new grant from the Danish Council for Independent Research: Quantum Fluids beyond the Mean-Field Paradigm.

Quantum fluids play an important role in applications as well as fundamental research. Understanding their macroscopic properties is, however, particularly challenging and theoretical descriptions are often limited to the case of weak interactions, where a mean-field approach is sufficient. In a seminal result, the first correction term was obtained by Lee, Huang and Yang in 1957 (LHY), but it was only recently observed using ultracold atomic gases.

We have proposed a novel approach to study LHY quantum fluctuations by tuning the properties of a two-component Bose-Einstein condensate, such that the LHY correction is the only relevant interaction energy. Thus, it will be possible to obtain clear experimental signals beyond the mean-field paradigm and extend the current understanding of these quantum systems. Moreover, we will evaluate the use of this novel LHY fluid as a quantum simulator for highly nonlinear quantum systems.

View more details here.

(06/2020)


Simulation of XXZ Spin Models using Sideband Transitions in Trapped Bosonic Gases

We theoretically propose and experimentally demonstrate the use of motional sidebands in a trapped ensemble of 87Rb atoms to engineer tunable long-range XXZ spin models. We benchmark our simulator by probing a ferromagnetic to paramagnetic dynamical phase transition in the so called Lipkin-Meshkov-Glick (LMG) model, a collective XXZ model plus additional transverse and longitudinal fields, via Rabi spectroscopy. We experimentally reconstruct the boundary between the dynamical phases, which is in good agreement with mean-field theoretical predictions. In addition we analyze the achievable spin squeezing in our XXZ simulator theoretically, opening the possibilities of using motional sidebands as a tool to push the frontiers of metrology via quantum entanglement.

Read our paper on arXiv.

(04/2020)


Preparation of mesoscopic atomic ensembles with single-particle resolution

We contributed to the detection and stabilization of single atoms in collaboration with the Quantum Atom Optics group at Hannover University.

In our joint work we present an accurate fluorescence detection technique for atoms that is fully integrated into an experimental apparatus for the production of entangled quantum states. Single-particle resolving fluorescence measurements for 1 up to 30 atoms are presented. We utilize the accurate atom number detection for a number stabilization of the laser-cooled atomic ensemble. For a target ensemble size of 7 atoms prepared on demand, we achieve a 92% preparation fidelity and reach number fluctuations 18dB below the shot noise level using real-time feedback on the magneto-optical trap.   

See our publication on arXiv.

(12/2019)


Farewell to Nils

 

Nils has recently left the group to join Patentgruppen. 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.

(11/2019)



Funding

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

The Villum Foundation