Former Projects

Our experiments focused on the study of single Rydberg atoms embedded in an ultracold quantum gas at temperatures in the nanokelvin regime. Our ability to control such Rydberg impurities with principal quantum numbers up to n~200 allowed us to investigate situations where the size of the Rydberg atom by far extends the typical distance between the trapped atoms. This provided ways to investigate the interaction of the Rydberg electron with single, few, or many neutral perturbers residing within the electronic orbital. The quantum mechanical scattering process describing this interaction lead to a new molecular bond and the formation of giant so-called ultralong-range Rydberg molecules.

For many perturbers within the Rydberg orbit, which is realized when exciting the Rydberg state in a Bose-Einstein condensate, the contribution of higher-order partial waves in the electron-neutral scattering process could be studied. Specifically, a detailed modelling of the excitation lineshape, which is strongly shifted and broadened by the interaction with the condensate, revealed signatures of an electron-atom p-wave scattering resonance. Moreover, the lifetime of the Rydberg electron in the dense medium is determined by chemical reactions on micrometer length scales and microsecond time scales, leading to molecular ion formation and collision-induced changes of the electron’s orbital angular momentum. In this regime, we pursued to observe the backaction of the Rydberg electron on the condensate density distribution, a mechanism that may lead to a microscopy tool for electronic orbitals.

We focused on the interaction of the positively charged ionic Rydberg core with the Bose-Einstein condensate. This ion-atom interaction is much weaker than the electron-atom interaction and typically fully disguised by the latter. However, the contribution of the electron could be largely reduced once its orbit exceeds the typical size of the atomic sample. At the same time, the presence of the electron serves as a protective Faraday shield for the ion from detrimental electric stray fields. We demonstrated this regime using a micron-sized optical tweezer trap and observe evidence for ion-atom interaction. These experiments may lead to a new way for studying ionic impurities in a BEC at temperatures where quantum effects in the scattering process start to play a role, with prospects for the exploration of associated polaron physics.

We studied a new approach to reach quantum degeneracy for situations when standard laser cooling mechanisms are not applicable, e.g. due to the lack of a closed cycling transition. An optical dipole trap (ODT) is continously loaded from a guided atomic beam with a mechanism that can be compared to a diode for atoms. An ODT is therefore placed within an atomic beam. A state dependent potential barrier superimposed to the trap removes the directed kinetic energy from the atoms, while they are traveling through the ODT's focus. This dissipative mechanism is in principle a single photon process, and not depending on a closed cycling transition. Loading the trap until it reaches the collisionally dense regime then allows further cooling by evaporation.

With this method, we managed to produce a BEC with chromium atoms within less than 5s. While we did not use laser cooling to generate our beam of chromium atoms, there are methods to generate intense beams with molecules that have recently been established and which are not based on cycling transitions.

In our first generation Rubidium Rydberg experiment, we have started our long-standing endeavor to probe and control giant Rydberg atoms in an ultracold atomic gas. The apparatus used a single chamber approach, where MOT and magnetic trap are located at the same spot. The MOT was loaded from a Zeeman slower, followed by a magnetic cloverleaf trap, in which BECs containing ~10⁵ atoms have been produced. The magnetic trap is located at the center of 8 electric field plates (see figure) which can produce strong electric fields for tuning and ionization of the Rydberg atoms.

Our experiments started with detailed studies of interactions between Rydberg atoms (dipole-dipole and van der Waals interaction), tuning of interactions via Förster resonances, and the demonstration of the first excitation of Rydberg atoms in a Bose-Einstein condensate. The excitation of Rydberg atoms in a dense an ultracold atomic ensemble lead to the experimental discovery of a new type of molecular bond between Rydberg and ground state atoms, forming an ultralong-range Rydberg molecule. We have investigated the nature of this binding mechanism in detail, demonstrated coherent control of the long-range dimers, and studied the transition from few-body bound states to many-body density shifts.

The Rydberg Quantum Optics Group led by Sebastian Hofferberth moved to Syddansk University (Odense, Denmark). The website of Sebastian's group can be found here.