Rydberg Lab
In our lab, we study the interactions between Rydberg atoms with each other and with ground state atoms. Rydberg atoms are atoms excited to a state of high principal quantum number. Due to the large orbital radius of the valence electron, the atom has distinctive properties such as a large geometric scattering cross section and high polarizability. In particular, the interaction between two Rydberg atoms is so strong that it can still be measured on a micrometer scale.
As a result of the interaction between a Rydberg atom and a ground state atom, novel molecules can be formed. In these molecules, a ground state atom is located in the region of the wave function of the Rydberg electron. Such Rydberg molecules have bond lengths that can be several orders of magnitude larger than the sizes of classical molecules. Rydberg molecules can be divided into different classes depending on the state of the Rydberg electron. The picture on the right shows the wave function for a so-called butterfly molecule.
The interaction between several Rydberg atoms allows to study interesting quantum mechanical interactions. The most fundamental effect is the Rydberg blockade. The blockade describes that within a volume around an existing excitation the transition frequency for further Rydberg excitations is shifted. As a result, there is a minimum distance for the excitation of Rydberg atoms. This effect is now used for first applications in quantum computing and quantum simulations. An active research field of ours is the temporal dynamics with which Rydberg excitations arise in the system and propagate epidemically.
Technical
Our experiments take place in a high vacuum chamber at pressures around 10-10 mbar. We prepare 87Rb atoms at temperatures from 10 nK to several µK via a multi-step process. We use a magneto-optical trap (MOT) followed by evaporative cooling in an optical dipole trap. In this way, we can produce Bose-Einstein condensates (BEC) of about 100,000 atoms.
Two independent laser systems are available in the laboratory for excitation of Rydberg states. A laser in the UV range at 295-300 nm can be used to address Rydberg P states. Via a two-photon process with lasers at 420 nm and 1010-1030 nm we excite Rydberg S and D states.
On the one hand, we use absorption imaging as a measurement signal. In addition, we can use time-resolved detection of ions, which are produced by the continuous decay of Rydberg excitations into the continuum. In addition, a special feature of our laboratory is an electron beam microscope, which allows us to image the atomic cloud with high spatial resolution.
T Klas, J Bender, P Mischke, T Niederpüm, H Ott
Engineering long-range molecular potentials by external drive
We report the engineering of molecular potentials at large interatomic distances. The molecular states are generated by off-resonant optical coupling to a highly excited, long-range Rydberg molecular potential. The coupling produces a potential well in the low-lying molecular potential, which supports a bound state. The depth of the potential well, and thus the binding energy of the molecule, can be tuned by the coupling parameters. We characterize these molecules and find good agreement with a theoretical model based on the coupling of the two involved adiabatic potential energy curves. Our results open numerous possibilities to create long-range molecules between ultracold ground state atoms and to use them for ultracold chemistry and applications such as Feshbach resonances, Efimov physics or the study of halo molecules.
C Lippe, T Klas, J Bender, P Mischke, T Niederprüm, H Ott
Experimental realization of a 3D random hopping model
Nature Communications 12, 6976 (2021)
Scientific advance is often driven by identifying conceptually simple models underlying complex phenomena. This process commonly ignores imperfections which, however, might give rise to non-trivial collective behavior. For example, already a small amount of disorder can dramatically change the transport properties of a system compared to the underlying simple model. While systems with disordered potentials were already studied in detail, experimental investigations on systems with disordered hopping are still in its infancy. To this end, we experimentally study a dipole–dipole-interacting three-dimensional Rydberg system and map it onto a simple XY model with random couplings by spectroscopic evidence. We discuss the localization–delocalization crossover emerging in the model and present experimental signatures of it. Our results demonstrate that Rydberg systems are a useful platform to study random hopping models with the ability to access the microscopic degrees of freedom. This will allow to study transport processes and localization phenomena in random hopping models with a high level of control.
O. Thomas, C. Lippe, T. Eichert and H. Ott
Photoassociation of rotating ultra-long range Rydberg molecules
J. Phys. B: At. Mol. Opt. Phys. 51, 155201 (2018)
We discuss the rotational structure of ultra-long range Rydberg molecules and their angular nuclear wave function. Expressing the complete molecular wave function in a laboratory fixed frame, we derive the transition matrix elements for the photoassociation of free ground state atoms. The coupling strength depends on the angular momentum coupling in the Rydberg molecule, which differs for the various types of Rydberg molecules. This work explains the different steps to calculate the wave functions and the transition matrix elements in a way, that they can be transferred to other Rydberg molecules involving different atomic species or molecular coupling cases.
O. Thomas, C. Lippe, T. Eichert and H. Ott
Experimental realization of a Rydberg optical Feshbach resonance in a quantum many-body system
Nature Communications 9, 2238 (2018)
Feshbach resonances are a powerful tool to tune the interaction in an ultracold atomic gas. The commonly used magnetic Feshbach resonances are specific for each species and are restricted with respect to their temporal and spatial modulation. Optical Feshbach resonances are an alternative which can overcome this limitation. Here, we show that ultra-long-range Rydberg molecules can be used to implement an optical Feshbach resonance. Tuning the on-site interaction of a degenerate Bose gas in a 3D optical lattice, we demonstrate a similar performance compared to recent realizations of optical Feshbach resonances using intercombination transitions. Our results open up a class of optical Feshbach resonances with a plenitude of available lines for many atomic species and the possibility to further increase the performance by carefully selecting the underlying Rydberg state.