Many-body effects in strongly interacting systems of Rydberg atoms

Hendrik Weimer

Jovica Stanojevic

In this thesis I present several theoretical methods to explore the excitation dynamics of Ultracold Rydberg systems in various regimes. For some applications the details of interactions between Rydberg atoms have to be carefully examined, while for others, the many-body aspect of Rydberg excitation may be crucial. In both cases, the basic prerequisite is to know the interactions sufficiently well. I will review the basic concepts how to evaluate molecular potentials in various Hund's cases and approximations. Long-range Rydberg-Rydberg interactions between Rydberg atoms induce ℓ-mixing which in certain situations gives rise to molecular resonances. For these resonances, we calculate long-range potentials in Hund's case (c) by diagonalization of an interaction matrix. The excitation dynamics of the resonances is always modeled as pair excitation. At high principal quantum number n, the interactions between Rydberg atoms can blockade the excitation of many surrounding atoms in the range of few μm. The atoms within this range are strongly correlated so that many-body treatments are, in general, needed. To describe this blockading effect and other manifestations of the collective behavior of Rydberg excitation, such as the many-body Rabi oscillations, spatial correlations between atoms and the fluctuations of the number of excited atoms, I have used three different approaches. In the mean-field approach the interactions between different atoms is modeled by a distribution of mean-field shifts for which a distribution of probabilities is calculated, A good agreement between the theoretical model and experimental measurements is found. To study the correlations between atoms the many-body wavefunction is numerically computed. The possibility of observing the many-body Rabi oscillations of Rydberg excitation and other aspects of many-body dynamics is analyzed. Although strong interactions cannot be treated perturbatively other expansions may be feasible. I will show that the expansion in powers of the single atom Rabi frequency can be evaluated in the interaction and Heisenberg picture. The expansion is expected to behave well for arbitrary strong interactions. ^

Robin C ot 'e, Jovica Stanojevic

Alban Urvoy, Fabian Ripka, Igor Lesanovsky +4 more

The observation of strongly interacting many-body phenomena in atomic gases typically requires ultracold samples. Here we show that the strong interaction potentials between Rydberg atoms enable the observation of many-body effects in an atomic vapor, even at room temperature. We excite Rydberg atoms in cesium vapor and observe in real time an out-of-equilibrium excitation dynamics that is consistent with an aggregation mechanism. The experimental observations show qualitative and quantitative agreement with a microscopic theoretical model. Numerical simulations reveal that the strongly correlated growth of the emerging aggregates is reminiscent of soft-matter type systems.

C. S. Hofmann

This thesis experimentally explores the application of strongly interacting states of Rydberg atoms as means to introduce and control correlations in many-body systems. A new apparatus has been built, which combines high atomic densities possible using optical dipole traps with the ability to detect and probe Rydberg atoms via both field ionization detection and electromagnetically induced transparency. In the first study, the interaction induced excitation of Rydberg atom pairs ('antiblockade') is demonstrated using detuned laser excitation. This provides \nthe possibility to engineer spatial correlations and to control interactions between Rydberg atoms. We then investigate the effects of Rydberg-Rydberg interactions \nand correlations on the spontaneous evolution of the system to an ultracold plasma. The initial correlations between Rydberg atoms should be preserved, suggesting a new route to overcome disorder-induced heating and to enter new strongly coupled regimes. Finally the back-action of Rydberg-Rydberg interactions on propagating light fields is explored. Dissipative interactions between dark-state polaritons give rise to a strongly nonlinear optical response and sub-Poissonian statistics of polaritons \nwhich reflect the emergence of correlations in both the atomic and light fields. Combined, these experiments help elucidate the effects of Rydberg induced correlations on diverse physical systems involving atoms, ions and photons.

Jing Qian, Zhou Lü, Xing-Dong Zhao +1 more