B2
Dissipative dynamics of strongly interacting, optically driven Rydberg gases
Thomas Niederprüm, Herwig Ott & Michael Fleischhauer
The joint experimental and theoretical project B2 focuses on open manybody systems with a particular emphasis on strongly interacting systems. A highly controllable physical platform to study systems with inherently open character and strong longrange interactions is comprised by ultracold Rydberg gases. Due to their high polarizability, Rydberg atoms exhibit strong longrange interactions which are accompanied by inherent dissipative processes, such as spontaneous decay, ionization and motional decoherence. Adding a continous drive, the interplay between longrange interaction and intrinsic dissipation leads to nontrivial spatiotemporal correlations and complex collective phenomena in the nonequilibrium steadystate.
Facilitation and Selforganized Criticality As such, an offresonantly driven Rydberg system shows socalled facilitation where the Rydberg interaction leads to cooparativelyenhaned excitation of two or more Rydberg atoms, ultimately leading to the creation of excitation clusters. The competition of the interactionenhanced drive with the inherent decay of the involved Rydberg states leads to critical behavior reminiscent of the directed percolation problem. Due to additional particle loss, the Rydberg system can selforganize to the critical point. Project B2 aims to investigate the criticality of the system as well as the selforganization process by analyzing the temporal excitation correlations. Microscopic Facilitation Process Universal behavior emerges, when the details of the underlying microscopic processes are not relevant for the global system dynamics. However, given a specifc process, it is not a priori clear, what renders the system into a situation where these details do not matter any more. Therefore, project B2 aims to study the microscopic processes underlying facilitation. A special emphasis is put on the question how motion and coherence in the microscopic dynamics influence the system dynamics on a larger scale. 

Figure 2: The resonant dipoledipole interaction between Rydberg states of opposite partiy allows to study discrete quantum XY spin models. 
Dissipative Spin Models with Spin Transport Extending its view, project B2 complements the percolationtype transport studied through the faciliation process by looking into transport processes in discrete spin systems in the presence of dephasing and disorder. Such a system is readily available in ultracold Rydberg systems since XY spin models are realized by the resonant dipoledipole interaction between Rydberg states of opposite parity (e.g. Rydberg S and Pstate). Motivated by the advent of optical tweezer arrays as an experimental platform, the project will study how dephasing, local loss and gain, as well as disorder alter the transport in the spin system. 