Category: Publications

Signatures of self-organized criticality (SOC) have recently been observed in an ultracold atomic gas under continuous laser excitation to strongly interacting Rydberg states [S. Helmrich et al., Nature, 577, 481–486 (2020)]. This creates unique possibilities to study this intriguing dynamical phenomenon under controlled experimental conditions. Here we theoretically and experimentally examine the self-organizing dynamics of a driven ultracold gas and identify an unanticipated feedback mechanism originating from the interaction of the system with a thermal reservoir. Transport of particles from the flanks of the cloud toward the center compensates avalanche-induced atom loss. This mechanism sustains an extended critical region in the trap center for timescales much longer than the initial self-organization dynamics. The characteristic flattop density profile provides an additional experimental signature for SOC while simultaneously enabling studies of SOC under almost homogeneous conditions. We present a hydrodynamic description for the reorganization of the atom density, which very accurately describes the experimentally observed features on intermediate and long timescales, and which is applicable to both collisional hydrodynamic and chaotic ballistic regimes. K. Klocke, T. M. Wintermantel, G. Lochead, S. Whitlock, and M. BuchholdHydrodynamic Stabilization of Self-Organized Criticality in a Driven Rydberg GasPhys. Rev. Lett. 126, 123401 (2021)

We have discovered a striking correspondence between the excitation dynamics of a laser driven gas of Rydberg atoms and the spreading of diseases, which in turn opens up a controllable platform for studying non-equilibrium dynamics on complex networks. We show that the competition between facilitated excitation and spontaneous decay of Rydberg excitations results in sub-exponential growth of the excitation number, which is empirically observed in real epidemics. The observed dynamics follow a power-law time dependence that parallels that which is empirically observed in real-world epidemics, providing a powerful demonstration of universality reaching beyond physics; (ii) a full description and interpretation of the experiment in terms of an emergent susceptible-infected-susceptible network linking the observed macroscopic dynamics to the microscopic physics; and (iii) the unexpected presence of rare region effects and a dynamical Griffiths phase associated to the emergent network structure, which gives rise to critical dynamics over an extended parameter regime and explains the appearance of power-law growth and relaxation, but with non-universal exponents. T. M. Wintermantel, M. Buchhold, S. Shevate et al. Epidemic growth and Griffiths effects on an emergent network of excited atoms. Nat. Commun. 12, 103 (2021).