Observation of scale invariance and universal quench dynamics of Townes soliton formation in two-dimensional Bose gases

We experimentally study universal non-equilibrium dynamics of two-dimensional (2D) Bose gases quenched from repulsive to attractive interactions. We observe the manifestation of a modulational instability which fragments a 2D sample into multiple wave packets universally around a threshold atom number necessary for the formation of a scale-invariant stationary state -- the Townes solitons. Our measurements reveal the early coherent dynamics of modulational instability, the formation of Townes solitons, and the subsequent collision and collapse dynamics, demonstrating multiple universal behaviors. Furthermore, we test the scale invariance in 2D solitons by observing the collapse of soliton density profiles of different sizes and peak densities onto a single curve in a rescaled, dimensionless coordinate. We confirm that the scale-invariant profiles measured at different atomic interactions can further collapse onto the universal profile of Townes solitons. Our experiment demonstrates remarkable manifestation of 2D scale invariance and universal quench dynamics in the formation of a many-body quasi-stationary state.