Many body systems generally become more disordered, as more energy is pumped into them. A curious exception to this rule was predicted in connection with the turbulent flow of the physical chemist Lars Onsager. He suggested that the entropy of certain two-dimensional (2D) systems may decrease with increasing energy corresponding to an effective negative temperature. By using 2D Bose-Einstein condensates of atoms, Gauthier et al. and Johnstone et al. put Onsager's theory to the test. They gave energy to the system by disturbing the condensate, creating vortices and antivortice. With increasing energy, the system became more organized as clusters containing only vertebrae or only antivortice.
Science this problem p. 1
No-vigibrium interacting systems can evolve to exhibit large-scale structure and order. In two-dimensional turbulent flow, the seemingly random swirling motion of a fluid can develop into sustained large-scale vortices. To explain such behavior, Lars Onsager proposed a statistical hydrodynamic model based on quantized vertebrae. Here we report on the experimental confirmation of the Onsager model. We dragged a grid barrier through a wafer superfluid Bose-Einstein condensate to create equilibrium distribution of vertebrae. We observed the signatures of an inverse energy cascade driven by evaporation heating of vertebrae, which led to steady state configurations characterized by negative absolute temperatures. Our results open the way for quantitative studies of emergent structures in interactive quantum systems that are driven far from equilibrium.