Experiments on Turbulence in the Pure Quantum Limit

As Richard Feynman stated it Turbulence is the last great unsolved problem of classical physics . Despite being observed at all length scales from the subnuclear to the cosmological, turbulence is one of the least understood phenomena. Turbulent motion impacts on the behaviour of cytoplasm in cells, on the atmosphere, on the oceans, aviation, hydraulics, industrial processing, even on the simple process of water running out of the bath. Despite its ubiquity, physicists have great difficulty in describing and studying the constantly changing mix of eddies that constitute turbulent flow. We see turbulence everywhere but our understanding of it is still very rudimentary. This proposal aims to further our understanding of turbulence in general by addressing the simpler problem of turbulence in pure quantum fluids. We approach the elucidation of turbulence by starting with the most ideal simple model system possible, a superfluid. Although superfluids are widely known for their ability to flow without dissipation, they still form turbulent flow patterns when sufficiently agitated. In a superfluid, the atoms are constrained to move according to the dictates of quantum mechanics since in the superfluid component all the constituent atoms are in the same quantum mechanical state. The crucial point here is that while vortices in a conventional fluid have infinite variability, in a superfluid the circulation is quantized and all the vortices are identical. Quantum turbulence is the sum total of a random tangle of these similar quantized vortex lines.Clearly, the key to investigating quantum turbulence is how to detect the vorticity and measure the distribution and evolution of a turbulent tangle. Despite requiring temperatures well below 1 mK, it turns out that superfluid 3He is most suitable for this purpose. At low enough temperatures, where the normal fluid component is negligible, we have essentially pure quantum turbulence with virtually no frictional dissipation. These are absolute ideal conditions for studying turbulence. The proposed research programmes aims to address the following problems. While we can create turbulence in superfluid 3He, we first need to determine the absolute line densities of the vortex tangle to gain a quantitative description to work with. Secondly, we want to investigate the decay processes of quantum turbulence to contrast the decay with that of classical turbulence, as well as that of high temperature superfluid turbulence. Thirdly, we need to understand the homogeneity of turbulence generated by a vibrating grid resonator, since turbulence decays in time but also can disperse in space and we need to distinguish between the two processes. Finally we want to measure the energy stored in turbulence by following decay of a quantum tangle in a black-body radiator. If we can achieve these goals we will be much further on the road of understanding quantum turbulence.