Superflow in Solid Helium

The small mass and significant zero point motion of solid He make it very different from its conventional counterparts. In the seventies it was suggested that at very low temperatures solid helium might flow like a superfluid while remaining a crystal. This hidden “supersolid” phase was subsequently almost ruled out both by experiments and theoretical calculations. However, in 2004 Kim and Chan reported that when rotated at temperatures below 230mK a fraction of solid He does indeed refuse to be dragged around by its walls, a famous effect hitherto only seen in its superfluid counterpart. In this paper, I will describe the experiment, its consequences and possible theoretical explanation.