Below 2.176K, liquid 4He enters into a superfluid state and flows without any friction. The onset of superfluidity is associated with Bose-Einstein Condensation where 4He atoms, which are bosons, condense into a single momentum state and acquire quantum mechanical coherence over a macroscopic length scale. In 1972, superfluidity in liquid 3He through p-wave pairing was discovered below 2mK. Bose-Einstein condensation in alkali atoms in the vapor phase was achieved in 1995 and there is strong evidence for superfluidity in the system. Perhaps counter to intuition, superfluid-like behavior is thought possible even in solid 4He. First experimental confirmation of this supersolid state has not been achieved until the experiments of Kim and Chan. [1] Despite many efforts to uncover the mechanism of supersolidity since the first observation of non-classical rotational inertia in solid helium, the microscopic origin and physical interpretation of the observed phenomena is still a subject of considerable intrigue. In this talk, I will review and correlate recent experimental results to provide an up-to-date picture of what we do and still do not understand. I will also take the opportunity to highlight the recent progress in solid helium experiment under rotation.

1. E. Kim and M. H. W. Chan, Nature 427, 225 (2004)