Liquid helium-4 becomes superfluid when it is cooled to temperatures below 2.176 Kelvin. However, theory predicts that it should also be possible to observe superfluid behaviour in solid helium-4 under certain conditions.

One way to observe superfluidity in solid helium is to measure the resonant period of a sample of the material in a piece of apparatus called a torsional oscillator. This period depends on the moment of inertia of the sample, and this moment changes when helium enters the superfluid state.

Kim and Chan measured a total of 17 samples of solid helium-4 at pressures between 26 and 66 bars and found that they all became superfluid at temperatures below 230 millikelvin. "Our experiment shows that the superfluid-like behaviour is a general and intrinsic property of solid helium," they write, "and not the result of confinement in any particular medium."

However, many of the details of the experiment are not yet understood. In an accompanying article Tony Leggett of the University of Illinois in Urbana writes that the experiment "will force theorists to revise dramatically the generally accepted picture of crystalline solid helium-4."