A quantum fluid pipes up.

Helium never ceases to amaze. Physicists have now heard a quantum-mechanical whistle emanating from two reservoirs of liquid helium-4 that were separated by a perforated membrane.

According to theory, when liquid helium is pushed through a tiny hole at ultracold temperatures, the substance oscillates at a frequency that, when amplified electronically, sounds like a whistle.

Other than confirming a quantum talent first predicted for helium-4 some 40 years ago, the new findings by Emile M. Hoskinson of the University of California, Berkeley and his colleagues may lead to ultra-precise devices that sense helium flow through a ring of punctured membranes to measure an object's rotation. Those devices could prove useful for such tasks as measuring earthquakes and Earth's spin, the scientists report in the Jan. 27 Nature.

In prior, unsuccessful attempts by other teams to detect helium-4's whistle, scientists used membranes with single holes. The membrane in the new experiment was riddled with more than 4,000 holes.

In 1997, scientists in the same Berkeley lab, which is headed by Richard E. Packard, heard a whistle from reservoirs of helium-3 separated by a sieve like membrane (SN: 8/2/97, p. 69). Helium-3 is the rarer of helium's two isotopes.

Helium's melodic oscillations are a quantum behavior that shows up at temperatures so cold that the material is a superfluid--a liquid that flows without friction. Because helium-3 attains that state at a lower temperature than helium-4 does, researchers surmised that there are too few thermal fluctuations to stifle its musical expressions, Hoskinson explains. Now, he adds, it seems that having an array of holes between reservoirs also helps helium sing out.--P.W.