Superfluid helium-4 whistles just the right tune

University of California, Berkeley, physicists can now tune in to and hear normally inaudible quantum vibrations, called quantum whistles, enabling them to build very sensitive detectors of rotation or very precise gyroscopes.
A quantum whistle is a peculiar characteristic of supercold condensed fluids, in this case superfluid helium-4, which vibrate when you try to push them through a tiny hole. Richard Packard, professor of physics at UC Berkeley, and graduate student Emile Hoskinson knew that many other researchers had failed to produce a quantum whistle by pushing helium-4 through a tiny aperture, which must be no bigger than a few tens of nanometers across - the size of the smallest viruses and about 1,000 times smaller than the diameter of a human hair.