It may not sound as pretty as a harp, but a new instrument that picks up on vibrations in liquid helium is setting the laws of quantum mechanics to music. On a more practical note, the sounds emitted by the device, described this week at the Acoustical Society of America's annual meeting in State College, Pennsylvania, may provide a more precise method for defining pressure.
When chilled to near absolute zero, helium-4 becomes a "superfluid." Quantum mechanics forces it to stop acting like an ordinary liquid, endowing it with such outlandish properties as a complete lack of viscosity. Superfluid helium, for example, readily oozes up and over the side of containers.
Hoping to exploit this weird phenomenon, physicists Scott Backhaus and Richard Packard of the University of California, Berkeley, engraved in a silicon wafer a millimeterwide channel for helium to flow through. At one end, the channel was capped with a wall that had a 100-nanometer-wide hole, creating a bottleneck. Just as forcing air through a small hole in your lips produces a whistle, helium squeezed through the bottleneck whistles. "If you listen, you should hear the thing ringing," says Packard. Sure enough, when tuned to the proper frequency--a few thousand hertz--the pipe acts as a resonating cavity and seems to pick up the sound, just as an organ's pipe projects a tone.
While ordinary musical notes waver a little, superhelium's sounds stem from quantum vortices, in which energy is shed in identically sized packets: Thus, the researchers could generate exactly the same tone every time they forced the helium through the bottleneck under the same pressure. An instrument with perfect pitch, in theory, could be used to define pressure standards, just as an orchestra tunes to an oboe. If temperature fluctuations that appear to spur extra vortices could be ironed out of the system, says Packard, "you could define a pascal as the pressure which causes a frequency of so many hertz across [an aperture]." A pressure standard set by superfluid helium would be analogous to the voltage standard set by a quantum-mechanical circuit called a Josephson junction, says Stanford physicist Francis Everitt. "It's a very interesting, cute idea--one I've never thought of," he says.