Quantum Oscillations Ring Out Loud and Clear

Abstract

At the Symposium on Quantum Fluids and Solids in Paris this past July, Richard Packard of the University of California, Berkeley, entertained the audience by playing a brief recording: The sound on the tape began as a high‐pitched whistle and slid down the frequency scale over a period of a few seconds. What people were hearing was the sound of a superfluid surging rapidly back and forth through the holes in a membrane1 in response to a pressure difference applied across the membrane. This phenomenon is the superfluid analog of the AC Josephson effect for superconductors, according to which a supercurrent will oscillate across a thin tunnel junction under an applied voltage. (A French group had earlier reported evidence of this phenomenon.) In addition to recording the sounds of the mass oscillations, Packard and Seamus Davis and their respective groups at Berkeley found that the measured frequencies agreed with those predicted by the Josephson equations.