Quantum oscillations between two weakly coupled reservoirs of superfluid ³He

Arguments first proposed over thirty years ago, based on fundamental quantum-mechanical principles, led to the prediction that if macroscopic quantum systems are weakly coupled together, particle currents should oscillate between the two systems. The conditions for these quantum oscillations to occur are that the two systems must both have a well defined quantum phase, φ, and a different average energy per particle, μ: the term 'weakly coupled' means that the wavefunctions describing the systems must overlap slightly. The frequency of the resulting oscillations is then given by f=(μ₂ - μ₁)/h, where h is Planck's constant. To date, the only observed example of this phenomenon is the oscillation of electric current between two superconductors coupled by a Josephson tunnelling weak link. Here we report the observation of oscillating mass currents between two reservoirs of superfluid ³He, the weak link being provided by an array of submicrometre apertures in a membrane separating the reservoirs. An applied pressure difference creates mass-current oscillations, which are detected as sound in a nearby microphone. The sound frequency (typically 6,000-200 Hz) is precisely proportional to the applied pressure difference, in accordance with the above equation. These superfluid quantum oscillations were first detected while monitoring an amplified microphone signal with the human ear.