Observation of the ideal Josephson effect in superfluid 4He

Superfluids and superconductors are the only states of condensed matter that can be described by a single wavefunction, with a coherent quantum phase Phi. The mass flow in a superfluid can be described by classical hydrodynamics for small flow velocity, but above a critical velocity, quantized vortices are created and the classical picture breaks down. This can be observed for a superfluid flowing through a microscopic aperture when the mass flow is measured as a function of the phase difference across the aperture; the curve resembles a hysteretic sawtooth where each jump corresponds to the creation of a vortex(1-3). When the aperture is made small enough, the system can enter the so-called 'ideal' Josephson regime(1,4), where the superfluid mass flow becomes a continuous function of the phase difference. This regime has been detected(1,5,6) in superfluid He-3, but was hitherto believed to be unobservable, owing to fluctuations(7), in He-4. Here we report the observation of the ideal Josephson effect in He-4. We study the flow of He-4 through an array of micro-apertures and observe a transition to the ideal Josephson regime as the temperature is increased towards the superfluid transition temperature, T-lambda.