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 Φ. 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 vortex1,2,3. When the aperture is made small enough, the system can enter the so-called ‘ideal’ Josephson regime1,4, where the superfluid mass flow becomes a continuous function of the phase difference. This regime has been detected1,5,6 in superfluid 3He, but was hitherto believed to be unobservable, owing to fluctuations7, in 4He. Here we report the observation of the ideal Josephson effect in 4He. We study the flow of 4He 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, Tλ.