Observation of the ideal Josephson effect in superfluid ⁴He

Abstract

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 vortex^1,2,3. 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 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, T_λ.