Observation of Singly Quantized Dissipation Events Obeying the Josephson Frequency Relation in the Critical Flow of Superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ through an Aperture

We have observed highly reproducible isolated dissipation events in the flow of pure superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ through an aperture of submicronic size at 10 mK, when the flow velocity just exceeds a well-defined critical threshold in agreement with Feynman's criterion for vortex emission. These events, which we interpret as phase slippages by $2\pi$ in the sense of Anderson, occur at a rate given by the Josephson frequency relation.