Double-quantum vortex in superfluid 3He-A

Linear defects are generic in continuous media 1 . In quantum systems they appear as topological line defects which are associated with a circulating persistent current. In relativistic quantum field theories they are known as cosmic strings 2 , in superconductors as quantized flux lines 3 , and in superfluids 3 , 4 and low-density Bose–Einstein condensates 5 as quantized vortex lines. A conventional quantized vortex line consists of a central core around which the phase of the order parameter winds by 2π n , while within the core the order parameter vanishes or is depleted from the bulk value. Usually vortices are singly quantized (that is, have n = 1). But it has been theoretically predicted that, in superfluid 3 He-A, vortex lines are possible that have n = 2 and continuous structure, so that the orientation of the multi-component order parameter changes smoothly throughout the vortex while the amplitude remains constant. Here we report direct proof, based on high-resolution nuclear magnetic resonance measurements, that the most common vortex line in 3 He-A has n = 2. One vortex line after another is observed to form in a regular periodic process, similar to a phase-slip in the Josephson effect.