Detection of Quantized Vortices using Fourth Sound Attenuation

Superfluid helium confined to nanofluidic systems is emerging as an important system for studies of two-dimensional turbulence and as the basis for novel quantum technologies. In fully enclosed nanofluidic geometries only the fourth sound can propagate, which we show can be used for probing quantized vortices pinned in well-defined slab geometry. We show that similarly to well-established second sound attenuation, fourth sound attenuation can be used to infer the number of quantized vortices in a unit area. We experimentally verify fourth sound attenuation as a probe of vortex line density by injecting a known number of quantized vortices into a nanofluidic slab using a rotating cryostat.