A stretched string sounds: the stronger the tension, the faster the sound travels. In a superfluid Bose-Einstein condensate, the role of tension is played by a special elasticity from the coherent motion of particles. When heated in a two-dimensional layer, vortices appear — knots on the string. They weaken the elasticity, and sound slows down. When there are many vortices, the string sags, and sound vanishes.

In superconductors, such vortex fluctuations are noticeable over a range of millionths of a degree, but in exciton bilayers, they appear over full degrees. A gift for experimentalists. Calculations with quantum field theory and simulations confirm: in double graphene, sound falls silent at temperatures ten degrees above absolute zero. This can be detected by spectroscopy (light scattering) and interferometry (wave superposition).

Back in the mid-20th century, Richard Feynman, Lev Landau, and John Bardeen linked superfluidity with vortices and the phase of the quantum state. The new work gives this direct, measurable confirmation.