From their data, the researchers observed clear resonances through the fluid, particularly at low frequencies. From the distribution of these resonances, they calculated the fluid’s sound diffusion. This value, they found, could also be calculated very simply via Planck’s constant and the mass of the average fermion in the gas.
This told the researchers that the gas was a perfect fluid, and fundamental in nature: Its sound diffusion, and therefore its viscosity, was at the lowest possible limit set by quantum mechanics.