Figure 1
Illustration: Alan Stonebraker

Figure 1: (Left) In 1946, Andronikashvili carried out a classic experiment on superfluid liquid helium by measuring the period and damping rate of a torsional oscillator consisting of a stack of closely spaced cylindrical plates hanging by an elastic thread and immersed in the liquid helium. The normal component of the fluid was dragged along by the rotating disk surfaces, while the superfluid took no part in the rotational motion, which allowed measurement of the fraction of the superfluid component as a function of temperature (bottom panel). (Right) Cooper and Hadzibabic [2] propose an optical method to measure the superfluid fraction in ultracold atomic gases. The pair of copropagating laser beams with different orbital angular momenta create an azimuthal vector potential that imparts angular momentum to the normal component but leaves the superfluid one at rest. Spectroscopic analysis permits measurement of the induced angular momentum and hence the amount of normal vs superfluid gas.