There are a couple nice old PRLs from the Packard Group at Berkeley demonstrating the existence of quantized vortices and the vortex pattern in superfluid 4He. See here (pdf!) and here (paywall). Just as a little bit of background for those who are unfamiliar: below 2.17K helium undergoes a liquid to liquid phase transition from a “normal” liquid to a superfluid. The superfluid is characterized by several properties including zero viscosity (similar to electrons in a superconductor), second sound (effectively, temperature waves), and quantized vorticity.
The observation of this latter property was captured vividly in the series of images taken by the Packard Group shown below. To induce vortex formation, the authors rotated the bucket in which the superfluid had been placed. Because of the zero viscosity, the superfluid remained still until a critical velocity was reached where a single vortex formed in the center of the bucket. As the angular velocity was increased, more and more vortices started to form and the authors show the pattern formed by these vortices in the presence of 1-11 vortices.
Interestingly, there are two different stable configurations for which 6 vortices can form as shown in the figure. I happen to know that Richard Feynman, who had done a lot of the prior theoretical work on vortices in superfluid 4He, sent a personal letter to the authors of these papers to thank them for their elegant experiment.