  Matter at low temperatures exhibits unusual properties such as superfluidity,
superconductivity, Bose-Einstein condensation, and supersolidity. These states
display quantum mechanical behaviours at scales much larger than atomic
dimensions. As in many phase transitions, defects can occur during the
transition to the low temperature state. The study of these defects yields
useful information about the nature of the transitions and of the macroscopic
states themselves. When cooled below the lambda temperature (~2.172 K), liquid
helium acquires superfluid properties, and the defects in the superfluid take
the form of line vortices with quantized circulation. The formation of these
vortices has been suggested as a model for cosmological structure formation.
Here we visualize these superfluid (or quantized) vortices by suspending
micron-sized solid particles of hydrogen in the fluid. The particles at low
concentrations arrange themselves with nearly equal spacing along vortex lines.
The number density of the vortex lines in a steadily rotating state compares
well with Feynman's prediction. For the first time, it is possible to observe
superfluid turbulence in which the superfluid vortices interact, and exist in
networks with complex branching. Our method makes it possible to characterize
the vortices during the transition across the lambda point and in the turbulent
state itself.
