  When a system consisting of many interacting particles is set rotating, it
may form vortices. This is familiar to us from every-day life: you can observe
vortices while stirring your coffee or watching a hurricane. In the world of
quantum mechanics, famous examples of vortices are superconducting films and
rotating bosonic $^4$He or fermionic $^3$He liquids. Vortices are also observed
in rotating Bose-Einstein condensates in atomic traps and are predicted to
exist for paired fermionic atoms. Here we show that the rotation of trapped
particles with a repulsive interaction leads to a similar vortex formation,
regardless of whether the particles are bosons or (unpaired) fermions. The
exact, quantum mechanical many-particle wave function provides evidence that in
fact, the mechanism of this vortex formation is the same for boson and fermion
systems.
