Comparison of the Hanbury Brown–Twiss effect for bosons and fermions

A tale of two heliums Helium-3 is a fermion, a particle, like protons, electrons and neutrons, obeying statistical rules requiring that not more than one in a set of identical particles may occupy a particular quantum state. Fermions avoid one another (a phenomenon called anti-bunching). Helium-4, though, is a boson. Bosons, like photons, pions and alpha particles, stick together and obey statistical rules that allow any number of identical particles to occupy a quantum state. Evidence for both types of quantum statistical behaviour has been observed separately, but until now no single experiment has compared the two directly. By exploiting the physical similarities of the two heliums, a team from Vrije Universiteit Amsterdam and Laboratoire Charles Fabry in Paris has succeeded in demonstrating bunching and anti-bunching behaviour of atoms in a single experiment. This is a spectacular demonstration of the role of quantum statistical effects, and could also lead to some exotic new areas of physics with cold atoms.