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. A stream of bosons tends to bunch together, whereas fermions avoid each other. Although evidence for each type of behaviour has been observed in various settings, no single experiment has been able to directly compare the two types of quantum statistics until now, where this paper reveals the contrasting behaviour of 3 He (a fermion) and 4 He (a boson) in the same apparatus. Fifty years ago, Hanbury Brown and Twiss (HBT) discovered photon bunching in light emitted by a chaotic source 1 , highlighting the importance of two-photon correlations 2 and stimulating the development of modern quantum optics 3 . The quantum interpretation of bunching relies on the constructive interference between amplitudes involving two indistinguishable photons, and its additive character is intimately linked to the Bose nature of photons. Advances in atom cooling and detection have led to the observation and full characterization of the atomic analogue of the HBT effect with bosonic atoms 4 , 5 , 6 . By contrast, fermions should reveal an antibunching effect (a tendency to avoid each other). Antibunching of fermions is associated with destructive two-particle interference, and is related to the Pauli principle forbidding more than one identical fermion to occupy the same quantum state. Here we report an experimental comparison of the fermionic and bosonic HBT effects in the same apparatus, using two different isotopes of helium: 3 He (a fermion) and 4 He (a boson). Ord