1. Laser Centre Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands
2. Laboratoire Charles Fabry de l'Institut d'Optique, CNRS, Univ. Paris-sud, Campus Polytechnique RD 128, 91127 Palaiseau Cedex, France

Correspondence to: W. Vassen¹C. I. Westbrook² Correspondence and requests for materials should be addressed to C.I.W. (Email: christoph.westbrook@institutoptique.fr) or W.V. (Email: w.vassen@few.vu.nl).

Abstract

Fifty years ago, Hanbury Brown and Twiss (HBT) discovered photon bunching in light emitted by a chaotic source¹, highlighting the importance of two-photon correlations² and stimulating the development of modern quantum optics³. 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: ³He (a fermion) and ⁴He (a boson). Ordinary attractive or repulsive interactions between atoms are negligible; therefore, the contrasting bunching and antibunching behaviour that we observe can be fully attributed to the different quantum statistics of each atomic species. Our results show how atom–atom correlation measurements can be used to reveal details in the spatial density^{7, 8} or momentum correlations⁹ in an atomic ensemble. They also enable the direct observation of phase effects linked to the quantum statistics of a many-body system, which may facilitate the study of more exotic situations¹⁰.

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