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 fermion and the boson HBT effects realised in the same apparatus with two different isotopes of helium, 3 He (a fermion) and 4 He (a boson). Ordinary attractive or repulsive interactions between atoms are negligible, and the contrasting bunching and antibunching behaviours can be fully attributed to the different quantum statistics. Our result shows how atom-atom correlation measurements can be used not only for revealing details in the spatial density 7,8 or momentum correlations 9 in an atomic ensemble, but also to directly observe phase 2 effects linked to the quantum statistics in a many body system. It may thus find applications to study more exotic situations 10 .Two-particle correlation analysis is an increasingly important method for studying complex quantum phases of ultracold atoms 7,8,9,10,11,12,13 . It goes back to the discovery by Hanbury Brown and Twiss 1 , that photons emitted by a chaotic (incoherent) light source tend to be bunched: the joint detection probability is enhanced, compared to that of statistically independent particles, when the two detectors are close together.Although the effect is easily understood in the context of classical wave optics 14 , it took some time to find a clear quantum interpretation 3,15 . The explanation relies upon interference between the quantum amplitude for two particles, emitted from two source points S 1 and S 2 , to be detected at two detection points D 1 and D 2 (see fig. 1). For bosons, the two amplitudes D S D S must be added, which yields a factor of 2 excess in the joint detection probability, if the two amplitudes have the same phase. The sum over all pairs (S 1 ,S 2 ) of source points washes out the interference, unless the distance between the detectors is small enough that the phase difference between the amplitudes is less than one radian, or equivalently if the two detectors are separated by a distance less than the coherence length. Study of the joint detection rates vs. detector separation along the i-direction then reveals a bump whose width l i is the coherence length along that axis 1,5,16,17,18,19 . For a source size s i along i (standard half width at e -1/2 of a Gaussian density profile), one has a half width at 1/e of l i = ht / 2πms i , where m is the mass of the particle, t the time of flight from the source to the detector, and h Planck's constant. This formula is the analogue of the formula l i = Lλ / 2πs i for photons if one identifies λ = h / mv with the de Broglie wavelength for particles travelling at velocity v = L / t from the source to the detector.For indistinguishable fermions, the two-body wave function is antisymmetric, and the two amplitudes must be subtracted, yielding a null probability for joint detection in the same coherence volume. In the language of particles, it means th...

We report the observation of simultaneous quantum degeneracy in a dilute gaseous Bose-Fermi mixture of metastable atoms. Sympathetic cooling of helium-3 (fermion) by helium-4 (boson), both in the lowest triplet state, allows us to produce ensembles containing more than 10(6) atoms of each isotope at temperatures below 1 microK, and achieve a fermionic degeneracy parameter of T/TF = 0.45. Because of their high internal energy, the detection of individual metastable atoms with subnanosecond time resolution is possible, permitting the study of bosonic and fermionic quantum gases with unprecedented precision. This may lead to metastable helium becoming the mainstay of quantum atom optics.

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We describe a powerful scheme which combines laser cooling on two transitions of metastable helium to obtain a high phase-space density. By running a sequence of a large 1083 nm magneto-optical trap (MOT) and a compressed 389 nm MOT, a density increase of more than one order of magnitude is achieved within 5 ms. After compression, 8 ϫ 10 8 atoms at a central density of 5 ϫ 10 10 cm −3 remain, while the temperature of the cloud has been reduced from 1 mK to 0.4 mK. The resulting phase-space density ͑4.1ϫ 10 −6 ͒ is more than one order of magnitude higher than what we achieved by 1083 nm laser cooling only.

We have studied a cloud of cold metastable helium (He*) atoms interacting with near-resonant light at 1083 nm and 389 nm. The 1083 nm light allows for efficient loading of a large magneto-optical trap (MOT) and the 389 nm light is subsequently used to increase the density and reduce the temperature of the He* cloud during a brief compression stage. Cold collisions in the cloud yield ions and fast metastables which are monitored separately using calibrated microchannel plate (MCP) detectors. We thus measure absolute production rates of ions and fast metastables escaping from the MOT during the various stages of the experiment. We observe that 389 nm optical collisions, apart from Penning ionization, produce a relatively large flux of fast metastables, which we relate to the short-range behaviour of the molecular potentials involved. Furthermore, by rapidly switching between 389 nm and 1083 nm the ratio between the respective two-body loss rate constants is determined. Using these values, together with the observed time dependence of the cloud size, the temporal behaviour of the absolute ion production rate during the compression stage is well reproduced.