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...

Abstract. We present measured scattering matrices as functions of the scattering angle in the range 5ø-173 ø and at wavelengths of 441.6 nra and 632.8 nra for seven distinct irregularly shaped mineral aerosol samples with properties representative of mineral aerosols present in the Earth's atmosphere. The aerosol samples, i.e., feldspar, red clay, quartz, loess, Pinatubo and Lokon volcanic ash, and Sahara sand, represent a wide variety of particle size (typical diameters between 0.1 and 100 pra) and composition (mainly silicates). We investigate the effects of differences in size and complex refractive index on the light-scattering properties of these irregular particles. In particular, we find that the measured scattering matrix elements when plotted as functions of the scattering angle are confined to rather limited domains. This similarity in scattering behavior justifies the construction of an average aerosol scattering matrix as a function of scattering angle to facilitate, for example, the use of our results for the interpretation of remote sensing data. We show that results of ray-optics calculations, using Gaussian random shapes, are able to describe the experimental data well when taking into account the high irregularity in shape of the aerosols, even when these aerosols are rather small. Using the results of ray-optics calculations, we interpret the differences found between the measured aerosol scattering matrices in terms of differences in complex refractive index and particle size relative to the wavelength. The importance of our results for studies of astronomical objects, such as planets, comets, asteroids, and circumstellar dust shells is discussed.

Precision spectroscopy of simple atomic systems has refined our understanding of the fundamental laws of quantum physics. In particular, helium spectroscopy has played a crucial role in describing two-electron interactions, determining the fine-structure constant and extracting the size of the helium nucleus. Here we present a measurement of the doubly-forbidden 1557-nanometer transition connecting the two metastable states of helium (the lowest energy triplet state 2 3 S 1 and first excited singlet state 2 1 S 0 ), for which quantum electrodynamic and nuclear size effects are very strong. This transition is fourteen orders of magnitude weaker than the most predominantly measured transition in helium. Ultracold, sub-microkelvin, fermionic 3 He and bosonic 4 He atoms are used to obtain a precision of 8×10 −12 , providing a stringent test of two-electron quantum electrodynamic theory and of nuclear few-body theory.

With a phase-modulated extreme ultraviolet pulsed laser source the frequency of the 1 1 S-2 1 P transition of helium at 58 nm has been measured. The phase modulation scheme enabled measurement and reduction of frequency chirp, usually limiting pulsed precision spectroscopy. From the measured transition frequency of 5 130 495 083͑45͒ MHz, a fourfold improved value of the ground state Lamb shift of 41 224͑45͒ MHz is deduced, in good agreement with a theoretical value of 41 233͑35͒ MHz based on QED calculations up to order ␣ 5 Z 6 . From these measurements, the well-known binding energy of the 2 1 P state and the previously determined 4 He-3 He isotope shift, accurate values for the ionization energies of the helium atom follow: 198 310.6672 (15) cm Ϫ1 for 4 He and 198 301.8808(15) cm Ϫ1 for 3 He. ͓S1050-2947͑97͒05403-6͔ PACS number͑s͒: 32.30.Jc, 12.20.Fv, 42.65.Ky

We review experimental work on cold, trapped metastable noble gases. We emphasize the aspects which distinguish work with these atoms from the large body of work on cold, trapped atoms in general. These aspects include detection techniques and collision processes unique to metastable atoms. We describe several experiments exploiting these unique features in fields including atom optics and statistical physics. We also discuss precision measurements on these atoms including fine structure splittings, isotope shifts, and atomic lifetimes.