Interference with atomic and molecular matter waves is a rich branch of atomic physics and quantum optics. It started with atom diffraction from crystal surfaces and the separated oscillatory fields technique used in atomic clocks. Atom interferometry is now reaching maturity as a powerful art with many applications in modern science. In this review the basic tools for coherent atom optics are described including diffraction by nanostructures and laser light, three-grating interferometers, and double wells on atom chips. Scientific advances in a broad range of fields that have resulted from the application of atom interferometers are reviewed. These are grouped in three categories: ͑i͒ fundamental quantum science, ͑ii͒ precision metrology, and ͑iii͒ atomic and molecular physics. Although some experiments with Bose-Einstein condensates are included, the focus of the review is on linear matter wave optics, i.e., phenomena where each single atom interferes with itself.

Properties of a Bose-Einstein condensate were studied by stimulated, two-photon Bragg scattering. The high momentum and energy resolution of this method allowed a spectroscopic measurement of the mean-field energy and of the intrinsic momentum uncertainty of the condensate. The coherence length of the condensate was shown to be equal to its size. Bragg spectroscopy can be used to determine the dynamic structure factor over a wide range of energy and momentum transfers.[S0031-9007(99)

We report the confinement and cooling of an optically dense cloud of neutral sodium atoms by radiation pressure. The trapping and damping forces were provided by three retroreflected laser beams propagating along orthogonal axes, with a weak magnetic field used to distinguish between the beams. We have trapped as many as 10 7 atoms for 2 min at densities exceeding lOll atoms cm-3. The trap was =0.4 K deep and the atoms, once trapped, were cooled to less than a millikelvin and compacted into a region less than 0.5 mm in diameter.

Rayleigh scattering off a Bose-Einstein condensate was studied. Exposing an elongated condensate to a single off-resonant laser beam resulted in the observation of highly directional scattering of light and atoms. This collective light scattering is caused by the coherent center-of-mass motion of the atoms in the condensate. A directional beam of recoiling atoms was built up by matter wave amplification.

Magnetism is a macroscopic phenomenon with its origin deeply rooted in quantum mechanics. In condensed matter physics, there are two paradigms for magnetism: localized spins interacting via tunnelling, and delocalized spins interacting via an exchange energy. The latter gives rise to itinerant ferromagnetism which is responsible for the properties of transition metals 1