U ltracold gases in optical lattices are of great interest, because these systems bear great potential for applications in quantum simulations and quantum information processing, in particular when using particles with a longrange dipole-dipole interaction, such as polar molecules 1-5 . Here we show the preparation of a quantum state with exactly one molecule at each site of an optical lattice. The molecules are produced from an atomic Mott insulator 6 with a density profile chosen such that the central region of the gas contains two atoms per lattice site. A Feshbach resonance is used to associate the atom pairs to molecules 7-14 . The remaining atoms can be removed with blast light 13,15 . The technique does not rely on the moleculemolecule interaction properties and is therefore applicable to many systems.A variety of interesting proposals for quantum information processing and quantum simulations 1-5 require as a prerequisite a quantum state of ultracold polar molecules in an optical lattice, where each lattice site is occupied by exactly one molecule. A promising strategy for the creation of such molecules is based on the association of ultracold atoms using a Feshbach resonance, or photoassociation and subsequent transfer to a much lower rovibrational level using Raman transitions 16 . If the moleculemolecule interactions are predominantly elastic and effectively repulsive, then a state with one molecule per lattice site can finally be obtained using a quantum phase transition from a superfluid to a Mott insulator by ramping up the depth of an optical lattice 6 . However, many molecular species do not have such convenient interaction properties, so alternative strategies are needed. Here, we demonstrate a technique that is independent of the molecule-molecule interaction properties. The technique relies on first forming an atomic Mott insulator and then associating molecules. Several previous experiments 15,17-20 associated molecules in an optical lattice, but none of them demonstrated the production of a quantum state with exactly one molecule per lattice site. Another interesting perspective of the state prepared here is that after Raman transitions to the rovibrational ground state, the lattice potential can be lowered to obtain a Bose-Einstein condensate (BEC) of molecules in the rovibrational ground state 21,22 . Figure 1 Schematic diagram of the molecular n = 1 state. In the core of the cloud, each lattice site is occupied by exactly n = 1 molecule (shown in green). In the surrounding shell, each site is occupied by exactly one atom (shown in red). The atoms can be removed with a blast laser. In the experiment, the number of occupied lattice sites is much larger than shown here.The behaviour of bosons in an optical lattice is described by the Bose-Hubbard hamiltonian 23 . The relevant parameters are the amplitude J for tunnelling between neighbouring lattice sites and the on-site interaction matrix element U. We create a Mott insulator 6 of atomic 87 Rb starting from an atomic BEC in an optical dipol...

We report on the trapping of long-lived strongly magnetized Rydberg atoms. 85Rb atoms are laser cooled and collected in a superconducting magnetic trap with a strong bias field (2.9 T) and laser excited to Rydberg states. Collisions scatter a small fraction of the Rydberg atoms into long-lived high-angular momentum "guiding-center" Rydberg states, which are magnetically trapped. The Rydberg atomic cloud is examined using a time-delayed, position-sensitive probe. We observe magnetic trapping of these Rydberg atoms for times up to 200 ms. Oscillations of the Rydberg-atom cloud in the trap reveal an average magnetic moment of the trapped Rydberg atoms of approximately -8microB. These results provide guidance for other Rydberg-atom trapping schemes and illuminate a possible route for trapping antihydrogen.

Laser cooling and magnetic trapping of (85)Rb atoms have been performed in extremely strong and tunable magnetic fields, extending these techniques to a new regime and setting the stage for a variety of cold atom and plasma experiments. Using a superconducting Ioffe-Pritchard trap and an optical molasses, 2.4 x 10(7) atoms were laser cooled to the Doppler limit and magnetically trapped at bias fields up to 2.9 T. At magnetic fields up to 6 T, 3 x 10(6) cold atoms were laser cooled in a pulsed loading scheme. These bias fields are well beyond an order of magnitude larger than those in previous experiments. Loading rates, molasses lifetimes, magnetic-trapping times, and temperatures were measured using photoionization and electron detection.

A simple, compact and inexpensive pressure-tuned Fabry-Pérot interferometer is presented. It is used as a laser locking reference for optical frequencies where the use of an atomic reference is impractical. The scanning range is several GHz. Absolute positioning of the interferometer with an accuracy of 7 MHz rms over a range of 2 GHz is possible. The instrument is temperature stabilized and shows long-term drift of 16 MHz rms over 48 h.

The electric-field-ionization and autoionization behavior of cold Rydberg atoms of 85Rb in magnetic fields up to 6 T is investigated. Multiple ionization potentials and field-ionization bands reflecting the Landau energy quantization of the quasifree Rydberg electron are observed. The time-resolved and state-selective field-ionization study provides evidence of mixing and spin flips of the Rydberg electron. Spin-orbit coupling combined with mixing gives rise to a Feshbach-type autoionization of metastable positive-energy atoms.