Throughout physics, stable composite objects are usually formed via attractive forces, which allow the constituents to lower their energy by binding together. Repulsive forces separate particles in free space. However, in a structured environment such as a periodic potential and in the absence of dissipation, stable composite objects can exist even for repulsive interactions. Here we report on the first observation of such an exotic bound state, comprised of a pair of ultracold atoms in an optical lattice. Consistent with our theoretical analysis, these repulsively bound pairs exhibit long lifetimes, even under collisions with one another. Signatures of the pairs are also recognised in the characteristic momentum distribution and through spectroscopic measurements. There is no analogue in traditional condensed matter systems of such repulsively bound pairs, due to the presence of strong decay channels. These results exemplify on a new level the strong correspondence between the optical lattice physics of ultracold bosonic atoms and the Bose-Hubbard model [1,2], a correspondence which is vital for future applications of these systems to the study of strongly correlated condensed matter systems and to quantum information.Cold atoms loaded into a 3D optical lattice provide a realisation of a quantum lattice gas [1,2]. An optical lattice can be generated by pairs of counterpropagating laser beams, where the resulting standing wave intensity pattern forms a periodic array of microtraps for the cold atoms, with period a given by half the wavelength of the light, λ /2. The periodicity of the potential gives rise to a bandstructure for the atom dynamics with Bloch bands separated by band gaps, which can be controlled via the laser parameters and beam configuration. The dynamics of ultracold atoms loaded into the lowest band of a sufficiently deep optical lattice is well described by the BoseHubbard model with Hamiltonian[1, 3]are destruction (creation) operators for the bosonic atoms at site i. J/h denotes the nearest neighbour tunnelling rate, U the on-site collisional energy shift, and ε i the background potential. The high degree of control available over the parameters in this system, e.g., changing the relative values of U and J by varying the lattice depth, V 0 , has led to seminal experiments on strongly correlated gases in optical lattices, e.g., the study of the superfluidMott insulator transition[4], the realisation of 1D quantum liquids with atomic gases [5,6] (see also [7, 8]), and the investigation of disordered systems [9]. 3D optical lattices have also opened new avenues in cold collision physics and chemistry [10,11,12,13].A striking prediction of the Bose-Hubbard Hamiltonian (1) is the existence of stable repulsively bound atom pairs. These are most intuitively understood for strong repulsive interaction |U| ≫ J, U > 0, where an example of such a pair is a state of two atoms occupying a single site,This state has a potential energy offset U with respect to states where the atoms are separated (see Fig. ...

We report here on the production of an ultracold gas of tightly bound Rb2 triplet molecules in the rovibrational ground state, close to quantum degeneracy. This is achieved by optically transferring weakly bound Rb2 molecules to the absolute lowest level of the ground triplet potential with a transfer efficiency of about 90%. The transfer takes place in a 3D optical lattice which traps a sizeable fraction of the tightly bound molecules with a lifetime exceeding 200 ms.

Transport of lactate and other monocarboxylates in mammalian cells is mediated by a family of transporters, designated monocarboxylate transporters (MCTs). The MCT4 member of this family has recently been identified as the major isoform of white muscle cells, mediating lactate efflux out of glycolytically active myocytes [Wilson, Jackson, Heddle, Price, Pilegaard, Juel, Bonen, Montgomery, Hutter and Halestrap (1998) J. Biol. Chem. 273, 15920-15926]. To analyse the functional properties of this transporter, rat MCT4 was expressed in Xenopus laevis oocytes and transport activity was monitored by flux measurements with radioactive tracers and by changes of the cytosolic pH using pH-sensitive microelectrodes. Similar to other members of this family, monocarboxylate transport via MCT4 is accompanied by the transport of H(+) across the plasma membrane. Uptake of lactate strongly increased with decreasing extracellular pH, which resulted from a concomitant drop in the K(m) value. MCT4 could be distinguished from the other isoforms mainly in two respects. First, MCT4 is a low-affinity MCT: for L-lactate K(m) values of 17+/-3 mM (pH-electrode) and 34+/-5 mM (flux measurements with L-[U-(14)C]lactate) were determined. Secondly, lactate is the preferred substrate of MCT4. K(m) values of other monocarboxylates were either similar to the K(m) value for lactate (pyruvate, 2-oxoisohexanoate, 2-oxoisopentanoate, acetoacetate) or displayed much lower affinity for the transporter (beta-hydroxybutyrate and short-chain fatty acids). Under physiological conditions, rat MCT will therefore preferentially transport lactate. Monocarboxylate transport via MCT4 could be competitively inhibited by alpha-cyano-4-hydroxycinnamate, phloretin and partly by 4, 4'-di-isothiocyanostilbene-2,2'-disulphonic acid. Similar to MCT1, monocarboxylate transport via MCT4 was sensitive to inhibition by the thiol reagent p-chloromercuribenzoesulphonic acid.

We have created and trapped a pure sample of Feshbach molecules in a three-dimensional optical lattice. Compared to previous experiments without a lattice, we find dramatic improvements such as long lifetimes of up to 700 ms and a near unit efficiency for converting tightly confined atom pairs into molecules. The lattice shields the trapped molecules from collisions and, thus, overcomes the problem of inelastic decay by vibrational quenching. Furthermore, we have developed an advanced purification scheme that removes residual atoms, resulting in a lattice in which individual sites are either empty or filled with a single molecule in the vibrational ground state of the lattice.

We have performed high-resolution two-photon dark-state spectroscopy of an ultracold gas of 87 Rb2 molecules in the a 3 Σ + u state at a magnetic field of about 1000 G. The vibrational ladder as well as the hyperfine and low-lying rotational structure is mapped out. Energy shifts in the spectrum are observed due to singlet-triplet mixing at binding energies as deep as a few hundred GHz×h. This information together with data from other sources is used to optimize the potentials of the a 3 Σ + u and X 1 Σ + g states in a coupled-channel model. We find that the hyperfine structure depends weakly on the vibrational level. This provides a possible explanation for inaccuracies in recent Feshbach resonance calculations.