Abstract. We report the experimental observation of the disruption of the superfluid atomic current flowing through an array of weakly linked BoseEinstein condensates. The condensates are trapped in an optical lattice superimposed on a harmonic magnetic potential. The dynamical response of the system to a change of the magnetic potential minimum along the optical lattice axis goes from a coherent oscillation (superfluid regime) to a localization of the condensates in the harmonic trap ('classical' insulator regime). The localization occurs when the initial displacement is larger than a critical value or, equivalently, when the velocity of the wavepacket's centre of mass is larger than a critical velocity dependent on the rate of tunnelling between adjacent sites. Atomic Bose-Einstein condensates have been either loaded or produced in periodic potentials opening up the possibility of investigating new phenomena tuning the degree of coherence in the system. Experiments have explored regimes ranging from the coherent matter wave emission from a condensate loaded on a vertical standing wave [1], to the observation of number squeezed states [2], the demonstration of a one-dimensional Josephson junction array with a linear chain of condensates produced in an optical lattice [3], and the recent observation of a quantum phase transition in a condensate loaded in a 3D optical lattice [4]. The dynamical behaviour of coherent matter waves in periodic potentials has also been the subject of extensive 4

We have trapped 40 K in a vapor-cell magneto-optical trap starting from a natural abundance sample. This rare, weakly radioactive atom is a fermion. The number of trapped atoms is approximately 8000, with a density of ϳ10 8 atoms/cm 3. Contrary to what is observed for the other isotopes of potassium, our data indicate that temperatures lower than the Doppler-cooling limit can be reached for 40 K. We discuss the interest of these results in view of possible experiments to study a degenerate Fermi gas. ͓S1050-2947͑98͒05601-7͔

We report on the efficient operation of a double-magneto-optical-trap ͑MOT͒ apparatus for potassium isotopes. Mechanisms for the cooling and the transfer between the two MOTs are studied. A magnetic quadrupole trap has been loaded from the second MOT; density and temperature appear to be promising for starting runaway evaporative cooling. ͓S1050-2947͑99͒03201-1͔

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