We report on the direct observation of an oscillating atomic current in a one-dimensional array of Josephson junctions realized with an atomic Bose-Einstein condensate. The array is created by a laser standing wave, with the condensates trapped in the valleys of the periodic potential and weakly coupled by the interwell barriers. The coherence of multiple tunneling between adjacent wells is continuously probed by atomic interference. The square of the small-amplitude oscillation frequency is proportional to the microscopic tunneling rate of each condensate through the barriers and provides a direct measurement of the Josephson critical current as a function of the intermediate barrier heights. Our superfluid array may allow investigation of phenomena so far inaccessible to superconducting Josephson junctions and lays a bridge between the condensate dynamics and the physics of discrete nonlinear media.

We create Bose-Einstein condensates of 87 Rb in a static magnetic trap with a superimposed blue-detuned 1D optical lattice. By displacing the magnetic trap center we are able to control the condensate evolution. We observe a change in the frequency of the center-of-mass oscillation in the harmonic trapping potential, in analogy with an increase in effective mass. For fluid velocities greater than a local speed of sound, we observe the onset of dissipative processes up to full removal of the superfluid component. A parallel simulation study visualizes the dynamics of the BEC and accounts for the main features of the observed behavior. 03.75.Fi, 32.80.Pj, 67.57.De Bose-Einstein condensates (BEC) in dilute atomic gases are macroscopic quantum systems which can be manipulated by a variety of experimental techniques [1]. The current development of such techniques is opening up a wealth of possibilities to explore new physics, e.g., in non-linear atom optics [2], and to study various aspects of superfluid behavior in the precisely controllable context of atomic physics [3].Atoms confined in a periodic potential share some properties with systems of electrons in crystals. Effects known from solid state physics, like Bloch oscillations and Wannier-Stark ladders, have been observed by exposing cold atoms to the dipole potential of far detuned optical lattices [4]. Macroscopic quantum interference has been observed in an experiment on a BEC confined to the antinodes of a far detuned optical lattice [5]. Bragg diffraction from a condensate has been induced in moving optical lattices [6]. This has been used, e.g., as an atom-laser outcoupler [7] and as a tool for spectroscopy of the momentum in BEC's [8]. Applications of BEC's in periodic potentials range from matter-wave transport [9] to interferometry [5] and quantum computing [10]. The question of the stability of the BEC during the evolution in optical potentials is crucial for these applications and has been addressed in theoretical works [11].In this Letter we report on some novel aspects of superfluidity in BEC's by studying their center-of-mass oscillations inside the harmonic potential of a magnetic trap in presence of a one-dimensional (1D) optical lattice. We identify different dynamical regimes by varying the initial displacement of the BEC from the bottom of the trap. For small displacements the BEC performs undamped oscillations in the harmonic potential and feels the periodic potential only through a shift in the oscillation frequency. At larger displacements we observe the onset of dissipative processes appearing through a damping in the oscillations. We can describe the experimental results in terms of an inhomogeneous superfluid having a density-dependent critical velocity. In parallel we report numerical studies of the Gross-Pitaevskii equation (GPE), which capture the main features of the observed dynamics.In our experimental setup [12] we now produce BEC's of 87 Rb atoms in the (F=1,m F = −1) state. The fundamental frequencies of our Ioffe-type magne...

In the present work we demonstrate how to realize a 1D closed optical lattice experimentally, including a tunable boundary phase twist. The latter may induce "persistent currents" visible by studying the atoms' momentum distribution. We show how important phenomena in 1D physics can be studied by physical realization of systems of trapped atoms in ring-shaped optical lattices. A mixture of bosonic and/or fermionic atoms can be loaded into the lattice, realizing a generic quantum system of many interacting particles.

We investigate the properties of a coherent array containing about 200 Bose-Einstein condensates produced in a far detuned 1D optical lattice. The density profile of the gas, imaged after releasing the trap, provides information about the coherence of the ground-state wavefunction. The measured atomic distribution is characterized by interference peaks. The time evolution of the peaks, their relative population as well as the radial size of the expanding cloud are in good agreement with the predictions of theory. The 2D nature of the trapped condensates and the conditions required to observe the effects of coherence are also discussed.