We study the metastability and decay of multiply-charged superflow in a ring-shaped atomic Bose-Einstein condensate. Supercurrent corresponding to a giant vortex with topological charge up to q = 10 is phaseimprinted optically and detected both interferometrically and kinematically. We observe q = 3 superflow persisting for up to a minute and clearly resolve a cascade of quantised steps in its decay. These stochastic decay events, associated with vortex-induced 2π phase slips, correspond to collective jumps of atoms between discrete q values. We demonstrate the ability to detect quantised rotational states with > 99 % fidelity, which allows a detailed quantitative study of time-resolved phase-slip dynamics. We find that the supercurrent decays rapidly if the superflow speed exceeds a critical velocity in good agreement with numerical simulations, and we also observe rare stochastic phase slips for superflow speeds below the critical velocity.

We create and study persistent currents in a toroidal two-component Bose gas, consisting of 87Rb atoms in two different spin states. For a large spin-population imbalance we observe supercurrents persisting for over two minutes. However, we find that the supercurrent is unstable for spin polarization below a well-defined critical value. We also investigate the role of phase coherence between the two spin components and show that only the magnitude of the spin-polarization vector, rather than its orientation in spin space, is relevant for supercurrent stability.

We scrutinize the concept of saturation of the thermal component in a partially condensed trapped Bose gas. Using a 39K gas with tunable interactions, we demonstrate strong deviation from Einstein's textbook concept of a saturated vapor. However, the saturation picture can be recovered by extrapolation to the strictly noninteracting limit. We provide evidence for the universality of our observations through additional measurements with a different atomic species, 87Rb.

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By quenching the strength of interactions in a partially condensed Bose gas we create a "super-saturated" vapor which has more thermal atoms than it can contain in equilibrium. Subsequently, the number of condensed atoms (N0) grows even though the temperature (T ) rises and the total atom number decays. We show that the non-equilibrium evolution of the system is isoenergetic and for small initial N0 observe a clear separation between T and N0 dynamics, thus explicitly demonstrating the theoretically expected "two-step" picture of condensate growth. For increasing initial N0 values we observe a crossover to classical relaxation dynamics. The size of the observed quench-induced effects can be explained using a simple equation of state for an interacting harmonically-trapped atomic gas.PACS numbers: 03.75. Kk, 67.85.De, Non-equilibrium dynamics of interacting quantum systems are generally far less understood than the corresponding equilibrium many-body states [1]. Of particular interest are manybody dynamics of both the order parameter and the excitations in a system close to a phase transition. From a theoretical point of view, a clean and well defined way to induce and study non-equilibrium quantum dynamics is a rapid "quantum quench" [2] of a single Hamiltonian parameter. Ultracold atomic gases are very well suited for such quantum quench experiments. In addition to the possibility to dynamically vary microscopic Hamiltonian parameters, they feature near-perfect isolation from the environment and characteristic many-body timescales (ranging from milliseconds to seconds) that are experimentally resolvable and allow real-time non-equilibrium studies.In this Letter, we introduce a quantum quench of the interaction strength in an atomic Bose gas as a tool to study the dynamics of Bose-Einstein condensation [3][4][5][6][7][8][9][10][11][12][13][14][15]. Earlier experiments highlighted the importance of bosonic stimulation in condensate formation [10], but could not quantitatively address the theoretically debated interplay of energy redistribution and coherence development in the system [3][4][5][6][7][8][9]. The use of a quantum quench of the interaction strength allows us to study these two processes in parallel. The quench induces a growth of the condensed atom number in a degenerate gas without any removal of thermal energy; we explain this effect with a simple theoretical model and experimentally study its real-time dynamics. We explicitly show that the post-quench non-equilibrium evolution of the system is isoenergetic, and directly reveal the theoretically postulated "two-step" picture of condensation [4][5][6][7]. As expected, close to the critical point the growth of the condensed atom number lags behind the energy redistribution in the thermal component of the gas. Moving away from the critical point, we also observe a crossover to effectively one-step condensation dynamics governed by a classical relaxation process.In an ideal Bose gas the number of condensed atoms, N 0 , depends only on the total atom num...