SuperRing

The aim of this project is the study of superfluidity, angular momentum transfer and phase-slips in ring-trapped quantum gases of ultracold atoms. We will develop a quantum simulator capable of answering questions related to condensed matter phenomena, in particular to the physics of superconducting rings, used both in metrology and quantum computing. The expected outcomes of this project include the experimental demonstration of a controlled transfer of angular momentum to a quantum gas confined in an annulus and the understanding of its decay mechanisms throughout the dimensional crossover between two-dimensional (2D) and one-dimensional (1D) regimes, as well as analytic description of this dynamics. The expected results have applications in atom interferometry and will pave the way to the realization of macroscopic superpositions of current states with ultracold atoms, of major interest for quantum computation and quantum simulation. A remarkable feature of superfluids is their ability to flow without dissipation below a critical velocity. A superfluid quantum gas thus exhibits a persistent current flow around a ring trap, and the circulation of its velocity is quantised (described by a single integer number l). We propose to study the mechanisms at work as the quantum gas is set into rotation, and conversely as the flow eventually decays. Low-dimensional quantum gases are of particular interest for this study. In contrast to three dimensional weakly interacting gases, superfluids are not Bose-Einstein condensed. In 2D, superfluidity is governed by the Berezinskii-Kosterlitz-Thouless mechanism of vortex-antivortex pairing. In 1D, the physics is described by the Luttinger liquid theory. At strong interactions, only imperfect superfluidity is expected due to quantum fluctuations. While vortices play an essential role in 2D, and are expected to also take an important part in the superfluid dynamics, they are not supported in 1D, where topological defects are solitons. The project combines theory and experiment. The experiments will make use of a specially designed setup for the production of a rubidium degenerate gas in a ring trap, with widely tunable parameters: ring radius, ellipticity, vertical and radial trapping frequencies can be adjusted independently, which allows to explore both the 2D and the 1D regimes. All these parameters can be made time-dependent. With this setup, we will measure both in 2D and in 1D the critical angular velocity to set the gas into rotation when a defect is rotated. The defect will be either the spot of a focused laser or an elliptic deformation of the ring. We will study the possibility to prepare a given circulation state l with this kind of excitation procedure, combined with evaporative cooling. The theory will provide predictions for the critical velocity and the drag force, and numerical simulations of the dynamics at finite temperature. At the same time, we will study the effect of phase-slips on the flow, as the circulation changes by one quantum of angular momentum, eg from l to l-1. To this aim, we will develop a laser system allowing to prepare the superfluid in a given circulation state l. In 2D, the change from l to l-1 can be due to the expulsion of a vortex outside the outer edge of the ring. We will consider then tunnel-coupled concentric rings in a quasi-1D multi-mode geometry under a gauge field induced by rotation. We will theoretically analyze the state of the multicomponent system at arbitrary interactions, the corresponding ground-state current and vortex configurations and possible transitions as a function of the applied gauge field and interaction strength. In the strictly 1D regime, we expect solitons to play a major role. In reduced dimensionality, quantum phase-slips are expected to occur, leading to macroscopic superpositions of circulation states. To date, quantum phase-slips have not been observed in quantum gases.