Giulia Del Pace (CNR-INO and LENS, Italy) talks about “Superflow in ultracold atomic rings: from persistent currents to Kelvin-Helmholtz instability”.

The emergence of persistent currents in rings is one of the most striking manifestations of a quantum system coherence. The periodic boundary of such a geometry constrains the wavefunction phase to wind in a loop of an integer multiple of 2π, which, when non-zero, gives rise to a particularly stable current, protected by its topological nature. Persistent currents have been observed in materials with a macroscopic coherence, like superconductors or neutral superfluids, but also in mesoscopic metallic rings. In this webinar, I will report on our recent experimental studies on the stability of persistent currents in homogeneous fermionic superfluid rings of ultracold Lithium-6 atoms. In a first work [1], we demonstrate a fast and accurate technique to excite persistent currents of circulation as high as 9 and we probe their stability in the presence of a controlled impurity in the ring. All currents are observed to be unperturbed by the defect up to a critical criculation, above which vortices enters the superfluid bulk, tearing away circulation quanta and therefore inducing the current decay. In a second work [2], we scale up our system to a concentric double ring geometry and investigate the stability of the interface between counterflowing currents when we merge the two rings. The circular vortex array that constitute the interface is observed to be unstable, undergoing the rolling-up dynamics characteristic of the Kelvin-Helmholtz instability (KHI) of classical fluids. In fact, we find the measured growth rate of the instability to show the universal scaling predicted for KHI by classical hydrodynamic and by a microscopic point vortex model. Our results demonstrate atomic superfluids as a versatile testbed to study superflow and its interplay with vortices.

[1] G. Del Pace, et al., Imprinting persistent currents in tunable fermionic rings, Phys. Rev. X 12, 041037 (2022). [2] D. Hernandez-Rajkov, et al., Universality of the superfluid Kelvin-Helmholtz instability by single-vortex tracking, arXiv:2303.12631 (2023)