Persistent currents in a ring are one of the most striking manifestations of quantum system coherence. The periodic boundary constrains the wavefunction phase to wind in an integer number of complete loops, which gives rise to a current. This happens in materials with a macroscopic coherence, like superconductors or neutral superfluids, but also in mesoscopic metallic rings. Besides being a proxy of quantum phase coherence, persistent currents represent a cornerstone for many applications, from precision sensing to quantum computing, that require a fast and controlled current injection and a reliable readout of its magnitude. In our work, we realize a fast and on-demand generation of persistent currents in atomic Fermi superfluid rings and investigate their connection with vortex nucleation.
We excite persistent current states of on-demand winding number by dynamically imprinting the phase winding in the ring with a tailored laser beam. Using an interferometric probe, we directly accesses the ring phase profile and we consequently readout the current state. We apply our method to an atomic Fermi gas in different interaction regimes, ranging from a bosonic to a fermionic superfluid. Persistent currents in these rotating neutral superfluids are metastable states, interrupted only by a phase slippage that tears out the phase winding. We finally induce the current decay by inserting a small defect in the ring. For currents higher than a critical value, the obstacle triggers the emission of vortices, which reduce the phase winding.
Our work demonstrates fast and accurate control of persistent currents in strongly interacting fermionic superfluids, opening the route for their application in quantum technologies.

G. Del Pace et al.
Imprinting Persistent Currents in Tunable Fermionic Rings
Phys. Rev. X 12, 041037 (2022)