  Persistent currents in annular geometries have played an important role in
disclosing the quantum phase coherence of superconductors and mesoscopic
electronic systems. Ultracold atomic gases in multiply connected traps also
exhibit long-lived supercurrents, and have attracted much interest both for
fundamental studies of superfluid dynamics and as prototypes for atomtronic
circuits. Here, we report on the realization of supercurrents in homogeneous,
tunable fermionic rings. We gain exquisite, rapid control over quantized
persistent currents in all regimes of the BCS-BEC crossover through a universal
phase-imprinting technique, attaining on-demand circulations $w$ as high as 9.
High-fidelity read-out of the superfluid circulation state is achieved by
exploiting an interferometric protocol, which also yields local information
about the superfluid phase around the ring. In the absence of externally
introduced perturbations, we find the induced metastable supercurrents to be as
long-lived as the atomic sample. Conversely, we trigger and inspect the
supercurrent decay by inserting a single small obstacle within the ring. For
circulations higher than a critical value, the quantized current is observed to
dissipate via the emission of vortices, i.e., quantized phase slips, which we
directly image, in good agreement with numerical simulations. The critical
circulation at which the superflow becomes unstable is found to depend starkly
on the interaction strength, taking its maximum value for the unitary Fermi
gas. Our results demonstrate fast and accurate control of quantized collective
excitations in a macroscopic quantum system, and establish strongly interacting
fermionic superfluids as excellent candidates for atomtronic applications.
