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Description
Abrikosov vortices strongly influence dissipation and coherence in superconducting devices, yet their nucleation and pinning dynamics in mesoscopic structures remain difficult to probe experimentally. Here we employ a cavity-optomechanical platform to detect vortex entry in a superconducting aluminum nanostring resonator integrated into a SQUID-terminated microwave cavity. By monitoring the mechanical resonance frequency using displacement-noise spectroscopy at millikelvin temperatures, we observe discrete frequency jumps attributed to individual vortex-entry events. These steps are superimposed on a smooth background consistent with vortex-pinning elasticity in the Campbell regime. From the collective stiffening we extract a Labusch parameter of order $10^{14}\,\mathrm{N\,m^{-4}}$, while individual steps correspond to attonewton-scale force changes and single-vortex pinning energies of $0.1$--$0.4\,\mathrm{eV}$. The stochastic field positions of the jumps across thermal cycles reflect the underlying disorder-defined pinning landscape. Our results demonstrate that nanomechanical cavity-optomechanical devices provide a highly sensitive probe of vortex physics in mesoscopic superconductors and offer a new route to investigate flux-induced dissipation mechanisms relevant for superconducting quantum circuits.