  Vortices trapped in thin-film superconducting microwave resonators can have a
significant influence on the resonator performance. Using a variable-linewidth
geometry for a weakly coupled resonator we are able to observe the effects of a
single vortex trapped in the resonator through field cooling. For resonant
modes where the vortex is near a current antinode, the presence of even a
single vortex leads to a measurable decrease in the quality factor and a
dispersive shift of the resonant frequency. For modes with the vortex located
at a current node, the presence of the vortex results in no detectable excess
loss and, in fact, produces an increase in the quality factor. We attribute
this enhancement to a reduction in the density of nonequilibrium quasiparticles
in the resonator due to the suppressed gap from the vortex.
