of an individual resonator mode. This was achieved by preparing the atoms in the 1s55s3S1 Rydberg level by resonance enhanced two-color two-photon excitation from the metastable 1s2s3S1 level. The atoms then travelled over the resonator in which the third harmonic microwave field, at a frequency of ωres=2π×19.556 GHz, drove the two-photon 1s55s3S1→1s56s3S1 transition. By injecting a sequence of Ramsey pulses into the resonator, and monitoring the coherent evolution of the Rydberg state population by state-selective pulsed electric field ionization as the frequency of the microwave field was tuned, spectra were recorded that allowed the resonator resonance frequency and quality factor to be determined with the atoms acting as microscopic quantum sensors.