  The coherent interaction of Rydberg helium atoms with microwave fields in a
$\lambda/4$ superconducting coplanar waveguide resonator has been exploited to
probe the spectral characteristics of an individual resonator mode. This was
achieved by preparing the atoms in the 1s55s$^3$S$_1$ Rydberg level by
resonance enhanced two-color two-photon excitation from the metastable
1s2s$^3$S$_1$ level. The atoms then travelled over the resonator in which the
third harmonic microwave field, at a frequency of
$\omega_{\mathrm{res}}=2\pi\times19.556$ GHz, drove the two-photon
1s55s$^3$S$_1\rightarrow$1s56s$^3$S$_1$ 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.
