  The coherent interaction between ensembles of helium Rydberg atoms and
microwave fields in the vicinity of a solid-state co-planar waveguide is
reported. Rydberg-Rydberg transitions, at frequencies between 25 GHz and 38
GHz, have been studied for states with principal quantum numbers in the range
30 - 35 by selective electric-field ionization. An experimental apparatus
cooled to 100 K was used to reduce effects of blackbody radiation.
Inhomogeneous, stray electric fields emanating from the surface of the
waveguide have been characterized in frequency- and time-resolved measurements
and coherence times of the Rydberg atoms on the order of 250 ns have been
determined.
