Superconducting microwave resonators for physics experiments

Superconducting resonators at K-band frequencies have been developed for different applications in general physics. Niobium pillbox cavities have been built for the one-atom maser experiment by which the interaction of Rydberg atoms with single microwave photons has been investigated. At 21.5 GHz and 1.3 K, quality factors of up to 10 ¹¹ were obtained. Coating of the cavity with Nb₃Sn resulted in quality factors of 6×10⁸ at 4.2 K and 6×10⁹ at 2 K. A superconducting Fabry-Perot resonator consisting of two spherically curved niobium mirrors was also investigated. The quality factor of 1.8×10⁷ measured at 25 GHz and 4.2 K was found to be two orders of magnitude higher than for a corresponding copper resonator. Fabry-Perot resonators can be used for detecting small position changes for one mirror with respect to another caused by gravitational forces. First experiments with copper Fabry-Perot mirrors suspended in a vacuum chamber provided a maximum sensitivity for a gravitational acceleration of one mirror of 4×10 ^-11 m/s². These results are promising for a possible fifth force detector based on a superconducting Fabry-Perot resonator