  This is a Snowmass white paper on the utility of existing and future
superconducting cavities to probe fundamental physics. Superconducting radio
frequency (SRF) cavity technology has seen tremendous progress in the past
decades, as a tool for accelerator science. With advances spear-headed by the
SQMS center at Fermilab, they are now being brought to the quantum regime
becoming a tool in quantum science thanks to the high degree of coherence. The
same high quality factor can be leveraged in the search for new physics,
including searches for new particles, dark matter, including the QCD axion, and
gravitational waves. We survey some of the physics opportunities and the
required directions of R&D. Given the already demonstrated integration of SRF
cavities in large accelerator systems, this R&D may enable larger scale
searches by dedicated experiments.

High Energy Physics - Phenomenology;High Energy Physics - Experiment