  We conduct the first ``light-shining-through-wall" (LSW) search for dark
photons using two state-of-the-art high quality-factor superconducting radio
frequency (SRF) cavities and report the results of its pathfinder run. Our new
experimental setup enables improvements in sensitivity over previous searches
and covers new dark photon parameter space. We design delicate calibration and
measurement protocols to utilize the high-$Q$ setup at Dark SRF. Using cavities
operating at $1.3 \ \text{GHz}$, we establish a new exclusion limit for kinetic
mixing as small as {$\epsilon= 1.6\times 10^{-9}$} and provide the world's best
constraints on dark photons in the $2.1\times 10^{-7} \ \text{eV} -
5.7\times10^{-6} \ \text{eV}$ mass range. Our result is the first
proof-of-concept for the enabling role of SRF cavities in LSW setups, with
ample opportunities for further improvements. In addition, our data sets a
competitive lab-based limit on the Standard Model photon mass by searching for
longitudinal photon polarization.
