  Uncovering the nature of dark matter is one of the most important goals of
particle physics. Light bosonic particles, such as the dark photon, are
well-motivated candidates: they are generally long-lived, weakly-interacting,
and naturally produced in the early universe. In this work, we report on
LAMPOST (Light $A'$ Multilayer Periodic Optical SNSPD Target), a
proof-of-concept experiment searching for dark photon dark matter in the eV
mass range, via coherent absorption in a multi-layer dielectric haloscope.
Using a superconducting nanowire single-photon detector (SNSPD), we achieve
efficient photon detection with a dark count rate (DCR) of $\sim
6\times10^{-6}$ counts/s. We find no evidence for dark photon dark matter in
the mass range of $\sim 0.7$-$0.8$ eV with kinetic mixing $\epsilon \gtrsim
10^{-12}$, improving existing limits in $\epsilon$ by up to a factor of two.
With future improvements to SNSPDs, our architecture could probe significant
new parameter space for dark photon and axion dark matter in the meV to 10 eV
mass range.
