First Constraints on Dark Photon Dark Matter with Superconducting Nanowire Detectors in an Optical Haloscope

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 $\sim$ 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 x 10$^{6}$ counts/sec. The observed count rate in our detector differed insignificantly from a reference SNSPD, enabling our prototype experiment to set new limits for the dark photon dark matter kinetic mixing parameter $\epsilon$ $_{\sim}^{<}$ 10$^{-12}$ and find no evidence for dark photon dark matter over a mass range of $\sim$ 0.7-0.8 eV (photon wavelength $\sim$ 1550-1770 nm). This performance demonstrates that, with feasible upgrades, our architecture could probe significant new parameter space for dark photon and axion dark matter in the meV to 10 eV mass range.