Scalar-Magnetometer Search for Ultralight Dark Photon Dark Matter with a Single-Site, Two-Sensor Array: A 6-Channel DTFT Likelihood Analysis with Scalar Optically Pumped Magnetometers

We report on a laboratory search for ultralight dark photon dark matter using a single-site, two-sensor scalar magnetometer array. The experiment employs two scalar optically pumped magnetometers (OPMs) operated in a differential configuration to suppress common-mode noise and enhance sensitivity to spatially coherent dark photon fields. We analyze 10.5 hours of continuous data with a six-channel complex data vector evaluated at the three physical frequencies of the expected dark photon signal triplet. Assuming Gaussian noise, we develop a likelihood framework to set robust, frequency-resolved upper limits on the kinetic-mixing parameter $\epsilon$, which governs the coupling between Standard Model photons and dark photons. Within the mass range $4\times {10}^{-15}\mathrm{e}\mathrm{V}\le m_{A^{\mathrm{\prime }}}\le 3\times {10}^{-14}\mathrm{e}\mathrm{V}$, we obtain the most stringent direct laboratory limits to date on $\epsilon$, complementing existing astrophysical bounds including those inferred from observations of the Leo-T dwarf galaxy.