Search for dark photons with a shielded room and noise subtraction

The dark photon is a well-motivated ultralight dark-matter candidate that may couple to the Standard Model through kinetic mixing. We search for dark-photon dark matter in the mass range 4 × 10⁻¹² – 2 × 10⁻⁹ eV (1-500kHz) using a three-axis magnetometer inside a large conductive shielded room. We set new laboratory limits on the kinetic-mixing parameter, improving upon previous laboratory bounds by up to two orders of magnitude. Our search exploits a geometry-defined null response along one axis as a noise reference to implement a subtraction procedure, lowering noise and enhancing sensitivity. These results establish the strongest laboratory constraints in this mass range and illustrate how null-axis magnetometry can broaden terrestrial searches for ultralight vector dark matter.