UPLOAD-HELIX: A High-Helicity Single-Mode Microwave Haloscope with Low-Noise Interferometric Readout for Ultralight Axion Dark Matter

We propose a superconducting single-mode microwave haloscope based on chiral cavityresonators for the detection of ultralight dark matter axions over the mass range4 × 10−19– 4 × 10−14 eV. Building on the single-mode chiral-cavity haloscope for detectingultra light dark matter (ULDM) axions we develop a resonator geometry compatible withsubtractive manufacturing from high-purity bulk niobium, taking advantage of the substantiallylower surface resistance achievable relative to the additively manufactured Möbiuscavity proposed in the earlier work. An inverse-design framework is then used to maximisea figure of merit derived to minimise the measurement time required to achieve a fixedexperimental sensitivity. The resulting optimised bulk-niobium design achieves a figure ofmerit more than three orders of magnitude larger than the additively manufactured Möbiusbenchmark. An experimentally informed microwave interferometric readout model incorporatingmeasured electronics noise and active suppression of pump amplitude noise isused to project the sensitivity of the proposed experiment. For an acquisition time of threemonths, the haloscope is projected to reach gaγγ < 10−11 GeV−1 across more than fourorders of magnitude in axion mass. The projected sensitivity extends approximately oneorder of magnitude below the current exclusion limits set by CAST, providing a practicalpathway towards a high-sensitivity direct search for ultralight dark matter axions.