Integration of a Transmon-Based Single Microwave Photon Counter into the QUAX a-e Ferrimagnetic Haloscope

The search for the QCD axion, a hypothetical particle proposed to solve the strong CP problem and a well-motivated cDM candidate, requires detecting extremely weak electromagnetic signals generated by its interaction with standard matter. Among the various experimental approaches, ferrimagnetic haloscope detectors exploit the axion– electron coupling, through collective spin waves excitations, whose quantum is called magnon, in a resonant microwave cavity. This thesis presents the integration and characterization of a transmon-based Single Microwave Photon Detector (SMPD) designed for the new generation of QUAXa–e ferrimagnetic haloscope experiment. After a review of axion physics, we discuss the theoretical framework of circuit QED (cQED) leading from superconducting circuits to single-photon counting devices. Comprehensive measurements of the SMPD performance, carried out in this work, demonstrate the possibility of below-SQL operation and low dark-count rates, establishing a reliable method for detecting microwave photons in the single-quantum regime: a fully operative quantum sensor. The second part of the work details the haloscope detector, where theoretical background along with full carachterization of the magnon-photon hybrid system is laid down in depth. The next experimental milestones will consist of completing the SMPD characterization through a measurement of the dark-count rate and overall detection efficiency, followed by the installation of the 12T magnet and qualification of the SC detector under magnetic field, that will enable the high-field operation required for a realistic axion search. With these developments, the experiment will be positioned to explore unexplored regions of the axion parameter space of the QCD axion parameter space with unprecedent speed and efficiency.