A haloscope of the QUAX–$a\gamma$ experiment, composed of an high-$Q$ resonant cavity immersed in a 8 T magnet and cooled to $\sim 4.5\mathrm{K}$ is operated to search for galactic axions with mass $m_a\simeq 42.8\mu \mathrm{eV}$, not accessible to other running experiments. The design of the cavity with hollow dielectric cylinders concentrically inserted in a OFHC Cu cavity, allowed us to maintain a loaded quality-factor $\mathrm{Q}\sim 300000$ during the measurements in presence of magnetic field. Through the cavity tuning mechanism it was possible to modulate the resonance frequency of the haloscope in the region $10.35337-10.35345\mathrm{GHz}$ and thus acquire different datasets at different resonance frequencies. Acquiring each datasets for about 50 minutes, combining them and correcting for the axion’s signal estimation-efficiency, we set a limit on the axion-photon coupling $g_{a\gamma \gamma }<0.731\times {10}^{-13}{\mathrm{GeV}}^{-1}$ with the confidence level set at 90%.

We report the first experimental results obtained with the new haloscope of the QUAX experiment located at Laboratori Nazionali di Frascati of INFN (LNF). The haloscope is composed of a OFHC Cu resonant cavity cooled down to about 30 mK and immersed in a magnetic field of 8 T. The cavity frequency was varied in a 6 MHz range between 8.831496 and 8.83803 GHz. This corresponds to a previously unprobed mass range between 36.52413 and $36.5511\mathrm{\mu }\mathrm{eV}$. We don’t observe any excess in the power spectrum and set limits on the axion-photon coupling in this mass range down to $g_{a\gamma \gamma }<0.882\times {10}^{-13}{\mathrm{GeV}}^{-1}$ with the confidence level set to 90%.

The hypothetical axion particle is an excellent cold dark matter candidate. Models predict an axion mass of tens of μeV (GHz frequency range). It is in principle possible to convert galactic axions into a monochromatic microwave signal or to detect them with an electron-spin flip. In both cases, to measure the resulting vanishing signal ( $P\approx10^{-23} W$ ) a high-Q cavity is needed, with $Q\sim10^6$ . Superconducting RF technology can satisfy the requirement of operation in moderate-to-high DC magnetic fields. In this contribution we present an experimental study of the behavior of superconducting cavities in the vortex state. This study has been realized using 14 GHz Cu-NbTi and $Cu-NbTiN_x$ film cavities. A new cavity geometry is presented. We measured the magnetic field dependence of the Q-factor and $\Delta f_{res}/f_{res}$ where $f_{res}$ is resonant frequency, in the range B=0-6 T at 4.2 K. Also Q versus temperature at fixed B in ZFC and FC setups has been considered. A comparison was made with Cu and Nb cylindrical bulk cavities. The data for Q(B) were analyzed in terms of the elastic and dissipative motion of flux lines, thus including flux flow and flux pinning. We discuss whether the field-induced quasiparticle contribution is relevant in our field and temperature range.

Axions detection requires the ultimate sensitivity down to the single-photon limit. In the microwave region, this corresponds to energies in the yJ range. This extreme sensitivity has to be combined with an extremely low dark-count rate since the probability of axions conversion into microwave photons is supposed to be very low. To face this complicated task, we followed two promising approaches that both rely on the use of superconducting devices based on the Josephson effect. The first one is to use a single Josephson junction (JJ) as a switching detector (i.e., exploiting the superconducting to normal state transition in the presence of microwave photons). We designed a device composed of a coplanar waveguide terminated on a current-biased JJ. We tested its efficiency to pulsed (pulse duration 10 ns) microwave signals since this configuration is closer to an actual axions search experiment. We show how our device is able to reach detection capability of the order of ten photons with the frequency of 8 GHz. The second approach is based on an intrinsically quantum device formed by two resonators coupled only via a superconducting qubit network. This approach relies on quantum nondemolition measurements of the resonator photons. We show that by injecting radiofrequency power into the resonator, the frequency position of the resonant drop in the transmission coefficient ( S 21) can be modulated up to 4 MHz. We anticipate that, once optimized, both the devices have the potential to reach single-photon sensitivity.

Detection of light dark matter, such as axion-like particles, puts stringent requirements on the efficiency and dark-count rates of microwave-photon detectors. The possibility of operating a current-biased Josephson junction as a single-microwave photon-detector was investigated through numerical simulations, and through an initial characterization of two Al junctions fabricated by shadow mask evaporation, done in a dilution refrigerator by measuring escape currents at different temperatures, from 40 mK up to the Al transition temperature. The escape dynamics of the junctions were reproduced in the simulation, including the dissipative effects. Inhibition of thermal activation was observed, leaving the macroscopic quantum tunneling as the dominant effect well beyond the crossover temperature.

Quantum Sensing is a rapidly expanding research field that finds one of its applications in Fundamental Physics, as the search for Dark Matter. Devices based on superconducting qubits have already been successfully applied in detecting few-GHz single photons via Quantum Non-Demolition measurement (QND). This technique allows us to perform repeatable measurements, bringing remarkable sensitivity improvements and dark count rate suppression in experiments based on high-precision microwave photon detection, such as for Axions and Dark Photons search. In this context, the INFN Qub-IT project goal is to realize an itinerant single-photon counter based on superconducting qubits that will exploit QND for enhancing Axion search experiments. In this study, we present Qub-IT's status towards the realization of its first superconducting qubit device, illustrating design and simulation procedures and the characterization of fabricated Coplanar Waveguide Resonators (CPWs) for readout. We match target qubit parameters and assess a few-percent level agreement between lumped and distributed element simulation models. We reach a maximum internal quality factor of 9.2×10 5 for -92 dBm on-chip readout power.