To account for the dark-matter content in our Universe, postinflationary scenarios predict for the QCD axion a mass in the range $(10–{10}^3)\mu \mathrm{eV}$. Searches with haloscope experiments in this mass range require the monitoring of resonant cavity modes with frequency above 5 GHz, where several experimental limitations occur due to linear amplifiers, small volumes, and low quality factors of copper resonant cavities. In this paper, we deal with the last issue, presenting the result of a search for galactic axions using a haloscope based on a $36{\mathrm{cm}}^3$ NbTi superconducting cavity. The cavity worked at $T=4\mathrm{K}$ in a 2 T magnetic field and exhibited a quality factor $Q_0=4.5\times {10}^5$ for the TM010 mode at 9 GHz. With such values of $Q$, the axion signal is significantly increased with respect to copper cavity haloscopes. Operating this setup, we set the limit $g_{a\gamma \gamma }<1.03\times {10}^{-12}{\mathrm{GeV}}^{-1}$ on the axion photon coupling for a mass of about $37\mu \mathrm{eV}$. A comprehensive study of the NbTi cavity at different magnetic fields, temperatures, and frequencies is also presented.

A haloscope of the QUAX–$a\gamma$ experiment composed of an oxygen-free high thermal conductivity-Cu cavity inside an 8.1 T magnet and cooled to $\sim 200\mathrm{mK}$ is put in operation for the search of galactic axion with mass $m_a\simeq 43\mu \mathrm{eV}$. The power emitted by the resonant cavity is amplified with a Josephson parametric amplifier whose noise fluctuations are at the standard quantum limit. With the data collected in about 1 h at the cavity frequency ${\nu }_c=10.40176\mathrm{GHz}$, the experiment reaches the sensitivity necessary for the detection of galactic QCD-axion, setting the 90% confidence level limit to the axion-photon coupling $g_{a\gamma \gamma }<0.766\times {10}^{-13}{\mathrm{GeV}}^{-1}$.

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%.

We propose a scheme for the detection of microwave-induced photons through a current-biased Josephson junction, from the point of view of the statistical decision theory. Our analysis is based on the numerical study of the zero-voltage lifetime distribution in response to a periodic train of pulses, that mimics the absorption of photons. The statistical properties of the detection are retrieved comparing the thermally induced transitions with the distribution of the switchings to the finite voltage state due to the joint action of thermal noise and of the incident pulses. The capability to discriminate the photon arrival can be quantified through the Kumar-Caroll index, which is a good indicator of the signal-to-noise ratio. The index can be exploited to identify the system parameters best suited for the detection of weak microwave photons.

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.