Luca Gennaro Foggetta

The Beam Test Facility at the INFN National Laboratories of Frascati has undergone a major upgrade to meet the increasing demand for high-quality, flexible beam delivery for detector R&D, accelerator physics, and interdisciplinary applications. The new infrastructure includes a second beam line, a fully equipped experimental hall, and modernized beam diagnostics and control systems. This paper presents the reasons for the upgrade, the design and implementation of the new beam line, the introduction of the booking service and initial feedback from user experiments. Particular attention is given to the integration of a distributed EPICS-based control system and the deployment of OLOG, a web-accessible electronic logbook, which enhance the usability and traceability of beam operations. The upgraded BTF positions itself as a leading European facility for precision beam tests and applied accelerator science.

The PADME experiment at the Frascati DAΦNE LINAC has performed a search for the hypothetical X17 particle, with a mass of around 17 MeV, by scanning the energy of a positron beam striking a fixed target. The X17 should be produced from the resulting e ⁺ e ⁻ annihilation. Since the expected mass of this particle is only roughly known, data sidebands cannot be clearly defined. Furthermore, the need to keep the analysis blind to potentially sizable signal contributions prevents a clear assessment even of the quality of the data sample in this search. In light of these challenges, this paper presents an innovative strategy adopted by the PADME Collaboration to perform data quality checks without disclosing the X17 sample. Moreover, the procedure designed to eventually unblind the data is described, together with the statistical approach adopted to extract the limits on the coupling between the X17 and the Standard Model.

A bstract: This paper presents a detailed characterization of the positron beam delivered by the Beam Test Facility at Laboratori Nazionali of Frascati to the PADME experiment during Run III, which took place from October to December 2022. It showcases the methodology used to measure the main beam parameters such as the position in space, the absolute momentum scale, the beam energy spread, and its intensity through a combination of data analysis and Monte Carlo simulations. The results achieved include an absolute precision in the momentum of the beam to within ~1–2 MeV /c , a relative beam energy spread below 0.25%, and an absolute precision in the intensity of the beam at the level of 2%.

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.

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 presence of microwave photons). We designed a device composed of a coplanar waveguide terminated on a current biased Josephson junction. 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 10 photons with frequency 8 GHz. The second approach is based on an intrinsically quantum device formed by two resonators coupled only via a superconducting qubit network (SQN). This approach relies on quantum nondemolition measurements of the resonator photons. We show that injecting RF power into the resonator, the frequency position of the resonant drop in the transmission coefficient (S21) can be modulated up to 4 MHz. We anticipate that, once optimized, both the devices have the potential to reach single photon sensitivity.

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.

A growing interest in the detection of single microwave-photons has been stimulated by the search for light dark matter, such as Axions and Axion-like particles, together with the fast development of quantum technologies based on superconducting devices. Many solutions have been proposed in literature but most of them still fail to satisfy the tight requirements imposed by Dark Matter experiments. For instance, many have a large dark-count rate that still make parametric amplifiers a preferable choice. On the contrary, a device based on a current biased Josephson junction resonantly activated by the absorption of a microwave photon may have dark-counts rate down to the mHz level. Here, we report the experimental study of the resonant activation of an Al Josephson junction excited by continuous radiofrequency. The device under study consists of a chip with a transmission line terminated by an Al Josephson-junction, fabricated by shadow-mask evaporation technique. The sample-holder with the chip inside is hosted in a superconducting box and it is thermally anchored to the mixing-chamber plate of a dilution refrigerator at a temperature of about 15 mK. We measured the escape rates of the junction in different configurations of current bias and radiofrequency excitations. Response to continuous wave has been measured and interpreted within the RCSJ model. These results are used to optimize the design of a single microwave-photon detector.

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.