Development of Superconducting Single-Particle Detector Transition-Edge Sensor

This thesis presents the development and advancement of superconducting singleparticle detectors, specifically Transition-Edge Sensors (TESs), focusing on enhancing their performance for various scientific fields. TESs are highly sensitive microcalorimeters capable of detecting radiation across a wide spectrum, from submillimeter wavelengths to gamma rays, due to their intrinsic energy resolution. They are also known for their near-unit system detection efficiency, low dark count rate and photon-number resolution capabilities. This work was conducted at the Istituto Nazionale di Ricerca Metrologica (INRiM), the National Metrology Institute of Italy, where I worked in the Innovative Cryogenic Detectors Laboratory using TES devices fabricated at QR Laboratories, a micro and nanofabrication lab. The primary motivation for this research is to improve TES performance to enhance their use in physical and metrological experiments. Among these is the PontCorvo Tritium Observatory for Light, Early-Universe, Massive-neutrino Yield (PTOLEMY) experiment, which seeks to detect the Cosmic Neutrino Background (CNB). TES devices with an energy resolution of 0.11 eV are required to detect electrons produced by CNB via neutrino captures on beta-unstable nuclides, providing insights into the early universe and the nature of neutrinos. Two TiAu TES, with areas of 20 μm × 20 μm and 50 μm × 50 μm, were characterized, achieving energy resolutions of 0.114 eV and 0.158 eV, respectively. These results are particularly noteworthy as they match state-of-the-art energy resolutions reported in the literature but with TES devices of significantly larger area. This advancement is critical for the PTOLEMY project because it facilitates the implementation of large-area detectors based on an array of TESs. Moreover, this thesis demonstrates the first detection of electrons with kinetic energy in the 100 eV range using a TES. This was achieved with a 100 μm × 100 μm TiAu TES. Electrons were produced directly in the cryostat by a cold-cathode source vi based on field emission from vertically-aligned multiwall carbon nanotubes. The energy resolution obtained for fully-absorbed electrons in the (90−101) eV energy range was between 1.8 eV and 4 eV, compatible with the resolution for photons in the same energy range. This measurement opens new possibilities in electron detection, crucial for PTOLEMY, where TES devices must detect electrons with high precision, but also in electron spectroscopy. The Quantum Haloscope Search experiment (QHaloS) aims to search for light dark matter in the form of dark photons using a dielectric haloscope equipped with a TES for photon detection. This innovative detector aims to convert non-relativistic dark photons into Standard Model photons, which are then detected by the TES. Since the experiment searches for rare events, it requires a detector with high efficiency and low dark count rate (DCR). We characterized the intrinsic DCR of a TiAu TES having an area of 20 μm × 20 μm, finding it to be 3.6×10−4 Hz in the 0.8 eV to 3.2 eV range. Furthermore, a deeper study was conducted to categorize the types of dark counts in TES to improve its use in the experiment. The Single and Entangled Photon Sources for Quantum Metrology (SEQUME) project aims to develop high-purity single-photon sources and high-efficiency entangledphoton sources for quantum-enhanced measurements in quantum metrology. A TES with high efficiency is crucial for characterizing these single-photon sources. The results for the SEQUME project were obtained at the PTB in Braunschweig (Germany) during a four-month period abroad in my third year of PhD. In this thesis I present the measurement on a TiAu TES fabricated at AIST (Japan) with a system detection efficiency of 98%, a significant milestone for the SEQUME project’s objectives. Finally, the thesis describes preliminary results on the development of TES with fast recovery time to enhance operation above 1 MHz. Two distinct approaches are presented: one involving the use of Al TES with a high critical temperature, and the second utilizing Au pads to increase thermal conductance. Overall, this thesis encompasses the development, characterization and application of advanced TES devices, significantly contributing to their potential use in metrological and fundamental physics research, pushing the boundaries of their performance.