In the field of radiation instrumentation, there is a desire to reach a sub-10 ps FWHM timing resolution for applications such as time-of-flight positron emission tomography, time-of-flight positron computed tomography and time-resolved calorimetry. One of the key parts of the detection chain for these applications is a single-photon detector and, in recent years, the first single-photon avalanche diode (SPAD) with a sub-10 ps timing resolution was presented. To reach such a timing resolution, the SPAD was read out by an operational amplifier operated in open-loop as a comparator. This paper presents a comparison between comparators and inverters to determine which type of leading-edge discriminator can obtain the best single-photon timing resolution. Six different quenching circuits (QCs) implemented in TSMC 65 nm are tested with SPADs of the same architecture and in the same operation conditions. This allows us to compare experimental results between the different QCs. This paper also presents a method to measure the SPAD signal slope, the SPAD excess voltage variation and simulations to determine the added jitter of different leading-edge discriminators. For some discriminator architectures, a cascode transistor was required to increase the maximum excess voltage of the QC. This paper also presents the impact on the single-photon timing resolution of adding a cascode transistor for a comparator or an inverter-based discriminator. This paper reports a 6.3 ps FWHM SPTR for a SPAD read out by a low-threshold comparator and a 6.8 ps FWHM SPTR for an optimized 1 V inverter using a cascode transistor for a higher excess voltage.

The research on superconducting circuits for quantum information technologies progressively extends from the academic world towards the industry. Material sciences therefore become an essential lever for the development of technologies based on such circuits as they allow finding industry-compatible routes of improvements. In this context, we propose a study on the behavior of the niobium nitride (NbN), one of the most commonly used material for these devices, when exposed to strong magnetic fields. To this aim, the properties of NbN layers with the same composition but different microstructures, tuned by changing the deposition method, are compared. This study aims to establish interdependencies between the microstructure of the material and its behavior once exposed to a magnetic field. X-ray diffraction and Hall-effect characterizations are used to assess that the microstructure is significantly modified by the choice of the deposition technique. Pushing further these investigations also allowed to quantify and compare the level of disorder in both cases by extracting the Ioffe-regel and the Ginzburg-Landau parameters from characterizations of the superconducting transition temperature for several magnetic field amplitudes. This was used to conclude that in highly disordered NbN layers, the microstructure heterogeneities are responsible for a strong electron localization allowing to significantly enhance the resilience of their superconducting state under strong magnetic fields.