Photonic quantum technologies have rapidly progressed over the last years, moving from a laboratory scale to the industrial world. Such advancements have put more and more stringent requirements on single-photon detectors. For example, the development of a broad quantum network, and many quantum computing protocols require detectors able to precisely resolve the number of photons in the optical state. Moreover, quantum communication protocols such as Quantum Key Distribution (QKD) need fast detectors in order to exchange the secret key at the highest possible rate.

This thesis aims to use Superconducting Nanowire Single-Photon Detector (SNSPD) arrays to tackle these challenges, offering a solution for the further development of the photonic quantum world. SNSPDs have demonstrated remarkable performances in terms of efficiency, count rate and timing jitter, setting them apart from the different single-photon platforms.

In these years, we extensively worked on improving the Parallel SNSPD (PSNSPD) technology. We started from a high-efficiency 4-pixel device based on MoSi superconducting material, and we developed a generalized model to describe the multi-photon detection probability. We used this device to improve a Heralded Single-Photon Source (HSPS) by 27%, by filtering out the detected multi-photon pairs. We described the limitation of MoSi to achieve the best Photon-Number Resolving (PNR) capabilities, and we developed a new superconducting material, NbTiN.

Thanks to the material properties we were able to push the technology limits and we realized an interleaved 28-pixel P-SNSPD which exhibits state-of-the-art performances for a single coaxial cable read-out detector: Maximum Count Rate (MCR) of 250 Mcps, < 60 ps jitter at 100 Mcps, Photon-Number Resolving (PNR) operation at 40MHz, 2-photon efficiency of 75% and a 3-photon efficiency of 62%.

We also studied the Multi-Pixel SNSPD (MP-SNSPD), achieving ultra-high count rate (in the Gcps range) that enabled to exchange the secret key in QKD protocol at 64 Mbps over 10km. We demonstrated the suitability of MP-SNSPD as PNR detectors for ns-long light pulses, thus enabling operation with narrowband photons that are required for quantum repeaters protocols.

Additionally, we evaluated the performance of different PNR approaches to single-shot measurement. We defined the confidence values, and we showed that SNSPD arrays can perform as well as an intrinsic PNR detector, in the low photon-number regime. In the end, we drew a detailed comparison of the most used PNR approaches, proposing some properties and features that we believe define the ideal PNR detector.