Bruscino, Ciro (2025) Superconducting Nanostrip Single-Photon Detectors applications to Quantum Technologies. [Tesi di dottorato]

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Abstract

SPDs are widely employed across several applications, such as quantum communications, quantum optics, quantum computing, and LIDAR. The working principles underlying these devices can be leveraged to obtain unique features. Among the most relevant are ultrafast single-photon detection, with dead times of only a few nanoseconds and timing jitter on the order of picoseconds, and photon-number-resolving capability. These properties enable the investigation of phenomena with no classical counterpart, such as quantum interference effects and entanglement. Moreover, SPDs are among the fundamental components of QKD systems that, exploiting the quantum properties of light, enable secure communications over hundreds of kilometres without intermediate trusted nodes. Within the QKD framework, SNSPDs are among the most widely adopted detectors, thanks to their near-unity SDE and ultralow dark-count rates at telecom wavelengths, particularly around 1550 nm, where fibre propagation losses are minimal. First demonstrated in 2001, SNSPDs have been both optimised for performance and extended to the MIR spectrum. In parallel, nanostrip-based architectures have been engineered to provide photon-number-resolving functionality, yielding superconducting PNRDs. This thesis focuses on applying high-performance SNSPDs and PNRDs to characterize an SPDC source exhibiting entanglement in the photon-number mode. Then, the intrinsic randomness of PNRD dark counts has been harnessed to implement a light-source-less RNG. Moreover, SNSPDs with low DCR and high efficiency have been integrated with a commercial QKD device to demonstrate key exchange over channel losses equivalent to hundreds of kilometres of fibre, emulated via calibrated optical attenuators. Together, these results are a clear demonstration of how state-of-the-art SNSPDs are powerful tools that can be used to study quantum effects and exploit them to implement devices that guarantee intrinsic security.

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