Developing Superconducting Nanowire Single Photon Detectors with Photon Number Resolving Capability

Introduction: Superconducting nanowire single-photon detectors (SNSPDs) are pivotal to quantum information science due to their high detection efficiency and low timing jitter. However, traditional SNSPDs are limited by a binary response, necessitating the development of photon number resolving (PNR) capabilities for advanced quantum communication and sensing. While spatial multiplexing has enabled PNR resolution up to 24 photons, scaling these architectures to 32 pixels while maintaining detection uniformity and minimizing crosstalk remains a significant technical challenge.

Methods: We report the complete development cycle of a PNR-SNSPD, from design optimization to nanofabrication. The fabrication process depositing a 6 nm superconducting thin film via DC magnetron sputtering at room temperature. A three-step electron beam lithography (EBL) process was employed: PMMA for contact pads (10 nm Ti/60 nm Au), HSQ (hydrogen silsesquioxane) for the nanowire geometry, and a final PMMA layer as a resist for AuPd resistors (5 nm Ti/85 nm Au). The nanowire pattern was transferred using hybrid reactive ion etching (RIE), whereas the contact pad and resistor patterns were defined by electron beam evaporation followed by a lift-off procedure.

Results: SEM characterization successfully verified the morphological integrity of the 32-pixel PNR-SNSPD arrays. Material optimization of NbTiN films demonstrated that the superconducting transition temperature (Tc) is highly sensitive to the nitrogen concentration ratio. We achieved a peak Tc of approximately 11.4 K for thicker films and around 9 K for thinner films at 5% nitrogen concentration, followed by a monotonic decrease in Tc as the nitrogen concentration increased toward 25%.

Conclusions: Our results demonstrate a robust fabrication pathway for high-density 32-pixel SNSPD arrays. The optimization of NbTiN deposition parameters provides a viable route to improving device performance and operational stability. These advancements contribute to the scalable production of PNR-capable detectors essential for high-fidelity quantum state characterization.