Improving Photon Sensitivity of High Temperature Superconductor (HTS) Superconducting Nanowire Single Photon Detector (SNSPD) by Making it Dirty

Implementation of high temperature superconductors (HTS) in superconducting nanowire single photon detector (SNSPD) is subjected to intensive research; still the effect of different superconducting properties of HTS on the nanowire response is largely unexplored. Dirtiness of the HTS is expected to play a crucial role as it affects the effective coherence length and other superconducting properties. Considering YBa₂Cu₃O_7-δ (YBCO) here due to the maturity in the thin film synthesis, ultra-small coherence length (ξ₀ ~ 2 nm), controllable diffusion coefficient (D) and faster thermal interface with the substrate. While excellent timing response is observed in HTS nanowires with picosecond (ps) reset time (τ_reset), the minimum number of photons required to elicit a response is still much larger than low temperature superconductor (LTS). We developed a comprehensive model based on the two-fluid theory incorporating temperature dependent thermodynamic properties, Joule heating and time dependent thermal source from the incident photon (wavelength, λ = 1550 nm). Using the proposed model, we showed that increase in dirtiness of about 10% leads to a 14.5 fold reduction in the photon sensitivity, as lower D hinders fast diffusion of electrons, favoring fast temperature rise. However, it must also be noted that τ_reset is within ps limit thereby still providing an order of magnitude higher than the LTS equivalent. Thus, with proper control over dirtiness HTS can resolve single photon having reset time in ps regime.