Superconducting Isolator Design and HIPIMS Optimization of NbTiN/NbN Thin Films

The advancement of superconducting quantum computing technologies increasingly relies on the availability of low-loss, wide-bandwidth microwave amplifiers and non-reciprocal components such as isolators and circulators. This thesis addresses two principal challenges in this context: the optimization of superconducting nitride films through advanced deposition techniques and the realization of magnet-free non-reciprocal devices based on parametric modulation. On the materials side, the main focus is on ultrathin niobium nitride (NbN) and niobium titanium nitride (NbTiN) films, whose superconducting properties are strongly influenced by stoichiometry, disorder, and microstructure. High Power Impulse Magnetron Sputtering (HiPIMS), an advanced deposition technology, is employed to systematically investigate and optimize process parameters for NbN and NbTiN, with the goal of surpassing conventional DCMS methods. HiPIMS is shown to generate denser films with enhanced critical temperatures, thereby providing a solid foundation for the fabrication of superconducting devices. Building on these material advances, the work progresses toward non-reciprocal superconducting circuits. A travelling-wave parametric amplifier (TWPA) based on NbTiN transmission lines is adapted to operate as a parametric isolator, exploiting the nonlinear kinetic inductance of thin films under parametric pumping. In parallel, the high-impedance forward coupler introduced by Colangelo et al. is adopted as a compact and scalable element to interconnect isolators. A potential four-port circulator architecture is proposed by merging these devices, enabling magnet-free non-reciprocity. This thesis outlines a comprehensive framework that encompasses both HiPIMS-based thin-film optimization and the design of superconducting isolators and circulators. By merging advanced thin-film engineering with novel circuit topologies, it provides a new pathway for their integration into next-generation quantum computing and cryogenic detection systems.