Separation of Loss Mechanisms in Nb Superconducting Thin-Film Transmission Lines

Transmission losses in Nb superconducting coplanar waveguide (CPW) and microstrip lines were quantified at 2-mm wavelength and 3.3 K using a network analyzer. The loss measurement utilized both half-wavelength resonators and electrically long transmission lines. Validation of the measurement was achieved using a silicon membrane-based waveguide-to-CPW transition with low return loss across a broad bandwidth. Intrinsic losses were measured within a parametric space defined by frequency, temperature, signal power, and magnetic field. The multidimensional information allowed the breakdown of overall losses into major components, including quasiparticle loss, two-level system dielectric loss, vortex flow loss, and radiation loss. The performance of these transmission lines was extrapolated into the terahertz range based on these findings.