Embedded Niobium Using PI-2611 for Superconducting Flexible Cables

Dense, controlled-impedance, superconducting cables with small cross-sections are desirable, especially for quantum computing applications. In this study, superconductivity properties, rf microwave response and mechanical reliability performance of embedded Nb dc cables and Nb microstrip transmission line resonators with different thicknesses of polyimide PI-2611 encapsulation layers (0, 4 and 8 µm) have been investigated. Critical temperature (Tc) and critical current (Ic) of embedded Nb dc cables are ~ 8.2 K and ~ 0.2 A, respectively. Embedded Nb resonators yield high loaded quality factor (QL), with values as high as 14481 at ~ 1.2 K and at a fundamental resonance of ~ 2 GHz. From mechanical fatigue testing, we have observed that a polyimide encapsulation layer can effectively enhance the mechanical reliability of superconducting Nb flexible cables.