Flexible superconducting Nb transmission lines on thin film polyimide for quantum computing applications

Abstract

We describe progress and initial results achieved towards the goal of\ndeveloping integrated multi-conductor arrays of shielded controlled-impedance\nflexible superconducting transmission lines with ultra-miniature cross sections\nand wide bandwidths (dc to >10 GHz) over meter-scale lengths. Intended\nprimarily for use in future scaled-up quantum computing systems, such flexible\nthin-film Nb/polyimide ribbon cables provide a physically compact and ultra-low\nthermal conductance alternative to the rapidly increasing number of discrete\ncoaxial cables that are currently used by quantum computing experimentalists to\ntransmit signals between the low-temperature stages (from ~ 4 K down to ~ 20\nmK) of a dilution refrigerator. S-parameters are presented for 2-metal layer Nb\nmicrostrip structures with lengths ranging up to 550 mm. Weakly coupled\nopen-circuit microstrip resonators provided a sensitive measure of the overall\ntransmission line loss as a function of frequency, temperature, and power. Two\ncommon polyimide dielectrics, one conventional and the other photo-definable\n(PI-2611 and HD-4100, respectively) were compared. Our most striking result,\nnot previously reported to our knowledge, was that the dielectric loss tangents\nof both polyimides are remarkably low at deep cryogenic temperatures, typically\n100$\\times$ smaller than corresponding room temperature values. This enables\nfairly long-distance transmission of microwave signals without excessive\nattenuation and permits usefully high rf power levels to be transmitted without\ncreating excessive dielectric heating. We observed loss tangents as low as\n2.2$\\times$10$^{-5}$ at 20 mK. Our fabrication techniques could be extended to\nmore complex structures such as multiconductor, multi-layer stripline or\nrectangular coax, and integrated attenuators and thermalization structures.\n