  Interconnects are a major discriminator for superconducting digital
technology, enabling energy efficient data transfer and high-bandwidth
heterogeneous integration. We report a method to simulate propagation of
picosecond pulses in superconducting passive transmission lines (PTLs). A
frequency-domain propagator model obtained from the Ansys High Frequency
Structure Simulator (HFSS) field solver is incorporated in a Cadence Spectre
circuit model, so that the particular PTL geometry can be simulated in the
time-domain. The Mattis-Bardeen complex conductivity of the superconductor is
encoded in the HFSS field solver as a complex-conductivity insulator.
Experimental and simulation results show that Nb 20 Ohm microstrip PTLs with
1um width can support propagation of a single-flux-quantum pulse up to 7mm and
a double-flux-quantum pulse up to 28mm.
