Ballistic SFQ signal propagation on-chip and chip-to-chip

We recently reported communication up to 60 Gb/s between digital superconductor chips mounted on a passive carrier, using a novel driver circuit that produces a double-flux-quantum pulse. Here, we answer various practical questions pertaining to chip-to-chip and on-chip communication in greater detail. On-chip, the traditional single-flux-quantum driver is adequate, although margins and bit error rate degrade significantly at the microstrip resonance frequency. Chip-to-chip, using the double-flux-quantum driver, shows little frequency dependence, in spite of 13% reduction of signal current in the chip-to-chip transitions. Appropriate microstrip impedance is in the range 4-8 Ω for a 4 kA/cm² Josephson junction IC process. Data includes bit errors rates for pseudo-random data in the range 3-27 Gb/s.