Waveforms of single flux quantum pulses imaged with a Josephson sampler circuit

We present electrically-sampled waveforms measured at 4 K of picosecond-width single flux quantum (SFQ) pulses output by different superconductor SFQ digital circuits galvanically connected on-chip to a Josephson sampler subcircuit.  The Josephson sampler consists of a single hysteretic (latching) Josephson junction (JJ) called the comparator, with three inputs: a dc current bias line, a JJ-based circuit that provides a picosecond-duration strobe current pulse with variable delay, and the output from the SFQ digital circuit (the device under test or DUT). Our SFQ and sampler circuit was designed and fabricated on silicon using niobium JJs with amorphous Si barriers (Nb/a-Si/Nb) and critical current density of J_c ~ 0.2 mA/μm². We show how the sampled waveform varies with the input bias parameters to the DUT and to the sampler subcircuit; we also show sampled SFQ pulses from the same DUT circuit but designed with a different critical current for the JJ generating the SFQ output. Finally, we compare the measured waveforms with simulations of the waveform sampling process, demonstrating the influence the sampler has on the perceived pulse waveform.