Response of a Josephson junction to a stepped voltage pulse

We have developed a fully cryogenic electro-optic sampler by integrating and immersing in superfluid helium both the photoconductive switch and the birefringent lithium tantalate sensor. The ultimate temporal response of the system as determined by measuring the transient onset of photoconductivity in a GaAs switch was less than 400 fs. The system was arranged into a coplanar transmission line geometry and used to probe the switching of a 30 × 30 μm²Josephson tunnel junction. Both time-dependent waveforms of the transmitted signal and current-voltage characteristics of the junction switched by a picosecond input pulse were measured. The results were compared to transient simulations based on the resistively and capacitively shunted junction model, and showed that: the initial junction response was limited only by the time integral of the input pulse, the switched voltage was proportional to the total charge delivered by the pulse, and there was no turn-on delay time. This represents the first, in situ measurement of the response of an unbiased Josephson tunnel junction to a stepped, picosecond pulse.