Direct Measurement of the Switching Waveform in a DC–SQUID

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Abstract
A high-resolution Josephson sampler using a single fluxoid is fabricated. The sampling pulse generator comprised a two-junction interferometer (DC–SQUID) with a damping resistance resistively coupled to a single junction as the sampling gate to reduce a turn-on delay. An ultra-fast sampling pulse was generated at a vortex-to-vortex transition in this sampler. The fabricated sampler had less than a 5ps time resolution and about a 5 µA current sensitivity. The dynamic waveform associated with two kinds of transitions was observed by the use of this sampler: a vortex-to-vortex transition and a vortex-to-voltage transition. The results show that the pulse corresponding to a single fluxoid has a nearly constant pulse height that is independent of the bias current and that setting the bias current near the critical point results in the long rise time of a DC–SQUID.