An application of picosecond electro-optic sampling to superconducting electronics

A picosecond electro-optic sampling technique has been used to measure the switching threshold of a single tunnel junction in a coplanar transmission line geometry. The optical system used, a colliding-pulse mode-locked laser, generated two 120 fs FWHM at 100 MHz. One pulse was used, via a photoconductive switch, to generate an electrical signal of adjustable height and width. The second pulse was used to detect the electrical transient by probing the change in the birefringence of a lithium tantalate crystal induced by the electrical signal. This optical sampling scheme had intrinsic temporal and voltage resolutions of < 500 fs and < 1 mV. In actual experiments, the connection-limited time resolution was 55 ps or 16 ps, depending on whether the detector was placed in room or cryogenic environment. The measured response of the tunnel junction was found to be consistent with the RSJ model.