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Ultrafast Electronics and Optoelectronics (UEO)

First Direct Observation of Single-Flux-Quantum Pulses

D. Jacobs-Perkins, Marc Currie, Chia-Chi Wang, Roman Sobolewski, M. J. Feldman, Thomas Y. Hsiang

Published Jan 1, 1995• 0 citations
Publisher

Abstract

We report the first direct and nonintrusive measurement of single-flux-quantum (SFQ) pulses propagating on a superconducting microstrip line. We present and discuss experimental results obtained with reflective electro-optic (EO) sampling.
DOI
10.1364/ueo.1995.umd5

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We report on the direct observation of a single-flux-quantum (SFQ) pulse. The response of a metal–semiconductor–metal photodiode to a femtosecond laser pulse was used to switch Josephson junctions and to generate an SFQ voltage pulse on a superconducting microstrip line. The detailed shape of the pulse was measured optoelectronically, using a cryogenic electro-optic sampling system. The measured SFQ pulse had a width of 3.2 ps, an amplitude of 0.67 mV, and a total pulse content of 2.1±0.2 mV×ps, corresponding to the quantum of magnetic flux h/2e. With larger excitation, multiple SFQ pulses were observed. Numerical simulations are shown to be qualitatively similar to our experimental results.
We have generated picosecond voltage pulses on a superconducting microstrip line by using a metal-semiconductor-metal photodiode as an optoelectronic switch. These pulses are fed into a two-Josephson-junction pulse shaper to generate single-flux quantum (SFQ) pulses. Using a reflective electro-optic sampling system, SFQ pulses are directly observed for the first time. This important demonstration of nonintrusively detecting electrical signals from superconducting microstrip lines at the level of rapid, single-flux quantum (RSFQ) circuits opens up a new way to test such circuits, on issues such as design verification, jitter, and failure-mode testing. Further, we propose a variable-rate optoelectronic clock for testing the functional speed of RSFQ logic circuits, with an adjustable clock rate up to 38 Gb/s.>
The ballistic transport of picosecond digital signals along terminated microstrip lines is a key feature of single flux quantum (SFQ) logic. In this work, we have used fast photoconductors driven by a femtosecond laser to measure directly for the first time the propagation and termination of picosecond pulses along Nb microstrip lines. Electrical pulses as short as 0.8 ps full-width-at-half-maximum (FWHM) have been measured. After propagating distances of 200 /spl mu/m, 1 mm, and 6.5 mm, the pulses broadened to 1.0, 1.2, and 1.8 ps (FWHM), respectively. In the frequency domain, attenuation is found to be negligible up to the gap frequency of Nb(0.7 THz), beyond which attenuation increases rapidly. We have also measured the reflections of the picosecond pulses off different terminations: open circuit, short circuit, and various resistors. The effective suppression of reflections with matched resistive loads is demonstrated up to frequencies approaching 1 THz.>
The authors analyzed, designed, and tested drivers and receivers which make it possible to connect distant circuits of the rapid single-flux-quantum (RSFQ) logic/memory family by passive superconducting microstrip lines. Using these circuits implemented with a niobium trilayer fabrication technology, reliable transmission and reception of the SFQ pulses over distances up to 1 cm, with margins of bias currents as wide as +or-30%, have been demonstrated. The pulses can be passed along wide lines (20 mu m for 3.5 mu Nb-Al/sub 2/O/sub 3/-Nb Josephson junction technology) over such distances and can be picked up by the RSFQ receiver.>