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Physica C Superconductivity

High-speed measurement method of quantized energy levels in Josephson junctions using SFQ circuits

Yuki Yamanashi, M. Ito, A. Tagami, Nobuyuki Yoshikawa

Published Jul 8, 2004• 4 citations
Publisher
DOI
10.1016/j.physc.2003.12.090

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A single flux quantum (SFQ) circuit to measure quantized energy levels in a Josephson junction has been proposed and implemented, which allows us to perform high-speed and low-noise measurements of the escape rate in Josephson junctions. By using this technique, the measurement of escape rate has been carried out at 4.2 K, the temperature three times higher than the crossover temperature. The test results show periodic dependence of the escape rate on the junction bias current, which is caused by the energy quantization effect in the Josephson junction.
We investigated high-current-density Josephson junctions with NbxSi1-xbarriers in single-flux-quantum (SFQ) digital circuits to evaluate their performance at clock frequencies of hundreds of gigahertz. We fabricated static divider SFQ circuits with a-NbxSi1-xand a-Si barriers for Jcranging from 3.5 to 85 kA/cm2. For junctions in the lower Jcrange (3.5-17 kA/cm2), we used optical lithography, and for the 85-kA/cm2junctions, we used electron-beam lithography. We measured SFQ circuits with a maximum operating speed of 300 GHz by use of intrinsically shunted junctions having submicrometer dimensions and an 85-kA/cm2critical current density. These results demonstrate that silicide-barrier junctions have the potential to achieve high-speed SFQ circuits and high-density circuits because the junctions are small and do not require external shunt resistors.