Fabrication of High-Speed and High-Density Single-Flux-Quantum Circuits at NIST

P. F. Hopkins, Manuel C. Castellanos Beltran, Christine A. Donnelly +4 more

Boyko Dimov, D Balashov, M. Khabipov +5 more

The reduction of the critical current density in rapid single-flux quantum (RSFQ) circuits enables new application fields, like quantum computing and photonic detector readout. The low current density fabrication process creates new design challenges, such as lower stability against thermal fluctuations, violation of the lumped elements condition for microstrip inductances and increased sensitivity to the technological spread. To overcome these issues, we suggest a passive phase shifter as a promising alternative technique for superconductive phase dropping in the RSFQ electronics. Here, we study experimentally their applicability in high-speed RSFQ digital circuits. Conclusions are drawn about the impact of the passive phase shifters on the complexity, the speed and the bit error rate of the investigated RSFQ circuits. We demonstrate the successful operation of different circuits with implemented passive phase shifters at low and high speeds.

Mutsuo Hidaka, Nagasawa Shuichi, K. Hinode +1 more

David I. Olaya, Paul D. Dresselhaus, Samuel P. Benz

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.

Shigeyuki Miyajima, Hirotaka Terai, Shigehito Miki

We demonstrated a coincidence circuit based on single flux quantum (SFQ) circuits with high-time accuracy for a two-photon interferometer using superconducting nanostrip photon detectors (SNSPDs). The coincidence circuit was designed to have 3 different time windows of 30 ps, 100 ps, and 200 ps, which can be selectable on demand. Furthermore, each time window can be fine-tuned by changing the applied bias currents. The circuit was fabricated by the AIST high-speed standard process based on Nb/AlOx/Nb junctions with critical current density of 10 kA/cm2. We experimentally evaluated the time window of the coincidence circuit at temperatures below 2.4 K in a 0.1-W Gifford-McMahon (GM) cryocooler.

Akira Fujimaki, Takahiro Yamada, Masamitsu Tanaka +4 more