New Single-Flux-Quantum Logic Circuits with SQUIDs

Hiroki Kodaka, Yoichi Okabe

Daiki Hasegawa, Yuto Takeshita, Feng Li +4 more

We demonstrated interface circuits between a half-flux-quantum (HFQ) circuit and a rapid single-flux-quantum (RSFQ) circuit to obtain SFQ/HFQ and HFQ/SFQ converters. These converters were created by simply inserting a resistor between an RSFQ circuit and the adjacent HFQ circuit, where both circuits are based on pulse logic. These converters enable us to create combinatory circuits that make the best use of the features of the two circuits. Using a DC/SFQ, SFQ/HFQ, HFQ/SFQ, and SFQ/DC converter, we measure an HFQ Josephson transmission line (JTL) composed of eight 0-0-π superconducting quantum interference devices (SQUIDs), which are the essential elements of an HFQ circuit. The 0-0-π SQUIDs are composed of two conventional Josephson junctions and a π-shifted ferromagnetic junction. The HFQ JTL demonstrated in this study is driven by a bias current 11-fold lower than that of a similar RSFQ JTL. These experimental results indicate that the HFQ circuit requires power that is two or more orders of magnitude lower than that of an RSFQ circuit.

V. K. Kaplunenko, V. P. Koshelets, M. Khabipov +2 more

The novel element of the rapid single flux quantum (RSFQ) logic family-the quasi digital voltage amplifier (QDVA)-which was recently proposed theoretically, has now been designed and experimentally investigated. The QDVA can be employed both as a logic element in digital circuits and as analogue voltage amplifier. The correct operation of the SFQ doubler has been demonstrated up to the output voltage 0.5 mV and with very large margins (up to 60%). The proposed QDVA can be integrated on the same chip with a SQUID sensor; it significantly simplifies the matching circuits and seems to be very attractive for the multichannel and digital SQUID systems.

Heinz Jäckel

Ying Ki Kwong, V. Nandakumar

Some basic cells in the rapid single-flux quantum (RSFQ) family of superconductive logic circuits were studied. An input stage, a buffer, and an OR gate were simulated and laid out for a standard niobium-based fabrication process. The resulting circuits perform properly for clock speeds up to 1 GHz. For the simpler circuits tested, the measured margins are wide, consistent with simulations, and not very dependent on clock speed. However, margin decreases with increased circuit complexity. The input stage converts a rising edge into an SFQ pulse, which has a small amplitude and narrow width in time. On the present circuits, these pulses are about 200 mu V in amplitude and 10 ps in width. To facilitate measurement, the authors have chosen as the output an asymmetrical superconducting quantum interference device (SQUID) consisting of two Josephson junctions. It converts SFQ pulses into 2.5 mV latching output levels on chip. By inductively coupling SFQ pulses into the SQUID loop, DC current isolation between the RSFQ circuit-under-test and the output stage is provided. The SQUID was optimized for 2 GHz operations.>

Yoichi Okabe, Hiroki Kodaka