Rapid Single Flux Quantum (RSFQ) Circuits

Fumihiro China, Tatsuya Narama, Naoki Takeuchi +3 more

We investigated interface circuits between rapid single-flux-quantum (RSFQ) circuits and adiabatic quantum-flux-parametron (AQFP) circuits for a high-speed and low-power hybrid computing system. In addition, the interface circuits allow us to use long-distance interconnections based on single-flux-quantum (SFQ) passive transmission lines between AQFP gates. The target frequency of interface circuits is several gigahertz. There are two types of interface circuits. The RSFQ/AQFP interface circuit, which transmits a signal from RSFQ circuits to AQFP gates, was realized by coupling an RSFQ toggle flip-flop (TFF) to an AQFP buffer gate. The simulated bias margins of the RSFQ TFF and the excitation current margin of the RSFQ/AQFP interface were 2.22 mA ± 24.7% and 1.71 mA ± 25.0%, respectively. The AQFP/RSFQ interface circuit, which transmits a signal from AQFP gates to RSFQ circuits, was designed by using a high-sensitive magnetically coupled dc/SFQ converter (MC-dc/SFQ) to detect an output current of an AQFP buffer gate. The simulated bias margins of the MC-dc/SFQ and the excitation current margin of the AQFP/RSFQ interface were 0.558 mA ± 20.5% and 1.95 mA ± 36.1%, respectively. We also fabricated and measured the interface circuits.

Gleb Krylov, Tahereh Jabbari, Eby G. Friedman

Juha Hassel, Leif Grönberg, P Helistö

It has been suggested that rapid single flux quantum (RSFQ) devices could be used as the classical interface of superconducting qubit systems. One problem is that the interface acts as a dissipative environment for a qubit. Recently, ways to modify the RSFQ damping to reduce the dissipation have been introduced. One of the solutions is to damp the Josephson junctions by a frequency-dependent linear circuit instead of the plain resistor. The approach has previously been experimentally tested with a simple SFQ comparator. In this paper we perform experiments with a full RSFQ circuit, and thus conclude that in terms of stable operation the approach is applicable to scalable RSFQ circuits. Realization and optimization issues are also discussed.

Pascal Febvre

Vinod Kumar Khanna

Oleg A. Mukhanov

Key digital circuits for signal processing with tens of gigahertz speed and a few microwatts power dissipation have been recently demonstrated using a simple, 2 /spl mu/m linewidth process. These are the fastest digital circuits reported to date. These circuits use novel rapid single flux quantum (RSFQ) technology that takes advantage of the fundamental properties of superconductivity. In this technology, data and clock are picosecond-long quantized voltage pulses corresponding to the transfer of single magnetic flux quanta, /spl Phisub 0/=h/2e=2 mVps. These SFQ pulses are regenerated at each gate and are processed by circuits comprised of overdamped Josephson junctions interconnected via microstrip inductances. The inherent internal memory of all RSFQ gates (primarily flip-flops), combined with high switching speed, makes serial, bit-pipelined circuit design advantageous. This paper presents experiments in RSFQ digital circuits, including an N/spl times/8b serial multiplier, and on-chip test systems based on RSFQ shift registers and on SFQ samplers. They enable test and study of RSFQ circuits at full GHz speed while communicating with semiconductor electronics at low speed.>