General design aspects of integrated superconductor electronics Original Research Article

The state-of-the-art Rapid Single Flux Quantum (RSFQ) circuits consists of Josephson junctions (JJ), inductances and bias current sources, whereat the inductances are practically used as functional elements. Even more, their value defines the functionality and therefore the conventional design process requires strict conditions for the wiring between JJs. The present design and optimization process does not take into account distributed microwave properties. This becomes an important issue for high-dense ultra fast applications above 100 GHz, but also for circuits dedicated to control quantum electronics with reduced critical current density. We discuss different wiring configurations and analyze their properties. We present our theoretical and experimental results about the influence of the line impedance to the operation of RSFQ circuits. The impedance matching between the connecting microstrip-lines and the JJ is a key issue for the correct operation. We performed circuit simulations and collected experimental data of correct and incorrect operating circuits to derive a design criteria. The critical reflection coefficient is 0.35 and the particular design must use a smaller value to ensure pulse transfer and a larger value for storage of SFQ pulses, respectively.