Single flux quantum circuit and packaging technology for sub-Kelvin temperature operation

Superconducting single flux quantum (SFQ) technology is well known for digital logic operations below 4.2 K, which are compatible with the Josephson junction qubit. Operation speed and signal sensitivity of circuits in SFQ are also sufficient to control qubits. Therefore, the SFQ circuit is a candidate technology for multiple qubit control. However, the self-heating effect of the SFQ circuit is a potential barrier to development of an SFQ-supported multiple qubit system because the qubit operating temperature is lower than the conventional 4.2 K. The multi-chip approach is a possible solution because the qubit chip can be thermally isolated from the SFQ circuitry. We report on recent progress in SFQ circuit and packaging technology for operation at sub-Kelvin temperatures. As for multi-chip module technology, a test sample of the multi-chip module shows good reliability of electrical connections between chips, and each solder bump has a bandwidth wide enough for signal transmission. We have designed lower power SFQ circuits for sub-Kelvin temperatures. The critical currents of Josephson junctions were scaled down, inductances were scaled up, and bias voltage was reduced to make power consumption lower. Based on our fabrication technology, critical current densities were optimized by considering trade-off relations between fabrication availability and operation speed. We successfully confirmed its correct operations at around 0.35 K. The combination of the multi-chip module and optimized SFQ circuits is very effective way of controlling qubit.