Low-Jc Rapid Single Flux Quantum (RSFQ) Qubit Control Circuit

We designed a 30 A/cm² RSFQ-qubit control circuit and tested operation at 4 K. The integration of RSFQ technology and qubits monolithically requires great attention to thermal budgeting and electromagnetic compatibility. These issues are of primary concern when developing an RSFQ-qubit experiment. The interface of an SFQ circuit to a qubit requires tunable coupling since RSFQ inherently limits logical elements to single-flux storage. By measuring the quantum-tunneling rate of a phase qubit, we can determine the effect of changing the coupling strength between the classical and quantum systems. By looking at the same system, we can diagnose the impact, whether thermal or electromagnetic, of an active digital circuit. These tests would set an upper bound on the deleterious effects of monolithic integration.