Comparative study of high-$J_{c}$ and low-$J_{c}$ SFQ microwave generators for transmon control

Abstract Scalability of superconducting quantum processors is increasingly constrained by the thermal load and wiring overhead associated with room-temperature control hardware. Cryogenic control interfaces based on single-flux-quantum (SFQ) logic offer a potential route to alleviating these bottlenecks, although their dissipation depends sensitively on the Josephson-junction critical current and the underlying fabrication process. In this work, we comparatively investigate SFQ-synthesized microwave generators implemented in high- and low-critical-current processes, and evaluate their suitability for transmon qubit control through a combination of experimental characterization and hybrid QuTiP–JoSIM analysis. By modulating SFQ pulse intervals, we synthesize coherent microwave drives and obtain single-qubit gate fidelities above 99.9% at 10 mK in QuTiP-based simulations for both implementations. The low- $J_{\mathrm{c}}$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>J</mml:mi> <mml:mi>c</mml:mi> </mml:msub> </mml:math> implementation exhibits reduced phase-noise-induced error, achieves higher gate fidelity within its valid bias window, and shows improved robustness against frequency detuning, while the high- $J_{\mathrm{c}}$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>J</mml:mi> <mml:mi>c</mml:mi> </mml:msub> </mml:math> implementation maintains a lower broadband noise floor. Using an explicit model of thermal dissipation and circuit footprint, we further estimate the integration limits of cryogenic control channels. The results show that reducing the switching current substantially relaxes the cooling-budget constraint and increases the number of channels that can be deployed simultaneously. These results clarify the trade-offs associated with implementing SFQ microwave generators in different fabrication processes and provide practical guidance for the design of scalable cryogenic qubit-control interfaces.