Single Flux Quantum-Based Digital Control of Superconducting Qubits in a Multi-Chip Module

The single flux quantum (SFQ) digital superconducting logic family has been proposed as a practical approach for controlling next-generation superconducting qubit arrays with more favorable scaling properties compared to conventional microwave-based control. In the initial implementation, the SFQ-based gate fidelity was limited by quasiparticle (QP) poisoning induced by the on-chip dissipative SFQ circuitry. In this work, we introduce a multi-chip module architecture to suppress phonon-mediated QP poisoning, where the SFQ elements and qubits are fabricated on separate chips that are joined with In bump bonds. In this work, we achieve an error per Clifford gate of 1.2(1)%, showing an order of magnitude reduction over the initial realization of SFQ-based qubit control. Additionally, we perform purity benchmarking of the SFQ-based gates to quantify the incoherent error; we measure 0.96(2)%, which can be attributed to photon-mediated QP poisoning through the electromagnetic coupling between the SFQ circuitry and the qubit. With a clear path to further reduce gate errors in future implementations, we thus demonstrate the feasibility of high-fidelity SFQ-based qubit control.