Superconducting Qubit Control with Single Flux Quantum Pulses in a Multi-chip Module*

Conventional techniques for controlling superconducting qubits are challenging to extend to large multi-qubit arrays. A promising candidate for scalable control is the Single Flux Quantum (SFQ) digital logic family. In an initial monolithic implementation with qubits and SFQ elements on the same chip, the fidelity of SFQ-based qubit gates was limited by poisoning of the qubit from quasiparticles (QPs) generated by SFQ phase slips. In order to suppress poisoning from QP diffusion and the propagation of phonons generated by QP recombination, we have developed a multi-chip module with an SFQ driver on a classical control chip that is flip-chip coupled to a superconducting transmon qubit on a separate quantum chip. We characterize the QP poisoning in these devices for different qubit geometries, and we compare with earlier measurements on single-chip implementations of SFQ-based qubit control. We also discuss strategies for further mitigation of QP poisoning, including superconductor bandgap engineering.