Measurement of leakage to higher qubit levels when using SFQ digital control.

The challenge to scale up superconducting quantum processors to thousands of qubits in a dilution refrigerator has been met with proposals to develop cryogenic quantum-classical interfaces, such as cryogenic CMOS, superconducting single-flux-quantum (SFQ) circuits and photonic circuits. SFQ circuits are a promising candidate for a scalable cryogenic quantum control system, especially for superconducting qubits, due to their speed and low power consumption. However, reduced gate fidelities attributed to quasiparticle poisoning have been observed in some SFQ implementations. In a recent publication, our group showed that by moving the SFQ control circuits to the 4 K stage of the dilution fridge, gate fidelities are significantly improved. While this qubit control scheme is simple and easy to implement, the fidelity of the digital gate operations is generally limited by leakage to higher qubit levels when operated in the traditional resonant mode using a train of periodic pulses. In this presentation, I will show an experimental analysis of the leakage when using SFQ resonant drive signals and discuss how we plan to mitigate this issue in future implementations.