Characterization and Suppression of Nonlinearities in a Quantum Control System Employing Direct Digital Synthesis*

With continual progress in superconducting qubit lifetimes and gate fidelities, the relevance of nonidealities in classical control signals must be reevaluated. Despite established practices for calibrating imperfections in nonlinear microwave components, modern high-fidelity quantum experiments place increased demand on the temporal stability, spectral purity, and channel count of the classical control signals. In recent years, direct digital synthesis has offered a promising solution by removing the mixer from the signal path entirely [1]. However, weak nonlinearities in the digital-to-analog converter (DAC) can produce undesired spectral content which is known to affect qubit gate fidelities [2]. Building on previous efforts in telecommunications, we demonstrate the characterization and suppression of spurious emissions in a modern mixed-signal microwave control system for superconducting qubits. We contextualize our results by examining the operational limits imposed by imperfect control signals both with and without suppression of undesired distortion.

[1] W. Kalfus, et al. IEEE Trans. Quantum Engineering (2020)

[2] J.P.G. van Dijk, et al. Phys. Rev. Appl. (2019)