Understanding and suppressing sources of decoherence is a leading challenge in building practical quantum computers. In superconducting qubits, low-frequency charge noise is a well-known decoherence mechanism that is effectively suppressed in the transmon qubit. Devices with multiple charge-sensitive modes can exhibit more complex behaviors, which can be exploited to study charge fluctuations in superconducting qubits. Here we characterize charge sensitivity in a superconducting qubit with two transmonlike modes, each of which is sensitive to multiple charge-parity configurations and charge-offset biases. Using Ramsey interferometry, we observe sensitivity to four charge-parity configurations and track two independent charge-offset drifts over hour time-scales. We provide a predictive theory for charge sensitivity in such multimode qubits which agrees with our results. Finally, we demonstrate the utility of a multimode qubit as a charge detector by spatially tracking local-charge drift.

• Received 11 August 2021
• Accepted 6 January 2022

DOI:https://doi.org/10.1103/PhysRevApplied.17.024058