Charge-Ramp Demodulated Sensing in an Offset-Charge Sensitive Transmon Array

Modern quantum error correction can be hampered by spatiotemporal-correlated errors, limiting the utility of superconducting qubit processors. The underlying dynamics of nonequilibrium quasiparticles during such an event can be investigated by monitoring the charge-parity in offset-charge-sensitive transmons. However, continuous monitoring is impeded by low-frequency charge noise; some protocols require frequent recalibration of the offset-charge bias point, obscuring visibility of the charge-parity following large discrete offset-charge jumps. In this work, we propose a novel charge-sensing scheme, using direct-dispersive readout in the ground-state manifold, while applying an external voltage modulation, for high-bandwidth extraction of the offset-charge (thus inferring the charge-parity state without interruption). We use this technique to investigate spatiotemporally correlated errors by simultaneously monitoring 4 qubits in an OCS transmon array, demonstrating an offset-charge resolution within 0.1e at a ~1kHz detection bandwidth.