Optimal Control for Correlated Measurements of Superconducting Candle Qubits with Crosstalk

Spurious interqubit couplings (crosstalk) present a major challenge in superconducting quantum processors. Crosstalk arises from static, ZZ, and capacitive couplings, leading to errors that degrade the fidelity of quantum operations. Mitigation strategies such as dynamical decoupling, tunable couplers, and frequency tuning help reduce these effects, but careful device design remains crucial.

In this study, we measured unwanted coupling in a "candle qubit" device with six high-coherence, weekly-coupled superconducting transmon qubits. While coherence times were measured as high as 200 microseconds, as expected, residual excited state population measurements indicated that the qubits were not in thermal equilibrium, likely due to interaction-induced heating. Cross-coupling between qubits as small as 500 Hz was observed. We developed an optimal quantum control scheme to minimize the impact of crosstalk, and multiplexed readout was performed to assess qubit behavior for correlation studies.