In situ Characterization and Compensation of Flux Pulse Distortions at Long Time Scales

Accurate flux control of superconducting transmon qubits in quantum processors is a key ingredient for achieving high-fidelity two-qubit gates. While controlling nanosecond-scale dynamics is crucial for the fidelity of individual gates, time ranges up to tens of microseconds are relevant for combating memory effects during sequences of gates, and also for enabling flux-pulse-assisted characterization measurements. To accurately compensate flux pulse distortions, the flux line response needs to be characterized in situ, i.e., by using the qubit itself as a measurement device, which guarantees that the flux line configuration is exactly the same as during the later operation of the quantum processor. However, existing in situ methods are limited by the coherence time of the qubits, which makes them inapplicable on long time scales. In this talk, we present an in situ method to characterize linear distortions in the flux line for time ranges going beyond the coherence time of the qubits. Moreover, we present a method to calibrate a higher-order infinite impulse response (IIR) filter based on a single series of measurements, enabling an efficient tune-up of corrections on time scales ranging over three orders of magnitude. We confirm the successful compensation of the long-time distortions in dedicated verification measurements and by measuring two-qubit gate fidelities in long randomized benchmarking sequences.