Characterization and Correction of 0.1%-Level Flux Pulse Distortion in Tunable Qubits

Flux pulse distortion is a source of control error for entangling gates implemented on tunable qubits with tunable couplers. In particular, the system's non-ideal step response introduces interference between gate operations, making gate tune-up cumbersome. Flux pulse distortion can be characterized using a Ramsey-based "cryoscope" measurement. This approach allows accurate measurement of step response; however, the measurement length is limited by the dephasing time of the qubit. We have improved the cryoscope protocol by using frame rotation and a rolling window, which enables us to characterize the distortion of flux pulses over a much longer timescale than the dephasing time of the qubit. We validated our approach using a commercial dilution refrigerator with standard semirigid coax wiring. We found a slow-settling response in the flux step at the 0.1% level over the course of 100 μs, which we then corrected. Our results provide insight into the frequency-dependent transfer function of standard coax wiring and will serve as an essential tool to realize high-fidelity gate operations with frequency-tunable qubits and couplers.