XY crosstalk in a 25-qubit flip-chip superconducting quantum processor

Abstract

Superconducting circuits are among the most promising physical platforms for building a quantum computer. Scaling superconducting quantum processors to thousands and eventually millions of qubits requires not only high-fidelity quantum gates but also the ability to calibrate and execute them simultaneously. XY crosstalk, resulting from electromagnetic coupling between neighbouring qubits and control lines, is one of the main obstacles to this goal, as it introduces coherent errors and leakage that degrade gate performance during parallel operations. While several crosstalk characterisation and mitigation methods have been demonstrated in superconducting quantum processors, a systematic and scalable approach for integrating them into an automated calibration workflow remains a practical challenge, especially for fixed-frequency architectures, where qubit frequencies cannot be tuned away from one another. In this thesis, an experimental protocol has been developed to measure and compensate XY crosstalk at the signal level in Chalmers’ 25-qubit flip-chip fixed-frequency transmon quantum processor. It is shown that the amplitude and phase of the complex XY crosstalk coefficient can be extracted separately for both f01 −f01 and f01 −f12 crosstalk, and that these coefficients can be used for effective active compensation to improve simultaneous gate performance. Specifically, randomised benchmarking shows that by applying the crosstalk compensation scheme, the average single-qubit gate error during simultaneous measurements can be reduced to the same level as the value obtained from isolated operations. The same improvement in leakage rate is also observed for qubit pairs affected by f01 − f12 crosstalk. These results prove that crosstalk-induced coherent errors and leakage errors can be completely removed using calibration and control strategies and represent a step towards reliable parallel operations in large-scale quantum processors.