Diagnosing errors in superconducting two-qubit gates using continuous measurements

Improving the fidelity of two-qubit gates is essential for performing quantum algorithms on superconducting quantum computers. By using continuous weak measurements of superconducting qubits throughout a two-qubit gate, we can reveal applied pulse shapes and coherent gate errors with high time resolution. We treat potential coherent gate errors as unknown time-dependent parameters in the Hamiltonian and estimate them by fitting the measured voltage records to a master equation. We experimentally demonstrate this method on imperfect single-qubit and parametric entangling two-qubit gates, and show that we can accurately reconstruct errors such as over-rotations and leakage out of the computational subspace. The gate fidelity can be improved by designing a correction pulse that cancels out the reconstructed error.