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.

This is joint work with Debmalya Das, Karthik Siva, Gerwin Koolstra, William P. Livingston, Larry Chen, Christian Juenger, Noah J. Stevenson, Ravi K. Naik, David I. Santiago, Irfan Siddiqi, and Andrew N. Jordan. This work was supported by US Army Research Office grant no. W911NF-18-10178.