Diagnosing Errors in Qubit Gates Using Continuous Measurements (Theory)*

We propose a method of characterizing systematic, time-dependent errors in single- and two-qubit gates using continuous weak measurements. By tracking the qubit state throughout the gate operation, we can estimate the time dynamics of unknown parameters in the Hamiltonian. Measurement backaction affects the gate dynamics, but not the parameter estimation. We demonstrate this method on imperfect gates using simulated data, and show that we can reconstruct arbitrary coherent errors, as well as leakage to an unwanted third level.