Diagnosing Errors in Quantum Gates Using Continuous Measurements*

We use continuous weak measurements to track a quantum gate operation as it develops in time, which allows us to identify the full time-dependent dynamics of any systematic errors. We account for measurement backaction such that it has no bearing on the error estimate. This diagnostic method is tested on imperfect single- and two-qubit gates, and is shown to accurately extract known time-dependent error pulses. This offers significant advantages over traditional quantum process tomography, as it provides the ability to identify the origin and nature of any deviations from the desired evolution, which can give insight into how to modify the gate pulse.