Optimal Control of Superconducting Qubits

Fast and accurate two-qubit gates are a key requirement to perform complex algorithms on current quantum computers. Ideally, the duration of the gate should be much shorter than the coherence time of the system. However, shorter gates can result in unwanted loss of states from the computational subspace. Optimal control theory aims to design fast control pulses suppressing such side effects of the driving field. Even with an accurately calibrated system model, control pulses require a tune-up to accommodate for parameter-drifts and model-inaccuracies. Here we present our work on optimal control algorithms, using a closed loop approach with direct experimental feedback to design complex pulses. This approach avoids errors from an inaccurate initial system model and uses information gained during the pulse optimization to update the model. We improve the interplay of control instruments and multidimensional optimization algorithms to speed up the tune-up of feedback-loops, reducing evaluation times from several minutes to a few seconds . With these measures, we achieve a reduction of gate errors by more than a factor of five for short pulses.