NBIA/QDev Seminar: Simon Gustavosson

Improving quantum gate fidelities in superconducting qubits

We present a new method for determining pulse imperfections and improving the single-gate fidelity in a superconducting qubit. By applying consecutive positive and negative pi pulses, we amplify the qubit evolution due to microwave pulse distortions, which causes the qubit state to rotate around an axis perpendicular to the intended rotation axis. Measuring these rotations as a function of pulse period allows us to reconstruct the shape of the microwave pulse arriving at the sample. Using the extracted response to predistort the input signal, we are able to improve the pulse shapes and to reach an average single-qubit gate fidelity higher than 99.8%.

We also implement dynamical decoupling techniques to mitigate noise in the coupling between a superconducting flux qubit and a microscopic two-level system, thereby improving the coherence time of the entangled state. The coherence is further enhanced when applying multiple refocusing pulses. The results highlight the potential of decoupling techniques for improving two-qubit gate fidelities, an essential prerequisite for implementing fault-tolerant quantum computing.