Complete leakage removal in the surface code on superconducting qubits - Part 2

Removing excitations from non-computational (leakage) states is an essential challenge in achieving stable quantum error correction. We present the experimental realization of a distance-3 surface code with leakage removed on all qubits in each error correction cycle. We show that leakage populations over all qubits stabilize at 0.1%, and remain low as the code is extended in time. Furthermore, this leakage removal strategy also stabilizes the rate of detected errors and improves logical performance, resolving a major challenge for the field. Finally, when this leakage removal strategy is incorporated into a distance-21 repetition code, we minimize differences in logical performance under the influence of injected leakage errors when compared to injected Pauli errors, confirming that the correlated nature of leakage errors has been successfully mitigated.