Removing leakage-induced correlated errors in superconducting quantum error correction - Experiment

Removing excitations from non-computational states is an essential challenge in achieving stable quantum error correction. We present the experimental realisation of a multilevel reset protocol that produces the ground state with an error below 5e-3 within 250 ns, starting from the qubit being in any of the first three excited levels. We deploy this gate in the context of the bit-flip stabilizer code, and demonstrate a significant reduction in the population of leakage built up over time while running the code. We show that the removal of leakage reduces the incidence of time-correlated errors, and significantly improves the logical error rate as well as the error suppression factor Λ. This provides the first demonstration of error suppression that is stable over large numbers of rounds.