Fig. 2. Protecting entanglement from bit flips with repeated ZZ checks.

(A) The quantum circuit of Fig. 1A extended with M rounds of repeated ZZ checks. (B) Fidelity to ∣Φ⁺⟩ as a function of M. “No error” postselects the runs in which no bit flip is detected. “Final” applies PFU based on the last two outcomes (equivalent to MWPM). “First” uses the first parity outcome only. “Idling DD” are Bell states evolving under dynamical decoupling only (quantum circuit in fig. S4). (C) Corresponding 〈X ⊗ X〉. “Final” coincides with “First”. (D) Corresponding 〈Z ⊗ Z〉. The weak degradation observed for “Final” is the hallmark of leakage. Curves in (B) to (D) are best fits of a simple exponential decay.