Hardware-Efficient Leakage-Reduction Scheme for Quantum Error Correction with Superconducting Transmon Qubits

Leakage outside of the qubit computational subspace poses a threatening challenge to quantum error correction (QEC). We propose scheme using two leakage-reduction units (LRUs) that mitigate these issues for transmon-based surface code, without requiring an overhead in terms hardware or QEC-cycle time as previous proposals. For data qubits we consider microwave drive transfer leakage readout resonator, where it quickly decays, ensuring this negligibly affects coherence within realistic system parameters. ancilla apply $|1\rangle\leftrightarrow|2\rangle$ $\pi$ pulse conditioned on measurement outcome. Using density-matrix simulations distance-3 code show average lifetime is reduced almost 1 QEC cycle, even when LRUs are implemented with limited fidelity. Furthermore, leads significant reduction logical rate. This LRU opens prospect near-term scalable demonstrations.