A hardware-efficient leakage-reduction scheme for a transmon-based surface code

Leakage in superconducting transmon qubits poses a threat to quantum error correction (QEC) as leakage errors cannot be decomposed into standard Pauli errors. Furthermore, leakage can last for many QEC cycles, propagating many correlated errors through the code. Leakage-reduction units can shorten the average leakage lifetime by bringing the qubit back to the computational subspace. However, many of the units investigated so far for transmons either require changes in hardware or increase the QEC-cycle time.
We use a microwave pulse to transfer leakage from data-qubits to their readout resonator, where it quickly decays, without significantly affecting the coherence of the computational states. For ancilla qubits we use a |1>-|2> rotation conditioned on the measurement outcome.
First, we show the viability of these schemes using achievable experimental parameters, fitting within the QEC-cycle time. Second, via density-matrix simulations of the distance-3 surface code, we show that the average leakage lifetime is drastically shortened and that the logical fidelity is improved. This hardware-efficient scheme can be used in near-term QEC demonstrations and is scalable for large-scale QEC.