  Superconducting qubits are a promising platform for building fault-tolerant
quantum computers, with recent achievement showing the suppression of logical
error with increasing code size. However, leakage into non-computational
states, a common issue in practical quantum systems including superconducting
circuits, introduces correlated errors that undermine QEC scalability. Here, we
propose and demonstrate a leakage reduction scheme utilizing tunable couplers,
a widely adopted ingredient in large-scale superconducting quantum processors.
Leveraging the strong frequency tunability of the couplers and stray
interaction between the couplers and readout resonators, we eliminate state
leakage on the couplers, thus suppressing space-correlated errors caused by
population propagation among the couplers. Assisted by the couplers, we further
reduce leakage to higher qubit levels with high efficiency (98.1%) and low
error rate on the computational subspace (0.58%), suppressing time-correlated
errors during QEC cycles. The performance of our scheme demonstrates its
potential as an indispensable building block for scalable QEC with
superconducting qubits.
