Improved error correction in a superconducting quantum processor with leakage reduction units built into qubit measurement

Leakage to non-computational states is a source of correlated errors in both time and space that limits the effectiveness of quantum error correction (QEC) with superconducting circuits. We present and experimentally demonstrate a high-fidelity, leakage reduction unit (LRU) operating concurrently with transmon measurement without incurring time overhead. Adapted from double-drive reset of population (DDROP), the protocol utilizes simultaneous drives on the transmon and its readout resonator, leveraging the dispersive shift to create a directional process that returns the transmon to the computational subspace. The LRU achieves a 98.4% leakage removal fraction without compromising the computational-state assignment fidelity (99.2%). Furthermore, a high-fidelity three-level readout is also achieved with this protocol. We combine LRUs, three-level readouts and neural-network decoding to successfully suppress logical error rates in both memory and stability QEC experiments without any post-selection.