  Quantum computers will require quantum error correction to reach the low
error rates necessary for solving problems that surpass the capabilities of
conventional computers. One of the dominant errors limiting the performance of
quantum error correction codes across multiple technology platforms is leakage
out of the computational subspace arising from the multi-level structure of
qubit implementations. Here, we present a resource-efficient universal leakage
reduction unit for superconducting qubits using parametric flux modulation.
This operation removes leakage down to our measurement accuracy of $7\cdot
10^{-4}$ in approximately $50\, \mathrm{ns}$ with a low error of $2.5(1)\cdot
10^{-3}$ on the computational subspace, thereby reaching durations and
fidelities comparable to those of single-qubit gates. We demonstrate that using
the leakage reduction unit in repeated weight-two stabilizer measurements
reduces the total number of detected errors in a scalable fashion to close to
what can be achieved using leakage-rejection methods which do not scale. Our
approach does neither require additional control electronics nor on-chip
components and is applicable to both auxiliary and data qubits. These benefits
make our method particularly attractive for mitigating leakage in large-scale
quantum error correction circuits, a crucial requirement for the practical
implementation of fault-tolerant quantum computation.
