All-microwave leakage reduction units for quantum error correction with superconducting transmon qubits

Minimizing leakage from computational states is a challenge when using many-level systems like superconducting quantum circuits as qubits. We realize and extend the quantum-hardware-efficient, all-microwave leakage reduction unit (LRU) for transmons in a circuit QED architecture proposed by Battistel et al. This LRU effectively reduces leakage in the second- and third-excited transmon states with up to $99\mathrm{\%}$ efficacy in $220\mathrm{n}\mathrm{s}$, with minimum impact on the qubit subspace. As a first application in the context of quantum error correction, we demonstrate the ability of multiple simultaneous LRUs to reduce the error detection rate and to suppress leakage buildup within $1\mathrm{\%}$ in data and ancilla qubits over 50 cycles of a weight-2 parity measurement.