Fast, unconditional reset and leakage reduction in fixed-frequency transmon qubits toward fault-tolerant quantum computing*

Quantum error correction codes, such as the surface code, require rapid readout and reset of the ancilla qubits at the end of each cycle. During the operation, data and ancilla qubits are prone to accumulate leakage outside the computational subspace and compromise the relevant quantum information. Therefore, a fast and unconditional reset of the qubits is crucial in reducing cycle runtime and improving algorithm fidelity. Our platform comprises fixed-frequency transmon qubits pair-wise connected via tunable couplers. The fixed-frequency qubits have improved coherence times compared to tunable qubits; however, they lack the capabilities to run currently proposed reset protocols without additional circuit elements. To address these challenges, we present an unconditional reset scheme of the fixed-frequency qubits by sending a fast flux pulse to the existing tunable couplers between the qubits. We further investigate the effect on two-qubit gates following the reset protocol to fully characterize the impact of coupler reset on gate operations. In addition to a reduction in ancilla initialization errors, such an unconditional reset protocol circumvents the delays due to signal processing and, thus, enables a significant increase in the number of possible error correction cycles per computation.