Characterizing charge-parity detection based on an offset-charge-tunable transmonqubit via randomized benchmarking

Quantum parity detectors are promising for rare-event searches owing to their theoretical sensitiv-ity at the meV energy scale. Their efficacy, however, hinges on high-fidelity charge-parity detection.Superconducting qubits are compelling platforms for such detectors, given their high-fidelity quan-tum operations. In this work, we realize high-fidelity mapping of charge-parity states onto qubitstates using an offset-charge-tunable transmon qubit and efficiently characterize the fidelity of thecharge-parity detection via randomized benchmarking. Specifically, a gate control line is appliedto control offset charge, allowing us to achieve the single-qubit gate fidelity up to 99.96%. Wecombine a net-zero-based pulse on the gate line with a spin-echo-based sequence to realize charge-parity mapping, achieving a fidelity of 99.37%. Then, we demonstrate continuous monitoring of thecharge-parity state with over 93.4% fidelity at a 4-μs sampling interval. Finally, an error analysisof charge-parity detection is performed, and it is found that qubit readout is currently the largestsource of error. We believe this work lays the foundation for future exploration of ultra-low energyparticles.