Mitigation of Measurement-Induced State Transitions via a Fast-Load and Fast-Clear Readout

In superconducting quantum processors, high-fidelity readout is typically achieved via dispersive coupling between a qubit and a resonator. However, increasing the readout power to improve the signal-to-noise ratio (SNR) often triggers measurement-induced state transitions (MIST), where the qubit is inadvertently excited out of the computational subspace. This study investigates how to mitigate these transitions by engineering the temporal shape of the readout pulse. The authors use a semiclassical driven transmon model and Floquet branch analysis to map how the resonator's photon population trajectory influences transition probabilities, specifically focusing on the diabaticity of the passage through avoided crossings.

The researchers show that MIST is highly sensitive to the resonator's photon ring-up and ring-down dynamics. By implementing a three-step pulse—a fast-load segment to reach steady-state quickly, a steady-state hold, and a fast-clear segment to rapidly deplete the resonator—they force the system to traverse critical avoided crossings at higher speeds. This increased speed ensures a more diabatic passage, which effectively suppresses the probability of the qubit transitioning to unwanted states. The experimental results, validated on fixed-frequency transmon devices, confirm that this straightforward pulse shaping consistently minimizes total readout error compared to standard square pulses, providing a robust solution for scalable quantum architectures.

As quantum processors scale, the demand for rapid, high-fidelity syndrome extraction becomes critical. Traditional methods to mitigate MIST often require complex, real-time feedback loops or sophisticated hardware modifications. This study offers a practical, software-defined alternative that improves the trade-off between readout speed and QND (quantum non-demolition) preservation. By simply adjusting the pulse profile, researchers can enhance the reliability of their measurements without adding significant overhead to the control electronics.