Flexible Readout and Unconditional Reset for Superconducting Multiqubit Processors with Tunable Purcell Filters.

Abstract

Achieving high-fidelity qubit readout and reset while maintaining qubit coherence is crucial for quantum error correction and advanced quantum algorithms. Here, we design and experimentally demonstrate a scalable architecture based on frequency-tunable nonlinear Purcell filters, which enables flexible readout and rapid unconditional reset of multiple superconducting qubits. Our readout protocol dynamically adjusts the effective linewidth of the readout resonator through a tunable Purcell filter, optimizing the signal-to-noise ratio during measurement while suppressing photon noise during idle periods. Combined with a multilevel readout protocol, we achieve the highest readout fidelity of 99.3% without any quantum-limited amplifier, even with a small dispersive shift. Moreover, by leveraging a reset channel formed via the adjacent coupling between the filter and the coupler, we realize unconditional qubit reset of both leakage-induced |2⟩ and |1⟩ states within 200 ns and reset of the |1⟩ state alone within 75 ns, with error rates ≤1%. The filter also mitigates both photon-induced dephasing and the Purcell effect, thereby preserving qubit coherence. This scalable Purcell filter architecture shows exceptional performance in qubit readout, reset, and protection, marking it as a promising hardware component for advancing fault-tolerant quantum computing systems.