High-fidelity junction readout via nonperturbative cross-Kerr coupling with intrinsic purcell protection

Fast, high-fidelity, and quantum non-demolition (QND) qubit readout remains a major bottleneck for scalable superconducting quantum processors. We present the experimental realization of junction readout [1], a simple circuit that implements a nonperturbative cross-Kerr interaction by adding a Josephson junction in parallel with the conventional capacitive coupling between a transmon and its readout resonator. This architecture provides intrinsic Purcell protection and enhanced resilience against measurement-induced state transitions (MIST), allowing operation at higher photon number. We achieve 99.4% readout fidelity in 68ns and a QND fidelity of 98.4%, without using external Purcell filters or parametric amplifiers. These results establish junction readout as a scalable and promising alternative to dispersive readout, offering fast, high-fidelity operation with minimal circuit modification.

[1] A. A. Chapple, O. Benhayoune-Khadraoui, S. Richer, and A. Blais, Balanced cross-kerr coupling for superconducting qubit readout (2025),    arXiv:2501.09010